Galactic Nuclei (galactic + nucleus)

Distribution by Scientific Domains

Kinds of Galactic Nuclei

  • active galactic nucleus
  • central active galactic nucleus
  • radio-loud active galactic nucleus


  • Selected Abstracts


    On the anomalous silicate absorption feature of the prototypical Seyfert 2 galaxy NGC 1068

    MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY: LETTERS (ELECTRONIC), Issue 1 2010
    M. Khler
    ABSTRACT The first detection of the silicate absorption feature in Active Galactic Nuclei (AGNs) was made at 9.7 ,m for the prototypical Seyfert 2 galaxy NGC 1068 35 yr ago, indicating the presence of a large column of silicate dust in the line of sight to the nucleus. It is now well recognized that type 2 AGNs exhibit prominent silicate absorption bands, while the silicate bands of type 1 AGNs appear in emission. More recently, using the Mid-Infrared Interferometric Instrument on the Very Large Telescope Interferometer, Jaffe et al. for the first time spatially resolved the parsec-sized dust torus around NGC 1068 and found that the 10 ,m silicate absorption feature of the innermost hot component exhibits an anomalous profile differing from that of the interstellar medium and that of common olivine-type silicate dust. While they ascribed the anomalous absorption profile to gehlenite (Ca2Al2SiO7, a calcium aluminium silicate species), we propose a physical dust model and argue that, although the presence of gehlenite is not ruled out, the anomalous absorption feature mainly arises from silicon carbide. [source]


    General Relativity effects and line emission

    ASTRONOMISCHE NACHRICHTEN, Issue 10 2006
    G. Matt
    Abstract General Relativity effects (gravitational redshift, light bending, ,) strongly modify the characteristics of the lines emitted close to the Black Hole in Active Galactic Nuclei and Galactic Black Hole systems. These effects are reviewed and illustrated, with particular emphasis on line emission from the accretion disc. Methods, based on the iron line, to measure the two astrophysically relevant parameters of a Black Hole, the mass and spin, are briefly discussed. ( 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]


    A New X-Ray Flare from the Galactic Nucleus Detected with XMM-Newton

    ASTRONOMISCHE NACHRICHTEN, Issue S1 2003
    A. Goldwurm
    Abstract The compact radio source Sgr A*, believed to be the counterpart of the massive black hole at the Galactic nucleus, was observed to undergo rapid and intense flaring activity in X-rays with Chandra in October 2000. We report here the detection with XMM-Newton EPIC cameras of the early phase of a similar X-ray flare from this source, which occurred on 2001 September 4. The source 2,10 keV luminosity increased by a factor ,20 to reach a level of 4 1034 erg s,1 in a time interval of about 900 s, just before the end of the observation. The data indicate that the source spectrum was hard during the flare and can be described by simple power law of slope ,0.7. This XMM-Newton observation confirms the results obtained by Chandra, suggests that, in Sgr A* , rapid and intense X-ray flaring is not a rare event and therefore sets some constraints on the emission mechanism models proposed for this source. [source]


    Feedback under the microscope , I. Thermodynamic structure and AGN-driven shocks in M87

    MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2010
    E. T. Million
    ABSTRACT We present the first in a series of papers discussing the thermodynamic properties of M87 and the central regions of the Virgo Cluster in unprecedented detail. Using a deep Chandra exposure (574 ks), we present high-resolution thermodynamic maps created from the spectra of ,16 000 independent regions, each with ,1000 net counts. The excellent spatial resolution of the thermodynamic maps reveals the dramatic and complex temperature, pressure, entropy and metallicity structure of the system. The ,X-ray arms', driven outwards from M87 by the central active galactic nuclei (AGN), are prominent in the brightness, temperature and entropy maps. Excluding the ,X-ray arms', the diffuse cluster gas at a given radius is strikingly isothermal. This suggests either that the ambient cluster gas, beyond the arms, remains relatively undisturbed by AGN uplift or that conduction in the intracluster medium (ICM) is efficient along azimuthal directions, as expected under action of the heat-flux-driven buoyancy instability (HBI). We confirm the presence of a thick (,40 arcsec or ,3 kpc) ring of high-pressure gas at a radius of ,180 arcsec (,14 kpc) from the central AGN. We verify that this feature is associated with a classical shock front, with an average Mach number M= 1.25. Another, younger shock-like feature is observed at a radius of ,40 arcsec (,3 kpc) surrounding the central AGN, with an estimated Mach number M, 1.2. As shown previously, if repeated shocks occur every ,10 Myr, as suggested by these observations, then AGN-driven weak shocks could produce enough energy to offset radiative cooling of the ICM. A high significance enhancement of Fe abundance is observed at radii 350,400 arcsec (27,31 kpc). This ridge is likely formed in the wake of the rising bubbles filled with radio-emitting plasma that drag cool, metal-rich gas out of the central galaxy. We estimate that at least ,1.0 106 solar masses of Fe has been lifted and deposited at a radius of 350,400 arcsec; approximately the same mass of Fe is measured in the X-ray bright arms, suggesting that a single generation of buoyant radio bubbles may be responsible for the observed Fe excess at 350,400 arcsec. [source]


    Feedback under the microscope , II.

    MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2010
    Heating, gas uplift, mixing in the nearest cluster core
    ABSTRACT Using a combination of deep (574 ks) Chandra data, XMM,Newton high-resolution spectra and optical H,+[N ii] images, we study the nature and spatial distribution of the multi-phase plasma in M87. Our results provide direct observational evidence of ,radio-mode' active galactic nuclei (AGN) feedback in action, stripping the central galaxy of its lowest entropy gas and therefore preventing star formation. This low entropy gas was entrained with and uplifted by the buoyantly rising relativistic plasma, forming long ,arms'. A number of arguments suggest that these arms are oriented within 15,30 of our line-of-sight. The mass of the uplifted gas in the arms is comparable to the gas mass in the approximately spherically symmetric 3.8 kpc core, demonstrating that the AGN has a profound effect on its immediate surroundings. The coolest X-ray emitting gas in M87 has a temperature of ,0.5 keV and is spatially coincident with H,+[N ii] nebulae, forming a multi-phase medium where the cooler gas phases are arranged in magnetized filaments. We place strong upper limits of 0.06 M, yr,1 (at 95 per cent confidence) on the amount of plasma cooling radiatively from 0.5 to 0.25 keV and show that a uniform, volume-averaged heating mechanism could not be preventing the cool gas from further cooling. All of the bright H, filaments in M87 appear in the downstream region of the <3 Myr old shock front, at smaller radii than ,0.6 arcmin. We suggest that shocks induce shearing around the filaments, thereby promoting mixing of the cold gas with the ambient hot intra-cluster medium (ICM) via instabilities. By bringing hot thermal particles into contact with the cool, line-emitting gas, mixing can supply the power and ionizing particles needed to explain the observed optical spectra. Furthermore, mixing of the coolest X-ray emitting plasma with the cold optical line-emitting filamentary gas promotes efficient conduction between the two phases, allowing non-radiative cooling which could explain the lack of X-ray gas with temperatures under 0.5 keV. [source]


    Swimming against the current: simulations of central AGN evolution in dynamic galaxy clusters

    MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2010
    Brian J. Morsony
    ABSTRACT We present a series of three-dimensional hydrodynamical simulations of central active galactic nuclei (AGN)-driven jets in a dynamic, cosmologically evolved galaxy cluster. Extending previous work, we study jet powers ranging from Ljet= 1044 erg s,1 to Ljet= 1046 erg s,1 and in duration from 30 to 200 Myr. We find that large-scale motions of cluster gas disrupt the AGN jets, causing energy to be distributed throughout the centre of the cluster, rather than confined to a narrow angle around the jet axis. Disruption of the jet also leads to the appearance of multiple disconnected X-ray bubbles from a long-duration AGN with a constant luminosity. This implies that observations of multiple bubbles in a cluster are not necessarily an expression of the AGN duty cycle. We find that the ,sphere of influence' of the AGN, the radial scale within which the cluster is strongly affected by the jet, scales as R,L1/3jet. Increasing the duration of AGN activity does not increase the radius affected by the AGN significantly, but does change the magnitude of the AGN's effects. How an AGN delivers energy to a cluster will determine where that energy is deposited: a high luminosity is needed to heat material outside the core of the cluster, while a low-luminosity, long-duration AGN is more efficient at heating the inner few tens of kpc. [source]


    Enhanced star formation in narrow-line Seyfert 1 active galactic nuclei revealed by Spitzer

    MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2010
    E. Sani
    ABSTRACT We present new low-resolution Spitzer mid-infrared spectroscopy of a sample of 20 ROSAT -selected local narrow-line Seyfert 1 galaxies (NLS1s). We detect strong active galactic nucleus (AGN) continuum in all and clear polycyclic aromatic hydrocarbon (PAH) emission in 70 per cent of the sources. The 6.2 ,m PAH luminosity spans three orders of magnitude, from ,1039 to ,1042 erg s,1, providing strong evidence for intense ongoing star formation in the circumnuclear regions of these sources. Using the Infrared Spectrograph/Spitzer archive, we gathered a large number of additional NLS1s and their broad-line counterparts (BLS1s) and constructed NLS1 and BLS1 subsamples to compare them in various ways. The comparison shows a clear separation according to full width at half-maximum (H,) [FWHM(H,)] such that objects with narrower broad H, lines are the strongest PAH emitters. We test this division in various ways trying to remove biases due to luminosity and aperture size. Specifically, we find that star formation activity around NLS1 AGN is larger than around BLS1 of the same AGN luminosity. The above result seems to hold over the entire range of distance and luminosity. Moreover, the star formation rate is higher in low black hole mass and high L/LEdd systems indicating that black hole growth and star formation are occurring simultaneously. [source]


    Red star-forming and blue passive galaxies in clusters

    MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2009
    Smriti Mahajan
    ABSTRACT We explore the relation between colour (measured from photometry) and specific star formation rate (derived from optical spectra obtained by the Sloan Digital Sky Survey Data Release 4) of over 6000 galaxies (Mr,,20.5) in and around (<3 r200) low-redshift (z < 0.12) Abell clusters. Even though, as expected, most red sequence galaxies have little or no ongoing star formation, and most blue galaxies are currently forming stars, there are significant populations of red star-forming and blue passive galaxies. This paper examines various properties of galaxies belonging to the latter two categories, to understand why they deviate from the norm. These properties include morphological parameters, internal extinction, spectral features such as EW(H,) and the 4000 break, and metallicity. Our analysis shows that the blue passive galaxies have properties very similar to their star-forming counterparts, except that their large range in H, equivalent width indicates recent truncation of star formation. The red star-forming galaxies fall into two broad categories, one of them being massive galaxies in cluster cores dominated by an old stellar population, but with evidence of current star formation in the core (possibly linked with active galactic nuclei). For the remaining red star-forming galaxies, it is evident from spectral indices, stellar and gas-phase metallicities and mean stellar ages that their colours result from the predominance of a metal-rich stellar population. Only half of the red star-forming galaxies have extinction values consistent with a significant presence of dust. The implication of the properties of these star-forming galaxies on environmental studies, like that of the Butcher,Oemler effect, is discussed. [source]


    A QSO host galaxy and its Ly, emission at z= 6.43,

    MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2009
    Tomotsugu Goto
    ABSTRACT Host galaxies of highest redshift quasi-stellar objects (QSOs) are of interest; they provide us with a valuable opportunity to investigate physics relevant to the starburst,active galactic nuclei (AGN) connection at the earliest epoch of the Universe, with the most luminous black holes. Here, we report an optical detection of an extended structure around a QSO at z= 6.43 in deep z,- and zr -band images of the Subaru/Suprime-Cam. Our target is CFHQS J2329-0301 (z= 6.43), the highest redshift QSO currently known. We have carefully subtracted a point spread function (PSF) constructed using nearby stars from the images. After the PSF (QSO) subtraction, a structure in the z, band extends more than 4 arcsec on the sky (Re= 11 kpc), and, thus, is well resolved (16, detection). The PSF-subtracted zr -band structure is in a similar shape to that in the z, band, but less significant with a 3, detection. In the z, band, a radial profile of the QSO+host shows a clear excess over that of the averaged PSF in 0.8,3 arcsec radius. Since the z, band includes a Ly, emission at z= 6.43, we suggest the z, flux is a mixture of the host (continuum light) and its Ly, emission, whereas the zr -band flux is from the host. Through a SED modelling, we estimate 40 per cent of the PSF-subtracted z,-band light is from the host (continuum) and 60 per cent is from Ly, emission. The absolute magnitude of the host is M1450=,23.9 (cf. M1450=,26.4 for the QSO). A lower limit of the SFR(Ly,) is 1.6 M, yr,1 with stellar mass ranging from 6.2 108 to 1.1 1010 M, when 100 Myr of age is assumed. The detection shows that a luminous QSO is already harboured by a large, star-forming galaxy in the early Universe only after ,840 Myr after the big bang. The host may be a forming giant galaxy, co-evolving with a super-massive black hole. [source]


    Surprising evolution of the parsec-scale Faraday Rotation gradients in the jet of the BL Lac object B1803+784

    MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2009
    M. Mahmud
    ABSTRACT Several multifrequency polarization studies have shown the presence of systematic Faraday Rotation gradients across the parsec-scale jets of active galactic nuclei, taken to be due to the systematic variation of the line-of-sight component of a helical magnetic (B) field across the jet. Other studies have confirmed the presence and sense of these gradients in several sources, thus providing evidence that these gradients persist over time and over large distances from the core. However, we find surprising new evidence for a reversal in the direction of the Faraday Rotation gradient across the jet of B1803+784, for which multifrequency polarization observations are available at four epochs. At our three epochs and the epoch of Zavala & Taylor, we observe transverse rotation measure (RM) gradients across the jet, consistent with the presence of a helical magnetic field wrapped around the jet. However, we also observe a ,flip' in the direction of the gradient between 2000 June and 2002 August. Although the origins of this phenomenon are not entirely clear, possibly explanations include (i) the sense of rotation of the central supermassive black hole and accretion disc has remained the same, but the dominant magnetic pole facing the Earth has changed from north to south, (ii) a change in the direction of the azimuthal B field component as a result of torsional oscillations of the jet and (iii) a change in the relative contributions to the observed RMs of the ,inner' and ,outer' helical fields in a magnetic-tower model. Although we cannot entirely rule out the possibility that the observed changes in the RM distribution are associated instead with changes in the thermal-electron distribution in the vicinity of the jet, we argue that this explanation is unlikely. [source]


    Radio constraints on the volume filling factors of AGN winds

    MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2009
    A. J. Blustin
    ABSTRACT The calculation of mass outflow rates of active galactic nuclei (AGN) winds is of great importance in understanding the role that such winds play in AGN-galaxy feedback processes. The mass outflow rates are, however, difficult to estimate since the volume filling factors of the winds are unknown. In this paper, we use constraints imposed by the observed radio emission to obtain upper limits to the volume filling factors of wind components in certain nearby AGN. We do this by predicting the 1.4 GHz radio flux densities emitted by those components, assuming a uniform wind, and then comparing these with the observed flux densities for each AGN at this frequency. We find that the upper limits to the volume filling factors are in the range of 10,4,0.5. [source]


    Mapping low- and high-density clouds in astrophysical nebulae by imaging forbidden line emission,

    MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2009
    J. E. Steiner
    ABSTRACT Emission line ratios have been essential for determining physical parameters such as gas temperature and density in astrophysical gaseous nebulae. With the advent of panoramic spectroscopic devices, images of regions with emission lines related to these physical parameters can, in principle, also be produced. We show that, with observations from modern instruments, it is possible to transform images taken from density-sensitive forbidden lines into images of emission from high- and low-density clouds by applying a transformation matrix. In order to achieve this, images of the pairs of density-sensitive lines as well as the adjacent continuum have to be observed and combined. We have computed the critical densities for a series of pairs of lines in the infrared, optical, ultraviolet and X-rays bands, and calculated the pair line intensity ratios in the high- and low-density limit using a four- and five-level atom approximation. In order to illustrate the method, we applied it to Gemini Multi-Object Spectrograph (GMOS) Integral Field Unit (GMOS-IFU) data of two galactic nuclei. We conclude that this method provides new information of astrophysical interest, especially for mapping low- and high-density clouds; for this reason, we call it ,the ld/hd imaging method'. [source]


    Shock heating in the group atmosphere of the radio galaxy B2 0838+32A

    MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2008
    Nazirah N. Jetha
    ABSTRACT We present Chandra and radio observations, and analysis of Sloan Digital Sky Survey data, of the radio galaxy B2 0838+32A (4C 32.26) and its environment. The radio galaxy is at the centre of a nearby group that has often been identified with the cluster Abell 695, but we argue that the original Abell cluster is likely to be an unrelated and considerably more distant system. The radio source is a restarting radio galaxy and, using our Chandra data, we argue that the currently active lobes are expanding supersonically, driving a shock with Mach number 2.4+1.0,0.5 into the interstellar medium. This would be only the third strong shock round a young radio source to be discovered, after Centaurus A and NGC 3801. However, in contrast to both these systems, the host galaxy of B2 0838+32A shows no evidence for a recent merger, while the active galactic nuclei (AGN) spectrum shows no evidence for the dusty torus that would imply a large reservoir of cold gas close to the central black hole. On the contrary, the AGN spectrum is of a type that has been associated with the presence of a radiatively inefficient accretion flow that could be controlled by an AGN heating and subsequent cooling of the hot, X-ray emitting gas. If correct, this means that B2 0838+32A is the first source in which we can directly see entropy-increasing processes (shocks) driven by accretion from the hot phase of the interstellar medium. [source]


    A survey for redshifted molecular and atomic absorption lines , II.

    MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2008
    3 Parkes quarter-Jansky flat-spectrum sample, Associated H i, millimetre lines in the z
    ABSTRACT We present the results of a z, 2.9 survey for H i 21-cm and molecular absorption in the hosts of radio quasars using the Giant Metrewave Radio Telescope and the Tidbinbilla 70-m telescope. Although the atomic gas has been searched to limits capable of detecting most known absorption systems, no H i was detected in any of the 10 sources. Previously published searches, which are overwhelmingly at redshifts of z, 1, exhibit a 42 per cent detection rate (31 out of 73 sources), whereas the inclusion of our survey yields a 17 per cent detection rate (two out of 12 sources) at z > 2.5. We therefore believe that our high-redshift selection is responsible for our exclusive non-detections, and find that at ultraviolet (UV) luminosities of LUV, 1023 W Hz,1, 21-cm absorption has never been detected. We also find this to not only apply to our targets, but also those at low redshift exhibiting similar luminosities, giving zero detections out of a total of 16 sources over z= 0.24 to 3.8. This is in contrast to the LUV, 1023 W Hz,1 sources where there is a near 50 per cent detection rate of 21-cm absorption. The mix of 21-cm detections and non-detections is currently attributed to orientation effects, where according to unified schemes of active galactic nuclei, 21-cm absorption is more likely to occur in sources designated as radio galaxies (type 2 objects, where the nucleus is viewed through dense obscuring circumnuclear gas) than in quasars (type 1 objects, where we have a direct view to the nucleus). However, due to the exclusively high UV luminosities of our targets it is not clear whether orientation effects alone can wholly account for the distribution, although there exists the possibility that the large luminosities are indicative of a changing demographic of galaxy types. We also find that below luminosities of LUV, 1023 W Hz,1, both type 1 and type 2 objects have a 50 per cent likelihood of exhibiting 21-cm absorption. Finally, we do not detect molecular gas in any of the sources. The lack of H i absorption, combined with the results from Paper I, suggests these sources are not conducive to high molecular abundances. [source]


    Why are AGN found in high-mass galaxies?

    MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2008
    Lan Wang
    ABSTRACT There is a strong observed mass dependence of the fraction of nearby galaxies that contain either low-luminosity [low-ionization nuclear emission-line region (LINER) type] or higher luminosity (Seyfert or composite type) active galactic nuclei (AGN). This implies that either only a small fraction of low-mass galaxies contain black holes, or that the black holes in these systems only accrete rarely or at very low rates, and hence are generally not detectable as AGN. In this paper, we use semi-analytic models implemented in the Millennium Simulation to analyse the mass dependence of the merging histories of dark matter haloes and of the galaxies that reside in them. Only a few per cent of galaxies with stellar masses less than M* < 1010 M, are predicted to have experienced a major merger. The fraction of galaxies that have experienced major mergers increases steeply at larger stellar masses. We argue that if a major merger is required to form the initial seed black hole, the mass dependence of AGN activity in local galaxies can be understood quite naturally. We then investigate when the major mergers that first create these black holes are predicted to occur. High-mass galaxies are predicted to have formed their first black holes at very early epochs. The majority of low-mass galaxies never experience a major merger and hence may not contain a black hole, but a significant fraction of the supermassive black holes that do exist in low-mass galaxies are predicted to have formed recently. [source]


    Spitzer IRAC infrared colours of submillimetre-bright galaxies

    MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2008
    Min S. Yun
    ABSTRACT High-redshift submillimetre-bright galaxies identified by blank field surveys at millimetre and submillimetre wavelengths appear in the region of the Infra Red Array Camera (IRAC) colour,colour diagrams previously identified as the domain of luminous active galactic nuclei (AGNs). Our analysis using a set of empirical and theoretical dusty starburst spectral energy distribution (SED) models shows that power-law continuum sources associated with hot dust heated by young (,100 Myr old), extreme starbursts at z > 2 also occupy the same general area as AGNs in the IRAC colour,colour plots. A detailed comparison of the IRAC colours and SEDs demonstrates that the two populations are distinct from each other, with submillimetre-bright galaxies having a systematically flatter IRAC spectrum (,1 mag bluer in the observed [4.5],[8.0] colour). Only about 20 per cent of the objects overlap in the colour,colour plots, and this low fraction suggests that submillimetre galaxies powered by a dust-obscured AGN are not common. The red infrared colours of the submillimetre galaxies are distinct from those of the ubiquitous foreground IRAC sources, and we propose a set of infrared colour selection criteria for identifying SMG counterparts that can be used even in the absence of radio or Multiband Imaging Photometer for Spitzer (MIPS) 24 ,m data. [source]


    Exploring star formation using the filaments in the Sloan Digital Sky Survey Data Release Five

    MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2008
    Biswajit Pandey
    ABSTRACT We have quantified the average filamentarity of the galaxy distribution in seven nearly two-dimensional strips from the Sloan Digital Sky Survey Data Release Five (SDSS DR5) using a volume-limited sample in the absolute magnitude range ,21 ,Mr,,20. The average filamentarity of star-forming (SF) galaxies, which are predominantly blue, is found to be more than that of other galaxies which are predominantly red. This difference is possibly an outcome of the fact that blue galaxies have a more filamentary distribution. Comparing the SF galaxies with only the other blue galaxies, we find that the two show nearly equal filamentarity. Separately analyzing the galaxies with high star formation rates (SFR) and low SFR, we find that the latter has a more filamentary distribution. We interpret this in terms of two effects. (i) A correlation between the SFR and individual galaxy properties like luminosity with the high-SFR galaxies being more luminous. (ii) A relation between the SFR and environmental effects like the density with the high-SFR galaxies preferentially occurring in high-density regions. These two effects are possibly not independent and are operating simultaneously. We do not find any difference in the filamentarity of SF galaxies and active galactic nuclei. [source]


    Is AGN feedback necessary to form red elliptical galaxies?

    MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2008
    A. Khalatyan
    ABSTRACT We have used the smoothed particle hydrodynamics (SPH) code gadget-2 to simulate the formation of an elliptical galaxy in a group-size cosmological dark matter halo with mass Mhalo, 3 1012 h,1 M, at z= 0. The use of a stellar population synthesis model has allowed us to compute magnitudes, colours and surface brightness profiles. We have included a model to follow the growth of a central black hole and we have compared the results of simulations with and without feedback from active galactic nuclei (AGN). We have studied the interplay between cold gas accretion and merging in the development of galactic morphologies, the link between colour and morphology evolution, the effect of AGN feedback on the photometry of early-type galaxies, the redshift evolution in the properties of quasar hosts, and the impact of AGN winds on the chemical enrichment of the intergalactic medium (IGM). We have found that the early phases of galaxy formation are driven by the accretion of cold filamentary flows, which form a disc galaxy at the centre of the dark matter halo. Disc star formation rates in this mode of galaxy growth are about as high as the peak star formation rates attained at a later epoch in galaxy mergers. When the dark matter halo is sufficiently massive to support the propagation of a stable shock, the gas in the filaments is heated to the virial temperature, cold accretion is shut down, and the star formation rate begins to decline. Mergers transform the spiral galaxy into an elliptical one, but they also reactivate star formation by bringing gas into the galaxy. Without a mechanism that removes gas from the merger remnants, the galaxy ends up with blue colours, which are atypical for its elliptical morphology. We have demonstrated that AGN feedback can solve this problem even with a fairly low heating efficiency. Our simulations support a picture where AGN feedback is important for quenching star formation in the remnant of wet mergers and for moving them to the red sequence. This picture is consistent with recent observational results, which suggest that AGN hosts are galaxies in migration from the blue cloud to the red sequence on the colour,magnitude diagram. However, we have also seen a transition in the properties of AGN hosts from blue and star forming at z, 2 to mainly red and dead at z, 0. Ongoing merging is the primary but not the only triggering mechanism for luminous AGN activity. Quenching by AGN is only effective after the cold filaments have dried out, since otherwise the galaxy is constantly replenished with gas. AGN feedback also contributes to raising the entropy of the hot IGM by removing low-entropy tails vulnerable to developing cooling flows. We have also demonstrated that AGN winds are potentially important for the metal enrichment of the IGM a high redshift. [source]


    The SCUBA Half-Degree Extragalactic Survey (SHADES) , VIII.

    MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2008
    SWIRE, SXDF surveys, The nature of faint submillimetre galaxies in SHADES
    ABSTRACT We present the optical-to-submillimetre spectral energy distributions (SEDs) for 33 radio and mid-infrared (mid-IR) identified submillimetre galaxies discovered via the SHADES 850-,m SCUBA imaging in the Subaru- XMM Deep Field (SXDF). Optical data for the sources come from the SXDF and mid- and far-IR fluxes from SWIRE. We obtain photometric redshift estimates for our sources using optical and IRAC 3.6- and 4.5-,m fluxes. We then fit SED templates to the longer wavelength data to determine the nature of the far-IR emission that dominates the bolometric luminosity of these sources. The IR template fits are also used to resolve ambiguous identifications and cases of redshift aliasing. The redshift distribution obtained broadly matches previous results for submillimetre sources and on the SHADES SXDF field. Our template fitting finds that active galactic nuclei, while present in about 10 per cent of our sources, do not contribute significantly to their bolometric luminosity. Dust heating by starbursts, with either Arp220 or M82 type SEDs, appears to be responsible for the luminosity in most sources (23/33 are fitted by Arp220 templates, 2/33 by the warmer M82 templates). 8/33 sources, in contrast, are fitted by a cooler cirrus dust template, suggesting that cold dust has a role in some of these highly luminous objects. Three of our sources appear to have multiple identifications or components at the same redshift, but we find no statistical evidence that close associations are common among our SHADES sources. Examination of rest-frame K -band luminosity suggests that ,downsizing' is underway in the submillimetre galaxy population, with lower redshift systems lying in lower mass host galaxies. Of our 33 identifications six are found to be of lower reliability but their exclusion would not significantly alter our conclusions. [source]


    The impact of radio feedback from active galactic nuclei in cosmological simulations: formation of disc galaxies

    MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2008
    Takashi Okamoto
    ABSTRACT In this paper, we present a new implementation of feedback due to active galactic nuclei (AGN) in cosmological simulations of galaxy formation. We assume that a fraction of jet energy, which is generated by an AGN, is transferred to the surrounding gas as thermal energy. Combining a theoretical model of mass accretion on to black holes with a multiphase description of star-forming gas, we self-consistently follow evolution of both galaxies and their central black holes. The novelty in our model is that we consider two distinct accretion modes: standard radiatively efficient thin accretion discs and radiatively inefficient accretion flows which we will generically refer to as RIAFs; motivated by theoretical models for jet production in accretion discs, we assume that only the RIAF is responsible for the AGN feedback. The focus of this paper is to investigate the interplay between galaxies and their central black holes during the formation of a disc galaxy. We find that, after an initial episode of bursting star formation, the accretion rate on to the central black hole drops so that the accretion disc switches to a RIAF structure. At this point, the feedback from the AGN becomes efficient and slightly suppresses star formation in the galactic disc and almost completely halts star formation in the bulge. This suppression of the star formation regulates mass accretion on to the black hole and associated AGN feedback. As a result, the nucleus becomes a stochastically fuelled low-luminosity AGN (Seyfert galaxy) with recurrent short-lived episodes of activity after the star bursts. During the ,on' events, the AGN produces reasonably powerful jets (radio-loud state) and is less luminous than the host galaxy, while in the ,off' phase, the nucleus is inactive and ,radio quiet'. Our model predicts several properties of the low-luminosity AGN including the bolometric luminosity, jet powers, the effect on kpc scale of the radio jet and the AGN lifetime, which are in broad agreement with observations of Seyfert galaxies and their radio activity. We also find that the ratios between the central black hole mass and the mass of the host spheroid at z= 0 are ,10,3 regardless of the strength of either supernova feedback or AGN feedback because the radiation drag model directly relates the star formation activity in the Galactic Centre and the mass accretion rate on to the central black hole. [source]


    Environments of z > 5 quasars: searching for protoclusters at submillimetre wavelengths

    MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2008
    R. S. Priddey
    ABSTRACT We present submillimetre (submm) continuum images of the fields of three luminous quasars at z > 5, obtained at 850 and 450 ,m using the Submillimetre Common-User Bolometer Array on the James Clerk Maxwell Telescope (JCMT). N -body simulations predict that such quasars evolve to become the central dominant galaxies of massive clusters at z= 0, but at z= 5,6 they are actively forming stars and surrounded by a rich protofilamentary structure of young galaxies. Our purpose in taking these images was to search for other luminous, star-forming galaxies in the vicinity of the signpost active galactic nuclei and thus associated with such a protocluster. Two of the quasar host galaxies are luminous submm galaxies (SMGs) in their own right, implying star formation rates ,103 M, yr,1. Despite the coarse 850-,m beam of the JCMT, our images show evidence of extended emission on a scale of ,100 kpc from at least one quasar , indicative of a partially resolved merger or a colossal host galaxy. In addition, at >3, significance we detect 12 (5) SMGs at 850 ,m (450 ,m) in the surrounding fields. Number counts of these SMGs are comparable with those detected in the fields of z, 4 radio galaxies, and both samples are, at the bright end, overabundant by a factor of ,4 relative to blank-field submm surveys. Whilst the redshift-sensitive 850 ,m/450 ,m and 850 ,m/1.4 GHz flux density ratios indicate that some of these SMGs are likely foreground objects, the counts suggest that ,60 per cent lie in the same large-scale structures as the quasars. [source]


    Are galaxies with active galactic nuclei a transition population?

    MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2007
    P. B. Westoby
    ABSTRACT We present the results of an analysis of a well-selected sample of galaxies with active and inactive galactic nuclei from the Sloan Digital Sky Survey, in the range 0.01 < z < 0.16. The SDSS galaxy catalogue was split into two classes of active galaxies, Type 2 active galactic nuclei (AGN) and composites, and one set of inactive, star-forming/passive galaxies. For each active galaxy, two inactive control galaxies were selected by matching redshift, absolute magnitude, inclination, and radius. The sample of inactive galaxies naturally divides into a red and a blue sequence, while the vast majority of AGN hosts occur along the red sequence. In terms of H, equivalent width (EW), the population of composite galaxies peaks in the valley between the two modes, suggesting a transition population. However, this effect is not observed in other properties such as the colour,magnitude space or colour,concentration plane. Active galaxies are seen to be generally bulge-dominated systems, but with enhanced H, emission compared to inactive red-sequence galaxies. AGN and composites also occur in less dense environments than inactive red-sequence galaxies, implying that the fuelling of AGN is more restricted in high-density environments. These results are therefore inconsistent with theories in which AGN host galaxies are a ,transition' population. We also introduce a systematic 3D spectroscopic imaging survey, to quantify and compare the gaseous and stellar kinematics of a well-selected, distance-limited sample of up to 20 nearby Seyfert galaxies, and 20 inactive control galaxies with well-matched optical properties. The survey aims to search for dynamical triggers of nuclear activity and address outstanding controversies in optical/infrared imaging surveys. [source]


    VLT/SINFONI integral field spectroscopy of the Super-antennae,

    MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2007
    J. Reunanen
    ABSTRACT We present the results of H - and K -band very large telescope/Spectrograph for INtegral Field Observations in the Near Infrared (VLT/SINFONI) integral field spectroscopy of the Ultraluminous Infrared Galaxy (ULIRG) IRAS 19 254,7245 (the Super-antennae), an interacting double galaxy system containing an embedded active galactic nuclei. Deep K -band spectroscopy reveals Pa, arising in a warped disc with position angle of 330 and an inclination i= 40,55. The kinemetric parameters derived for H2 are similar to Pa,. Two high-ionization emission lines, [Si vi] and [Al ix], are detected and we identify as [Ni ii] the line observed at 1.94 ,m. Diluting non-stellar continuum, which was previously detected, has decayed, and the H -band continuum emission is consistent with pure stellar emission. Based on H2 emission-line ratios, it is likely that at the central 1-kpc region H2 is excited by ultraviolet fluorescence in dense clouds while shock excitation is dominant further out. This scenario is supported by very low Pa, to H2 line ratio detected outside the nuclear region and non-thermal ortho/para ratios (,2.0,2.5) close to the nucleus. [source]


    The UV properties of E+A galaxies: constraints on feedback-driven quenching of star formation

    MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2007
    S. Kaviraj
    ABSTRACT We present the first large-scale study of E+A galaxies that incorporates photometry in the ultraviolet (UV) wavelengths. E+A galaxies are ,post-starburst' systems, with strong Balmer absorption lines indicating significant recent star formation, but without [O ii] and H, emission lines which are characteristic of ongoing star formation. The starburst that creates the E+A galaxy typically takes place within the last Gyr and creates a high fraction (20,60 per cent) of the stellar mass in the remnant over a short time-scale (<0.1 Gyr). We find a tight correlation between the luminosity of our E+A galaxies and the implied star formation rate (SFR) during the starburst. While low-luminosity E+As [M(z) > ,20] exhibit implied SFRs of less than 50 M, yr,1, their luminous counterparts [M(z) < ,22] show SFRs greater than 300 and as high as 2000 M, yr,1, suggesting that luminous and ultra-luminous infrared galaxies in the low-redshift Universe could be the progenitors of massive nearby E+A galaxies. We perform a comprehensive study of the characteristics of the quenching that truncates the starburst in E+A systems. We find that, for galaxies less massive than 1010 M,, the quenching efficiency decreases as the galaxy mass increases. However, for galaxies more massive than 1010 M,, this trend is reversed and the quenching efficiency increases with galaxy mass. Noting that the mass threshold at which this reversal occurs is in excellent agreement with the mass above which active galactic nuclei (AGN) become significantly more abundant in nearby galaxies, we use simple energetic arguments to show that the bimodal behaviour of the quenching efficiency is consistent with AGN and supernovae (SN) being the principal sources of negative feedback above and below M, 1010 M,, respectively. The arguments assume that quenching occurs through the mechanical ejection or dispersal of the gas reservoir and that, in the high-mass regime (M > 1010 M,), the Eddington ratios in this sample of galaxies scale as M,, where 1 < , < 3. Finally, we use our E+A sample to estimate the time it takes for galaxies to migrate from the blue cloud to the red sequence. We find migration times between 1 and 5 Gyr, with a median value of 1.5 Gyr. [source]


    X-ray active galactic nuclei in the core of the Perseus cluster

    MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2007
    S. Santra
    ABSTRACT We present a study of the X-ray emission from the nuclei of galaxies observed in the core of the Perseus cluster in a deep exposure with Chandra. Point sources are found coincident with the nuclei of 13 early-type galaxies, as well as the central galaxy NGC 1275. This corresponds to all galaxies brighter than MB > ,18 in the Chandra field. All of these sources have a steep power-law spectral component and four have an additional thermal component. The unabsorbed power-law luminosities in the 0.5,7.0 keV band range from 8 1038 to 5 1040 erg s,1. We find no simple correlations between the K -band luminosity, or the FUV and NUV AB magnitudes of these galaxies and their X-ray properties. We have estimated the black hole masses of the nuclei using the K -band MBH,LKbol relation and again find no correlation between black hole mass and the X-ray luminosity. Bondi accretion on to the black holes in the galaxies with minihaloes should make them much more luminous than observed. [source]


    The XMM-SSC survey of hard-spectrum XMM,Newton sources , I. Optically bright sources

    MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2007
    M. J. Page
    ABSTRACT We present optical and X-ray data for a sample of serendipitous XMM,Newton sources that are selected to have 0.5,2 versus 2,4.5 keV X-ray hardness ratios which are harder than the X-ray background. The sources have 2,4.5 keV X-ray flux ,10,14 erg cm,2 s,1, and in this paper we examine a subsample of 42 optically bright (r < 21) sources; this subsample is 100 per cent spectroscopically identified. All but one of the optical counterparts are extragalactic, and we argue that the single exception, a Galactic M star, is probably a coincidental association rather than the correct identification of the X-ray source. The X-ray spectra of all the sources are consistent with heavily absorbed power laws (21.8 < log NH < 23.4), and all of them, including the two sources with 2,10 keV intrinsic luminosities of <1042 erg s,1, appear to be absorbed active galactic nuclei (AGN). The majority of the sources show only narrow emission lines in their optical spectra, implying that they are type 2 AGN. Three sources have 2,10 keV luminosities of >1044 erg s,1, and two of these sources have optical spectra which are dominated by narrow emission lines, that is, are type 2 QSOs. Only a small fraction of the sources (7/42) show broad optical emission lines, and all of these have NH < 1023 cm,2. This implies that ratios of X-ray absorption to optical/ultraviolet extinction equivalent to >100 times the Galactic gas-to-dust ratio are rare in AGN absorbers (at most a few per cent of the population), and may be restricted to broad absorption line QSOs. Seven objects appear to have an additional soft X-ray component in addition to the heavily absorbed power law; all seven are narrow emission-line objects with z < 0.3 and 2,10 keV intrinsic luminosities <1043 erg s,1. We consider the implications of our results in the light of the AGN unified scheme. We find that the soft components in narrow-line objects are consistent with the unified scheme provided that >4 per cent of broad-line AGN (BLAGN) have ionized absorbers that attenuate their soft X-ray flux by >50 per cent. In at least one of the X-ray-absorbed BLAGN in our sample the X-ray spectrum requires an ionized absorber, consistent with this picture. [source]


    Impact of tangled magnetic fields on fossil radio bubbles

    MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2007
    M. Ruszkowski
    ABSTRACT There is growing consensus that feedback from active galactic nuclei (AGN) is the main mechanism responsible for stopping cooling flows in clusters of galaxies. AGN are known to inflate buoyant bubbles that supply mechanical power to the intracluster gas [intracluster medium (ICM)]. High Reynolds number hydrodynamical simulations show that such bubbles get entirely disrupted within 100 Myr, as they rise in cluster atmospheres, which is contrary to observations. This artificial mixing has consequences for models trying to quantify the amount of heating and star formation in cool core clusters of galaxies. It has been suggested that magnetic fields can stabilize bubbles against disruption. We perform magnetohydrodynamical simulations of fossil bubbles in the presence of tangled magnetic fields using the high-order pencil code. We focus on the physically motivated case where thermal pressure dominates over magnetic pressure and consider randomly oriented fields with and without maximum helicity and a case where large-scale external fields drape the bubble. We find that helicity has some stabilizing effect. However, unless the coherence length of magnetic fields exceeds the bubble size, the bubbles are quickly shredded. As observations of Hydra A suggest that length-scale of magnetic fields may be smaller than typical bubble size, this may suggest that other mechanisms, such as viscosity, may be responsible for stabilizing the bubbles. However, since Faraday rotation observations of radio lobes do not constrain large-scale ICM fields well if they are aligned with the bubble surface, the draping case may be a viable alternative solution to the problem. A generic feature found in our simulations is the formation of magnetic wakes where fields are ordered and amplified. We suggest that this effect could prevent evaporation by thermal conduction of cold H, filaments observed in the Perseus cluster. [source]


    Hot and cold gas accretion and feedback in radio-loud active galaxies

    MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2007
    M. J. Hardcastle
    ABSTRACT We have recently shown that X-ray observations of the population of ,low-excitation' radio galaxies, which includes most low-power, Fanaroff,Riley class I sources as well as some more powerful Fanaroff,Riley class II objects, are consistent with a model in which the active nuclei of these objects are not radiatively efficient at any waveband. In another recent paper, Allen et al. have shown that Bondi accretion of the hot, X-ray emitting phase of the intergalactic medium (IGM) is sufficient to power the jets of several nearby, low-power radio galaxies at the centres of clusters. In this paper, we combine these ideas and suggest that accretion of the hot phase of the IGM is sufficient to power all low-excitation radio sources, while high-excitation sources are powered by accretion of cold gas that is in general unrelated to the hot IGM. This model explains a number of properties of the radio-loud active galaxy population, and has important implications for the energy input of radio-loud active galactic nuclei into the hot phase of the IGM: the energy supply of powerful high-excitation sources does not have a direct connection to the hot phase. [source]


    PVLAS experiment: some astrophysical consequences

    MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2007
    Yu. N. Gnedin
    ABSTRACT The birefringent effects of photon,pseudo-scalar boson (Goldstone) particle mixing in intergalactic magnetic field are calculated for cosmological objects. We use the recent results of PVLAS collaboration that reported recently the observation of a rotation of the polarization plane of light propagating through a transverse static magnetic field. Such result was interpreted as arising due to conversion of photon into pseudo-scalar with coupling strength ga,, 4 10,6 GeV,1. This result contradicts to data of stellar evolution that excluded standard axion model and seems to claim existence of supersymmetry (SUSY) pseudo-scalars. We estimate the intergalactic magnetic field magnitude as ,10,16 G based on Hatsemekers et al. observations of extreme-scale alignments of quasar polarization vectors. We analysed some additional results of astronomical observations that could be explained by axion interpretation of the PVLAS data: a sharp steepening of the quasi-stellar object (QSO) continuum shortward of ,1100 , observed circular polarization of active galactic nuclei (AGNs) and QSOs, discrepancy between observed intrinsic polarization of stars in the Local Bubble and stellar spectral classification. The observed polarization of stars in the Local Bubble cannot be explained by interstellar origin. [source]


    The clustering of narrow-line AGN in the local Universe

    MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2006
    Cheng Li
    ABSTRACT We have analysed the clustering of ,90 000 narrow-line active galactic nuclei (AGN) drawn from the Data Release 4 (DR4) of the Sloan Digital Sky Survey. Our analysis addresses the following questions. (i) How do the locations of galaxies within the large-scale distribution of dark matter influence ongoing accretion on to their central black holes? (ii) Is AGN activity triggered by interactions or mergers between galaxies? We compute the cross-correlation between AGN and a reference sample of galaxies drawn from the DR4. We compare this to results for control samples of inactive galaxies matched simultaneously in redshift, stellar mass, concentration, velocity dispersion and mean stellar age, as measured by the 4000- break strength. We also compare near-neighbour counts around AGN and around the control galaxies. On scales larger than a few Mpc, AGN have almost the same clustering amplitude as the control sample. This demonstrates that AGN host galaxies and inactive control galaxies populate dark matter haloes of similar mass. On scales between 100 kpc and 1 Mpc, AGN are clustered more weakly than the control galaxies. We use mock catalogues constructed from high-resolution N -body simulations to interpret this antibias, showing that the observed effect is easily understood if AGN are preferentially located at the centres of their dark matter haloes. On scales less than 70 kpc, AGN cluster marginally more strongly than the control sample, but the effect is weak. When compared to the control sample, we find that only one in 100 AGN has an extra neighbour within a radius of 70 kpc. This excess increases as a function of the accretion rate on to the black hole, but it does not rise above the few per cent level. Although interactions between galaxies may be responsible for triggering nuclear activity in a minority of nearby AGN, some other mechanism is required to explain the activity seen in the majority of the objects in our sample. [source]