MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2009
Minnie Y. Mao
ABSTRACT Using radio data at 1.4 GHz from the Australia Telescope Compact Array (ATCA), we identify five head,tail (HT) galaxies in the central region of the Horologium,Reticulum Supercluster (HRS). Physical parameters of the HT galaxies were determined along with substructure in the HRS to probe the relationship between environment and radio properties. Using a density enhancement technique applied to 582 spectroscopic measurements in the 2°× 2° region about A3125/A3128, we find all five HT galaxies reside in regions of extremely high density (>100 galaxies Mpc,3). In fact, the environments surrounding HT galaxies are statistically denser than those environments surrounding non-HT galaxies and among the densest environments in a cluster. Additionally, the HT galaxies are found in regions of enhanced X-ray emission and we show that the enhanced density continues out to substructure groups of 10 members. We propose that it is the high densities that allow ram pressure to bend the HT galaxies as opposed to previously proposed mechanisms relying on exceptionally high peculiar velocities. [source]

The evolution of the galaxy red sequence in simulated clusters and groups

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2008
A. D. Romeo
ABSTRACT N -body/hydrodynamical simulations of the formation and evolution of galaxy groups and clusters in a , cold dark matter (,CDM) cosmology are used in order to follow the building-up of the colour,magnitude relation in two clusters and in 12 groups. We have found that galaxies, starting from the more massive, move to the red sequence (RS) as they get aged over times and eventually set upon a ,dead sequence' (DS) once they have stopped their bulk star formation activity. Fainter galaxies keep having significant star formation out to very recent epochs and lie broader around the RS. Environment plays a role as galaxies in groups and cluster outskirts hold star formation activity longer than the central cluster regions. However, galaxies experiencing infall from the outskirts to the central parts keep star formation on until they settle on to the DS of the core galaxies. Merging contributes to mass assembly until z, 1, after which major events only involve the brightest cluster galaxies. The emerging scenario is that the evolution of the colour,magnitude properties of galaxies within the hierarchical framework is mainly driven by star formation activity during dark matter haloes assembly. Galaxies progressively quenching their star formation settle to a very sharp ,red and dead' sequence, which turns out to be universal, its slope and scatter being almost independent of the redshift (since at least z, 1.5) and environment. Differently from the DS, the operatively defined RS evolves more evidently with z, the epoch when it changes its slope being closely corresponding to that at which the passive galaxies population takes over the star-forming one: this goes from z, 1 in clusters down to 0.4 in normal groups. [source]

Photometric properties and scaling relations of early-type Brightest Cluster Galaxies

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2008
F. S. Liu
ABSTRACT We investigate the photometric properties of the early-type Brightest Cluster Galaxies (BCGs) using a carefully selected sample of 85 BCGs from the C4 cluster catalogue with a redshift of less than 0.1. We perform accurate background subtractions and surface photometry for these BCGs to 25 mag arcsec,2 in the Sloan r band. By quantitatively analysing the gradient of the Petrosian profiles of BCGs, we find that a large fraction of BCGs have extended stellar envelopes in their outskirts; more luminous BCGs tend to have more extended stellar haloes that are likely to be connected with mergers. A comparison sample of elliptical galaxies was chosen with similar apparent magnitude and redshift ranges, for which the same photometric analysis procedure is applied. We find that BCGs have steeper size,luminosity (R,L,) and Faber,Jackson (L,,,) relations than the bulk of early-type galaxies. Furthermore, the power-law indices (, and ,) in these relations increase as the isophotal limits become deeper. For isophotal limits from 22 to 25 mag arcsec,2, BCGs are usually larger than the bulk of early-type galaxies, and a large fraction (,49 per cent) of BCGs have discy isophotal shapes. The differences in the scaling relations are consistent with a scenario where the dynamical structure and formation route of BCGs may be different from the bulk of early-type galaxies; in particular dry (dissipationless) mergers may play a more important role in their formation. We highlight several possible dry merger candidates in our sample. [source]

The evolution of submillimetre galaxies: two populations and a redshift cut-off

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2008
J. V. Wall
ABSTRACT We explore the epoch dependence of number density and star formation rate for submillimetre galaxies (SMGs) found at 850 ,m. The study uses a sample of 38 SMG in the Great Observatories Origins Deep Survey (GOODS)-N field, for which cross-waveband identifications have been obtained for 35/38 members together with redshift measurements or estimates. A maximum-likelihood analysis is employed, along with the ,single-source-survey' technique. We find a diminution in both space-density and star formation rate at z > 3, closely mimicking the redshift cut-offs found for quasi-stellar objects (QSOs) selected in different wavebands. The diminution in redshift is particularly marked at a significance level too small to measure. The data further suggest, at a significance level of about 0.001, that two separately evolving populations may be present, with distinct luminosity functions. These results parallel the different evolutionary behaviours of Luminous Infrared Galaxies and Ultra-Luminous Infrared Galaxies, and represent another manifestation of ,cosmic down-sizing', suggesting that differential evolution extends to the most extreme star-forming galaxies. [source]

Where are z= 4 Lyman Break Galaxies?

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2006
Results from conditional luminosity function models of luminosity-dependent clustering
ABSTRACT Using the conditional luminosity function (CLF) , the luminosity distribution of galaxies in a dark matter halo , as a way to model galaxy statistics, we study how z= 4 Lyman Break Galaxies (LBGs) are distributed in dark matter haloes. For this purpose, we measure luminosity-dependent clustering of LBGs in the Subaru/XMM,Newton Deep Field by separating a sample of 16 920 galaxies to three magnitude bins in i, band between 24.5 and 27.5. Our model fits to data show a possible trend for more-luminous galaxies to appear as satellites in more-massive haloes; the minimum halo mass in which satellites appear is 3.9+4.1,3.5× 1012, 6.2+3.8,4.9× 1012 and 9.6+7.0,4.6× 1012 M, (1, errors) for galaxies with 26.5 < i, < 27.5, 25.5 < i, < 26.5 and 24.5 < i, < 25.5 mag, respectively. The satellite fraction of galaxies at z= 4 in these magnitude bins is 0.13,0.3, 0.09,0.22 and 0.03,0.14, respectively, where the 1, ranges account for differences coming from two different estimates of the z= 4 LF from the literature. To jointly explain the LF and the large-scale linear bias factor of z= 4 LBGs as a function of rest UV luminosity requires central galaxies to be brighter in UV at z= 4 than present-day galaxies in same dark matter mass haloes. Moreover, UV luminosity of central galaxies in haloes with total mass greater than roughly 1012 M, must decrease from z= 4 to today by an amount more than the luminosity change for galaxies in haloes below this mass. This mass-dependent luminosity evolution is preferred at more than 3, confidence level compared to a pure-luminosity evolution scenario where all galaxies decrease in luminosity by the same amount from z= 4 to today. The scenario preferred by the data is consistent with the ,downsizing' picture of galaxy evolution. [source]

Simultaneous ram pressure and tidal stripping; how dwarf spheroidals lost their gas

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2006
Lucio Mayer
ABSTRACT We perform high-resolution N -body+SPH (smoothed particle hydrodynamics) simulations of gas-rich dwarf galaxy satellites orbiting within a Milky Way-sized halo and study for the first time the combined effects of tides and ram pressure. The structure of the galaxy models and the orbital configurations are chosen in accordance with those expected in a Lambda cold dark matter (,CDM) universe. While tidal stirring of disky dwarfs produces objects whose stellar structure and kinematics resembles that of dwarf spheroidals after a few orbits, ram pressure stripping is needed to entirely remove their gas component. Gravitational tides can aid ram pressure stripping by diminishing the overall potential of the dwarf, but tides also induce bar formation which funnels gas inwards making subsequent stripping more difficult. This inflow is particularly effective when the gas can cool radiatively. Assuming a low density of the hot Galactic corona consistent with observational constraints, dwarfs with Vpeak < 30 km s,1 can be completely stripped of their gas content on orbits with pericenters of 50 kpc or less. Instead, dwarfs with more massive dark haloes and Vpeak > 30 km s,1 lose most or all of their gas content only if a heating source keeps the gas extended, partially counteracting the bar-driven inflow. We show that the ionizing radiation from the cosmic ultraviolet (UV) background at z > 2 can provide the required heating. In these objects, most of the gas is removed or becomes ionized at the first pericenter passage, explaining the early truncation of the star formation observed in Draco and Ursa Minor. Galaxies on orbits with larger pericenters and/or falling into the Milky Way halo at lower redshift can retain significant amounts of the centrally concentrated gas. These dwarfs would continue to form stars over a longer period of time, especially close to pericenter passages, as observed in Fornax and other dwarf spheroidal galaxies (dSphs) of the Local Group. The stripped gas breaks up into individual clouds pressure confined by the outer gaseous medium that have masses, sizes and densities comparable to the H i clouds recently discovered around M31. [source]

The 1,1000 ,m spectral energy distributions of far-infrared galaxies

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2006
A. Sajina
ABSTRACT Galaxies selected at 170 ,m by the Infrared Space Observatory (ISO) Far-IR BACKground (FIRBACK) survey represent the brightest ,10 per cent of the cosmic infrared background. Examining their nature in detail is therefore crucial for constraining models of galaxy evolution. Here, we combine Spitzer archival data with previous near-infrared (near-IR), far-IR, and submillimetre (submm) observations of a representative sample of 22 FIRBACK galaxies spanning three orders of magnitude in IR luminosity. We fit a flexible, multicomponent, empirical SED model of star-forming galaxies designed to model the entire ,1,1000 ,m wavelength range. The fits are performed with a Markov Chain Monte Carlo (MCMC) approach, allowing for meaningful uncertainties to be derived. This approach also highlights degeneracies such as between Td and ,, which we discuss in detail. From these fits and standard relations we derive: LIR, LPAH, star formation rate (SFR), ,V, M*, Mdust, Td, and ,. We look at a variety of correlations between these and combinations thereof in order to examine the physical nature of these galaxies. Our conclusions are supplemented by morphological examination of the sources, and comparison with local samples. We find the bulk of our sample to be consistent with fairly standard size and mass disc galaxies with somewhat enhanced star formation relative to local spirals, but likely not bona fide starbursts. A few higher- z luminous infrared galaxies (LIGs) and ultraluminous infrared galaxies (ULIGs) are also present, but contrary to expectation, they are weak mid-IR emitters and overall are consistent with star formation over an extended cold region rather than concentrated in the nuclear regions. We discuss the implications of this study for understanding populations detected at other wavelengths, such as the bright 850-,m Submillimetre Common-User Bolometer Array (SCUBA) sources or the faint Spitzer 24-,m sources. [source]

Galaxies as fluctuations in the ionizing background radiation at low redshift

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2003
Suzanne M. Linder
ABSTRACT Some Lyman continuum photons are likely to escape from most galaxies, and these can play an important role in ionizing gas around and between galaxies, including gas that gives rise to Lyman-alpha absorption. Thus the gas surrounding galaxies and in the intergalactic medium will be exposed to varying amounts of ionizing radiation depending upon the distances, orientations and luminosities of any nearby galaxies. The ionizing background can be recalculated at any point within a simulation by adding the flux from the galaxies to a uniform quasar contribution. Normal galaxies are found to almost always make some contribution to the ionizing background radiation at a redshift of zero, as seen by absorbers and at random points in space. Assuming that ,2 per cent of ionizing photons escape from a galaxy such as the Milky Way, we find that normal galaxies make a contribution of at least 30,40 per cent of the assumed quasar background. Lyman-alpha absorbers with a wide range of neutral column densities are found to be exposed to a wide range of ionization rates, although the distribution of photoionization rates for absorbers is found to be strongly peaked. On average, fewer highly ionized absorbers are found to arise further from luminous galaxies, while local fluctuations in the ionization rate are seen around galaxies having a wide range of properties. [source]

The redshift distribution of FIRST radio sources at 1 mJy

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2000
M. Magliocchetti
We present spectra for a sample of radio sources from the FIRST survey, and use them to define the form of the redshift distribution of radio sources at mJy levels. We targeted 365 sources and obtained 46 redshifts (13 per cent of the sample). We find that our sample is complete in redshift measurement to R,18.6, corresponding to z,0.2. Galaxies were assigned spectral types based on emission-line strengths. Early-type galaxies represent the largest subset (45 per cent) of the sample and have redshifts 0.15,z,0.5; late-type galaxies make up 15 per cent of the sample and have redshifts 0.05,z,0.2; starbursting galaxies are a small fraction (,6 per cent), and are very nearby (z,0.05). Some 9 per cent of the population have Seyfert 1/quasar-type spectra, all at z,0.8, and 4 per cent are Seyfert 2 type galaxies at intermediate redshifts (z,0.2). Using our measurements and data from the Phoenix survey (Hopkins et al.), we obtain an estimate for N(z) at S1.4 GHz,1 mJy and compare this with model predictions. At variance with previous conclusions, we find that the population of starbursting objects makes up ,5 per cent of the radio population at S,1 mJy. [source]

Disk galaxies and their environment

ASTRONOMISCHE NACHRICHTEN, Issue 9-10 2009
S.J. Kautsch
Abstract Environments of disk-dominated galaxies and simple disk systems-compared to systems with bulges-provide a means to explore how environment relates to galaxy morphology. Our approach focuses on systems with edge-on disks where disk-to-bulge ratios and disk flattening can be unambiguously determined and focuses on simple disks as evolutionary tracers. We study possible physical neighbors around the target disk galaxies and seek statistical relationships between local galaxy density and galaxy morphology. Galaxies consisting of simple stellar disks exist in environments ranging from the relatively the isolated field to moderate density galaxy groups. This distribution overlaps with that of systems with prominent bulges, although galaxies with large bulges are systematically rarer at low densities. The presence of simple disk galaxies in isolation and also in moderate density galaxy groups suggests that simple disks develop naturally in low density regions but have a limited ability to survive significant interactions with other galaxies. Simple disks thus are rare in denser galaxy systems where galaxy transformations are frequently driven by intense initial merging and later strong interactions (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Narrow-Line Seyfert 1 Galaxies with high-energy sharp spectral drops: reflection-versus absorption models

ASTRONOMISCHE NACHRICHTEN, Issue 10 2006
Th. Boller
Abstract With the launch of XMM-Newton in 1999, two Narrow-Line Seyfert 1 Galaxies (NLS1s) have been detected (IRAS 13224,3809 and 1H 0707,495) showing sharp spectral drops at energies equal or above the neutral Fe K edge at 7.1 keV without any narrow Fe K reemission. In this paper I summarize our present knowledge on the observed properties of sharp high-energy spectral drops. I list the problems presently arising from the reflection dominated and the optically thick disc models. Finally, I present an alternative solution which consists of a combination of the accretion disc model and the reflection dominated model. This might solve the problems of the standard accretion disc model. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

The Granada workshop on High Redshift Radio Galaxies: An overview

ASTRONOMISCHE NACHRICHTEN, Issue 2-3 2006
H. J. A. Röttgering
Abstract The Granada workshop on High Redshift Radio Galaxies (HzRGs) gave an excellent overview of the progress that has been made in this field during the last 3 years. Here we briefly review some of the results, with an emphasis on what studies of HzRGs can teach us about the formation and evolution of massive galaxies, clusters and active galactic nuclei (AGN). Of great relevance for this workshop are scenarios that describe certain aspects of the evolution of radio galaxies, including (i) the sequence of events after merging of galaxies that ultimately lead to extended powerful radio sources and (ii) the mass assembly and virialization of the hosting massive galaxies and their associated (proto-)clusters. Furthermore, I briefly discuss two projects that are important for a further understanding of AGN and high redshift radio galaxies. First, using the MIDI instrument mounted on the VLT Interferometer, the dusty tori of nearby AGN can be studied in the range of 8,13 micron at high angular resolution. The first result on the nearby AGN NGC 1068 as presented by Jaffe et al. (2004) indicated the presence of a hot (T > 800 K), compact (,1 pc) component, possible identified with the base of the jet and a warm (270 K), well-resolved (3 × 4 pc) component associated with the alleged torus. Second, LOFAR is a new low frequency radio telescope that is currently being build in the Netherlands and is expected to be operational in 2008. With 50 stations spread over an area of 100 km in diameter, its resolution and sensitivity will be unprecedented in the frequency range 10,240 MHz. LOFAR will be a unique instrument that will impact a broad range of astrophysical topics varying from the epoch of reionisation, to gamma ray bursts and cosmic rays. Surveys with LOFAR will be of paramount importance for studies of HzRGs: It will enable (i) defining samples of radio galaxies with redshifts higher than 6, (ii) observations of starbursting galaxies in proto-clusters, and (iii) mapping out the low-frequency radio emission of virtually all northern radio-loud AGN in revolutionary detail. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Chiral asymmetry in spiral galaxies?

CHIRALITY, Issue 7 2001
Dilip K. Kondepudi
Abstract Spiral galaxies are chiral entities when coupled with the direction of their recession velocity. As viewed from the Earth, the S-shaped and Z-shaped spiral galaxies are two chiral forms. What is the nature of chiral symmetry in spiral galaxies? In the Carnegie Atlas of Galaxies that lists photographs of a total of 1,168 galaxies, we found 540 galaxies, classified as normal or barred spirals, that are clearly identifiable as S- or Z- type. The recession velocities for 538 of these galaxies could be obtained from this atlas and other sources. A statistical analysis of this sample reveals no overall asymmetry but there is a significant asymmetry in certain subclasses: dominance of S-type galaxies in the Sb class of normal spiral galaxies and a dominance of Z-type in the SBb class of barred spiral galaxies. Both S- and Z-type galaxies seem to have similar velocity distribution, indicating no spatial segregation of the two chiral forms. Chirality 13:351,356, 2001. © 2001 Wiley-Liss, Inc. [source]

Galaxy,galaxy lensing by non-spherical haloes , I. Theoretical considerations

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2010
Paul J. Howell
ABSTRACT We use a series of Monte Carlo simulations to investigate the theory of galaxy,galaxy lensing by non-spherical dark matter haloes. The simulations include a careful accounting of the effects of multiple deflections on the galaxy,galaxy lensing signal. In a typical observational data set where the mean tangential shear of sources with redshifts zs, 0.6 is measured with respect to the observed symmetry axes of foreground galaxies with redshifts zl, 0.3, we find that the signature of anisotropic galaxy,galaxy lensing differs substantially from the simple expectation that one would have in the absence of multiple deflections. In general, the observed ratio of the mean tangential shears, ,+(,)/,,(,), is strongly suppressed compared to the function that one would measure if the intrinsic symmetry axes of the foreground galaxies were known. Depending upon the characteristic masses of the lenses, the observed ratio of the mean tangential shears may be consistent with an isotropic signal (despite the fact that the lenses are non-spherical), or it may even be reversed from the expected signal (i.e. the mean tangential shear for sources close to the observed minor axes of the lenses may exceed the mean tangential shear for sources close to the observed major axes of the lenses). These effects are caused primarily by the fact that the images of the lens galaxies have, themselves, been lensed and therefore the observed symmetry axes of the lens galaxies differ from their intrinsic symmetry axes. We show that the effects of lensing of the foreground galaxies on the observed function ,+(,)/,,(,) cannot be eliminated simply by the rejection of foreground galaxies with very small image ellipticities nor by simply focusing the analysis on sources that are located very close to the observed symmetry axes of the foreground galaxies. We conclude that any attempt to use a measurement of ,+(,)/,,(,) to constrain the shapes of dark matter galaxy haloes must include Monte Carlo simulations that take multiple deflections properly into account. [source]

The outburst duration and duty cycle of GRS 1915+105

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2009
Patrick Deegan
ABSTRACT The extraordinarily long outburst of GRS 1915+105 makes it one of the most remarkable low-mass X-ray binaries (LMXBs). It has been in a state of constant outburst since its discovery in 1992, an eruption which has persisted ,100 times longer than those of more typical LXMBs. The long orbital period of GRS 1915+105 implies that it contains large and massive accretion disc which is able to fuel its extreme outburst. In this paper, we address the longevity of the outburst and quiescence phases of GRS 1915+105 using smooth particle hydrodynamics (SPH) simulations of its accretion disc through many outburst cycles. Our model is set in the two-, framework and includes the effects of the thermoviscous instability, tidal torques, irradiation by central X-rays and wind mass loss. We explore the model parameter space and examine the impact of the various ingredients. We predict that the outburst of GRS 1915+105 should last a minimum of 20 yr and possibly up to ,100 yr if X-ray irradiation is very significant. The predicted recurrence times are of the order of 104 yr, making the X-ray duty cycle a few 0.1 per cent. Such a low duty cycle may mean that GRS 1915+105 is not an anomaly among the more standard LMXBs and that many similar, but quiescent, systems could be present in the Galaxy. [source]

The stellar population content of the thick disc and halo of the Milky Way analogue NGC 891

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2009
M. Rejkuba
ABSTRACT We present deep VI images obtained with the Advanced Camera for Surveys on board the Hubble Space Telescope, covering three fields in the north-east side of the edge-on disc galaxy NGC 891. The observed fields span a wide range of galactocentric distances along the eastern minor axis, extending from the plane of the disc to 12 kpc, and out to ,25 kpc along the major axis. The photometry of individual stars reaches ,2.5 mag below the tip of the red giant branch. We use the astrophotometric catalogue to probe the stellar content and metallicity distribution across the thick disc and spheroid of NGC 891. The colour,magnitude diagrams of thick disc and spheroid population are dominated by old red giant branch stars with a wide range of metallicities, from the sparsely populated metal-poor tail at [Fe/H],,2.4 dex, up to about half-solar metallicity. The peak of the metallicity distribution function of the thick disc is at ,0.9 dex. The inner parts of the thick disc, within ,14 kpc along the major axis show no vertical colour/metallicity gradient. In the outer parts, a mild vertical gradient of ,(V,I)0/,|Z| = 0.1 ± 0.05 kpc,1 or less than 0.1 dex kpc,1 is detected, with bluer colours or more metal-poor stars at larger distances from the plane. This gradient is, however, accounted for by the mixing with the metal-poor halo stars. No metallicity gradient along the major axis is present for thick-disc stars, but strong variations of about 0.35 dex around the mean of [Fe/H]=,1.13 dex are found. The properties of the asymmetric metallicity distribution functions of the thick-disc stars show no significant changes in both the radial and the vertical directions. The stellar populations situated within the solar-cylinder-like distances show strikingly different properties from those of the Galaxy populating similar distances. This suggests that the accretion histories of both galaxies have been different. The spheroid population, composed of the inner spheroid and the halo, shows remarkably uniform stellar population properties. The median metallicity of the halo stellar population shows a shallow gradient from about ,1.15 dex in the inner parts to ,1.27 dex at 24 kpc distance from the centre, corresponding to ,13reff. Similar to the thick-disc stars, large variations around the mean relation are present. [source]

Populating the Galaxy with pulsars , I. Stellar and binary evolution

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2008
Paul D. Kiel
ABSTRACT The computation of theoretical pulsar populations has been a major component of pulsar studies since the 1970s. However, the majority of pulsar population synthesis has only regarded isolated pulsar evolution. Those that have examined pulsar evolution within binary systems tend to either treat binary evolution poorly or evolve the pulsar population in an ad hoc manner. Thus, no complete and direct comparison with observations of the pulsar population within the Galactic disc has been possible to date. Described here is the first component of what will be a complete synthetic pulsar population survey code. This component is used to evolve both isolated and binary pulsars. Synthetic observational surveys can then be performed on this population for a variety of radio telescopes. The final tool used for completing this work will be a code comprised of three components: stellar/binary evolution, Galactic kinematics and survey selection effects. Results provided here support the need for further (apparent) pulsar magnetic field decay during accretion, while they conversely suggest the need for a re-evaluation of the assumed typical millisecond pulsar formation process. Results also focus on reproducing the observed diagram for Galactic pulsars and how this precludes short time-scales for standard pulsar exponential magnetic field decay. Finally, comparisons of bulk pulsar population characteristics are made to observations displaying the predictive power of this code, while we also show that under standard binary evolutionary assumption binary pulsars may accrete much mass. [source]

Dust mass-loss rates from asymptotic giant branch stars in the Fornax and Sagittarius dwarf spheroidal galaxies

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2008
Eric Lagadec
ABSTRACT To study the effect of metallicity on the mass-loss rate of asymptotic giant branch (AGB) stars, we have conducted mid-infrared photometric measurements of such stars in the Sagittarius and Fornax dwarf spheroidal galaxies with the 10-,m camera VISIR at the Very Large Telescope. We derive mass-loss rates for 29 AGB stars in Sgr dSph and two in Fornax. The dust mass-loss rates are estimated from the K,[9] and K,[11] colours. Radiative transfer models are used to check the consistency of the method. Published IRAS and Spitzer data confirm that the same tight correlation between K,[12] colour and dust mass-loss rates is observed for AGB stars from galaxies with different metallicities, i.e., the Galaxy, the Large Magellanic Clouds and the Small Magellanic Clouds. The derived dust mass-loss rates are in the range 5 × 10,10 to 3 × 10,8 M, yr,1 for the observed AGB stars in Sgr dSph and around 5 × 10,9 M, yr,1 for those in Fornax; while values obtained with the two different methods are of the same order of magnitude. The mass-loss rates for these stars are higher than the nuclear burning rates, so they will terminate their AGB phase by the depletion of their stellar mantles before their core can grow significantly. Some observed stars have lower mass-loss rates than the minimum value predicted by theoretical models. [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]

Probing dark matter substructure with pulsar timing

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2007
E. R. Siegel
ABSTRACT We demonstrate that pulsar timing measurements may potentially be able to detect the presence of dark matter substructure within our own Galaxy. As dark matter substructure transits near the line of sight between a pulsar and an observer, the change in the gravitational field will result in a delay of the light travel-time of photons. We calculate the effect of this delay due to transiting dark matter substructure and find that the effect on pulsar timing ought to be observable over decadal time-scales for a wide range of substructure masses and density profiles. We find that transiting dark matter substructure with masses above 10,2 M, ought to be detectable at present by these means. With improved measurements, this method may be able to distinguish between baryonic, thermal non-baryonic, and non-thermal non-baryonic types of dark matter. Additionally, information about structure formation on small scales and the density profiles of Galactic dark matter substructure can be extracted via this method. [source]

A large-scale extinction map of the Galactic Anticentre from 2MASS

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2007
D. Froebrich
ABSTRACT We present a 127 × 63 -deg2 extinction map of the Anticentre of the Galaxy, based on ,J,H, and ,H,K, colour excess maps from the Two-Micron All-Sky Survey. This 8001-deg2 map with a resolution of 4 arcmin is provided as online material. The colour excess ratio ,J,H,/,H,K, is used to determine the power-law index of the reddening law (,) for individual regions contained in the area (e.g. Orion, Perseus, Taurus, Auriga, Monoceros, Camelopardalis, Cassiopeia). On average we find a dominant value of ,= 1.8 ± 0.2 for the individual clouds, in agreement with the canonical value for the interstellar medium. We also show that there is an internal scatter of , values in these regions, and that in some areas more than one dominant , values are present. This indicates large-scale variations in the dust properties. The analysis of the AV values within individual regions shows a change in the slope of the column density distribution with distance. This can be attributed either to a change in the governing physical processes in molecular clouds on spatial scales of about 1 pc or to an AV dilution with distance in our map. [source]

Cold dark matter microhalo survival in the Milky Way

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2007
G. W. Angus
ABSTRACT A special purpose N -body simulation has been built to understand the tidal heating of the smallest dark matter substructures (10,6 M, and 0.01 pc) from the grainy potential of the Milky Way due to individual stars in the disc and the bulge. To test the method, we first run simulations of single encounters of microhaloes with an isolated star, and compare with analytical predictions of the dark particle bound fraction as a function of impact parameter. We then follow the orbits of a set of microhaloes in a realistic flattened Milky Way potential. We concentrate on (detectable) microhaloes passing near the Sun with a range of pericentre and apocentre. Stellar perturbers near the orbital path of a microhalo would exert stochastic impulses, which we apply in a Monte Carlo fashion according to the Besançon model for the distribution of stars of different masses and ages in our Galaxy. Also incorporated are the usual pericentre tidal heating and disc shocking. We give a detailed diagnosis of typical microhaloes and find microhaloes with internal tangential anisotropy are slightly more robust than the ones with radial anisotropy. In addition, the dark particles generally go through of a random walk in velocity space and diffuse out of the microhaloes. We show that the typical destruction time-scales are strongly correlated with the stellar density averaged along a microhalo's orbit over the age of the stellar disc. We also present the morphology of a microhalo at several epochs which may hold the key to dark matter detections. We checked our results against different choices of microhalo mass, virial radius and anisotropy. [source]

A systematic survey for infrared star clusters with |b| < 20° using 2MASS

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2007
D. Froebrich
ABSTRACT We used star density maps obtained from the Two-Micron All-Sky Survey (2MASS) to obtain a sample of star clusters in the entire Galactic Plane with |b| < 20°. A total of 1788 star cluster candidates are identified in this survey. Among those are 681 previously known open clusters and 86 globular clusters. A statistical analysis indicates that our sample of 1021 new cluster candidates has a contamination of about 50 per cent. Star cluster parameters are obtained by fitting a King profile to the star density. These parameters are used to statistically identify probable new globular cluster candidates in our sample. A detailed investigation of the projected distribution of star clusters in the Galaxy demonstrates that they show a clear tendency to cluster on spatial scales in the order of 12,25 pc, a typical size for molecular clouds. [source]

Populating the Galaxy with low-mass X-ray binaries

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2006
Paul D. Kiel
ABSTRACT We perform binary population-synthesis calculations to investigate the incidence of low-mass X-ray binaries (LMXBs) and their birth rate in the Galaxy. We use a binary-evolution algorithm that models all the relevant processes including tidal circularization and synchronization. Parameters in the evolution algorithm that are uncertain and may affect X-ray binary formation are allowed to vary during the investigation. We agree with previous studies that under standard assumptions of binary evolution the formation rate and number of black hole (BH) LMXBs predicted by the model are more than an order of magnitude less than what is indicated by observations. We find that the common-envelope process cannot be manipulated to produce significant numbers of BH LMXBs. However, by simply reducing the mass-loss rate from helium stars adopted in the standard model, to a rate that agrees with the latest data, we produce a good match to the observations. Including LMXBs that evolve from intermediate-mass systems also leads to favourable results. We stress that constraints on the X-ray binary population provided by observations are used here merely as a guide as surveys suffer from incompleteness and much uncertainty is involved in the interpretation of results. [source]

Weighing the young stellar discs around Sgr A*

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2006
Sergei Nayakshin
ABSTRACT It is believed that young massive stars orbiting Sgr A* in two stellar discs on scales of , 0.1,0.2 parsec were formed either farther out in the Galaxy and then quickly migrated inwards or in situ in a massive self-gravitating disc. Comparing N -body evolution of stellar orbits with observational constraints, we set upper limits on the masses of the two stellar systems. These masses turn out to be a few times lower than the expected total stellar mass estimated from the observed young high-mass stellar population and the standard galactic initial mass function (IMF). If these stars were formed in situ, in a massive self-gravitating disc, our results suggest that the formation of low-mass stars was suppressed by a factor of at least a few, requiring a top-heavy IMF for stars formed near Sgr A*. [source]

Monitoring lensed starlight emitted close to the Galactic centre

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2004
Adi Nusser
ABSTRACT We describe the feasibility of detecting the gravitational deflection of light emitted by stars moving around the massive object at the Galactic centre. Light reaching us from an orbiting star can pass closer to the large central mass than the star itself, so the central potential is in principle constrained to a smaller radius by lensing rather than by orbit fitting. A mass of 4.3 × 106 M, causes a 0.1,2 mas deflection in the apparent position of orbiting stars in a cone of diameter 10° as seen from the central mass. The distance to the centre of the Galaxy is uniquely constrained from such measurements because lensing deflections depend on the ratio rg/R0 of the Schwarzschild radius to the distance to the black hole, R0, whereas the ratio rg/R30 is obtained by fitting the shapes of the orbits. Deflection measurements are complimentary to observations of radial velocities of stars which constrain the ratio rg/R0 in the framework of Newtonian gravity. [source]

Star cluster ecology , V. Dissection of an open star cluster: spectroscopy

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2004
Simon F. Portegies Zwart
ABSTRACT We have modelled in detail the evolution of rich open star clusters such as NGC 2516, NGC 2287, Pleiades, Praesepe, Hyades, NGC 2660 and 3680, using simulations that include stellar dynamics as well as the effects of stellar evolution. The dynamics is modelled via direct N -body integration, while the evolution of single stars and binaries is followed through the use of fitting formulae and recipes. The feedback of stellar and binary evolution on the dynamical evolution of the stellar system is taken into account self-consistently. Our model clusters dissolve in the tidal field of the Galaxy in a time-span of the order of a billion years. The rate of mass loss is rather constant, ,1 M, per million years. The binary fraction at first is nearly constant in time, then increases slowly near the end of a cluster's lifetime. For clusters which are more than about 108 yr old the fractions of stars in the form of binaries, giants and merger products in the inner few core radii are considerably higher than in the outer regions, beyond the cluster's half-mass radius. When stars with masses ,2 M, escape from the cluster, they tend to do so with velocities higher than average. The stellar merger rate in our models is roughly one per 30 million years. Most mergers are the result of unstable mass transfer in close binaries (,70 per cent), but a significant minority are caused by direct encounters between single and binary stars. While most mergers occur within the cluster core, even beyond the half-mass radius stellar mergers occasionally take place. We notice a significant birth rate of X-ray binaries, most containing a white dwarf as the mass acceptor. We also find one high-mass X-ray binary with a neutron-star accretor. If formed and retained, black holes participate in many (higher-order) encounters in the cluster centre, resulting in a large variety of exotic binaries. The persistent triple and higher-order systems formed in our models by dynamical encounters between binaries and single stars are not representative for the multiple systems observed in the Galactic disc. We conclude that the majority of multiples in the disc probably formed when the stars were born, rather than through later dynamical interactions. [source]

Isochrone ages for field dwarfs: method and application to the age,metallicity relation

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2004
Frédéric Pont
ABSTRACT A new method is presented to compute age estimates from theoretical isochrones using temperature, luminosity and metallicity data for individual stars. Based on Bayesian probability theory, this method avoids the systematic biases affecting simpler strategies and provides reliable estimates of the age probability distribution function for late-type dwarfs. Basic assumptions concerning the a priori parameter distribution suitable for the solar neighbourhood are combined with the likelihood assigned to the observed data to yield the complete posterior age probability. This method is especially relevant for G dwarfs in the 3,15 Gyr range of ages, crucial to the study of the chemical and dynamical history of the Galaxy. In many cases, it yields markedly different results from the traditional approach of reading the derived age from the isochrone nearest to the data point. We show that the strongest process affecting the traditional approach is that of strongly favouring computed ages near the end-of-main-sequence lifetime. The Bayesian method compensates for this potential bias and generally assigns much higher probabilities to lower main-sequence ages, compared with short-lived evolved stages. This has a strong influence on any application to galactic studies, especially given the present uncertainties on the absolute temperature scale of the stellar evolution models. In particular, the known mismatch between the model predictions and the observations for moderately metal-poor dwarfs (,1 < [Fe/H] < ,0.3) has a dramatic effect on the traditional age determination. We apply our method to the classic sample of Edvardsson et al., who derived the age,metallicity relation (AMR) of 189 field dwarfs with precisely determined abundances. We show how much of the observed scatter in the AMR is caused by the interplay between the systematic biases affecting the traditional age determination, the colour mismatch with the evolution models and the presence of undetected binaries. Using new parallax, temperature and metallicity data, our age determination for the same sample indicates that the intrinsic dispersion in the AMR is at most 0.15 dex and probably lower. In particular, we show that old, metal-rich objects ([Fe/H], 0.0 dex, age > 5 Gyr) and young, metal-poor objects ([Fe/H] < ,0.5 dex, age < 6 Gyr) in many observed AMR plots are artefacts caused by too simple a treatment of the age determination. The incompatibility of those AMR plots with a well-mixed interstellar medium may therefore only be apparent. Incidentally, our results tend to restore confidence in the method of age determination from the chromospheric activity for field dwarfs. [source]

Is the dark halo of our Galaxy spherical?

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2004
Amina Helmi
ABSTRACT It has been recently claimed that the confined structure of the debris from the Sagittarius dwarf implies that the dark matter halo of our Galaxy should be nearly spherical, in strong contrast with predictions from cold dark matter simulations, where dark haloes are found to have typical density axis ratios of 0.6,0.8. In this paper, numerical simulations are used to show that the Sagittarius streams discovered thus far are too young dynamically to be sensitive to the shape of the dark halo of the Milky Way. The data presently available are entirely consistent with a Galactic dark matter halo that could be either oblate or prolate, with minor-to-major density axis ratios as low as 0.6 within the region probed by the orbit of the Sagittarius dwarf. [source]

Formation of , Centauri from an ancient nucleated dwarf galaxy in the young Galactic disc

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2003
K. Bekki
ABSTRACT We first present a self-consistent dynamical model in which , Cen is formed from an ancient nucleated dwarf galaxy merging with the first generation of the Galactic thin disc in a retrograde manner with respect to the Galactic rotation. Our numerical simulations demonstrate that during merging between the Galaxy and the , Cen host dwarf with MB,,14 mag and its nucleus mass of 107 M,, the outer stellar envelope of the dwarf is nearly completely stripped, whereas the central nucleus can survive from the tidal stripping because of its compactness. The developed naked nucleus has a very bound retrograde orbit around the young Galactic disc, as observed for , Cen, with apocentre and pericentre distances of ,8 and ,1 kpc, respectively. The Galactic tidal force can induce radial inflow of gas to the centre of the dwarf and consequently triggers moderately strong nuclear starbursts in a repetitive manner. This result implies that efficient nuclear chemical enrichment resulting from the later starbursts can be closely associated with the origin of the observed relatively young and metal-rich stars in , Cen. Dynamical heating by the , Cen host can transform the young thin disc into the thick disc during merging. [source]