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Stellar Mass (stellar + mass)
Terms modified by Stellar Mass Selected AbstractsLong-term monitoring in IC4665: fast rotation and weak variability in very low mass objectsMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2009Alexander Scholz ABSTRACT We present the combined results of three photometric monitoring campaigns targeting very low mass (VLM) stars and brown dwarfs in the young open cluster IC4665 (age ,40 Myr). Each of our observing runs covers time-scales of ,5 d in the seasons 1999, 2001 and 2002, respectively. In all three runs, we observe ,100 cluster members, allowing us for the first time to put limits on the evolution of spots and magnetic activity in fully convective objects on time-scales of a few years. For 20 objects covering masses from 0.05 to 0.5 M,, we detect a periodic flux modulation, indicating the presence of magnetic spots co-rotating with the objects. The detection rate of photometric periods (,20 per cent) is significantly lower than in solar-mass stars at the same age, which points to a mass dependence in the spot properties. With two exceptions, none of the objects exhibits variability and thus spot activity in more than one season. This is contrary to what is seen in solar-mass stars and indicates that spot configurations capable of producing photometric modulations occur relatively rarely and are transient in VLM objects. The rotation periods derived in this paper range from 3 to 30 h, arguing for a lack of slow rotators among VLM objects. The periods fit into a rotational evolution scenario with pre-main sequence contraction and moderate (40,50 per cent) angular momentum losses due to wind braking. By combining our findings with literature results, we identify two regimes of rotational and magnetic properties, called C- and I-sequence. Main properties on the C-sequence are fast rotation, weak wind braking, H, emission and saturated activity levels, while the I-sequence is characterized by slow rotation, strong wind braking, no H, emission and linear activity-rotation relationship. Rotation rate and stellar mass are the primary parameters that determine in which regime an object is found. We outline a general scheme to understand rotational evolution for low-mass objects in the context of these two regimes and discuss the potential as well as the problems of this picture. [source] A QSO host galaxy and its Ly, emission at z= 6.43,MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2009Tomotsugu 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] The disc mass of spiral galaxiesMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2008Paolo Salucci ABSTRACT We derive the disc masses of 18 spiral galaxies of different luminosity and Hubble type, both by mass modelling their rotation curves and by fitting their spectral energy distribution with spectrophotometric models. The good agreement of the estimates obtained from these two different methods allows us to quantify the reliability of their performance and to derive very accurate stellar mass-to-light ratio versus colour (and stellar mass) relationships. [source] Galaxy growth in the concordance ,CDM cosmologyMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2008Q. Guo ABSTRACT We use galaxy and dark halo data from the public database for the Millennium Simulation to study the growth of galaxies in the De Lucia et al. model for galaxy formation. Previous work has shown this model to reproduce many aspects of the systematic properties and the clustering of real galaxies, both in the nearby universe and at high redshift. It assumes the stellar masses of galaxies to increase through three processes, major mergers, the accretion of smaller satellite systems and star formation. We show the relative importance of these three modes to be a strong function of stellar mass and redshift. Galaxy growth through major mergers depends strongly on stellar mass, but only weakly on redshift. Except for massive systems, minor mergers contribute more to galaxy growth than major mergers at all redshifts and stellar masses. For galaxies significantly less massive than the Milky Way, star formation dominates the growth at all epochs. For galaxies significantly more massive than the Milky Way, growth through mergers is the dominant process at all epochs. At a stellar mass of 6 × 1010 M,, about that of the Milk Way, star formation dominates at z > 1 and mergers at later times. At every stellar mass, the growth rates through star formation increase rapidly with increasing redshift. Specific star formation rates are the decreasing function of stellar mass not only at z= 0 but also at all higher redshifts. For comparison, we carry out a similar analysis of the growth of dark matter haloes. In contrast to the galaxies, growth rates depend strongly on redshift, but only weakly on mass. They agree qualitatively with analytic predictions for halo growth. [source] A census of metals and baryons in stars in the local UniverseMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2008Anna Gallazzi ABSTRACT We combine stellar metallicity and stellar mass estimates for a large sample of galaxies drawn from the Sloan Digital Sky Survey Data Release 2 (SDSS DR2) spanning wide ranges in physical properties, in order to derive an inventory of the total mass of metals and baryons locked up in stars in the local Universe. Physical parameter estimates are derived from galaxy spectra with high signal-to-noise ratio (S/N) (of at least 20). Co-added spectra of galaxies with similar velocity dispersions, absolute r -band magnitudes and 4000-Ĺ break values are used for those regions of parameter space where individual spectra have lower S/N. We estimate the total density of metals ,Z and of baryons ,* in stars and, from these two quantities, we obtain a mass- and volume-averaged stellar metallicity of ,Z*,= 1.04 ± 0.14 Z,, i.e. consistent with solar. We also study how metals are distributed in galaxies according to different properties, such as mass, morphology, mass- and light-weighted age, and we then compare these distributions with the corresponding distributions of stellar mass. We find that the bulk of metals locked up in stars in the local Universe reside in massive, bulge-dominated galaxies, with red colours and high 4000-Ĺ break values corresponding to old stellar populations. Bulge-dominated and disc-dominated galaxies contribute similar amounts to the total stellar mass density, but have different fractional contributions to the mass density of metals in stars, in agreement with the mass,metallicity relation. Bulge-dominated galaxies contain roughly 40 per cent of the total amount of metals in stars, while disc-dominated galaxies less than 25 per cent. Finally, at a given galaxy stellar mass, we define two characteristic ages as the median of the distributions of mass and metals as a function of age. These characteristic ages decrease progressively from high-mass to low-mass galaxies, consistent with the high formation epochs of stars in massive galaxies. [source] The UV properties of E+A galaxies: constraints on feedback-driven quenching of star formationMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2007S. 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] The dynamical formation of LMXBs in dense stellar environments: globular clusters and the inner bulge of M31MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2007R. Voss ABSTRACT The radial distribution of luminous (LX > 1036 erg s,1) X-ray point sources in the bulge of M31 is investigated using archival Chandra observations. We find a significant increase in the specific frequency of X-ray sources, per unit stellar mass, within 1 arcmin from the centre of the galaxy. The radial distribution of surplus sources in this region follows the ,2* law, suggesting that they are low-mass X-ray binaries (LMXBs) formed dynamically in the dense inner bulge. We investigate dynamical formation of LMXBs, paying particular attention to the high-velocity regime characteristic for galactic bulges, which has not been explored previously. Our calculations suggest that the majority of the surplus sources are formed in tidal captures of black holes by main-sequence stars of low mass, M*, 0.3,0.4 M,, with some contribution of neutron star (NS) systems of same type. Due to the small size of the accretion discs, a fraction of such systems may be persistent X-ray sources. Some of the sources may be ultracompact X-ray binaries with helium star/white dwarf companions. We also predict a large number of faint transients, both NS and BH systems, within ,1 arcmin from the M31 galactic centre. Finally, we consider the population of dynamically formed binaries in Galactic globular clusters, emphasizing the differences between these two types of stellar environments. [source] Constraints on the merging time-scale of luminous red galaxies, or, where do all the haloes go?MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2007Charlie Conroy ABSTRACT In the , cold dark matter cosmology, dark matter haloes grow primarily through the accretion of smaller haloes. Much of the mass in a halo of 1014 M, comes in through accretion of ,1013 M, haloes. If each such halo hosted one luminous red galaxy (LRG) then the accretion of so many haloes is at odds with the observed number of LRGs in clusters unless these accreted LRGs merge or disrupt on relatively short time-scales (,2 Gyr). These time-scales are consistent with classical dynamical friction arguments, and imply that two to three LRGs have merged or disrupted within each halo more massive than 1014 M, by z= 0. The total amount of stellar mass brought into these massive haloes by z= 0 is consistent with observations once the intracluster light (ICL) is included. If disrupted LRGs build up the ICL, then the hierarchical growth of massive haloes implies that a substantial amount of ICL should also surround satellite LRGs, as suggested by recent observations of the Virgo cluster. Finally, we point out that these results are entirely consistent with a non-evolving clustering strength and halo occupation distribution, and note that observations of the latter in fact support the hypothesis that merging/disruption of massive galaxies does indeed take place at late times. [source] The first appearance of the red sequence of galaxies in proto-clusters at 2 ,z, 3MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2007Tadayuki Kodama ABSTRACT We explore the evolved galaxy population in the proto-clusters around four high- z radio galaxies at 2 ,z, 3 based on wide-field near-infrared (NIR) imaging. Three of the four fields are known proto-clusters as demonstrated by overdensities of line-emitting galaxies at the same redshifts as the radio galaxies found by narrow-band surveys and spectroscopic follow-up observations. We imaged the fields of three targets (PKS 1138,262, USS 0943,242 and MRC 0316,257) to a depth of Ks, 22 (Vega magnitude, 5,) over a 4 × 7 arcmin2 area centred on the radio galaxies with a new wide-field NIR camera, Multi-Object Infra-Red Camera and Spectrograph (MOIRCS), on the Subaru Telescope. Another target (USS 1558,003) was observed with Son of ISAAC on the New technology Telescope (NTT) to a depth of Ks= 20.5 (5,) over a 5 × 5 arcmin2 area. We apply colour cuts in J,Ks and/or JHKs in order to exclusively search for galaxies located at high redshifts: z > 2. To the 5, limiting magnitudes, we see a significant excess of NIR-selected galaxies by a factor of 2 to 3 compared to those found in the field of GOODS-South. The spatial distribution of these NIR-selected galaxies is not uniform and traces structures similar to those of emission-line galaxies, although the samples of NIR-selected galaxies and emitters show little overlap, from which we conclude that the former tend to be an evolved population with much higher stellar mass than the latter, young and active emitters. We focus on the NIR colour,magnitude sequence of the evolved population and find that the bright-end (Mstars > 1011 M,) of the red sequence is well populated by z, 2 but much less so in the z, 3 proto-clusters. This may imply that the bright-end of the colour,magnitude sequence first appeared between z= 3 and 2, an era coinciding with the appearance of sub-mm galaxies and the peak of the cosmic star formation rate. Our observations show that during the same epoch, massive galaxies are forming in high-density environments by vigorous star formation and assembly. [source] The clustering of narrow-line AGN in the local UniverseMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2006Cheng 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-Ĺ 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] Emission-line diagnostics of low-metallicity active galactic nucleiMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2006Brent A. Groves ABSTRACT Current emission-line-based estimates of the metallicity of active galactic nuclei (AGN) at both high and low redshifts indicate that AGN have predominantly solar-to-supersolar metallicities. This leads to the question: do low-metallicity AGN exist? In this paper, we use photoionization models to examine the effects of metallicity variations on the narrow emission-lines from an AGN. We explore a variety of emission-line diagnostics that are useful for identifying AGN with low-metallicity gas. We find that line ratios involving [N ii] are the most robust metallicity indicators in galaxies where the primary source of ionization is from the active nucleus. Ratios involving [S ii] and [O i] are strongly affected by uncertainties in modelling the density structure of the narrow-line clouds. To test our diagnostics, we turn to an analysis of AGN in the Sloan Digital Sky Survey (SDSS). We find a clear trend in the relative strength of [N ii] with the mass of the AGN-host galaxy. The metallicity of the ISM is known to be correlated with stellar mass in star-forming galaxies; our results indicate that a similar trend exists for AGN. We also find that the best-fitting models for typical Seyfert narrow-line regions (NLRs) have supersolar abundances. Although there is a mass-dependent range of a factor of 2,3 in the NLR metallicities of the AGN in our sample, AGN with subsolar metallicities are very rare in the SDSS. Out of a sample of ,23 000 Seyfert 2 galaxies, we find only ,40 clear candidates for AGN with NLR abundances that are below solar. [source] Fragmentation of star-forming clouds enriched with the first dustMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2006Raffaella Schneider ABSTRACT The thermal and fragmentation properties of star forming clouds have important consequences on the corresponding characteristic stellar mass. The initial composition of the gas within these clouds is a record of the nucleosynthetic products of previous stellar generations. In this paper, we present a model for the evolution of star forming clouds enriched by metals and dust from the first supernovae (SNe), resulting from the explosions of metal-free progenitors with masses in the range 12,30 M, and 140,260 M,. Using a self-consistent approach, we show that: (i) metals depleted on to dust grains play a fundamental role, enabling fragmentation to solar or subsolar mass scales already at metallicities Zcr= 10,6 Z,; (ii) even at metallicities as high as 10,2 Z,, metals diffused in the gas phase lead to fragment mass scales which are ,100 M,; (iii) C atoms are strongly depleted on to amorphous carbon grains and CO molecules so that C ii plays a minor role in gas cooling, leaving O i as the main gas-phase cooling agent in low-metallicity clouds. These conclusions hold independently of the assumed SN progenitors and suggest that the onset of low-mass star formation is conditioned to the presence of dust in the parent clouds. [source] Weighing the young stellar discs around Sgr A*MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2006Sergei 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] The properties of Ly, emitting galaxies in hierarchical galaxy formation modelsMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2006M. Le Delliou ABSTRACT We present detailed predictions for the properties of Ly,-emitting galaxies in the framework of the , cold dark matter cosmology, calculated using the semi-analytical galaxy formation model galform. We explore a model that assumes a top-heavy initial mass function in starbursts and that has previously been shown to explain the sub-millimetre number counts and the luminosity function of Lyman-break galaxies at high redshift. We show that this model, with the simple assumption that a fixed fraction of Ly, photons escape from each galaxy, is remarkably successful at explaining the observed luminosity function of Ly, emitters (LAEs) over the redshift range 3 < z < 6.6. We also examine the distribution of Ly, equivalent widths and the broad-band continuum magnitudes of emitters, which are in good agreement with the available observations. We look more deeply into the nature of LAEs, presenting predictions for fundamental properties such as the stellar mass and radius of the emitting galaxy and the mass of the host dark matter halo. The model predicts that the clustering of LAEs at high redshifts should be strongly biased relative to the dark matter, in agreement with observational estimates. We also present predictions for the luminosity function of LAEs at z > 7, a redshift range that is starting to be be probed by near-infrared surveys and using new instruments such as the Dark Ages Z Lyman Explorer (DAzLE). [source] Extracting star formation histories from medium-resolution galaxy spectraMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2006H. Mathis ABSTRACT We adapt an existing data compression algorithm, moped, to the extraction of median-likelihood star formation histories from medium-resolution galaxy spectra. By focusing on the high-pass components of galaxy spectra, we minimize potential uncertainties arising from the spectrophotometric calibration and intrinsic attenuation by dust. We validate our approach using model high-pass spectra of galaxies with different star formation histories covering the wavelength range 3650,8500 Ĺ at a resolving power of ,2000. We show that the method can recover the full star formation histories of these models, without prior knowledge of the metallicity, to within an accuracy that depends sensitively on the signal-to-noise ratio. The investigation of the sensitivity of the flux at each wavelength to the mass fraction of stars of different ages allows us to identify new age-sensitive features in galaxy spectra. We also highlight a fundamental limitation in the recovery of the star formation histories of galaxies for which the optical signatures of intermediate-age stars are masked by those of younger and older stars. As an example of application, we use this method to derive average star formation histories from the highest-quality spectra of typical (in terms of their stellar mass), morphologically identified early- and late-type galaxies in the Early Data Release (EDR) of the Sloan Digital Sky Survey (SDSS). We find that, in agreement with the common expectation, early-type galaxies must have formed most of their stars over 8 Gyr ago, although a small fraction of the total stellar mass of these galaxies may be accounted for by stars with ages down to 4 Gyr. In contrast, late-type galaxies appear to have formed stars at a roughly constant rate. We also investigate the constraints set by the high-pass signal in the stacked spectra of a magnitude-limited sample of 20 623 SDSS-EDR galaxies on the global star formation history of the Universe and its distribution among galaxies in different mass ranges. We confirm that the stellar populations in the most massive galaxies today appear to have formed on average earlier than those in the least massive galaxies. Our results do not support the recent suggestion of a statistically significant peak in the star formation activity of the Universe at redshifts below unity, although such a peak is not ruled out. [source] LO Peg in 1998: star-spot patterns and differential rotationMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2005J. R. Barnes ABSTRACT We present Doppler images of the young K5V,K7V rapid rotator LO Peg from seven nights of continuous spectroscopy obtained in 1998 from July 04 to July 10. The images reveal the presence of a strong polar cap with appendages extending to mid-latitudes, but no star-spots are seen below 15°. We briefly discuss the distribution of spots in light of recent flux transport simulations, which are able to reproduce the observed latitude dependence. With the full time series of spectra, of which 314 are useful, many phases are observed three times over the seven nights of observations. Using star-spots as tracers of a solar-like latitudinal differential rotation in our image reconstructions, we find that the equatorial regions complete one more rotation than the polar regions every 181 ± 35 d. LO Peg is the second coolest star for which such a measurement has been made using indirect imaging methods. The degree of latitudinal shear is less than that seen in G and early K dwarfs, suggesting a trend in which differential rotation decreases with stellar mass in (pre-)main-sequence objects. [source] Thin discs, thick dwarfs and the effects of stellar feedbackMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY: LETTERS (ELECTRONIC), Issue 1 2010R. Sánchez-Janssen ABSTRACT We investigate the role of stellar mass in shaping the intrinsic thickness of galaxy discs by determining the probability distribution of apparent axial ratios (b/a) for two different samples that probe the faint end of the galaxy luminosity function. We find that the b/a distribution has a characteristic ,U-shape' and identify a limiting mass M*, 2 × 109 M, below which low-mass galaxies start to be systematically thicker. This tendency holds for very faint (MB,,8) dwarfs in the local volume, which are essentially spheroidal systems. We argue that galaxy shape is the result of the complex interplay between mass, specific angular momentum and stellar feedback effects. Thus, the increasing importance of turbulent motions in lower mass galaxies leads to the formation of thicker systems, a result supported by the latest hydrodynamical simulations of dwarf galaxy formation and other theoretical expectations. We discuss several implications of this finding, including the formation of bars in faint galaxies, the deprojection of H i line profiles and simulations of environmental effects on the dwarf galaxy population. [source] The ,,M relationship in pre-main sequence starsMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY: LETTERS (ELECTRONIC), Issue 1 2006C. J. Clarke ABSTRACT We examine the recent data and analysis of Natta et al. concerning the accretion rate on to young stars as a function of stellar mass, and conclude that the apparently steep dependence of accretion rate on mass is strongly driven by selection/detection thresholds. We argue that a convincing demonstration of a physical relationship between accretion and stellar mass requires further studies which, as is the case for Natta et al., include information on upper limits, and which quantify the possible incompleteness of the sample, at both low and high accretion rates. We point out that the distribution of detections in the -plane can in principle be used to test conventional accretion disc evolutionary models, and that higher sensitivity observations might be able to test the hypothesis of accelerated disc clearing at late times. [source] Helium pre-enrichment in the first starsMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY: LETTERS (ELECTRONIC), Issue 1 2006Leonid Chuzhoy ABSTRACT We show that element diffusion can produce large fluctuations in the initial helium abundance of stars. Diffusion time-scale, which in stellar cores is much larger than the Hubble time, can fall below 108 yr in the neutral gas clouds of stellar mass, dominated by collisionless dark matter or with dynamically important radiation or magnetic pressure. Helium diffusion may therefore explain the recent observations of globular clusters, which are inconsistent with initially homogeneous helium distribution. [source] Dust attenuation in starburst galaxies determined by measuring the dependence of the optical color indices on galaxy inclinationASTRONOMISCHE NACHRICHTEN, Issue 7 2010O. Vince Abstract We use optical color indices (colors) from the SDSS database to study the effect of dust in starburst galaxies by mea-suring the dependence of colors on galaxy inclination. Starburst galaxies with ongoing star formation, are rich with metals/dust and are, therefore, an excellent objects for studying the effect of dust in galaxies. They are selected using the [O III], 5007/H, vs. [N II], 6584/H, diagram, that is, the BPT-diagram. We use Kauffmann's empirical demarcation line in the BPT-diagram to exclude galaxies with active galactic nuclei (AGN) from the sample because they have different physical and dust properties from normal galaxies. The sample is divided into bins according to galaxy stellar mass and 4000 Ĺ break (which is a coarse measure of a galaxy star formation history; SFH) and the reddening with inclination is studied as a function of these two physical parameters. Assuming that the dust effect is negligible in the SDSS z -band, we derive the attenuation curves for these galaxies. We fit the attenuation curves with a simple power law and use power law index to interpret the relative distribution of dust and stars in the starburst galaxies (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] The giant Ly, nebula associated with a z = 2.5 radio galaxyASTRONOMISCHE NACHRICHTEN, Issue 2-3 2006M. Villar-Martín Abstract We present a kinematic and morphological study of the giant Ly, nebula associated with the radio galaxy MRC 2104,242 (z = 2.49) based on integral field spectroscopic VIMOS data from VLT. The galaxy appears to be embedded in a giant (,120 kpc) gas reservoir that surrounds it completely. The kinematic properties of the nebula suggest that it is a rotating structure, which would imply a lower limit to the dynamical mass of ,3 × 1011 M,. An alternate scenario is that the gas is infalling. Such a process would be able to initiate and sustain significant central starburst activity, although it is likely to contribute with less than 10% of the total stellar mass of the galaxy. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] On the interstellar medium and star formation demographics of galaxies in the local universeMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2009Matthew S. Bothwell ABSTRACT We present a demographic analysis of integrated star formation and gas properties for a sample of galaxies representative of the overall population at z, 0. This research was undertaken in order to characterize the nature of star formation and interstellar medium (ISM) behaviour in the local Universe, and test the extent to which global star formation rates (SFRs) can be seen as dependent on the interstellar gas content. Archival 21-cm derived H i data are compiled from the literature, and are combined with CO (J =1 , 0) derived H2 masses to calculate and characterize the total gas content for a large sample of local galaxies. The distribution in stellar mass-normalized H i content is found to exhibit the noted characteristic transition at stellar masses of ,3 × 1010 M,, turning off towards low values, but no such transition is observed in the equivalent distribution of molecular gas. H, based SFRs and specific star formation rates (SSFRs) are also compiled for a large (1110) sample of local galaxies. We confirm two transitions as found in previous work: a turnover towards low SFRs at high luminosities, indicative of the quenching of SF characteristic of the red sequence; and a broadening of the SF distribution in low-luminosity dwarf galaxies, again to extremely low SFRs of <10,3 M, yr,1. However, a new finding is that while the upper luminosity transition is mirrored by the turnover in H i content, suggesting that the low SFRs of the red sequence result from a lack of available gas supply, the transition towards a large spread of SFRs in the least luminous dwarf galaxies is not matched by a prominent increase in scatter in gas content. Possible mass-dependent quenching mechanisms are discussed, along with speculations that in low-mass galaxies, the H, luminosity may not faithfully trace the SFR. [source] Why are AGN found in high-mass galaxies?MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2008Lan 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] The star formation efficiency and its relation to variations in the initial mass functionMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2008Paul C. Clark ABSTRACT We investigate how the dynamical state of a turbulently supported, 1000 M,, molecular cloud affects the properties of the cluster it forms, focusing our discussion on the star formation efficiency (SFE) and the initial mass function (IMF). A variety of initial energy states are examined in this paper, ranging from clouds with |Egrav| = 0.1 Ekin to clouds with |Egrav| = 10 Ekin, and for both isothermal and piece-wise polytropic equations of state (similar to that suggested by Larson). It is found that arbitrary SFEs are possible, with strongly unbound clouds yielding very low SFEs. We suggest that the low SFE in the Maddelena cloud may be a consequence of the relatively unbound state of its internal structure. It is also found that competitive accretion results in the observed IMF when the clouds have initial energy states of |Egrav| ,Ekin. We show that under such conditions the shape of the IMF is independent of time in the calculations. This demonstrates that the global accretion process can be terminated at any stage in the cluster's evolution, while still yielding a distribution of stellar masses that is consistent with the observed IMF. As the clouds become progressively more unbound, competitive accretion is less important and the protostellar mass function flattens. These results predict that molecular clouds should be permeated with a distributed population of stars that follow a flatter than Salpeter IMF. [source] Galaxy growth in the concordance ,CDM cosmologyMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2008Q. Guo ABSTRACT We use galaxy and dark halo data from the public database for the Millennium Simulation to study the growth of galaxies in the De Lucia et al. model for galaxy formation. Previous work has shown this model to reproduce many aspects of the systematic properties and the clustering of real galaxies, both in the nearby universe and at high redshift. It assumes the stellar masses of galaxies to increase through three processes, major mergers, the accretion of smaller satellite systems and star formation. We show the relative importance of these three modes to be a strong function of stellar mass and redshift. Galaxy growth through major mergers depends strongly on stellar mass, but only weakly on redshift. Except for massive systems, minor mergers contribute more to galaxy growth than major mergers at all redshifts and stellar masses. For galaxies significantly less massive than the Milky Way, star formation dominates the growth at all epochs. For galaxies significantly more massive than the Milky Way, growth through mergers is the dominant process at all epochs. At a stellar mass of 6 × 1010 M,, about that of the Milk Way, star formation dominates at z > 1 and mergers at later times. At every stellar mass, the growth rates through star formation increase rapidly with increasing redshift. Specific star formation rates are the decreasing function of stellar mass not only at z= 0 but also at all higher redshifts. For comparison, we carry out a similar analysis of the growth of dark matter haloes. In contrast to the galaxies, growth rates depend strongly on redshift, but only weakly on mass. They agree qualitatively with analytic predictions for halo growth. [source] The stellar mass density at z, 6 from Spitzer imaging of i,-drop galaxiesMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2007Laurence P. Eyles ABSTRACT We measure the ages, stellar masses, and star formation histories of z, 6 galaxies, observed within 1 Gyr of the big bang. We use imaging from the Hubble Space Telescope (HST) and the Spitzer Space Telescope from the public ,Great Observatories Origins Deep Survey' (GOODS), coupled with ground-based near-infrared imaging, to measure their spectral energy distributions (SEDs) from 0.8,5 ,m, spanning the rest-frame ultraviolet (UV) and optical. From our sample of ,50 ,i,-drop' Lyman-break star-forming galaxies in GOODS-South with z,AB < 27, we focus on ,30 with reliable photometric or spectroscopic redshifts. Half of these are confused with foreground sources at Spitzer resolution, but from the 16 with clean photometry we find that a surprisingly large fraction (40 per cent) have evidence for substantial Balmer/4000-Ĺ spectral breaks. This indicates the presence of old underlying stellar populations that dominate the stellar masses. For these objects, we find ages of ,200,700 Myr, implying formation redshifts of 7 ,zf, 18, and large stellar masses in the range ,1,3 × 1010 M,. Analysis of seven i,-drops that are undetected at 3.6 ,m indicates that these are younger, considerably less massive systems. We calculate that emission line contamination should not severely affect our photometry or derived results. Using SED fits out to 8 ,m, we find little evidence for substantial intrinsic dust reddening in our sources. We use our individual galaxy results to obtain an estimate of the global stellar mass density at z, 6. Correcting for incompleteness in our sample, we find the z, 6 comoving stellar mass density to be 2.5 × 106 M, Mpc,3. This is a lower limit, as post-starburst and dust-obscured objects, and also galaxies below our selection thresholds, are not accounted for. From our results, we are able to explore the star formation histories of our selected galaxies, and we suggest that the past global star formation rate may have been much higher than that observed at the z, 6 epoch. The associated UV flux we infer at z > 7 could have played a major role in reionizing the Universe. [source] A weak lensing estimate from GEMS of the virial to stellar mass ratio in massive galaxies to z, 0.8MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY: LETTERS (ELECTRONIC), Issue 1 2006Catherine Heymans ABSTRACT We present constraints on the evolution of the virial to stellar mass ratio of galaxies with high stellar masses in the redshift range 0.2 < z < 0.8, by comparing weak lensing measurements of virial mass Mvir with estimates of stellar mass Mstar. For a complete sample of galaxies with log (Mstar/M,) > 10.5, where the majority show an early-type morphology, we find that the virial mass to stellar mass ratio is given by Mvir/Mstar= 53+13,16. Assuming a baryon fraction from the concordance cosmology, this corresponds to a stellar fraction of baryons in massive galaxies of ,*b/,b= 0.10 ± 0.03. Analysing the galaxy sample in different redshift slices, we find little or no evolution in the virial to stellar mass ratio, and place an upper limit of ,2.5 on the growth of massive galaxies through the conversion of gas into stars from z= 0.8 to the present day. [source] | ||||||||||