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1.
We use the 2dF Galaxy Redshift Survey to measure the dependence of the b J-band galaxy luminosity function on large-scale environment, defined by density contrast in spheres of radius  8  h −1 Mpc  , and on spectral type, determined from principal component analysis. We find that the galaxy populations at both extremes of density differ significantly from that at the mean density. The population in voids is dominated by late types and shows, relative to the mean, a deficit of galaxies that becomes increasingly pronounced at magnitudes brighter than   M b J−5log10 h ≲−18.5  . In contrast, cluster regions have a relative excess of very bright early-type galaxies with   M b J−5log10 h ≲−21  . Differences in the mid- to faint-end population between environments are significant: at   M b J−5log10 h =−18  early- and late-type cluster galaxies show comparable abundances, whereas in voids the late types dominate by almost an order of magnitude. We find that the luminosity functions measured in all density environments, from voids to clusters, can be approximated by Schechter functions with parameters that vary smoothly with local density, but in a fashion that differs strikingly for early- and late-type galaxies. These observed variations, combined with our finding that the faint-end slope of the overall luminosity function depends at most weakly on density environment, may prove to be a significant challenge for models of galaxy formation.  相似文献   

2.
Photometric redshifts are used to determine the rest-frame luminosity function (LF) of both early- and late-type galaxies to  MB∼−17.6  for the cluster Cl 1601+42 at  z=0.54  . The total LF shows a steep faint-end slope   α ∼−1.4  , indicating the existence of a population of numerous dwarf galaxies. Luminous galaxies, with  MB≲−19.5  are mostly red, early-type galaxies, with a LF best described by a Gaussian. Faint galaxies are predominantly blue, late-type galaxies, well fitted by a Schechter function with   α ∼−1.7  . Compared with clusters at lower redshift, the steepening of the faint end starts at brighter magnitudes for Cl 1601+42, which may indicate a brightening of the present-day dwarf population relative to the giant population with increasing redshift. Early-type galaxies are centrally concentrated, and dominate the core region, implying that the radial gradient of early-type galaxies seen in local clusters is already established at  z∼0.5  . Bright, late-type galaxies are rare, consistent with a decrease in star formation in field galaxies as they are accreted on to the cluster, while faint, blue galaxies are evenly distributed across the cluster, except for a depletion in the core region. The blue fraction is  fB∼0.15  , which is somewhat lower than the Butcher–Oemler average at  z∼0.5  . The value of f B is found to increase with limiting magnitude and with radius from the centre.  相似文献   

3.
We investigate the properties of satellite galaxies formed in N -body/SPH simulations of galaxy formation in the ΛCDM cosmology. The simulations include the main physical effects thought to be important in galaxy formation and, in several cases, produce realistic spiral discs. In total, a sample of nine galaxies of luminosity comparable to the Milky Way was obtained. At magnitudes brighter than the resolution limit,   MV =−12  , the luminosity function of the satellite galaxies in the simulations is in excellent agreement with data for the Local Group. The radial number density profile of the model satellites, as well as their gas fractions also match observations very well. In agreement with previous N -body studies, we find that the satellites tend to be distributed in highly flattened configurations whose major axis is aligned with the major axis of the (generally triaxial) dark halo. In two out of three systems with sufficiently large satellite populations, the satellite system is nearly perpendicular to the plane of the galactic disc, a configuration analogous to that observed in the Milk Way. The discs themselves are perpendicular to the minor axis of their host haloes in the inner parts, and the correlation between the orientation of the galaxy and the shape of the halo persists even out to the virial radius. However, in one case the disc's minor axis ends up, at the virial radius, perpendicular to the minor axis of the halo. The angular momenta of the galaxies and their host halo tend to be well aligned.  相似文献   

4.
We investigate the behaviour of the asymmetry parameter A as a morphological parameter using a 'volume-limited' sample of 349 galaxies  (distance ≤25 Mpc,  MV ≤−18.5 mag)  and a larger magnitude-limited sample of 707 nearby galaxies. We confirm the correlation of A with morphological type. The late-type galaxies (Sdm, Sm and Im) have larger A than early-type galaxies, and they tend to have larger A than spiral galaxies. We investigate the usefulness of the A versus concentration index C in diagram as a tool for the regular–irregular and early–late classification. The diagram is not very useful to the regular versus late-type irregular classification, as inferred previously, but it is found to be useful to the early–late classification.  相似文献   

5.
We present velocity dispersion measurements for 69 faint early-type galaxies in the core of the Coma cluster, spanning  −22.0 ≲ MR ≲−17.5 mag  . We examine the   L –σ  relation for our sample and compare it to that of bright elliptical galaxies (Es) from the literature. The distribution of the the faint early-type galaxies in the   L –σ  plane follows the relation   L ∝σ2.01±0.36  , which is significantly shallower from   L ∝σ4  as defined for the bright Es. While increased rotational support for fainter early-type galaxies could account for some of the difference in slope, we show that it cannot explain it. We also investigate the colour–σ relation for our Coma galaxies. Using the scatter in this relation, we constrain the range of galaxy ages as a function of their formation epoch for different formation scenarios. Assuming a strong coordination in the formation epoch of faint early-type systems in Coma, we find that most had to be formed at least 6 Gyr ago and over a short 1-Gyr period.  相似文献   

6.
The kinematics of satellite galaxies reflect the masses of the extended dark matter haloes in which they orbit, and thus shed light on the mass–luminosity relation (MLR) of their corresponding central galaxies. In this paper, we select a large sample of centrals and satellites from the Sloan Digital Sky Survey and measure the kinematics (velocity dispersions) of the satellite galaxies as a function of the r -band luminosity of the central galaxies. Using the analytical framework presented in More, van den Bosch & Cacciato, we use these data to infer both the mean and the scatter of the MLR of central galaxies, carefully taking account of selection effects and biases introduced by the stacking procedure. As expected, brighter centrals on average reside in more massive haloes. In addition, we find that the scatter in halo masses for centrals of a given luminosity,  σlog  M   , also increases with increasing luminosity. As we demonstrate, this is consistent with  σlog  L   , which reflects the scatter in the conditional probability function   P ( L c| M )  , being independent of halo mass. Our analysis of the satellite kinematics yields  σlog  L = 0.16  ±  0.04  , in excellent agreement with constraints from clustering and group catalogues, and with predictions from a semi-analytical model of galaxy formation. We thus conclude that the amount of stochasticity in galaxy formation, which is characterized by  σlog  L   , is well constrained, independent of halo mass and in a good agreement with current models of galaxy formation.  相似文献   

7.
We consider the luminosity and environmental dependence of structural parameters of lenticular galaxies in the near-infrared K band. Using a 2D galaxy image decomposition technique, we extract bulge and disc structural parameters for a sample of 36 lenticular galaxies observed by us in the K band. By combining data from the literature for field and cluster lenticulars with our data, we study correlations between parameters that characterize the bulge and the disc as a function of luminosity and environment. We find that scaling relations such as the Kormendy relation, photometric plane and other correlations involving bulge and disc parameters show a luminosity dependence. This dependence can be explained in terms of galaxy formation models in which faint lenticulars  ( M T > −24.5)  formed via secular formation processes that likely formed the pseudo-bulges of late-type disc galaxies, while brighter lenticulars  ( M T < −24.5)  formed through a different formation mechanism most likely involving major mergers. On probing variations in lenticular properties as a function of environment, we find that faint cluster lenticulars show systematic differences with respect to faint field lenticulars. These differences support the idea that the bulge and disc components fade after the galaxy falls into a cluster, while simultaneously undergoing a transformation from spiral to lenticular morphologies.  相似文献   

8.
We have derived the X-ray luminosities of a sample of galaxies in groups, making careful allowance for contaminating intragroup emission. The L X: L B and L X: L FIR relations of spiral galaxies in groups appear to be indistinguishable from those in other environments, however the elliptical galaxies fall into two distinct classes. The first class is central-dominant group galaxies, which are very X-ray luminous and may be the focus of group cooling flows. All other early-type galaxies in groups belong to the second class, which populates an almost constant band of L X/ L B over the range 9.8< log  L B<11.3 . The X-ray emission from these galaxies can be explained by a superposition of discrete galactic X-ray sources together with a contribution from hot gas lost by stars, which varies a great deal from galaxy to galaxy. In the region where the optical luminosity of the non-central group galaxies overlaps with the dominant galaxies, the dominant galaxies are over an order of magnitude more luminous in X-rays.
We also compared these group galaxies with a sample of isolated early-type galaxies, and used previously published work to derive L X: L B relations as a function of environment. The non-dominant group galaxies have mean L X/ L B ratios very similar to those of isolated galaxies, and we see no significant correlation between L X/ L B and environment. We suggest that previous findings of a steep L X: L B relation for early-type galaxies result largely from the inclusion of group-dominant galaxies in samples.  相似文献   

9.
We investigate pure luminosity evolution models for early-type (elliptical and S0) galaxies (i.e. no number density change or morphological transition), and examine whether these models are consistent with observed number counts in the B , I and K bands, and redshift distributions of two samples of faint galaxies selected in the I and K bands. The models are characterized by the star formation time-scale τ SF and the time t gw when the galactic wind starts to blow, in addition to several other conventional parameters. We find that the single-burst model ( τ SF=0.1 Gyr and t gw=0.353 Gyr), which is known to reproduce the photometric properties of early-type galaxies in clusters, is inconsistent with the redshift distributions of early-type galaxies in the field environment, owing to overpredictions of the number of galaxies at z ≳1.4 even with strong extinction which is at work until t gw. In order for dust extinction to be more effective, we treat τ SF and t gw as free parameters, and find that models with τ SF≳0.5 Gyr and t gw>1.0 Gyr can be made consistent with both the observed redshift distributions and the number counts, if we introduce strong extinction [ E ( B − V )≥1 as a peak value]. These results suggest that early-type galaxies in the field environment do not have the same evolutionary history as described by the single-burst model.  相似文献   

10.
We determine the companion galaxy luminosity function (LF) for regions around isolated spiral galaxies. If we assume that any excess in the galaxy number counts in the vicinity of a spiral galaxy is due to galaxies at the same distance, then a system LF can be determined from the variation of excess numbers with apparent magnitude. By studying the excess over many field 'centre' galaxies, a good statistical accuracy can be obtained for the companion galaxy LF. Since redshift information is not required for the faint galaxies, it is possible to sample further down the LF as compared with redshift surveys. For 23 primary galaxies of known redshift, we find a dwarf satellite Schechter LF with a characteristic magnitude M V *( D )≃−19 and a faint-end slope α=−1.7, down to MV =−14 ( H 0=50 km s−1 Mpc−1).  相似文献   

11.
The first spectroscopic census of active galactic nuclei (AGNs) associated with late-type galaxies in the Virgo cluster was carried out by observing 213 out of a complete set of 237 galaxies more massive than   M dyn > 108.5 M⊙  . Among them, 77 are classified as AGNs [including 21 transition objects, 47 low-ionization nuclear emission regions (LINERs) and nine Seyferts] and comprise 32 per cent of the late-type galaxies in Virgo. Due to spectroscopic incompleteness, at most 21 AGNs are missed in the survey, so that the fraction would increase up to 41 per cent. Using corollary near-infrared observations that enable us to estimate galaxy dynamical masses, it is found that AGNs are hosted exclusively in massive galaxies, i.e.   M dyn≳ 1010 M⊙  . Their frequency increases steeply with the dynamical mass from zero at   M dyn≈ 109.5 M⊙  to virtually 1 at   M dyn > 1011.5 M⊙  . These frequencies are consistent with those of low-luminosity AGNs found in the general field by the Sloan Digital Sky Survey. Massive galaxies that harbour AGNs commonly show conspicuous r -band star-like nuclear enhancements. Conversely, they often, but not necessarily, contain massive bulges. A few well-known AGNs (e.g. M61, M100, NGC 4535) are found in massive Sc galaxies with little or no bulge. The AGN fraction seems to be only marginally sensitive to galaxy environment. We infer the black hole masses using the known scaling relations of quiescent black holes. No black holes lighter than  ∼106 M⊙  are found active in our sample.  相似文献   

12.
We present XMM data for the supercluster A901/2, at   z ∼ 0.17  , which is combined with deep imaging and 17-band photometric redshifts (from the COMBO-17 survey), two degree field (2dF) spectra and Spitzer 24 μm data, to identify active galactic nuclei (AGN) in the supercluster. The 90 ksec XMM image contains 139 point sources, of which 11 are identified as supercluster AGN with   L X(0.5−7.5 keV) > 1.7 × 1041 erg cm−2 s−1  . The host galaxies have   M R < −20  and only two of eight sources with spectra could have been identified as AGN by the detected optical emission lines. Using a large sample of 795 supercluster galaxies, we define control samples of massive galaxies with no detected AGN. The local environments of the AGN and control samples differ at ≳98 per cent significance. The AGN host galaxies lie predominantly in areas of moderate projected galaxy density and with more local blue galaxies than the control sample, with the exception of one very bright type I AGN very near the centre of a cluster. These environments are similar to, but not limited to, cluster outskirts and blue groups. Despite the large number of potential host galaxies, no AGN are found in regions with the highest galaxy density (excluding some cluster cores where emission from the intra-cluster medium obscures moderate luminosity AGN). AGN are also absent from the areas with lowest galaxy density. We conclude that the prevalence of cluster AGN is linked to their environment.  相似文献   

13.
We investigate the old globular cluster (GC) population of 68 faint  ( M V > −16 mag)  dwarf galaxies located in the halo regions of nearby (≲12 Mpc) loose galaxy groups and in the field environment based on archival Hubble Space Telescope ( HST )/Advanced Camera for Surveys (ACS) images in F606W and F814W filters. The combined colour distribution of 175 GC candidates peaks at  ( V − I ) = 0.96 ± 0.07 mag  and the GC luminosity function turnover for the entire sample is found at   M V ,TO=−7.6 ± 0.11 mag  , similar to the old metal-poor Large Magellanic Cloud (LMC) GC population. Our data reveal a tentative trend of   M V ,TO  becoming fainter from late- to early-type galaxies. The luminosity and colour distributions of GCs in dIrrs show a lack of faint blue GCs (bGCs). Our analysis reveals that this might reflect a relatively younger GC system than typically found in luminous early-type galaxies. If verified by spectroscopy, this would suggest a later formation epoch of the first metal-poor star clusters in dwarf galaxies. We find several bright (massive) GCs which reside in the nuclear regions of their host galaxies. These nuclear clusters have similar luminosities and structural parameters as the peculiar Galactic clusters suspected of being the remnant nuclei of accreted dwarf galaxies, such as M54 and ωCen. Except for these nuclear clusters, the distribution of GCs in dIrrs in the half-light radius versus cluster mass plane is very similar to that of Galactic young halo clusters, which suggests comparable formation and dynamical evolution histories. A comparison with theoretical models of cluster disruption indicates that GCs in low-mass galaxies evolve dynamically as self-gravitating systems in a benign tidal environment.  相似文献   

14.
We present optical and infrared broad-band images, radio maps, and optical spectroscopy for the nuclear region of a sample of nearby galaxies. The galaxies have been drawn from a complete volume-limited sample for which we have already presented X-ray imaging. We modelled the stellar component of the spectroscopic observations to determine the star formation history of our targets. Diagnostic diagrams were used to classify the emission-line spectra and determine the ionizing mechanism driving the nuclear regions. All those sources classified as active galactic nuclei present small Eddington ratios  (∼10−3–10−6)  , implying a very slow growth rate of their black holes. We finally investigate the relative numbers of active and normal nuclei as a function of host galaxy luminosity and find that the fraction of active galaxies slowly rises as a function of host absolute magnitude in the   M B ∼−12  to −22 range.  相似文献   

15.
We present the first results from a major HST WFPC2 imaging study aimed at providing the first statistically meaningful comparison of the morphologies, luminosities, scalelengths and colours of the host galaxies of radio-quiet quasars, radio-loud quasars and radio galaxies. We describe the design of this study and present the images that have been obtained for the first half of our 33-source sample. We find that the hosts of all three classes of luminous AGN are massive elliptical galaxies, with scalelengths ≃10 kpc, and R − K colours consistent with mature stellar populations. Most importantly, this is first unambiguous evidence that, just like radio-loud quasars, essentially all radio-quiet quasars brighter than M R =−24 reside in massive ellipticals. This result removes the possibility that radio 'loudness' is directly linked to host galaxy morphology, but is however in excellent accord with the black hole/spheroid mass correlation recently highlighted by Magorrian et al. We apply the relations given by Magorrian et al. to infer the expected Eddington luminosity of the putative black hole at the centre of each of the spheroidal host galaxies we have uncovered. Comparison with the actual nuclear R -band luminosities suggests that the black holes in most of these galaxies are radiating at a few per cent of the Eddington luminosity; the brightest host galaxies in our low- z sample are capable of hosting quasars with M R ≃− 28, comparable to the most luminous quasars at z ≃3. Finally, we discuss our host-derived black hole masses in the context of the radio luminosity:black hole mass correlation recently uncovered for nearby galaxies by Franceschini et al., and consider the resulting implications for the physical origin of radio loudness.  相似文献   

16.
We have investigated the dynamics of the merging process in the minor merger hypothesis for active galactic nuclei. We find that for a satellite galaxy to be able to merge directly with the nucleus of the host galaxy (for example, to give rise to the compact dust discs which are seen in early-type active galaxies), the initial orbit of the satellite is required to be well aimed. For the case of the host galaxy being a disc galaxy, if the initial orbits of the satellites are randomly oriented with respect to the host galaxy, then the orbits of those which reach the host nuclear regions in a reasonable time are also fairly randomly oriented once they reach the nucleus. We note that this result might be able to provide an explanation of why the jet directions in the nuclei of Seyfert galaxies are apparently unrelated to the plane of the galaxy discs.  相似文献   

17.
We present the luminosity function and measurements of the scalelengths, colours and radial distribution of dwarf galaxies in the Coma cluster down to R =24. Our survey area is 674 arcmin2; this is the deepest and most detailed survey covering such a large area.
Our measurements agree with those of most previous authors at bright and intermediate magnitudes. The new results are as follows.
(1) Galaxies in the Coma cluster have a luminosity function φ( L )∝ L α that is steep (α∼−1.7) for −15< MR <−11, and is shallower brighter than this. The curvature in the luminosity function at MR ∼−15 is statistically significant.
(2) The galaxies that contribute most strongly to the luminosity function at −14< MR <−12 have colours and scalelengths that are consistent with those of local dwarf spheroidal galaxies placed at the distance of Coma.
(3) These galaxies with −14< MR <−12 have a colour distribution that is very strongly peaked at B − R =1.3. This is suggestive of a substantial degree of homogeneity in their star formation histories and metallicities.
(4) These galaxies with −14< MR <−12 also appear to be more confined to the cluster core ( r ∼200 kpc) than the brighter galaxies. Alternatively, this observation may be explained in part or whole by the presence of an anomalously high number of background galaxies behind the cluster core. Velocity measurements of these galaxies would distinguish between these two possibilities.  相似文献   

18.
In the 2dF Galaxy Redshift Survey, we study the properties of voids and of fainter galaxies within voids that are defined by brighter galaxies. Our results are compared with simulated galaxy catalogues from the Millennium simulation coupled with a semi-analytical galaxy formation recipe. We derive the void size distribution and discuss its dependence on the faint magnitude limit of the galaxies defining the voids. While voids among faint galaxies are typically smaller than those among bright galaxies, the ratio of the void sizes to the mean galaxy separation reaches larger values. This is well reproduced in the mock galaxy samples studied. We provide analytic fitting functions for the void size distribution. Furthermore, we study the galaxy population inside voids defined by galaxies with   B J− 5 log  h < −20  and diameter larger than  10  h −1 Mpc  . We find a clear bimodality of galaxies inside voids and in the average field but with different characteristics. The abundance of blue cloud galaxies inside voids is enhanced. There is an indication of a slight blueshift of the blue cloud. Furthermore, galaxies in void centres have slightly higher specific star formation rates as measured by the η parameter. We determine the radial distribution of the ratio of early- and late-type galaxies through the voids. We find and discuss some differences between observations and the Millennium catalogues.  相似文献   

19.
Understanding the origin and evolution of dwarf early-type galaxies remains an important open issue in modern astrophysics. Internal kinematics of a galaxy contains signatures of violent phenomena which may have occurred, e.g. mergers or tidal interactions, while stellar population keeps a fossil record of the star formation history; therefore studying connection between them becomes crucial for understanding galaxy evolution. Here, in the first paper of the series, we present the data on spatially resolved stellar populations and internal kinematics for a large sample of dwarf elliptical (dE) and lenticular (dS0) galaxies in the Virgo cluster. We obtained radial velocities, velocity dispersions, stellar ages and metallicities out to 1–2 half-light radii by reanalysing already published long-slit and integral-field spectroscopic data sets using the nbursts full spectral fitting technique. Surprisingly, bright representatives of the dE/dS0 class (   MB =−18.0  to −16.0 mag) look very similar to intermediate-mass and giant lenticulars and ellipticals: (1) their nuclear regions often harbour young metal-rich stellar populations always associated with the drops in the velocity dispersion profiles; (2) metallicity gradients in the main discs/spheroids vary significantly from nearly flat profiles to −0.9 dex   r −1e  , i.e. somewhat three times steeper than for typical bulges; (3) kinematically decoupled cores were discovered in four galaxies, including two with very little, if any, large-scale rotation. These results suggest similarities in the evolutionary paths of dwarf and giant early-type galaxies and call for reconsidering the role of major mergers in the dE/dS0 evolution.  相似文献   

20.
Near-infrared images have been obtained of nine BL Lacertae objects in order to investigate their host galaxy properties. From numerical modelling of the data we find that five of the nine BL Lacs have contributions from extended galaxy emission in their profiles. Tentative morphologies were derived for two of the BL Lacs, namely B2 0752+258, for which a disc morphology is slightly preferred, and S4 0954+65, for which an elliptical morphology is preferred. From our modelling we derive host galaxy absolute magnitudes of MH =−25.6±0.5 for B2 0752+258, and MH =−26.3±0.8 for S4 0954+65. We also find an average K -band absolute K -corrected host galaxy magnitude, for the BL Lacs, of 〈 MK 〉=−26.3±0.6 asssuming an elliptical galaxy model, and 〈 MK 〉=−26.1±0.9 assuming a disc galaxy model. The derived absolute magnitudes are similar to those found for the putative parent population of FRI radio galaxies, predicted by unified schemes. For those BL Lacs in which host galaxies have been previously detected at optical wavelengths we derive rest frame colours which are generally consistent with those of 'normal' galaxies. However, for TEX 0836+182 we find a rather blue colour which, together with the elongated morphology, may be indicative of interaction or spiral structure. Our results suggest that the host galaxies of BL Lacs are indeed similar to those of FRI radio galaxies, adding further weight to the unification scenario. In our present infrared frames of relatively modest resolution, we do not find that infrared observations are any more or less effective than optical observations at revealing the underlying host galaxy.  相似文献   

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