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1.
We present the result of a photometric and Keck low-resolution imaging spectrometer (LRIS) spectroscopic study of dwarf galaxies in the core of the Perseus Cluster, down to a magnitude of   M B =−12.5  . Spectra were obtained for 23 dwarf-galaxy candidates, from which we measure radial velocities and stellar population characteristics from absorption line indices. From radial velocities obtained using these spectra, we confirm 12 systems as cluster members, with the remaining 11 as non-members. Using these newly confirmed cluster members, we are able to extend the confirmed colour–magnitude relation for the Perseus Cluster down to   M B =−12.5  . We confirm an increase in the scatter about the colour–magnitude relationship below   M B =−15.5  , but reject the hypothesis that very red dwarfs are cluster members. We measure the faint-end slope of the luminosity function between   M B =−18  and −12.5, finding  α=−1.26 ± 0.06  , which is similar to that of the field. This implies that an overabundance of dwarf galaxies does not exist in the core of the Perseus Cluster. By comparing metal and Balmer absorption line indices with α-enhanced single stellar population models, we derive ages and metallicities for these newly confirmed cluster members. We find two distinct dwarf elliptical populations: an old, metal-poor population with ages ∼8 Gyr and metallicities  [Fe/H] < −0.33  , and a young, metal-rich population with ages <5 Gyr and metallicities  [Fe/H] > −0.33  . Dwarf galaxies in the Perseus Cluster are therefore not a simple homogeneous population, but rather exhibit a range in age and metallicity.  相似文献   

2.
Recent spectroscopic observations of galaxies in the Fornax Cluster reveal nearly unresolved 'star-like' objects with redshifts appropriate to the Fornax Cluster. These objects have intrinsic sizes of ≈100 pc and absolute B -band magnitudes in the range  −14< M B<−11.5 mag  and lower limits for the central surface brightness   μ B≳23 mag arcsec−2  , and so appear to constitute a new population of ultracompact dwarf galaxies (UCDs). Such compact dwarfs were predicted to form from the amalgamation of stellar superclusters (by Kroupa) , which are rich aggregates of young massive star clusters (YMCs) that can form in collisions between gas-rich galaxies. Here we present the evolution of superclusters in a tidal field. The YMCs merge on a few supercluster crossing times. Superclusters that are initially as concentrated and massive as knot S in the interacting Antennae galaxies evolve to merger objects that are long-lived and show properties comparable to the newly discovered UCDs. Less massive superclusters resembling knot 430 in the Antennae may evolve to ω Cen-type systems. Low-concentration superclusters are disrupted by the tidal field, dispersing their surviving star clusters while the remaining merger objects rapidly evolve into the   μ B− M B  region populated by low-mass Milky Way dSph satellites.  相似文献   

3.
This paper investigates the possibility that ultra-compact dwarf (UCD) galaxies in the Fornax cluster are formed by the threshing of nucleated, early-type dwarf galaxies (hereafter dwarf galaxies).
Similar to the results of Côté et al. for the Virgo cluster, we show that the Fornax cluster observations are consistent with a single population in which all dwarfs are nucleated, with a ratio of nuclear to total magnitude that varies slowly with magnitude. Importantly, the magnitude distribution of the UCD population is similar to that of the dwarf nuclei in the Fornax cluster.
The joint population of UCDs and the dwarfs from which they may originate is modelled and shown to be consistent with a Navarro, Frenk & White (NFW) profile with a characteristic radius of 5 kpc. Furthermore, a steady-state dynamical model reproduces the known mass profile of Fornax. However, there are a number of peculiarities in the velocity dispersion data that remain unexplained.
The simplest possible threshing model is tested, in which dwarf galaxies move on orbits in a static cluster potential and are threshed if they pass within a radius at which the tidal force from the cluster exceeds the internal gravity at the core of their dark matter halo. This fails to reproduce the observed fraction of UCDs at radii greater than 30 kpc from the core of Fornax.  相似文献   

4.
We have obtained spectroscopic redshifts of colour-selected point sources in four wide area VLT-FLAMES (Very Large Telescope-Fibre Large Array Multi Element Spectrograph) fields around the Fornax cluster giant elliptical galaxy NGC 1399, identifying as cluster members 27 previously unknown faint     compact stellar systems (CSS), and improving redshift accuracy for 23 previously catalogued CSS.
By amalgamating our results with CSS from previous 2dF observations and excluding CSS dynamically associated with prominent (non-dwarf) galaxies surrounding NGC 1399, we have isolated 80 'unbound' systems that are either part of NGC  1399's globular cluster (GC) system or intracluster GCs. For these unbound systems, we find (i) they are mostly located off the main stellar locus in colour–colour space; (ii) their projected distribution about NGC  1399 is anisotropic, following the Fornax cluster galaxy distribution, and there is weak evidence for group rotation about NGC  1399; (iii) their completeness-adjusted radial surface density profile has a slope similar to that of NGC  1399's inner GC system; (iv) their mean heliocentric recessional velocity is between that of NGC  1399's inner GCs and that of the surrounding dwarf galaxies, but their velocity dispersion is significantly lower; (v) bright CSS  ( M V < −11)  are slightly redder than the fainter systems, suggesting they have higher metallicity; (vi) CSS show no significant trend in   g '− i '  colour index with radial distance from NGC  1399.  相似文献   

5.
In order to investigate the nature of dwarf low surface brightness (LSB) galaxies we have undertaken a deep B - and I -band CCD survey of a 14-deg2 strip in the Virgo Cluster and applied a Fourier convolution technique to explore its dwarf galaxy population down to a central surface brightness of ∼26 B mag arcsec−2 and a total absolute B mag of  ∼−10  . In this paper we carry out an analysis of their morphology, ( B − I ) colours and atomic hydrogen content. We compare these properties with those of dwarf galaxies in other environments to try and assess how the cluster environment has influenced their evolution. Field dwarfs are generally of a more irregular morphology, are bluer and contain relatively more gas. We assess the importance that various physical processes have on the evolution of cluster dwarf galaxies (ram-pressure stripping, tidal interactions, supernova-driven gas loss). We suggest that enhanced star formation triggered by tidal interactions is the major reason for the very different general properties of cluster dwarfs: they have undergone accelerated evolution.  相似文献   

6.
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7.
Near-infrared spectra for a sample of 31 elliptical galaxies in the Coma cluster are obtained. The galaxies are selected to be ellipticals (no lenticulars), with a large spatial distribution, covering both the core and outskirts of the cluster (i.e. corresponding to regions with large density contrasts). COsp (2.3-μm) absorption indices, measuring the contribution from intermediate-age red giant and supergiant stars to the near-infrared light of the ellipticals, are then estimated.
It is found that the strength of COsp features in elliptical galaxies increases from the core ( r <02) to the outskirts ( r >02) of the Coma cluster. Using the Mg2 strengths, it is shown that the observed effect is not caused by metallicity and is mostly caused by the presence of a younger population (giant and supergiant stars) in ellipticals in the outskirts (low-density region) of the cluster.
Using the COsp features, the origin of the scatter on the near-infrared Fundamental Plane (the relation between the effective diameter, effective surface brightness and velocity dispersion) of elliptical galaxies is studied. Correcting this relation for contributions from the red giant and supergiant stars, the rms scatter reduces from 0.077 to 0.073 dex. Although measurable, the contribution from these intermediate-age stars to the scatter on the near-infrared Fundamental Plane of ellipticals is only marginal.
A relation is found between the COsp and V − K colours of ellipticals, corresponding to a slope of 0.036±0.016, significantly shallower than that from the Mg2–( V − K ) relation. This is studied using stellar synthesis models.  相似文献   

8.
Using images from a charge-coupled device survey with the Wide Field Camera on the Isaac Newton Telescope, we performed B - and I -band photometry on 156 Virgo cluster dwarf elliptical (dE) galaxies, 25 candidate new cluster dwarfs, and nine candidate field dwarfs. Galaxies were modelled with Sérsic profiles, using both 1D χ2 and 2D cross-correlation methods, with nuclei modelled as point sources. The intensity profiles of 50 galaxies previously classified as dE, dE?, or ? are more accurately fitted if a nucleus is included, and this results in the majority of dwarfs now being classified as nucleated dwarf ellipticals (dE,N). Some faint galaxies with B magnitudes of 18–21 have particularly large relative nuclei, while a small number have apparent central dimmings. For cluster dE,N galaxies the nucleus magnitude is correlated with the magnitude of the host galaxy. The profile parameters of dE and dE,N galaxies are not significantly different, and there is no evident discontinuity in relative nucleus size between non-nucleated and nucleated dwarfs, suggesting that they may form a continuum. Nuclei are on average redder than their underlying galaxies, though a spread of relative colours was found, and two-fifths of nuclei are bluer. Formation mechanisms of nuclei are discussed: at least some appear to have formed in an already existing non-nucleated galaxy, though others may have formed simultaneously with their galaxies and subsequently evolved within them.  相似文献   

9.
We present a new photometric catalogue of the Coma galaxy cluster in the Johnson U and V bands. We cover an area of 3360 arcmin2 of sky, to a depth of     in a 13-arcsec diameter aperture, and produce magnitudes for ∼1400 extended objects in metric apertures from 8.8- to 26-arcsec diameters. The mean internal rms scatter in the photometry is 0.014 mag in V , and 0.026 mag in U , for     .
We place new limits on the levels of scatter in the colour–magnitude relation (CMR) in the Coma cluster, and investigate how the slope and scatter of the CMR depend on galaxy morphology, luminosity and position within the cluster. As expected, the lowest levels of scatter are found in the elliptical galaxies, while the late-type galaxies have the highest numbers of galaxies bluewards of the CMR. We investigate whether the slope of the CMR is an artefact of colour gradients within galaxies, and show that it persists when the colours are measured within a diameter that scales with galaxy size. Looking at the environmental dependence of the CMR, we find a trend of systematically bluer galaxy colours with increasing projected radius from the centre of the cluster. Surprisingly, this is accompanied by a decreased scatter of the CMR. We investigate whether this gradient could be caused by dust in the cluster potential, however the reddening required would produce too large a scatter in the colours of the central galaxies. The gradient appears to be better reproduced by a gradient in the mean galactic ages with projected radius.  相似文献   

10.
We have determined a dust-free colour–magnitude (CM) relation for spiral galaxies, by using I  −  K colours in edge-on galaxies above the plane. We find that the scatter in this relation is small and approximately as large as can be explained by observational uncertainties. The slope of the near-IR CM relation is steeper for spirals than for elliptical galaxies. We suggest two possible explanations. First, the difference could be caused by vertical colour gradients in spiral galaxies. In that case these gradients should be similar for all galaxies, on average ∼0.15 dex in [Fe/H] per scaleheight, and should increase for later galaxy types. The most likely explanation, however, is that spirals and ellipticals have intrinsically different CM relations. This means that the stars in spirals are younger than those in ellipticals. The age, however, or the fraction of young stars in spiral galaxies would be determined solely by the luminosity of the galaxy, and not by its environment.  相似文献   

11.
We have discovered a new type of galaxy in the Fornax Cluster: `ultra-compact' dwarfs (UCDs). The UCDs are unresolved in ground-based imaging and have spectra typical of old stellar systems. Although the UCDs resemble overgrown globular clusters, based on VLT UVES echelle spectroscopy, they appear to be dynamically distinct systems with higher internal velocity dispersions and M/L ratios for a given luminosity than Milky Way or M31 globulars. Our preferred explanation for their origin is that they are the remnant nuclei of dwarf elliptical galaxies which have been tidally stripped, or `threshed' by repeated encounters with the central cluster galaxy, NGC1399. If correct, then tidal stripping of nucleated dwarfs to form UCDs may, over a Hubble time, be an important source of the plentiful globular cluster population in the halo of NGC1399, and, by implication, other cD galaxies. In this picture, the dwarf elliptical halo contents, up to 99% of the original dwarf luminosity, contribute a significant fraction of the populations of intergalactic stars, globulars, and gas in galaxy clusters. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

12.
We present K -band observations of the low-luminosity galaxies in the Coma cluster, which are responsible for the steep upturn in the optical luminosity function at M R∼−16, discovered recently. The main results of this study are as follows.
(i) The optical–near-infrared colours of these galaxies imply that they are dwarf spheroidal galaxies. The median B − K colour for galaxies with −19.3< MK <−16.3 is 3.6 mag.
(ii) The K -band luminosity function in the Coma cluster is not well constrained, because of the uncertainties due to the field-to-field variance of the background. However, within the estimated large errors, this is consistent with the R -band luminosity function, shifted by ∼3 mag.
(iii) Many of the cluster dwarfs lie in a region of the B − K versus B − R colour–colour diagram where background galaxies are rare ( B − K <5; 1.2< B − R <1.6). Local dwarf spheroidal galaxies lie in this region too. This suggests that a better measurement of the K -band cluster luminosity can be made if the field-to-field variance of the background can be measured as a function of colour, even if it is large.
(iv) If we assume that none of the galaxies in the region of the B − K versus B − R plane given in (iii) in our cluster fields are background, and that all the cluster galaxies with 15.5< K <18.5 lie in this region of the plane, then we measure α=−1.41+0.34−0.37 for −19.3< MK −16.3, where α is the logarithmic slope of the luminosity function. The uncertainties in this number come from counting statistics.  相似文献   

13.
We have measured central line strengths for a complete sample of early-type galaxies in the Fornax cluster, comprising 11 elliptical and 11 lenticular galaxies, more luminous than M B  = −17. In contrast to the elliptical galaxies in the sample studied by González (and recently revisited by Trager) we find that the Fornax ellipticals follow the locus of galaxies of fixed age in Worthey's models and have metallicities varying from roughly solar to three times solar. The lenticular galaxies, however, exhibit a substantial spread to younger luminosity-weighted ages, indicating a more extended star formation history. We present measurements of the more sensitive indices: C4668 and HγA; these confirm and reinforce the conclusions that the elliptical galaxies are coeval and that only the lenticular galaxies show symptoms of late star formation. The inferred difference in the age distribution between lenticular and elliptical galaxies is a robust conclusion as the models generate consistent relative ages using different age and metallicity indicators even though the absolute ages remain uncertain. The young luminosity-weighted ages of the S0s in the Fornax cluster are consistent with the recent discovery that the fraction of S0 galaxies in intermediate-redshift clusters is a factor of 2–3 lower than found locally, and suggest that a fraction of the cluster spiral galaxy population has evolved to quiescence in the 5-Gyr interval from z  = 0.5 to the present. Two of the faintest lenticular galaxies in our sample have blue continua and strong Balmer-line absorption, suggesting starbursts ≲2 Gyr ago. These may be the low-redshift analogues of the starburst or post-starburst galaxies seen in clusters at z  = 0.3, similar to the Hδ-strong galaxies in the Coma cluster.  相似文献   

14.
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   

15.
We put upper limits on the secondary burst of star formation in elliptical galaxies of the González sample, based on the colour dispersion around the U  −  V versus central velocity dispersion relation, and the equivalent width of Hβ absorption. Note that most of these galaxies locate in small groups. There is a significant number of Hβ strong galaxies that have EW(Hβ) > 2 Å, but they do not always have bluer colours in U  −  V . To be consistent with the small colour dispersion of U  −  V , the mass fraction of the secondary burst to the total mass should be less than 10 per cent at the maximum within the most recent 2 Gyr. This result suggests that even if recent galaxy merging has produced some ellipticals, it should not have been accompanied by an intensive starburst, and hence it could not involve large gas-rich systems. The capture of a dwarf galaxy is more likely to explain the dynamical disturbances observed in some elliptical galaxies. The above analysis, based on the U  −  V , is not compatible with the one based on the line indices, which requires that more than 10 per cent of mass is present in a 2-Gyr-old starburst to cover the full range of the observed Hβ (de Jong &38; Davies). The discrepancy might be partly explained by the internal extinction localized at the region where young stars form. However, considering that the Hβ index might have great uncertainties both in models and in observational data, we basically rely on U − V analysis.  相似文献   

16.
The discovery of a flux excess in the far-ultraviolet (FUV) spectrum of elliptical galaxies was a major surprise in 1969. While it is now clear that this UV excess is caused by an old population of hot helium-burning stars without large hydrogen-rich envelopes, rather than young stars, their origin has remained a mystery. Here we show that these stars most likely lost their envelopes because of binary interactions, similar to the hot subdwarf population in our own Galaxy. We have developed an evolutionary population synthesis model for the FUV excess of elliptical galaxies based on the binary model developed by Han et al. for the formation of hot subdwarfs in our Galaxy. Despite its simplicity, it successfully reproduces most of the properties of elliptical galaxies with a UV excess: the range of observed UV excesses, both in  (1550 − V )  and  (2000 − V )  , and their evolution with redshift. We also present colour–colour diagrams for use as diagnostic tools in the study of elliptical galaxies. The model has major implications for understanding the evolution of the UV excess and of elliptical galaxies in general. In particular, it implies that the UV excess is not a sign of age, as had been postulated previously, and predicts that it should not be strongly dependent on the metallicity of the population, but exists universally from dwarf ellipticals to giant ellipticals.  相似文献   

17.
We have conducted ultra-deep optical and deep near-infrared observations of a field around the z =1.226 radio-quiet quasar 104420.8+055739 from the Clowes–Campusano LQG of 18 quasars at z ∼1.3, in search of associated galaxy clustering. Galaxies at these redshifts are distinguished by their extremely red colours, with I − K >3.75, and we find a factor ∼11 overdensity of such galaxies in a 2.25×2.25 arcmin2 field centred on the quasar. In particular, we find 15–18 galaxies that have colours consistent with being a population of passively evolving massive ellipticals at the quasar redshift. They form 'fingers' in the V − K K , I − K K colour–magnitude plots at V − K ≃6.9, I − K ≃4.3 comparable to the red sequences observed in other z ≃1.2 clusters. We find suggestive evidence for substructure among the red sequence galaxies in the K image, in the form of two compact groups, 40 arcsec to the north, and 60 arcsec to the south-east of the quasar. An examination of the wider optical images indicates that this substructure is significant, and that the clustering extends to form a large-scale structure 2–3  h −1 Mpc across. We find evidence for a high (≳50 per cent) fraction of blue galaxies in this system, in the form of 15–20 'red outlier' galaxies with I − K >3.75 and V − I <2.00, which we suggest are dusty, star-forming galaxies at the quasar redshift. Within 30 arcsec of the quasar we find a concentration of blue ( V − I <1) galaxies in a band that bisects the two groups of red sequence galaxies. This band of blue galaxies is presumed to correspond to a region of enhanced star formation. We explain this distribution of galaxies as the early stages of a cluster merger which has triggered both the star formation and the quasar.  相似文献   

18.
We present the results of a study of the morphology of the dwarf galaxy population in Abell 868, a rich, intermediate-redshift     cluster which has a galaxy luminosity function (LF) with a steep faint-end slope     . A statistical background subtraction method is employed to study the     colour distribution of the cluster galaxies. This distribution suggests that the galaxies contributing to the faint-end of the measured cluster LF can be split into three populations: dwarf irregular galaxies (dIrrs) with     dwarf elliptical galaxies (dEs) with     and contaminating background giant ellipticals (gEs) with     . The removal of the contribution of the background gEs from the counts only marginally lessens the faint-end slope     . However, the removal of the contribution of the dIrrs from the counts produces a flat LF     . The dEs and the dIrrs have similar spatial distributions within the cluster, except that the dIrrs appear to be totally absent within a central projected radius of about 0.2 Mpc     . The number densities of both dEs and dIrrs appear to fall off beyond a projected radius of ≃ 0.35 Mpc. We suggest that the dE and dIrr populations of A868 have been associated with the cluster for similar time-scales, but evolutionary processes such as 'galaxy harassment' tend to fade the dIrr galaxies while having a much smaller effect on the dE galaxies. The harassment would be expected to have the greatest effect on dwarfs residing in the central parts of the cluster.  相似文献   

19.
An analysis of the environments around a sample of 28 3CR radio galaxies with redshifts 0.6< z <1.8 is presented, based primarily upon K -band images down to K ∼20 taken using the UK Infrared Telescope (UKIRT). A net overdensity of K -band galaxies is found in the fields of the radio galaxies, with the mean excess counts being comparable to that expected for clusters of Abell Class 0 richness. A sharp peak is found in the angular cross-correlation amplitude centred on the radio galaxies that, for reasonable assumptions about the luminosity function of the galaxies, corresponds to a spatial cross-correlation amplitude between those determined for low-redshift Abell Class 0 and 1 clusters.
These data are complemented by J -band images also from UKIRT, and by optical images from the Hubble Space Telescope . The fields of the lower redshift ( z ≲0.9) radio galaxies in the sample generally show well-defined near-infrared colour–magnitude relations with little scatter, indicating a significant number of galaxies at the redshift of the radio galaxy; the relations involving colours at shorter wavelengths than the 4000 Å break show considerably greater scatter, suggesting that many of the cluster galaxies have low levels of recent or on-going star formation. At higher redshifts the colour–magnitude sequences are less prominent owing to the increased field galaxy contribution at faint magnitudes, but there is a statistical excess of galaxies with the very red infrared colours ( J − K ≳1.75) expected of old cluster galaxies at these redshifts.
Although these results are appropriate for the mean of all of the radio galaxy fields, there exist large field-to-field variations in the richness of the environments. Many, but certainly not all, powerful z ∼1 radio galaxies lie in (proto)cluster environments.  相似文献   

20.
We present and discuss optical, near-infrared and H  i measurements of the galaxy Markarian 1460 at a distance of 19 Mpc in the Ursa Major Cluster. This low-luminosity ( M B =−14) galaxy is unusual because (i) it is blue ( B − R =0.8) and has the spectrum of an H  ii galaxy, (ii) it has a light profile that is smooth and well fitted by an r 1/4 and not an exponential function at all radii larger than the seeing, and (iii) it has an observed central brightness of about μ B =20 mag arcsec−2 , intermediate between those of elliptical galaxies (on the bright μ B side) and normal low-luminosity dwarf irregular (on the low μ B side) galaxies. No other known galaxy exhibits all these properties in conjunction. On morphological grounds this galaxy looks like a normal distant luminous elliptical galaxy, since the Fundamental Plane tells us that higher luminosity normal elliptical galaxies tend to have lower surface-brightnesses. Markarian 1460 has 2×107 M⊙ of H  i and a ratio M (H  i )/ L B of 0.2, which is low compared to the typical values for star-forming dwarf galaxies. From the high surface-brightness and r 1/4 profile, we infer that the baryonic component of Markarian 1460 has become self-gravitating through dissipative processes. From the colours, radio continuum, H  i and optical emission line properties, and yet smooth texture, we infer that Markarian 1460 has had significant star formation as recently as ∼1 Gyr ago but not today.  相似文献   

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