Luminous Early-type Galaxies Research Articles

Circumnuclear Dust in Luminous Early-type Galaxies. I. Sample Properties and Stellar Luminosity Models

Dusty circumnuclear disks (CNDs) in luminous early-type galaxies (ETGs) show regular, dynamically cold molecular gas kinematics. For a growing number of ETGs, Atacama Large Millimeter/sub-millimeter Array (ALMA) CO imaging and detailed gas-dynamical modeling facilitate moderate-to-high precision black hole (BH) mass (M BH) determinations. From the ALMA archive, we identified a subset of 26 ETGs with estimated M BH/M ⊙ ≳ 108 to a few × 109 and clean CO kinematics but that previously did not have sufficiently high-angular-resolution near-IR observations to mitigate dust obscuration when constructing stellar luminosity models. We present new optical and near-IR Hubble Space Telescope (HST) images of this sample to supplement the archival HST data, detailing the sample properties and data-analysis techniques. After masking the most apparent dust features, we measure stellar surface-brightness profiles and model the luminosities using the multi-Gaussian expansion (MGE) formalism. Some of these MGEs have already been used in CO dynamical modeling efforts to secure quality M BH determinations, and the remaining ETG targets here are expected to significantly improve the high-mass end of the current BH census, facilitating new scrutiny of local BH mass–host galaxy scaling relationships. We also explore stellar isophotal behavior and general dust properties, finding these CNDs generally become optically thick in the near-IR (A H ≳ 1 mag). These CNDs are typically well aligned with the larger-scale stellar photometric axes, with a few notable exceptions. Uncertain dust impact on the MGE often dominates the BH mass error budget, so extensions of this work will focus on constraining CND dust attenuation.

The MASSIVE survey – XVIII. Deep wide-field K-band photometry and local scaling relations for massive early-type galaxies

ABSTRACT We present wide-field, deep K-band photometry of 98 luminous early-type galaxies (ETGs) from the MASSIVE survey based on observations taken with the WIRCam instrument on the Canada–France–Hawaii Telescope. Using these images, we extract accurate total K-band luminosities (LK) and half-light radii (Re) for this sample of galaxies. We use these new values to explore the size–luminosity and Faber–Jackson relations for massive ETGs. Within this volume-limited sample, we find clear evidence for curvature in both relations, indicating that the most luminous galaxies tend to have larger sizes and smaller velocity dispersions than expected from a simple power-law fit to less luminous galaxies. Our measured relations are qualitatively consistent with the most massive elliptical galaxies forming largely through dissipationless mergers. When the sample is separated into fast and slow rotators, we find the slow rotators to exhibit similar changes in slope with increasing LK, suggesting that low-mass and high-mass slow rotators have different formation histories. The curvatures in the Re–LK and σ–LK relations cancel, leading to a relation between dynamical mass and luminosity that is well described by a single power law: Reσ2 ∝ LKb with b ≈ 1.2. This is consistent with the tilt of the fundamental plane observed in lower mass elliptical galaxies.

The extended Local Supercluster

ABSTRACT It has long been established but seldom noticed that we are in a region at least 170 Mpc across in which different types of galaxies show different degrees of alignment with the plane of the de Vaucouleurs Local Supercluster. While clusters of galaxies and radio galaxies at redshifts z < 0.02 are concentrated at low supergalactic latitudes, the most luminous galaxies in the infrared, LIRGs, show little correlation with this plane. The most luminous early-type galaxies are concentrated at low supergalactic latitudes, but similarly luminous spirals are not noticeably so. The cross-correlations of the positions of what might be considered galaxies selected for their stellar mass with positions of clusters and LIRGs offer a measure of the situation. The mean density at distance ∼0.5 Mpc from an LIRG is comparable to the mean density at that distance from a cluster of galaxies, but the mean density 5 Mpc from an LIRG is well below the mean density at that distance from a cluster and not much greater than the cosmic mean density. Discussion of issues arising is brief.

Early-type galaxies with recent star formation have low metallicities – evidence for star formation triggered by external processes?

ABSTRACTOn the basis of the Galaxy Evolution Explore (GALEX) ultraviolet (UV) data, many studies have demonstrated that recent star formation (RSF) is common in early-type galaxies. In particular, near-ultraviolet (NUV) light is used to investigate star formation activity in early-type galaxies, owing to its high sensitivity to the presence of young stars. This study characterized the stellar population properties of luminous early-type galaxies (Mr ≤ −20.65 mag) in the redshift range 0.005 ≤ z ≤ 0.09 on the basis of the Sloan Digital Sky Survey (SDSS) data. Initially, the most frequently used criterion of NUV $-\, r$ ≤ 5.4 was applied to identify early-type galaxies with RSF, and 19 per cent (172/913) of the sample galaxies met this criterion, in agreement with previous studies. A more robust sample of galaxies with RSF along with a stricter criterion (70 galaxies with NUV $-\, r$ ≤ 5.0) were then used for further analysis, and consequently 7.7 per cent (70/913; lower limit of the RSF fraction) of the sample galaxies were classified as early-type galaxies with RSF. These galaxies tended to exhibit higher H β absorption-line strengths and stronger emission lines than quiescent (QST, NUV $-\, r$ > 5.4) early-type galaxies. The most prominent feature of early-type galaxies with RSF identified in this study was that they were more metal-poor than QST galaxies owing to metal-poor stellar populations that were possibly formed from materials accreted from gas-rich satellites during (minor) mergers or interactions. The results strongly indicate that the observed RSF in the sample galaxies at the present epoch is mainly driven by external processes (i.e. mergers or interactions).

Distance and mass of the M 104 (Sombrero) group

Aims. Distances and radial velocities of galaxies in the vicinity of the luminous early-type galaxy M 104 (Sombrero) are used to derive its dark matter mass. Methods. Two dwarf galaxies: UGCA 307 and KKSG 30 situated near M 104 were observed with the Advanced Camera for Surveys on the Hubble Space Telescope. The distances 9.03−0.51+0.84 Mpc (UGCA 307) and 9.72−0.41+0.44 Mpc (KKSG 30) were determined using the tip of the red giant branch method. These distances are consistent with the dwarf galaxies being satellites of Sombrero. Results. Using radial velocities and projected separations of UGCA 307, KKSG 30, and a third galaxy with an accurate distance (KKSG 29), as well as 12 other assumed companions with less accurate distances, the total mass of M 104 is estimated to be (1.55 ± 0.49) × 1013 M⊙. At the K-band luminosity of the Sombrero galaxy of 2.4 × 1011 L⊙, its total mass-to-luminosity ratio is MT/LK = (65 ± 20) M⊙/L⊙, which is about three times higher than that of luminous bulgeless galaxies.

Cosmological information content in redshift-space power spectrum of SDSS-like galaxies in the quasinonlinear regime up to k=0.3 h Mpc−1

Clustering properties and peculiar velocities of halos in large-scale structure carry a wealth of cosmological information over a wide range of scales from linear to nonlinear scales. We use halo catalogs in a suite of high-resolution $N$-body simulations to construct mock catalogs of galaxies that resemble the SDSS-like luminous early-type galaxies at three redshift bins in the range $0.15\le z\le 0.7$. To do this we include 10 nuisance parameters to model variations in halo-galaxy connections for each redshift bin; the halo occupation distribution, and the spatial and velocity distributions of galaxies in the host halos. We evaluate the Fisher information matrix for the redshift-space power spectrum of SDSS-like galaxies using different sets of the mock catalogs that are generated from changes in each of model parameters; cosmological parameters ($\sigma_8$ and $\Omega_{\rm m}$), the halo-galaxy connection parameters, and the cosmological distances ($D_{\rm A}$ and $H$ parameters at each redshift bin) for modeling an apparent geometrical distortion of the redshift-space power spectrum (the Alcock-Paczynski effect). We show that combining the monopole and quadrupole power spectra of galaxies allows for accurate estimations of the cosmological parameters and the cosmological distances, even after marginalization over the halo-galaxy parameters, by lifting the parameter degeneracies that are otherwise inevitable if either of the two spectra alone is used. When including the galaxy power spectrum information up to $k=0.3\,h\,$Mpc$^{-1}$, we find about factor of 6 gain in the cosmological information content of ($\sigma_8,\Omega_{\rm m},D_{\rm A}$'s and $H$'s) compared to $k=0.2\,h\,$Mpc$^{-1}$.

Simple interpolation functions for the galaxy-wide stellar initial mass function and its effects in early-type galaxies

ABSTRACT The galaxy-wide stellar initial mass function (IGIMF) of a galaxy is thought to depend on its star formation rate (SFR). Using a catalogue of observational properties of early-type galaxies (ETGs) and a relation that correlates the formation time-scales of ETGs with their stellar masses, the dependencies of the IGIMF on the SFR are translated into dependencies on more intuitive parameters like present-day luminosities in different passbands. It is found that up to a luminosity of approximately 109 L⊙ (quite independent of the considered passband), the total masses of the stellar populations of ETGs are slightly lower than expected from the canonical stellar initial mass function (IMF). However, the actual mass of the stellar populations of the most luminous ETGs may be up to two times higher than expected from a simple stellar population model with the canonical IMF. The variation of the IGIMF with the mass of ETGs is presented here also as convenient functions of the luminosity in various passbands.

The SAMI Galaxy Survey: Kinematic Alignments of Early-type Galaxies in A119 and A168

Abstract We investigate the kinematic alignments of luminous early-type galaxies (M r ≤ −19.5 mag) in A119 and A168 using the kinematic position angles ( ) from the Sydney-AAO Multi-object Integral-field spectrograph (SAMI) survey data, motivated by the implication of the galaxy spin alignment in a cosmological context. To increase the size of our sample for statistical significance, we also use the photometric position angles ( ) for galaxies that have not been observed by SAMI, if their ellipticities are higher than 0.15. Our luminous early-type galaxies tend to prefer the specific position angles in both clusters, confirming the results of Kim et al., who recently found the kinematic alignment of early-type galaxies in the Virgo cluster based on the ATLAS 3D integral-field spectroscopic data. This alignment signal is more prominent for galaxies in the projected phase-space regions dominated by infalling populations. Furthermore, the alignment angles are closely related to the directions of the filamentary structures around clusters. The results lead us to conclude that many cluster early-type galaxies are likely to be accreted along filaments while maintaining their spin axes, which are predetermined before cluster infall.

Eight luminous early-type galaxies in nearby pairs and sparse groups. I. Stellar populations spatially analysed

We present a detailed spatial analysis of stellar populations based on long-slit optical spectra in a sample of eight luminous early-type galaxies selected from nearby sparse groups and pairs, three of them may be interacting with a galaxy of similar mass. We have measured luminosity-weighted averages of age, [M/H], [Fe/H], and [$\alpha$/Fe] to add empirical data relative to the influence of galaxy mass, environment, interaction, and AGN feedback in their formation and evolution. The stellar population of the individual galaxies were determined through the stellar population synthesis code STARLIGHT using semi-empirical simple stellar population models. Radial variations of luminosity-weighted means of age, [M/H], [Fe/H], and [$\alpha$/Fe] were measured up to half of the effective radius of each galaxy. We found trends between these values and the nuclear stellar velocity dispersion. There are also relations between the metallicity/age gradients and the velocity dispersion. Contributions of 1-4 Gyr old stellar populations were found in IC5328 and NGC6758 as well as 4-8 Gyr old ones in NGC5812. Extended gas is present in IC5328, NGC1052, NGC1209, and NGC6758, and the presence of a LINER is identified in all these galaxies. The regions up to one effective radius of all galaxies are dominated by $\alpha$-enhanced metal-rich old stellar populations likely due to rapid star formation episodes that induced efficient chemical enrichment. On average, the age and [$\alpha$/Fe] gradients are null and the [M/H] gradients are negative, although discordant cases were found. We found no correlation between the stellar population properties and the LINER presence as well as between the stellar properties and environment or gravitational interaction, suggesting that the influence of progenitor mass can-not be discarded in the formation and evolution of early-type galaxies.

THE QUEST FOR THE LARGEST DEPLETED GALAXY CORE: SUPERMASSIVE BLACK HOLE BINARIES AND STALLED INFALLING SATELLITES

ABSTRACT Partially depleted cores are practically ubiquitous in luminous early-type galaxies (M B ≲ −20.5 mag) and are typically smaller than 1 kpc. In one popular scenario, supermassive black hole (SMBH) binaries—established during dry (i.e., gas-poor) galaxy mergers—kick out the stars from a galaxy’s central region via three-body interactions. Here, this “binary black hole scouring scenario” is probed at its extremes by investigating the two galaxies reported to have the largest partially depleted cores found to date: 2MASX J09194427+5622012 and 2MASX J17222717+3207571 (the brightest galaxy in Abell 2261). We have fit these galaxy’s two-dimensional light distribution using the core-Sérsic model and found that the former galaxy has a core-Sérsic break radius , which is three times smaller than the published value. We use this galaxy to caution that other reportedly large break radii may too have been overestimated if they were derived using the “sharp-transition” (inner core)-to-(outer Sérsic) model. In the case of 2MASX J17222717+3207571, we obtain R b,cS = 3.6 kpc. While we confirm that this is the biggest known partially depleted core of any galaxy, we stress that it is larger than expected from the evolution of SMBH binaries—unless one invokes substantial gravitational-wave-induced (black hole-)recoil events. Given the presence of multiple nuclei located (in projection) within the core radius of this galaxy, we explored and found support for the alternative “stalled infalling perturber” core-formation scenario, in which this galaxy’s core could have been excavated by the action of an infalling massive perturber.

Depleted cores, multicomponent fits, and structural parameter relations for luminous early-type galaxies

New surface brightness profiles from 26 early-type galaxies with partially depleted cores have been extracted from the full radial extent of HST images, giving us a total sample of 31 such core-Sersic galaxies. We have carefully quantified the radial stellar distributions of the elliptical galaxies using the core-Sersic model whereas for the lenticular galaxies a core-Sersic bulge plus an exponential disc model gives the best representation. We additionally caution about the excessive use of multiple Sersic functions for decomposing galaxies and compare with past fits in the literature. The structural parameters obtained from our fitted models are in general, in good agreement with our initial study using radially limited (R < 10") profiles, and are used here to update several galaxy scaling relations. We find near-linear relations between the break radius R_b and the spheroid luminosity L such that R_b ~ L^(1.13 +/- 0.13), and with the supermassive black hole mass M_BH such that R_b ~ M_BH^(0.83+/- 0.21). This is internally consistent with the notion that major, dry mergers add the stellar and black hole mass in equal proportion, i.e., M_BH ~ L. In addition, we observe a linear relation R_b ~ R_e^(0.98 +/- 0.15) for the core-Sersic elliptical galaxies, where R_e is the galaxies' effective half light radii, which is collectively consistent with the approximately-linear, bright-end of the curved L-R_e relation. Finally, we measure accurate stellar mass deficits M_def that are in general 0.5-4 M_BH, and we identify two galaxies (NGC 1399, NGC 5061) that, due to their high M_def/M_BH ratio, may have experienced oscillatory core-passage by a kicked black hole. The galaxy scaling relations and stellar mass deficits favor core-Sersic galaxy formation through a few `dry' major merger events involving supermassive black holes such that M_def ~ M_BH^(3.70 +/- 0.76), for M_BH > 2 X 10^8 M_sun.

HOT GAS HALOS IN EARLY-TYPE GALAXIES AND ENVIRONMENTS

We investigate the dependence of the extended X-ray emission from the halos of optically luminous early-type galaxies on the small-scale (the nearest neighbor distance) and large-scale (the average density inside the 20 nearest galaxies) environments. We cross-match the 3rd Data Release of the Second XMMNewton Serendipitous Source Catalog (2XMMi-DR3) to a volume-limited sample of the Sloan Digital Sky Survey (SDSS) Data Release 7 with <TEX>$M_r$</TEX> < -19.5 and 0.020 < z < 0.085, and find 20 early-type galaxies that have extended X-ray detections. The X-ray luminosity of the galaxies is found to have a tighter correlation with the optical and near infrared luminosities when the galaxy is situated in the low large-scale density region than in the high large-scale density region. Furthermore, the X-ray to optical (r-band) luminosity ratio, <TEX>$L_X/L_r$</TEX>, shows a clear correlation with the distance to the nearest neighbor and with large-scale density environment only where the galaxies in pair interact hydrodynamically with seperations of <TEX>$r_p$</TEX> < <TEX>$r_{vir}$</TEX>. These findings indicate that the galaxies in the high local density region have other mechanisms that are responsible for their halo X-ray luminosities than the current presence of a close encounter, or alternatively, in the high local density region the cooling time of the heated gas halo is longer than the typical time between the subsequent encounters.

EXTENDED HOT HALOS AROUND ISOLATED GALAXIES OBSERVED IN THEROSATALL-SKY SURVEY

We place general constraints on the luminosity and mass of hot X-ray emitting gas residing in extended "hot halos" around nearby massive galaxies. We examine stacked images of 2165 galaxies from the 2MASS Very Isolated Galaxy Catalog (2MVIG), as well as subsets of this sample based on galaxy morphology and K-band luminosity. We detect X-ray emission at high confidence (ranging up to nearly 10\sigma) for each subsample of galaxies. The average L_X within 50 kpc is 1.0\pm0.1 (statistical) \pm0.2 (systematic) x10^40 erg/s, although the early-type galaxies are more than twice as luminous as the late-type galaxies. Using a spatial analysis, we also find evidence for extended emission around five out of seven subsamples (the full sample, the luminous galaxies, early-type galaxies, luminous late-type galaxies, and luminous early-type galaxies) at 92.7%, 99.3%, 89.3%, 98.7%, and 92.1% confidence, respectively. Several additional lines of evidence also support this conclusion and suggest that about 1/2 of the total emission is extended, and about 1/3 of the extended emission comes from hot gas. For the sample of luminous galaxies, which has the strongest evidence for extended emission, the average hot gas mass is 4x10^9 Msun within 50 kpc and the implied accretion rate is 0.4 Msun/yr.

OBSERVATIONAL SELECTION EFFECTS AND THEM-σ RELATION

We examine the possibility that the observed relation between black-hole mass and host-galaxy stellar velocity dispersion (the M-sigma relation) is biased by an observational selection effect, the difficulty of detecting a black hole whose sphere of influence is smaller than the telescope resolution. In particular, we critically investigate recent claims that the M-sigma relation only represents the upper limit to a broad distribution of black-hole masses in galaxies of a given velocity dispersion. We find that this hypothesis can be rejected at a high confidence level, at least for the early-type galaxies with relatively high velocity dispersions (median 268 km/s) that comprise most of our sample. We also describe a general procedure for incorporating observational selection effects in estimates of the properties of the M-sigma relation. Applying this procedure we find results that are consistent with earlier estimates that did not account for selection effects, although with larger error bars. In particular, (i) the width of the M-sigma relation is not significantly increased; (ii) the slope and normalization of the M-sigma relation are not significantly changed; (iii) most or all luminous early-type galaxies contain central black holes at zero redshift. Our results may not apply to late-type or small galaxies, which are not well-represented in our sample.

Surface photometry and radial color gradients of nearby luminous early-type galaxies in SDSS Stripe 82

We make use of the images from the Sloan Digital Sky Survey Stripe 82 (Stripe 82) to present an analysis of r band surface brightness profiles and radial color gradients (g-r, u-r) in our sample of 111 nearby early-type galaxies (ETGs). Thanks to the Stripe 82 images, each of which is co-added from about 50 single frames, we are able to pay special attention to the low-surface-brightness areas (LSB areas) of the galaxies. The LSB areas affect the Sérsic fittings and concentration indices by making both of the indices less than the typical values for ETGs. In the Sérsic fits to all the surface brightness profiles, we found some Sérsic indices that range from 1.5 to 2.5, much smaller than those of typical de Vaucouleur profiles and relatively close to those of exponential disks, and some others much larger than four but still with accurate fitting. Two galaxies cannot be fitted with a single Sérsic profile, but once we try double Sérsic profiles, the fittings are improved: one with a profile relatively close to the de Vaucouleur law in the inner area and a profile relatively close to an exponential law in the LSB area, the other with a nice fitting in the inner area but still having a failed fitting in the outer area. About 60% of the sample has negative color gradients (red-core) within 1.5 Re, much more than the approximately 10% positive ones (blue-core) within the same radius. However, taking into account the LSB areas, we find that the color gradients are not necessarily monotonic: about one third of the red-core (or blue-core) galaxies have positive (or negative) color gradients in the outer areas. So LSB areas not only make ETGs' Sérsic profiles deviate from de Vaucouleur ones and shift to the disk end, but also reveal that quite a number of ETGs have opposite color gradients in inner and outer areas. These outcomes remind us of the necessity of double-Sérsic fitting. These LSB phenomena may be interpreted by mergers and thus have different metallicity in the outer areas. Isophotal parameters are also discussed briefly in this paper with the following conclusion: there are more disky nearby ETGs that are identified than boxy ones.

A Complex Stellar Line-of-Sight Velocity Distribution in the Lenticular Galaxy NGC 524

AbstractWe present a detailed study of the stellar and gaseous kinematics in the luminous early-type galaxy NGC 524, derived from the long-slit spectroscopic observations obtained with the Russian 6 m telescope and the IFU data from the SAURON survey. The stellar line-of-sight velocity distribution (LOSVD) of NGC 524 exhibits strong asymmetry. We performed a comprehensive analysis of the LOSVD using two complementary approaches by the nbursts full spectral fitting technique: (a) a non-parametric LOSVD recovery and (b) a parametric recovery of two Gaussian kinematical components having different stellar populations. We discuss the origin of the complex stellar LOSVD of NGC 524.

Peculiar early-type galaxies in the Sloan Digital Sky Survey Stripe82

We explore the properties of `peculiar' early-type galaxies (ETGs) in the local Universe, that show (faint) morphological signatures of recent interactions such as tidal tails, shells and dust lanes. Standard-depth (51s exposure) multi-colour galaxy images from the Sloan Digital Sky Survey (SDSS) are combined with the significantly (2 mags) deeper monochromatic images from the public SDSS Stripe82 to extract, through careful visual inspection, a robust sample of nearby, luminous ETGs, including a subset of ~70 peculiar systems. 18% of ETGs exhibit signs of disturbed morphologies (e.g. shells), while 7% show evidence of dust lanes and patches. The peculiar ETG population is found to preferentially inhabit low-density environments (outskirts of clusters, groups or the field). An analysis of optical emission-line ratios indicates that the fraction of peculiar ETGs that are Seyferts or LINERs (19.4%) is twice the corresponding values in their relaxed counterparts (10.1%). LINER-like emission is the dominant type of nebular activity in all ETG classes, plausibly driven by stellar photoionisation associated with recent star formation. An analysis of UV-optical colours indicates that, regardless of the luminosity range being considered, the fraction of peculiar ETGs that have experienced star formation in the last Gyr is a factor of ~1.5 higher than that in their relaxed counterparts. The spectro-photometric results strongly suggest that the interactions that produce the morphological peculiarities also induce low-level recent star formation which, based on the recent literature, are likely to contribute a few percent of the stellar mass over the last 1 Gyr. The catalogue of galaxies that forms the basis of this paper can be obtained at: http://www.mssl.ucl.ac.uk/~ska/stripe82/skaviraj_stripe82.dat or on request from the author.

HALO OCCUPATION DISTRIBUTION MODELING OF CLUSTERING OF LUMINOUS RED GALAXIES

We perform Halo Occupation Distribution (HOD) modeling to interpret small-scale and intermediate-scale clustering of 35,000 luminous early-type galaxies and their cross-correlation with a reference imaging sample of normal L* galaxies in the Sloan Digital Sky Survey. The modeling results show that most of these luminous red galaxies (LRGs) are central galaxies residing in massive halos of typical mass M ~ a few times 10^13 to 10^14 Msun/h, while a few percent of them have to be satellites within halos in order to produce the strong auto-correlations exhibited on smaller scales. The mean luminosity Lc of central LRGs increases with the host halo mass, with a rough scaling relation of Lc \propto M^0.5. The halo mass required to host on average one satellite LRG above a luminosity threshold is found to be about 10 times higher than that required to host a central LRG above the same threshold. We find that in massive halos the distribution of L* galaxies roughly follows that of the dark matter and their mean occupation number scales with halo mass as M^1.5. The HOD modeling results also allows for an intuitive understanding of the scale-dependent luminosity dependence of the cross-correlation between LRGs and L_* galaxies. Constraints on the LRG HOD provide tests to models of formation and evolution of massive galaxies, and they are also useful for cosmological parameter investigations. In one of the appendices, we provide LRG HOD parameters with dependence on cosmology inferred from modeling the two-point auto-correlation functions of LRGs.

Stellar Populations of the Most Massive Galaxies

Within the hierarchical CDM framework, gas-poor mergers contribute substantially to the building of the most massive galaxies (Faber et al. 2007). We want to test this scenario by studying the fundamental plane (FP) and the stellar populations of the most massive galaxies. We investigate a well-defined sample of massive early-type galaxies at 0.1&lt;z&lt;0.4, identified from the SDSS database. Out of 42,000 possible targets in the SDSS database, we extracted 23 luminous early-type galaxies with bona fide high velocity dispersions of σ&gt;350 km s−1. These systems are located either in high or low-density environments and show a variety of small surface-brightness structure. Using archival HST/ACS images and Gemini/GMOS spectroscopy, we will explore the photometric and spectroscopic properties of these galaxies.

EVOLUTION OF THE UV EXCESS IN EARLY-TYPE GALAXIES

We examine the UV emission from luminous early-type galaxies as a function of redshift. We perform a stacking analysis using Galaxy Evolution Explorer (GALEX) images of galaxies in the NOAO Deep Wide Field Survey (NDWFS) Bo\"otes field and examine the evolution in the UV colors of the average galaxy. Our sample, selected to have minimal ongoing star formation based on the optical to mid-IR SEDs of the galaxies, includes 1843 galaxies spanning the redshift range $0.05\leq z\leq0.65$. We find evidence that the strength of the UV excess decreases, on average, with redshift, and our measurements also show moderate disagreement with previous models of the UV excess. Our results show little evolution in the shape of the UV continuum with redshift, consistent either with the binary model for the formation of Extreme Horizontal Branch (EHB) stars or with no evolution in EHB morphology with look-back time. However, the binary formation model predicts that the strength of the UV excess should also be relatively constant, in contradiction with our measured results. Finally, we see no significant influence of a galaxy's environment on the strength of its UV excess.