arXiv subjects

Mark Seibert

Publications and source records attributed to Mark Seibert.

At least 127 records · Page 7Linked to original sources

arXiv2007

Clustering Properties of restframe UV selected galaxies I: the correlation length derived from GALEX data in the local Universe

We present the first measurements of the angular correlation function of galaxies selected in the far (1530 A) and near (2310 A) Ultraviolet from the GALEX survey fields overlapping SDSS DR5 in low galactic extinction regions. The area used covers 120 sqdeg (GALEX - MIS) down to magnitude AB = 22, yielding a total of 100,000 galaxies. The mean correlation length is ~ 3.7 \pm 0.6 Mpc and no significant trend is seen for this value as a function of the limiting apparent magnitude or between the GALEX bands. This estimate is close to that found from samples of blue galaxies in the local universe selected in the visible, and similar to that derived at z ~ 3 for LBGs with similar rest frame selection criteria. This result supports models that predict anti-biasing of star forming galaxies at low redshift, and brings an additional clue to the downsizing of star formation at z<1.

astro-ph↗

arXiv2007

The Star Formation and Extinction Co-Evolution of UV-Selected Galaxies over 0.05<z<1.2

We use a new stacking technique to obtain mean mid IR and far IR to far UV flux ratios over the rest near-UV/near-IR color-magnitude diagram. We employ COMBO-17 redshifts and COMBO-17 optical, GALEX far and near UV, Spitzer IRAC and MIPS Mid IR photometry. This technique permits us to probe infrared excess (IRX), the ratio of far IR to far UV luminosity, and specific star formation rate (SSFR) and their co-evolution over two orders of magnitude of stellar mass and redshift 0.1<z<1.2. We find that the SSFR and the characteristic mass (M_0) above which the SSFR drops increase with redshift (downsizing). At any given epoch, IRX is an increasing function of mass up to M_0. Above this mass IRX falls, suggesting gas exhaustion. In a given mass bin below M_0 IRX increases with time in a fashion consistent with enrichment. We interpret these trends using a simple model with a Schmidt-Kennicutt law and extinction that tracks gas density and enrichment. We find that the average IRX and SSFR follows a galaxy age parameter which is determined mainly by the galaxy mass and time since formation. We conclude that blue sequence galaxies have properties which show simple, systematic trends with mass and time such as the steady build-up of heavy elements in the interstellar media of evolving galaxies and the exhaustion of gas in galaxies that are evolving off the blue sequence. The IRX represents a tool for selecting galaxies at various stages of evolution.

astro-ph↗

arXiv2007

Extinction Corrected Star Formation Rates Empirically Derived from Ultraviolet-Optical Colors

Using a sample of galaxies from the Sloan Digital Sky Survey spectroscopic catalog with measured star-formation rates (SFRs) and ultraviolet (UV) photometry from the GALEX Medium Imaging Survey, we derived empirical linear correlations between the SFR to UV luminosity ratio and the UV-optical colors of blue sequence galaxies. The relations provide a simple prescription to correct UV data for dust attenuation that best reconciles the SFRs derived from UV and emission line data. The method breaks down for the red sequence population as well as for very blue galaxies such as the local ``supercompact'' UV luminous galaxies and the majority of high redshift Lyman Break Galaxies which form a low attenuation sequence of their own.

astro-ph↗

arXiv2007

The Young and the Dustless: Interpreting Radio Observations of UltraViolet Luminous Galaxies

Ultraviolet Luminous Galaxies (UVLGs) have been identified as intensely star-forming, nearby galaxies. A subset of these, the supercompact UVLGs, are believed to be local analogs of high redshift Lyman Break Galaxies. Here we investigate the radio continuum properties of this important population for the first time. We have observed 42 supercompact UVLGs with the VLA, all of which have extensive coverage in the UV/optical by GALEX and SDSS. Our analysis includes comparison samples of multiwavelength data from the Spitzer First Look Survey and from the SDSS-Galex matched catalogs. In addition we have Spitzer MIPS data for 24 of our galaxies and find that they fall on the radio-FIR correlation of normal star-forming galaxies. We find that our galaxies have lower radio-to-UV ratios and lower Balmer decrements than other local galaxies with similar (high) star formation rates. Optical spectra show they have lower Dn(4000) and HdeltaA indices, higher Hbeta emission-line equivalents widths, and higher [OIII]5007/Hbeta emission-line ratios than normal star forming galaxies. Comparing these results to galaxy spectral evolution models we conclude that supercompact UVLGs are distinguished from normal star forming galaxies firstly by their high specific star formation rates. Moreover, compared to other types of galaxies with similar star formation rates, they have significantly less dust attenuation. In both regards they are similar to Lyman Break Galaxies. This suggests that the process that causes star formation in the supercompact UVLGs differs from other local star forming galaxies, but may be similar to Lyman Break Galaxies.

astro-ph↗

arXiv2007

Nitrogen Production in Starburst Galaxies Detected by GALEX

We investigate the production of nitrogen in star forming galaxies with ultraviolet (UV) radiation detected by the Galaxy Evolution Explorer Satellite (GALEX). We use a sample of 8,745 GALEX emission line galaxies matched to the Sloan Digital Sky Survey (SDSS) spectroscopic sample. We derive both gas-phase oxygen and nitrogen abundances for the sample, and apply stellar population synthesis models to derive stellar masses and star formation histories of the galaxies. We compare oxygen abundances derived using three different diagnostics. We derive the specific star formation rates of the galaxies by modeling the 7-band GALEX+SDSS photometry. We find that galaxies that have log SFR/M$_*$ > -10.0 typically have values of log N/O ~0.05 dex less than galaxies with log SFR/M$_*$ < -10.0 and similar oxygen abundances.

astro-ph↗

arXiv2007

The UV-optical Galaxy Color-Magnitude Diagram I: Basic Properties

We have analyzed the bivariate distribution of galaxies as a function of ultraviolet-optical colors and absolute magnitudes in the local universe. The sample consists of galaxies with redshifts and optical photometry from the Sloan Digital Sky Survey (SDSS) main galaxy sample matched with detections in the near-ultraviolet (NUV) and far-ultraviolet (FUV) bands in the Medium Imaging Survey being carried out by the Galaxy Evolution Explorer (GALEX) satellite. In the (NUV-r)_{0.1} vs. M_{r,0.1} galaxy color-magnitude diagram, the galaxies separate into two well-defined blue and red sequences. The (NUV-r)_{0.1} color distribution at each M_{r,0.1} is not well fit by the sum of two Gaussians due to an excess of galaxies in between the two sequences. The peaks of both sequences become redder with increasing luminosity with a distinct blue peak visible up to M_{r,0.1}\sim-23. The r_{0.1}-band luminosity functions vary systematically with color, with the faint end slope and characteristic luminosity gradually increasing with color. After correcting for attenuation due to dust, we find that approximately one quarter of the color variation along the blue sequence is due to dust with the remainder due to star formation history and metallicity. Finally, we present the distribution of galaxies as a function of specific star formation rate and stellar mass. The specific star formation rates imply that galaxies along the blue sequence progress from low mass galaxies with star formation rates that increase somewhat with time to more massive galaxies with a more or less constant star formation rate. Above a stellar mass of ~10^10.5 M_{sun}, galaxies with low ratios of current to past averaged star formation rate begin to dominate.

astro-ph↗

arXiv2007

Keck/Deimos Spectroscopy of a GALEX UV Selecte Sample from the Medium Imaging Survey

We report results from a pilot program to obtain spectroscopy for objects detected in the Galaxy Evolution Explorer (GALEX) Medium Imaging Survey (MIS). Our study examines the properties of galaxies detected by GALEX fainter than the Sloan Digital Sky Survey (SDSS) spectroscopic survey. This is the first study to extend the techinques of Salim et al. 2005 to estimate stellar masses, star formation rates (SFR) and the b (star formation history) parameter for star-forming galaxies out to z~0.7. We obtain redshifts for 50 GALEX MIS sources reaching NUV=23.9 (AB mag), having counterparts in the SDSS Data Release 4 (DR4). Of our sample, 43 are starforming galaxies with z<0.7, 3 have emission line ratios indicative of AGN with z<0.7, and 4 objects with z~1 are QSOs, 3 of which are not previously cataloged. We compare our sample to a much larger sample of ~50,000 matched GALEX/SDSS galaxies with SDSS spectroscopy; while our survey is shallow, the optical counterparts to our sources reach ~3 magnitudes fainter in SDSS r magnitude than the SDSS spectroscopic sample. We use emission line diagnostics for the galaxies to determine that the sample contains mostly star-forming galaxies. The galaxies in the sample populate the blue sequence in the NUV-r vs M_r color-magnitude diagram. Our sample has SFRs, luminosities, and velocity dispersions that are similar to the samples of faint compact blue galaxies studied previously in the same redshift range by Koo et. al 1995, Guzman et. al 1996 & Phillips et. al 1997. However, our sample is ~2 mag fainter in surface brightness than the compact blue galaxies. We find that the star-formation histories for a majority of the galaxies are consistent with a recent starburst within the last 100 Myr.

astro-ph↗

arXiv2007

Clustering Properties of restframe UV selected galaxies II: Migration of Star Formation sites with cosmic time from GALEX and CFHTLS

We analyze the clustering properties of ultraviolet selected galaxies by using GALEX-SDSS data at z<0.6 and CFHTLS deep u' imaging at z=1. These datasets provide a unique basis at z< 1 which can be directly compared with high redshift samples built with similar selection criteria. We discuss the dependence of the correlation function parameters (r0, delta) on the ultraviolet luminosity as well as the linear bias evolution. We find that the bias parameter shows a gradual decline from high (b > 2) to low redshift (b ~ 0.79^{+0.1}_{-0.08}). When accounting for the fraction of the star formation activity enclosed in the different samples, our results suggest that the bulk of star formation migrated from high mass dark matter halos at z>2 (10^12 < M_min < 10^13 M_sun, located in high density regions), to less massive halos at low redshift (M_min < 10^12 M_sun, located in low density regions). This result extends the ``downsizing'' picture (shift of the star formation activity from high stellar mass systems at high z to low stellar mass at low z) to the dark matter distribution.

astro-ph↗

arXiv2007

The Calibration and Data Products of the Galaxy Evolution Explorer

We describe the calibration status and data products pertaining to the GR2 and GR3 data releases of the Galaxy Evolution Explorer (GALEX). These releases have identical pipeline calibrations that are significantly improved over the GR1 data release. GALEX continues to survey the sky in the Far Ultraviolet (FUV, ~154 nm) and Near Ultraviolet (NUV, ~232 nm) bands, providing simultaneous imaging with a pair of photon counting, microchannel plate, delay line readout detectors. These 1.25 degree field-of-view detectors are well-suited to ultraviolet observations because of their excellent red rejection and negligible background. A dithered mode of observing and photon list output pose complex requirements on the data processing pipeline, entangling detector calibrations and aspect reconstruction algorithms. Recent improvements have achieved photometric repeatability of 0.05 and 0.03 mAB in the FUV and NUV, respectively. We have detected a long term drift of order 1% FUV and 6% NUV over the mission. Astrometric precision is of order 0.5" RMS in both bands. In this paper we provide the GALEX user with a broad overview of the calibration issues likely to be confronted in the current release. Improvements are likely as the GALEX mission continues into an extended phase with a healthy instrument, no consumables, and increased opportunities for guest investigations.

astro-ph↗

arXiv2007

UV Star Formation Rates in the Local Universe

We measure star formation rates of ~50,000 optically-selected galaxies in the local universe (z~0.1), spanning a range from gas-rich dwarfs to massive ellipticals. We obtain dust-corrected SFRs by fitting the GALEX (UV) and SDSS (optical) photometry to a library of population synthesis models that include dust attenuation. For star-forming galaxies, our UV-based SFRs compare remarkably well with those derived from SDSS H alpha. Deviations from perfect agreement between these two methods are due to differences in the dust attenuation estimates. In contrast to H alpha, UV provides reliable SFRs for galaxies with weak or no H alpha emission, and where H alpha is contaminated with an emission from an AGN. We use full-SED SFRs to calibrate a simple prescription that uses GALEX UV magnitudes to produce good SFRs for normal star-forming galaxies. The specific SFR is considered as a function of stellar mass for (1) star-forming galaxies with no AGN, (2) those hosting an AGN, and for (3) galaxies without H alpha emission. We find that the three have distinct star formation histories, with AGN lying intermediate between the star-forming and the quiescent galaxies. Normal star forming galaxies (without an AGN) lie on a relatively narrow linear sequence. Remarkably, galaxies hosting a strong AGN appear to represent the massive continuation of this sequence. Weak AGN, while also massive, have lower SFR, sometimes extending to the realm of quiescent galaxies. We propose an evolutionary sequence for massive galaxies that smoothly connects normal star-forming galaxies to quiescent (red sequence) galaxies via strong and weak AGN. We confirm that some galaxies with no H alpha emission show signs of SF in the UV. We derive a UV-based cosmic SFR density at z=0.1 with smaller total error than previous measurements (abridged).

astro-ph↗

arXiv2007

The Look-back Time Evolution of Far-Ultraviolet Flux from the Brightest Cluster Elliptical Galaxies at z < 0.2

We present the GALEX UV photometry of the elliptical galaxies in Abell clusters at moderate redshifts (z < 0.2) for the study of the look-back time evolution of the UV upturn phenomenon. The brightest elliptical galaxies (M_r < -22) in 12 remote clusters are compared with the nearby giant elliptical galaxies of comparable optical luminosity in the Fornax and Virgo clusters. The sample galaxies presented here appear to be quiescent without signs of massive star formation or strong nuclear activity, and show smooth, extended profiles in their UV images indicating that the far-UV (FUV) light is mostly produced by hot stars in the underlying old stellar population. Compared to their counterparts in nearby clusters, the FUV flux of cluster giant elliptical galaxies at moderate redshifts fades rapidly with ~ 2 Gyrs of look-back time, and the observed pace in FUV - V color evolution agrees reasonably well with the prediction from the population synthesis models where the dominant FUV source is hot horizontal-branch stars and their progeny. A similar amount of color spread (~ 1 mag) in FUV - V exists among the brightest cluster elliptical galaxies at z ~ 0.1, as observed among the nearby giant elliptical galaxies of comparable optical luminosity.

astro-ph↗

arXiv2007

The Ultraviolet-Optical Color-Magnitude Diagram III: Constraints on Evolution from the Blue to Red Sequence

We introduce a new quantity, the mass flux density of galaxies evolving from the blue sequence to the red sequence. We propose a simple technique for constraining this mass flux using the volume corrected number density in the extinction-corrected UV-optical color magnitude distribution, the stellar age indexes H-delta-a and D_n(4000), and a simple prescription for spectral evolution using a quenched star formation history. We exploit the excellent separation of red and blue sequences in the NUV-r band color-magnitude diagram. The final value we measure, 0.033 M_sun yr^-1 Mpc^-3, is strictly speaking an upper limit due to the possible contributions of bursting, composite, and extincted galaxies. However, it compares favorably with estimates of the average mass flux that we make based on the red luminosity function evolution derived from the DEEPII and COMBO-17 surveys (Bell et al 2004; Faber et al. 2005), 0.034 M_sun yr^-1 Mpc^-3. We find that the blue sequence mass has remained roughly constant since z=1 (mass flux 0.01 M_sun yr^-1 Mpc^-3) but the average on-going star formation of 0.037$ M_sun yr^-1 Mpc^-3 over 0<z<1 is balanced by mass flux off the blue sequence. We explore the nature of the galaxies in the transition zone with particular attention to the frequency and impact of AGNs. The AGN fraction peaks in the transition zone. We find circumstantial, albeit weak evidence that the quench rates are higher in higher luminosity AGNs.

astro-ph↗

arXiv2007

IR and UV Galaxies at z=0.6 -- Evolution of Dust Attenuation and Stellar Mass as Revealed by SWIRE and GALEX

We study dust attenuation and stellar mass of $\rm z\sim 0.6$ star-forming galaxies using new SWIRE observations in IR and GALEX observations in UV. Two samples are selected from the SWIRE and GALEX source catalogs in the SWIRE/GALEX field ELAIS-N1-00 ($Ω= 0.8$ deg$^2$). The UV selected sample has 600 galaxies with photometric redshift (hereafter photo-z) $0.5 \leq z \leq 0.7$ and NUV$\leq 23.5$ (corresponding to $\rm L_{FUV} \geq 10^{9.6} L_\sun$). The IR selected sample contains 430 galaxies with $f_{24μm} \geq 0.2$ mJy ($\rm L_{dust} \geq 10^{10.8} L_\sun$) in the same photo-z range. It is found that the mean $\rm L_{dust}/L_{FUV}$ ratios of the z=0.6 UV galaxies are consistent with that of their z=0 counterparts of the same $\rm L_{FUV}$. For IR galaxies, the mean $\rm L_{dust}/L_{FUV}$ ratios of the z=0.6 LIRGs ($\rm L_{dust} \sim 10^{11} L_\sun$) are about a factor of 2 lower than local LIRGs, whereas z=0.6 ULIRGs ($\rm L_{dust} \sim 10^{12} L_\sun$) have the same mean $\rm L_{dust}/L_{FUV}$ ratios as their local counterparts. This is consistent with the hypothesis that the dominant component of LIRG population has changed from large, gas rich spirals at z$>0.5$ to major-mergers at z=0. The stellar mass of z=0.6 UV galaxies of $\rm L_{FUV} \leq 10^{10.2} L_\sun$ is about a factor 2 less than their local counterparts of the same luminosity, indicating growth of these galaxies. The mass of z=0.6 UV lunmous galaxies (UVLGs: $\rm L_{FUV} > 10^{10.2} L_\sun$) and IR selected galaxies, which are nearly exclusively LIRGs and ULIRGs, is the same as their local counterparts.

astro-ph↗

arXiv2006

GALEX Ultraviolet Photometry of Globular Clusters in M31: Three Year Results and a Catalog

We present ultraviolet (UV) photometry of M31 globular clusters (GCs) found in 23 Galaxy Evolution Explorer (GALEX) images covering the entirety of M31. We detect 485 and 273 GCs (and GC candidates) in the near-ultraviolet (NUV; 2267 A) and far-ultraviolet (FUV; 1516 A), respectively. Comparing M31 data with those of Galactic GCs in the UV with the aid of population models, we find that the age ranges of old GCs in M31 and the Galactic halo are similar. Three metal-rich ([Fe/H]>-1) GCs in M31 produce significant FUV flux making their FUV-V colors unusually blue for their metallicities. These are thought to be analogs of the two peculiar Galactic GCs NGC 6388 and NGC 6441 with extended blue HB stars. Based on the models incorporating helium enriched subpopulations in addition to the majority of the population that have a normal helium abundance, we suggest that even small fraction of super-helium-rich subpopulations in GCs can reproduce the observed UV bright metal-rich GCs. Young clusters in M31 show distinct UV and optical properties from GCs in Milky Way. Population models indicate that their typical age is less than ~ 2 Gyrs. A large fraction of young GCs have the kinematics of the thin, rapidly rotating disk component. However, a subset of the old GCs also shares the thin-disk kinematics of the younger clusters. The existence of young GCs on the outskirts of M31 disk suggests the occurrence of a significant recent star formation in the thin-disk of M31. Old thin-disk GCs may set constraints on the epoch of early formation of the M31 thin-disk. We detect 12 (10) intermediate-age GC candidates in NUV (FUV). We suggest that some of spectroscopically identified intermediate-age GCs may not be truly intermediate in age, but rather older GCs that possess developed HB.

astro-ph↗

arXiv2006

Statistical Properties of the GALEX/SDSS matched source catalogs, and classification of the UV sources

We use the Galaxy Evolution Explorer (GALEX) Medium and All-Sky-Imaging Survey (MIS & AIS) data from the first public data release (GR1), matched to the Sloan Digital Sky Survey (SDSS) DR3 catalog, to perform source classification. The GALEX surveys provide photometry in far- and near-UV bands and the SDSS in five optical bands (u,g,r,i,z). The GR1/DR3 overlapping areas are 363[83]deg^2 for the GALEX AIS[MIS], for sources within the 0.5deg central area of the GALEX fields. Our sample covers mostly |b|>30deg galactic latitudes. We present statistical properties of the GALEX/SDSS matched sources catalog, containing >2x10^6 objects detected in at least one UV band. We classify the matched sources by comparing the seven-band photometry to model colors constructed for different classes of astrophysical objects. For sources with photometric errors <0.3 mag, the corresponding typical AB-magnitude limits are m_FUV~21.5, m_NUV~22.5 for AIS, and m_FUV~24, m_NUV~24.5 for MIS. At AIS depth, the number of Galactic and extragalactic objects are comparable, but the latter predominate in the MIS. Based on our stellar models, we estimate the GALEX surveys detect hot White Dwarfs throughout the Milky Way halo (down to a radius of 0.04 R_sun at MIS depth), providing an unprecedented improvement in the Galactic WD census. Their observed surface density is consistent with Milky Way model predictions. We also select low-redshift QSO candidates, extending the known QSO samples to lower magnitudes, and providing candidates for detailed z~1 follow-up investigations. SDSS optical spectra available for a large subsample confirm the classification for the photometrically selected candidates with 97% purity for single hot stars, ~45%(AIS)/31%(MIS) for binaries containing a hot star and a cooler companion, and about 85% for QSOs.

astro-ph↗

arXiv2006

Ongoing Formation of Bulges and Black Holes in the Local Universe: New Insights from GALEX

We analyze a volume-limited sample of massive bulge-dominated galaxies with data from both the Sloan Digital Sky Survey and the Galaxy Evolution Explorer (GALEX) satellite. The galaxies have central velocity dispersions greater than 100 km/s and stellar surface mass densities that lie above the value where galaxies transition from actively star forming to passive systems. The sample is limited to redshifts 0.03<z<0.07. At these distances, the SDSS spectra sample the light from the bulge-dominated central regions of the galaxies. The GALEX NUV data provide high sensitivity to low rates of global star formation in these systems. Our sample of bulge-dominated galaxies exhibits a much larger dispersion in NUV-r colour than in optical g-r colour. Nearly all of the galaxies with bluer NUV-r colours are AGN. Both GALEX images and SDSS colour profiles demonstrate that the excess UV light is associated with an extended disk. We find that galaxies with red outer regions almost never have a young bulge or a strong AGN. Galaxies with blue outer regions have bulges and black holes that span a wide range in age and accretion rate. Galaxies with young bulges and strongly accreting black holes almost always have blue outer disks. Our suggested scenario is one in which the source of gas that builds the bulge and black hole is a low mass reservoir of cold gas in the disk.The presence of this gas is a necessary, but not sufficient condition for bulge and black hole growth. Some mechanism must transport this gas inwards in a time variable way. As the gas in the disk is converted into stars, the galaxies will turn red, but further gas infall can bring them back into the blue NUV-r sequence.(Abridged)

astro-ph↗

arXiv2006

Radial variation of attenuation and star formation in the largest late-type disks observed with GALEX

For a sample of 43 nearby, late-type galaxies, we have investigated the radial variation of both the current star formation rate and the dust-induced UV light attenuation. To do this we have cross-correlated IRAS images and GALEX observations for each of these galaxies, and compiled observations of the gas (CO and HI) and metal-abundance gradients found in the literature. We find that attenuation correlates with metallicity. We then use the UV profiles, corrected for attenuation, to study several variants of the Schmidt law and conclude that our results are compatible with a simple law similar to the one of Kennicutt but extending smoothly to lower surface densities, but with considerable scatter. We do not detect an abrupt break in the UV light at the threshold radius derived from H-alpha data (at which the H-alpha profile shows a break and beyond which only a few HII regions are usually found). We interpret the H-alpha sudden break not as a change in the star formation regime (as often suggested) but as the vanishingly small number of ionizing stars corresponding to low levels of star formation.

astro-ph↗

arXiv2006

GALEX UV Color Relations for Nearby Early-Type Galaxies

We use GALEX/optical photometry to construct color-color relationships for early-type galaxies sorted by morphological type. We have matched objects in the GALEX GR1 public release and the first IR1.1 internal release, with the RC3 early-type galaxies having a morphological type -5.5<T<-1.5 with mean error in T<1.5, and mean error on (B-V)T<0.05. After visual inspection of each match, we are left with 130 galaxies with a reliable GALEX pipeline photometry in the far-UV and near-UV bands. This sample is divided into Ellipticals (-5.5<T<-3.5) and Lenticulars (-3.5<T<-1.5). After correction for the Galactic extinction, the color-color diagrams FUV-NUV vs. (B-V)_{Tc} are plotted for the two subsamples. We find a tight anti-correlation between the FUV-NUV and (B-V)_{Tc} colors for Ellipticals, the UV color getting bluer when the (B-V)_{Tc} get redder. This relationship very likely is an extension of the color-metallicity relationship into the GALEX NUV band. We suspect that the main source of the correlation is metal line blanketing in the NUV band. The FUV-NUV vs B-V correlation has larger scatter for lenticular galaxies; we speculate this reflects the presence of low level star formation. If the latter objects (i.e. those that are blue both in FUV-NUV and B-V) are interpreted as harboring recent star formation activity, this would be the case for a few percent (~4%) of Ellipticals and ~15% of Lenticulars; this would make about 10% of early-type galaxies with residual star formation in our full sample of 130 early-type galaxies. We also plot FUV-NUV vs. the Mg_2 index and central velocity dispersion. We find a tight anti-correlation between FUV-NUV and the Mg_2 index(...).

astro-ph↗