On The Extended Knotted Disks of Galaxies

The stellar disks of many spiral galaxies are twice as large as generally thought. We use archival data from the Galaxy Evolution Explorer mission (GALEX) to quantify the statistical properties of young stellar clusters in the outer, extended disks of a sample of eleven nearby galaxies. We find an excess of sources between 1.25 and 2 optical radii, R(25), for five of the galaxies, which statistically implies that at least a quarter of such galaxies have this cluster population (90% confidence level), and no significant statistical excess in the sample as a whole beyond 2 optical radii, even though one galaxy (M 83) individually shows such an excess. Although the excess is typically most pronounced for blue (FUV -NUV < 1, NUV < 25) sources, there is also an excess of sources with redder colors. Although from galaxy to galaxy the number of sources varies significantly, on average, the galaxies with such sources have 75 +- 10 blue sources at radii between 1.25 and 2 R(25). In addition, the radial distribution is consistent with the extended dust emission observed in the far IR and with the properties of H-alpha sources, assuming a constant cluster formation rate over the last few hundred Myrs. All of these results suggest that the phenomenon of low-level star formation well outside the apparent optical edges of disks (R ~ R(25)) is common and long-lasting.Comment: 8 pages, accepted for publication in A

Measuring Outer Disk Warps with Optical Spectroscopy

Warps in the outer gaseous disks of galaxies are a ubiquitous phenomenon, but it is unclear what generates them. One theory is that warps are generated internally through spontaneous bending instabilities. Other theories suggest that they result from the interaction of the outer disk with accreting extragalactic material. In this case, we expect to find cases where the circular velocity of the warp gas is poorly correlated with the rotational velocity of the galaxy disk at the same radius. Optical spectroscopy presents itself as an interesting alternative to 21-cm observations for testing this prediction, because (i) separating the kinematics of the warp from those of the disk requires a spatial resolution that is higher than what is achieved at 21 cm at low HI column density; (ii) optical spectroscopy also provides important information on star formation rates, gas excitation, and chemical abundances, which provide clues to the origin of the gas in warps. We present here preliminary results of a study of the kinematics of gas in the outer-disk warps of seven edge-on galaxies, using multi-hour VLT/FORS2 spectroscopy.Comment: 7 pages, 7 figures; to appear in the proceedings of IAU Symposium 254 "The Galaxy disk in a cosmological context", Copenhagen, June 200

The Dependence of the Galaxy Luminosity Function on Environment

We present luminosity functions for galaxies in loose groups in the Las Campanas Redshift Survey, differentiated by their environment (defined by the line-of-sight velocity dispersion sigma of the host groups) and also by their spectral type (emission or non-emission, defined by the equivalent width of the 3727-Angstrom [OII] line). We find systematic variations in the Schechter parameters alpha and M* for non-emission line galaxies over a range of 0 < sigma < 800 km/s. Alpha varies from 0.20 to -0.91, indicating an increase in the steepness of the faint end slope with increasing sigma. The accompanying variation in M* appears to be accounted for by the intrinsic correlation with alpha and does not indicate a significant physical variation in the bright end of the luminosity function. For emission line galaxies, we find no significant systematic variation of the luminosity function with the environment. Our results show that emission and non-emission galaxies generally occupy two distinct regions in the alpha-M* parameter space. From our luminosity functions, we derive the number ratios of emission to non-emission galaxies as a function of environment and absolute magnitude, showing that the relative abundance of non-emission line galaxies generally increases for all magnitudes -23 < M_R < -17.5 towards high-sigma environments, from ~80% to >90% at M_R = -22 and from ~10% to >50% at M_R = -18 (H_0 = 100 km s^{-1} Mpc^{-1} and q_0 = 0.5).Comment: 34 pages, 10 figures; accepted for publication in the Ap

Galaxy Luminosity Functions from Deep Spectroscopic Samples of Rich Clusters

Using a new spectroscopic sample and methods accounting for spectroscopic sampling fractions that vary in magnitude and surface brightness, we present R-band galaxy luminosity functions (GLFs) for six nearby galaxy clusters with redshifts 4000 < cz < 20000 km/s and velocity dispersions 700 < sigma < 1250 km/s. In the case of the nearest cluster, Abell 1060, our sample extends to M_R=-14 (7 magnitudes below M*), making this the deepest spectroscopic determination of the cluster GLF to date. Our methods also yield composite GLFs for cluster and field galaxies to M_R=-17 (M*+4), including the GLFs of subsamples of star forming and quiescent galaxies. The composite GLFs are consistent with Schechter functions (M*_R=-21.14^{+0.17}_{-0.17}, alpha=-1.21^{+0.08}_{-0.07} for the clusters, M*_R=-21.15^{+0.16}_{-0.16}, alpha=-1.28^{+0.12}_{-0.11} for the field). All six cluster samples are individually consistent with the composite GLF down to their respective absolute magnitude limits, but the GLF of the quiescent population in clusters is not universal. There are also significant variations in the GLF of quiescent galaxies between the field and clusters that can be described as a steepening of the faint end slope. The overall GLF in clusters is consistent with that of field galaxies, except for the most luminous tip, which is enhanced in clusters versus the field. The star formation properties of giant galaxies are more strongly correlated with the environment than those of fainter galaxies.Comment: 53 pages, 8 figures, 1 ASCII table; accepted for publication in Ap

Can Early Type Galaxies Evolve from Fading the Disks of Late Types?

We examine whether early-type galaxies in clusters may have evolved from later types by the fading of their disks (e.g., as a result of ram-pressure stripping or strangulation) or by enhancement of the bulge luminosity (e.g., due to tidal interactions and mergers). For this purpose, we compare the bulge and disk luminosities of early- and late-type galaxies and of galaxies at different radial distances from the cluster center. We find that, in order for early-type galaxies, including S0s, to have evolved from late-type galaxies, their bulge luminosities must have been physically enhanced. Disk fading models cannot explain the differences observed. We then show that galaxy bulges are systematically brighter at small projected distances from the cluster center, while disk luminosities are uncorrelated with cluster-centric distance. Our results suggest that bulge enhancement, not disk fading, distinguishes early from late types and is thus at least partially responsible for the morphology-environment relation of bright cluster galaxies.Comment: 20 pages, including 5 figures, accepted for publication in Ap

Disentangling Morphology, Star Formation, Stellar Mass, and Environment in Galaxy Evolution

We present a study of the spectroscopic and photometric properties of galaxies in six nearby clusters. We perform a partial correlation analysis on our dataset to investigate whether the correlation between star formation rates in galaxies and their environment is merely another aspect of correlations of morphology, stellar mass, or mean stellar age with environment, or whether star formation rates vary independently of these other correlations. We find a residual correlation of ongoing star formation with environment, indicating that even galaxies with similar morphologies, stellar masses, and mean stellar ages have lower star formation rates in denser environments. Thus, the current star formation gradient in clusters is not just another aspect of the morphology-density, stellar mass-density, or mean stellar age-density relations. Furthermore, the star formation gradient cannot be solely the result of initial conditions, but must partly be due to subsequent evolution through a mechanism (or mechanisms) sensitive to environment. Our results constitute a true ``smoking gun'' pointing to the effect of environment on the later evolution of galaxies.Comment: 31 pages, including 5 figures; accepted for publication in Ap

The Evolution of Early-type Galaxies Selected by Their Spatial Clustering

Aims: We present a new method that uses luminosity or stellar mass functions combined with clustering measurements to select samples of galaxies at different redshifts likely to follow a progenitor-to-descendant relationship. As the method uses clustering information, we refer to galaxy samples selected this way as clustering-selected samples. We apply this method to infer the number of mergers during the evolution of MUSYC early-type galaxies (ETGs) from z~1 to the present-day. Methods: The method consists in using clustering information to infer the typical dark-matter halo mass of the hosts of the selected progenitor galaxies. Using LambdaCDM predictions, it is then possible to follow these haloes to a later time where the sample of descendants will be that with the clustering of these descendant haloes. Results: This technique shows that ETGs at a given redshift evolve into brighter galaxies at lower redshifts (considering rest-frame, passively evolved optical luminosities). This indicates that the stellar mass of these galaxies increases with time and that, in principle, a stellar mass selection at different redshifts does not provide samples of galaxies in a progenitor-descendant relationship. Conclusions: The comparison between high redshift ETGs and their likely descendants at z=0 points to a higher number density for the progenitors by a factor 5.5+-4.0, implying the need for mergers to decrease their number density by today. Because the luminosity densities of progenitors and descendants are consistent, our results show no need for significant star-formation in ETGs since z=1, which indicates that the needed mergers are dry, i.e. gas free.Comment: 11 pages, 8 figures, accepted for publication in A&

The U-band Galaxy Luminosity Function of Nearby Clusters

Despite the great potential of the U-band galaxy luminosity function (GLF) to constrain the history of star formation in clusters, to clarify the question of variations of the GLF across filter bands, to provide a baseline for comparisons to high-redshift studies of the cluster GLF, and to estimate the contribution of bound systems of galaxies to the extragalactic near-UV background, determinations have so far been hampered by the generally low efficiency of detectors in the U-band and by the difficulty of constructing both deep and wide surveys. In this paper, we present U-band GLFs of three nearby, rich clusters to a limit of M_U=-17.5 (M*_U+2). Our analysis is based on a combination of separate spectroscopic and R-band and U-band photometric surveys. For this purpose, we have developed a new maximum-likelihood algorithm for calculating the luminosity function that is particularly useful for reconstructing the galaxy distribution function in multi-dimensional spaces (e.g., the number of galaxies as a simultaneous function of luminosity in different filter bands, surface brightness, star formation rate, morphology, etc.), because it requires no prior assumptions as to the shape of the distribution function. The composite luminosity function can be described by a Schechter function with characteristic magnitude M*_U=-19.82+/-0.27 and faint end slope alpha_U=-1.09+/-0.18. The total U-band GLF is slightly steeper than the R-band GLF, indicating that cluster galaxies are bluer at fainter magnitudes. Quiescent galaxies dominate the cumulative U-band flux for M_U<-14. The contribution of galaxies in nearby clusters to the U-band extragalactic background is <1% Gyr^-1 for clusters of masses ~3*10^14 to 2*10^15 M_solar.Comment: 44 pages, 11 figures, accepted for publication in Ap

The Kinematic Properties of the Extended Disks of Spiral Galaxies: A Sample of Edge-On Galaxies

We present a kinematic study of the outer regions (R_25<R<2 R_25) of 17 edge-on disk galaxies. Using deep long-slit spectroscopy (flux sensitivity a few 10^-19 erg s^-1 cm^-2 arcsec^-2), we search for H-alpha emission, which must be emitted at these flux levels by any accumulation of hydrogen due to the presence of the extragalactic UV background and any other, local source of UV flux. We present results from the individual galaxy spectra and a stacked composite. We detect H-alpha in many cases well beyond R_25 and sometimes as far as 2 R_25. The combination of sensitivity, spatial resolution, and kinematic resolution of this technique thus provides a powerful complement to 21-cm observations. Kinematics in the outer disk are generally disk-like (flat rotation curves, small velocity dispersions) at all radii, and there is no evidence for a change in the velocity dispersion with radius. We place strong limits, few percent, on the existence of counter-rotating gas out to 1.5 R_25. These results suggest that thin disks extend well beyond R_25; however, we also find a few puzzling anomalies. In ESO 323-G033 we find two emission regions that have velocities close to the systemic velocity rather than the expected rotation velocity. These low relative velocities are unlikely to be simply due to projection effects and so suggest that these regions are not on disk-plane, circular orbits. In MCG-01-31-002 we find emission from gas with a large velocity dispersion that is co-rotating with the inner disk.Comment: 18 pages, 14 figures, accepted for publication in Ap

The Surface Mass Density and Structure of the Outer Disk of NGC 628

We study the kinematics of GALEX-selected H_alpha knots in the outer disk (beyond R25) of NGC 628 (M74), a galaxy representative of large, undisturbed, extended UV (Type 1 XUV) disks. Our spectroscopic target sample of 235 of the bluest UV knots surrounding NGC 628 yielded 15 H_alpha detections (6%), roughly the number expected given the different mean ages of the two populations. The measured vertical velocity dispersion of the H_alpha knots between 1 - 1.8 R25 (13.5 - 23.2 kpc) is < 11 km/s. We assume that the H_alpha knots trace an 'intermediate' vertical mass density distribution (between the isothermal sech(z)^2 and exponential distributions) with a constant scaleheight across the outer disk (h_z = 700 pc) and estimate a total surface mass density of 7.5 solar masses/pc^2. This surface mass density can be accounted for by the observed gas and stars in the outer disk (little or no dark matter in the disk is required). The vertical velocity dispersion of the outer disk H_alpha knots nearly matches that measured from older planetary nebulae near the outskirts of the optical disk by Herrmann et al., suggesting a low level of scattering in the outer disk. A dynamically cold stellar component extending nearly twice as far as the traditional optical disk poses interesting constraints on the accretion history of the galaxy.Comment: Accepted for publication in The Astrophysical Journal