Fabio D. Barazza

History of Galaxy Interactions and Their Impact on Star Formation Over the Last 7 Gyr from GEMS

We perform a comprehensive estimate of the frequency of galaxy mergers and their impact on star formation over z similar to 0.24-0.80 (lookback time T(b) similar to 3-7 Gyr) using similar to 3600 (M >= 1 x 10(9) M(circle dot)) galaxies with GEMS Hubble Space Telescope, COMBO-17, and Spitzer data. Our results are as follows. (1) Among similar to 790 high-mass (M >= 2.5 x 10(10) M(circle dot)) galaxies, the visually based merger fraction over z similar to 0.24-0.80, ranges from 9% +/- 5% to 8% +/- 2%. Lower limits on the major merger and minor merger fraction over this interval range from 1.1% to 3.5%, and 3.6% to 7.5%, respectively. This is the first, albeit approximate, empirical estimate of the frequency of minor mergers over the last 7 Gyr. Assuming a visibility timescale of similar to 0.5 Gyr, it follows that over T(b) similar to 3-7 Gyr, similar to 68% of high-mass systems have undergone a merger of mass ratio > 1/10, with similar to 16%, 45%, and 7% of these corresponding respectively to major, minor, and ambiguous major or minor mergers. The average merger rate is similar to a few x 10(-4) galaxies Gyr(-1) Mpc(-3). Among similar to 2840 blue-cloud galaxies of mass M >= 1.0 x 10(9) M(circle dot), similar results hold. (2) We compare the empirical merger fraction and merger rate for high-mass galaxies to three. cold dark matter-based models: halo occupation distribution models, semi-analytic models, and hydrodynamic SPH simulations. We find qualitative agreement between observations and models such that the (major+minor) merger fraction or rate from different models bracket the observations, and show a factor of 5 dispersion. Near-future improvements can now start to rule out certain merger scenarios. (3) Among similar to 3698 M >= 1.0 x 10(9) M(circle dot) galaxies, we find that the mean star formation rate (SFR) of visibly merging systems is only modestly enhanced compared to non-interacting galaxies over z similar to 0.24-0.80. Visibly merging systems only account for a small fraction (< 30%) of the cosmic SFR density over T(b) similar to 3-7 Gyr. This complements the results of Wolf et al. over a shorter time interval of T(b) similar to 6.2-6.8 Gyr, and suggests that the behavior of the cosmic SFR density over the last 7 Gyr is predominantly shaped by non-interacting galaxies.

The STAGES view of red spirals and dusty red galaxies: mass-dependent quenching of star formation in cluster infall

We investigate the properties of optically passive spirals and dusty red galaxies in the A901/2 cluster complex at redshift ∼0.17 using rest-frame near-ultraviolet–optical spectral energy distributions, 24-μm infrared data and Hubble Space Telescope morphologies from the STAGES data set. The cluster sample is based on COMBO-17 redshifts with an rms precision of σcz ≈ 2000 km s −1 . We find that ‘dusty red galaxies’ and ‘optically passive spirals’ in A901/2 are largely the same phenomenon, and that they form stars at a substantial rate, which is only four times lower than that in blue spirals at fixed mass. This star formation is more obscured than in blue galaxies and its optical signatures are weak. They appear predominantly in the stellar mass range of log M∗/M� = [10, 11] where they constitute over half of the star-forming galaxies in the cluster; they are thus a vital ingredient for understanding the overall picture of star formation quenching in clusters. We find that the mean specific star formation rate (SFR) of star-forming galaxies in the cluster is clearly lower than in the field, in contrast to the specific SFR properties of blue galaxies alone, which appear similar in cluster and field. Such a rich red spiral population is best explained if quenching is a slow process and morphological transformation is delayed even more. At log M∗/M� < 10, such galaxies are rare, suggesting that their quenching is fast and accompanied by morphological change. We note that edge-on

Bars in Disk-dominated and Bulge-dominated Galaxies at z ~ 0: New Insights from ~3600 SDSS Galaxies

We present a study of large-scale bars in the local universe, based on a large sample of 3692 galaxies, with 18.5 ≤ Mg < − 22.0 mag and redshift 0.01 ≤ z < 0.03, drawn from the Sloan Digitized Sky Survey. Our sample includes many galaxies that are disk-dominated and of late Hubble types. Both color cuts and Sersic cuts yield a similar sample of ~2000 disk galaxies. We characterize bars and disks by ellipse-fitting r-band images and applying quantitative criteria. After excluding highly inclined (60°) systems, we find the following results. (1) The optical r-band fraction (fopt − r) of barred galaxies, when averaged over the whole sample, is ~48%-52%. (2) When galaxies are separated according to half light radius (re), or normalized re/R24, which is a measure of the bulge-to-disk (B/D) ratio, a remarkable result is seen: fopt − r rises sharply, from ~40% in galaxies that have small re/R24 and visually appear to host prominent bulges, to ~70% for galaxies that have large re/R24 and appear disk-dominated. (3) For galaxies with bluer colors, fopt − r rises significantly (by ~30%). A weaker rise (by ~15%-20%) is seen for lower luminosities or lower masses. (4) While hierarchical ΛCDM models of galaxy evolution models fail to produce galaxies without classical bulges, our study finds that ~20% of disk galaxies appear to be ``quasi-bulgeless. (5) We outline how the effect of a decreasing resolution and a rising obscuration of bars by gas and dust over z = 0.2–1.0 can cause a significant artificial loss of bars, and an artificial reduction in the optical bar fraction over z = 0.2–1.0.

Bar Evolution over the Last 8 Billion Years: A Constant Fraction of Strong Bars in the GEMS Survey

Original article can be found at: --http://www.journals.uchicago.edu/--Copyright The American Astronomical Society

Less than 10 percent of star formation in z=0.6 massive galaxies is triggered by major interactions

Both observations and simulations show that major tidal interactions or mergers between gas-rich galaxies can lead to intense bursts of star formation. Yet, the average enhancement in star formation rate (SFR) in major mergers and the contribution of such events to the cosmic SFR are not well estimated. Here we use photometric redshifts, stellar masses, and UV SFRs from COMBO-17, 24 mu m SFRs from Spitzer, and morphologies from two deep Hubble Space Telescope (HST) cosmological survey fields (ECDFS/GEMS and A901/STAGES) to study the enhancement in SFR as a function of projected galaxy separation. We apply two-point projected correlation function techniques, which we augment with morphologically selected very close pairs (separation = 10(10) M(circle dot)) star-forming galaxies at 0.4 < z < 0.8, we find that the SFRs of galaxies undergoing a major interaction (mass ratios <= 1:4 and separations <= 40 kpc) are only 1.80 +/- 0.30 times higher than the SFRs of non-interacting galaxies when averaged over all interactions and all stages of the interaction, in good agreement with other observational works. Our results also agree with hydrodynamical simulations of galaxy interactions, which produce some mergers with large bursts of star formation on similar to 100 Myr timescales, but only a modest SFR enhancement when averaged over the entire merger timescale. We demonstrate that these results imply that only less than or similar to 10% of star formation at 0.4 <= z <= 0.8 is triggered directly by major mergers and interactions; these events are not important factors in the build-up of stellar mass since z = 1.

OBSCURED STAR FORMATION IN INTERMEDIATE-DENSITY ENVIRONMENTS: A SPITZER STUDY OF THE ABELL 901/902 SUPERCLUSTER

We explore the amount of obscured star formation as a function of environment in the Abell 901/902 (A901/902) supercluster at z = 0.165 in conjunction with a field sample drawn from the A901 and CDFS fields, imaged with the Hubble Space Telescope as part of the Space Telescope A901/902 Galaxy Evolution Survey and Galaxy Evolution from Morphology and Spectral Energy Distributions (SEDs) Survey. We combine the combo-17 near-UV/optical SED with Spitzer 24 mu m photometry to estimate both the unobscured and obscured star formation in galaxies with M(*) > 10(10) M(circle dot). We find that the star formation activity in massive galaxies is suppressed in dense environments, in agreement with previous studies. Yet, nearly 40% of the star-forming (SF) galaxies have red optical colors at intermediate and high densities. These red systems are not starbursting; they have star formation rates (SFRs) per unit stellar mass similar to or lower than blue SF galaxies. More than half of the red SF galaxies have low infrared-to-ultraviolet (IR-to-UV) luminosity ratios, relatively high Sersicindices, and they are equally abundant at all densities. They might be gradually quenching their star formation, possibly but not necessarily under the influence of gas-removing environmental processes. The other greater than or similar to 40% of the red SF galaxies have high IR-to-UV luminosity ratios, indicative of high dust obscuration. They have relatively high specific SFRs and are more abundant at intermediate densities. Our results indicate that while there is an overall suppression in the SF galaxy fraction with density, the small amount of star formation surviving the cluster environment is to a large extent obscured, suggesting that environmental interactions trigger a phase of obscured star formation, before complete quenching.

Bars in disk-dominated and bulge-dominated galaxies at z similar to 0: New insights from similar to 3600 SDSS galaxies

We present a study of large-scale bars in the local universe, based on a large sample of 3692 galaxies, with 18.5 <= M-g < -22.0 mag and redshift 0.01 <= z < 0.03, drawn from the Sloan Digitized Sky Survey. Our sample includes many galaxies that are disk-dominated and of late Hubble types. Both color cuts and Se e rsic cuts yield a similar sample of similar to 2000 disk galaxies. We characterize bars and disks by ellipse-fitting r-band images and applying quantitative criteria. After excluding highly inclined (60 degrees) systems, we find the following results. (1) The optical r-band fraction (f(opt-r)) of barred galaxies, when averaged over the whole sample, is similar to 48%-52%. (2) When galaxies are separated according to half light radius (r(e)), or normalized r(e)/R-24, which is a measure of the bulge-to-disk (B/D) ratio, a remarkable result is seen: f(opt-r) rises sharply, from similar to 40% in galaxies that have small r(e)/R-24 and visually appear to host prominent bulges, to similar to 70% for galaxies that have large r(e)/R-24 and appear disk-dominated. (3) For galaxies with bluer colors, f(opt-r) rises significantly (by similar to 30%). A weaker rise (by similar to 15%-20%) is seen for lower luminosities or lower masses. (4) While hierarchical Lambda CDM models of galaxy evolution models fail to produce galaxies without classical bulges, our study finds that similar to 20% of disk galaxies appear to be quasi-bulgeless. (5) We outline how the effect of a decreasing resolution and a rising obscuration of bars by gas and dust over z = 0.2-1.0 can cause a significant artificial loss of bars, and an artificial reduction in the optical bar fraction over z = 0.2-1.0.

DISTANCES, METALLICITIES, AND AGES OF DWARF ELLIPTICAL GALAXIES IN THE VIRGO CLUSTER FROM SURFACE BRIGHTNESS FLUCTUATIONS

We have employed FORS1 and 2 at the Very Large Telescope at the European Southern Observatory (ESO) to acquire deep B and R-band CCD images of 16 dwarf elliptical (dE) galaxies in the direction of the Virgo cluster. For each dwarf, we measure the apparent R-band surface brightness fluctuation (SBF) magnitude R and the (B-R)0 color in a number of fields at different galactocentric distances. From the field-to-field variation of the two quantities, we determine the SBF distance by means of the (B-R)0–R relation. The derived distances of the dwarfs range from 14.9 to 21.3 Mpc, with a mean 1 σ uncertainty of 1.4 Mpc or 8% of the distance, which confirms that there is considerable depth in the distance distribution of early-type cluster members. For VCC 1104 (IC 3388), our SBF distance modulus of (m - M)SBF = 31.15 ± 0.19 (17.0 ± 1.5 Mpc) is in good agreement with the Harris et al result of (m - M)TRGB = 30.98 ± 0.19 mag (15.7 ± 1.5 Mpc) based on Hubble Space Telescope (HST) observations and the tip magnitude of the red giant branch. Combining our results with existing distances for giant Virgo ellipticals, we identify two major galaxy concentrations in the distance distribution: a broad primary clump around (M - m) = 31.0 mag (15.8 Mpc) and a narrow secondary clump around 31.33 mag (18.5 Mpc). An adaptive kernel analysis finds the two concentrations to be significant at the 99% (2.5 σ) and 89% (1.6 σ) levels, respectively. While the near-side clump of Virgo early-type galaxies can be associated with the subcluster centered on M87, the second clump is believed to be mainly due to the far side infalling group of galaxies around M86. The ages and metallicities of the dE stellar populations are estimated by combining the observed (B-R)0 colors with Wortheys stellar population synthesis models. It appears that the Virgo dE galaxies cover a wider range in metallicity, from [Fe/H] ≈ -1.4 (VCC 0815) to -0.5 (NGC 4415), than Fornax cluster dEs. The derived metallicities place the Virgo dEs on the extension of the metallicity-luminosity relation defined by the low-luminosity Local Group dEs. The data further suggest an age range from genuinely old (~17 Gyr) stellar systems such as IC 3019 and IC 0783 to intermediate-age (8–12 Gyr) dwarfs such as NGC 4431 and IC 3468.

More evidence for hidden spiral and bar features in bright early-type dwarf galaxies ?

Following the discovery of spiral structure in IC 3328 (Jerjen et al. 2000), we present further evidence that a sizable fraction of bright early-type dwarfs in the Virgo cluster are genuine disk galaxies, or are hosting a disk component. Among a sample of 23 nucleated dwarf ellipticals and dS0s observed with the Very Large Telescope in B and R, we found another four systems exhibiting non-axisymmetric structures, such as a bar and/or spiral arms, indicative of a disk (IC 0783, IC 3349, NGC 4431, IC 3468). Particularly remarkable are the two-armed spiral pattern in IC 0783 and the bar and trailing arms in NGC 4431. For both galaxies the disk nature has recently been confirmed by a rotation velocity measurement (Simien & Prugniel 2002). Our photometric search is based on a Fourier decomposition method and a specific version of unsharp masking. Some early-type dwarfs in the Virgo cluster seem to be former late-type galaxies which were transformed to early-type morphology, e.g. by harassment, during their infall to the cluster, while maintaining part of their disk structure.

The dark matter environment of the Abell 901/902 supercluster: a weak lensing analysis of the HST STAGES survey

We present a high-resolution dark matter reconstruction of the z = 0.165 Abell 901/902 supercluster from a weak lensing analysis of the Hubble Space Telescope STAGES survey. We detect the four main structures of the supercluster at high significance, resolving substructure within and between the clusters. We find that the distribution of dark matter is well traced by the cluster galaxies, with the brightest cluster galaxies marking out the strongest peaks in the dark matter distribution. We also find a significant extension of the dark matter distribution of Abell 901a in the direction of an infalling X-ray group Abell 901 alpha. We present mass, mass-to-light and mass-to-stellar mass ratio measurements of the structures and substructures that we detect. We find no evidence for variation of the mass-to-light and mass-to-stellar mass ratio between the different clusters. We compare our space-based lensing analysis with an earlier ground-based lensing analysis of the supercluster to demonstrate the importance of space-based imaging for future weak lensing dark matter observations.