Percy Luis Gomez

Galaxy Star Formation as a Function of Environment in the Early Data Release of the Sloan Digital Sky Survey

We study the galaxy star formation rate (SFR) as a function of environment using the SDSS EDR data. We nd that the SFR is depressed in dense environments (clusters and groups) compared to the eld. We nd that the suppression of the SFR starts to be noticeable at around 4 virial radii. We nd no evidence for SF triggering as galaxies fall into the clusters. We also present a project to study these eects in cluster pairs systems where the eects of lamen ts and large scale structure may be noticeable.

Galaxy ecology: groups and low-density environments in the SDSS and 2dFGRS

We analyse the observed correlation between galaxy environment and Halpha emission-line strength, using volume-limited samples and group catalogues of 24 968 galaxies at 0.05 < z < 0.095, drawn from the 2dF Galaxy Redshift Survey (M-bJ < -19.5) and the Sloan Digital Sky Survey (M-r < -20.6). We characterize the environment by: (1) Sigma(5), the surface number density of galaxies determined by the projected distance to the fifth nearest neighbour; and (2) rho(1.1) and rho(5.5), three-dimensional density estimates obtained by convolving the galaxy distribution with Gaussian kernels of dispersion 1.1 and 5.5 Mpc, respectively. We find that star-forming and quiescent galaxies form two distinct populations, as characterized by their H equivalent width, W-0(Halpha). The relative numbers of star-forming and quiescent galaxies vary strongly and continuously with local density. However, the distribution of W-0(Halpha) amongst the star-forming population is independent of environment. The fraction of star-forming galaxies shows strong sensitivity to the density on large scales, rho(5.5), which is likely independent of the trend with local density, rho(1.1). We use two differently selected group catalogues to demonstrate that the correlation with galaxy density is approximately independent of group velocity dispersion, for sigma = 200-1000 km s(-1). Even in the lowest-density environments, no more than similar to70 per cent of galaxies show significant Halpha emission. Based on these results, we conclude that the present-day correlation between star formation rate and environment is a result of short-time-scale mechanisms that take place preferentially at high redshift, such as starbursts induced by galaxy-galaxy interactions.

The morphology—density relation in the Sloan Digital Sky Survey

We have studied the morphology-density relation and morphology-cluster-centric-radius relation using a volume-limited sample (0.05 < z < 0.1, Mr* < -20.5) of the Sloan Digital Sky Survey (SDSS) data. Major improvements compared with previous work are: (i) automated galaxy morphology classification capable of separating galaxies into four types; (ii) three-dimensional local galaxy density estimation; and (iii) the extension of the morphology-density relation into the field region. We found that the morphology-density and morphology-cluster-centric-radius relation in the SDSS data for both of our automated morphological classifiers, Cin and Tauto, as fractions of early-type galaxies increase and late-type galaxies decrease toward increasing local galaxy density. In addition, we found that there are two characteristic changes in both the morphology-density and the morphology-radius relations, suggesting that two different mechanisms are responsible for the relations. In the sparsest regions (below 1 Mpc - 2 or outside of 1 virial radius), both relations become less noticeable, suggesting that the physical mechanisms responsible for galaxy morphological change require a denser environment. In the intermediate-density regions (density between 1 and 6 Mpc - 2 or virial radius between 0.3 and 1), intermediate-type fractions increase toward denser regions, whereas late-disc fractions decrease. Considering that the median size of intermediate-type galaxies is smaller than that of late-disc galaxies, we propose that the mechanism is likely to stop star formation in late-disc galaxies, eventually turning them into intermediate-type galaxies after their outer discs and spiral arms become invisible as stars die. For example, ram-pressure stripping is one of the candidate mechanisms. In the densest regions (above 6 Mpc - 2 or inside 0.3 virial radii), intermediate-type fractions decrease radically and early-type fractions increase in turn. This is a contrasting result to that in intermediate regions and it suggests that yet another mechanism is more responsible for the morphological change in these regions. We also compared the morphology-density relation from the SDSS (0.01 < z < 0.054) with that of the MORPHS data (z ∼ 0.5). Two relations lie on top of each other, suggesting that the morphology-density relation was already established at z ∼ 0.5 as in the present Universe. A slight sign of an excess elliptical/S0 fraction in the SDSS data in dense regions might suggest the additional formation of elliptical/S0 galaxies in the cluster core regions between z = 0.5 and 0.05.

Star Formation Rate Indicators in the Sloan Digital Sky Survey

The Sloan Digital Sky Survey (SDSS) first data release provides a database of ?106,000 unique galaxies in the main galaxy sample with measured spectra. A sample of star-forming (SF) galaxies are identified from among the 3079 of these having 1.4 GHz luminosities from FIRST, by using optical spectral diagnostics. Using 1.4 GHz luminosities as a reference star formation rate (SFR) estimator insensitive to obscuration effects, the SFRs derived from the measured SDSS H?, [O II], and u-band luminosities, as well as far-infrared luminosities from IRAS, are compared. It is established that straightforward corrections for obscuration and aperture effects reliably bring the SDSS emission line and photometric SFR estimates into agreement with those at 1.4 GHz, although considerable scatter (?60%) remains in the relations. It thus appears feasible to perform detailed investigations of star formation for large and varied samples of SF galaxies through the available spectroscopic and photometric measurements from the SDSS. We provide herein exact prescriptions for determining the SFR for SDSS galaxies. The expected strong correlation between [O II] and H? line fluxes for SF galaxies is seen, but with a median line flux ratio F/FH? = 0.23, about a factor of 2 smaller than that found in the sample of Kennicutt. This correlation, used in deriving the [O II] SFRs, is consistent with the luminosity-dependent relation found by Jansen and coworkers. The median obscuration for the SDSS SF systems is found to be AH? = 1.2 mag, while for the radio-detected sample the median obscuration is notably higher, 1.6 mag, and with a broader distribution.

The Environment of Active Galactic Nuclei in the Sloan Digital Sky Survey

We present the observed fraction of galaxies with an active galactic nucleus (AGN) as a function of environment in the early data release of the Sloan Digital Sky Survey (SDSS). Using 4921 galaxies in the redshift range 0.05 ≤ z ≤ 0.095 and brighter than M(r*) = -20.0 (or M* + 1.45), we find at least ~20% of these galaxies possess an unambiguous detection of an AGN, but this fraction could be as high as 40% after we model the ambiguous emission-line galaxies in our sample. We have studied the environmental dependence of galaxies, using the local galaxy density as determined from the distance to the 10th nearest neighbor. As expected, we observe that the fraction of star-forming galaxies decreases with density, while the fraction of passive galaxies (no emission lines) increases with density. In contrast, the fraction of galaxies with an AGN remains constant from the cores of galaxy clusters to the rarefied field population. We conclude that the presence of an AGN is independent of the disk component of a galaxy. We have extensively tested our results, and they are robust against measurement error, definition of an AGN, aperture bias, stellar absorption, survey geometry, and signal-to-noise ratio. Our observations are consistent with the hypothesis that a supermassive black hole resides in the bulge of all massive galaxies, and 40% of these black holes are seen as AGNs in our sample. A high fraction of local galaxies with an AGN suggests either that the mean lifetime of these AGNs is longer than previously thought (i.e., ≥108 yr) or that the AGNs burst more often than expected: ~40 times over the redshift range of our sample.

A Large, Uniform Sample of X-Ray-emitting AGNs: Selection Approach and an Initial Catalog from the ROSAT All-Sky and Sloan Digital Sky Surveys

Many open questions in X-ray astronomy are limited by the relatively small number of objects in uniform optically identified and observed samples, especially when rare subclasses are considered or when subsets are isolated to search for evolution or correlations between wavebands. We describe the initial results of a new program aimed to ultimately yield ~104 fully characterized X-ray source identifications?a sample about an order of magnitude larger than earlier efforts. The technique is detailed and employs X-ray data from the ROSAT All-Sky Survey (RASS) and optical imaging and spectroscopic follow-up from the Sloan Digital Sky Survey (SDSS); these two surveys prove to be serendipitously very well matched in sensitivity. As part of the SDSS software pipelines, optical objects in the SDSS photometric catalogs are automatically positionally cross-correlated with RASS X-ray sources. Then priorities for follow-on SDSS optical spectra of candidate counterparts are automatically assigned using an algorithm based on the known ratios of fx/fopt for various classes of X-ray emitters at typical RASS fluxes of ~10-13 ergs cm-2 s-1. SDSS photometric parameters for optical morphology, magnitude, and colors, plus FIRST radio information, serve as proxies for object class. Initial application of this approach to RASS/SDSS data from 1400 deg2 of sky provides a catalog of more than 1200 spectroscopically confirmed quasars and other AGNs that are probable RASS identifications. Most of these are new identifications, and only a few percent of the AGN counterparts are likely to be random superpositions. The magnitude and redshift ranges of the counterparts are very broad, extending over 15 < m < 21 and 0.03 < z < 3.6, respectively. Although most identifications are quasars and Seyfert 1 galaxies, a variety of other AGN subclasses are also sampled. Substantial numbers of rare AGN types are found, including more than 130 narrow-line Seyfert 1 galaxies and 45 BL Lac candidates. These early results already provide a very sizable set of source identifications, demonstrate the utility of the sample in multiwaveband investigations, and show the capability of the joint RASS/SDSS approach to efficiently proceed toward the largest homogeneously selected/observed sample of X-ray?emitting quasars and other kinds of AGNs.

The Environment of Passive Spiral Galaxies in the SDSS

In previous work on galaxy clusters, several authors reported the discovery of an unusual population of galaxies, which have spiral morphologies, but do not show any star-formation activity. These galaxies are called “passive spirals”, and have been interesting since it has been difficult to understand the existence of such galaxies. Using a volume-limited sample (0.05 <z <0. 1a ndMr ∗ < −20.5; 25813 galaxies) of the Sloan Digital Sky Survey data, we found 73 (0.28±0.03%) passivespiral galaxiesand studied their environments. It is found that passivespiral galaxies exist in a local galaxy density of 1–2Mpc −2 and have a 1–10 cluster-centric virial radius. Thus, the origins of passive spiral galaxies are likely to be cluster-related. These characteristic environments coincide with a previously reported environment where the galaxy star-formation rate suddenly declines and the so-called morphology–density relation turns. It is likely that the same physical mechanism is responsible for all of these observational results. The existence of passive spiral galaxies suggests that a physical mechanism that works calmly is preferred to dynamical origins such as major merger/interaction since such a mechanism would destroy the spiral-arm structures. Compared with the observed cluster galaxy evolution such as the Butcher–Oemler effect and the morphological Butcher–Oemler effect, passive spiral galaxies are likely to be a key galaxy population in transition between red, elliptical/S0 galaxies in low-redshift clusters and blue, spiral galaxies more numerous in higher-redshift clusters.

Composite Luminosity Functions Based on the Sloan Digital Sky Survey "Cut and Enhance" Galaxy Cluster Catalog

We present here results on the composite luminosity functions (LF) of galaxies in the clusters of galaxies selected from the Cut and Enhance cluster catalog (CE) of the Sloan Digital Sky Survey. We constructed composite LFs in the five SDSS bands, u, g, r , i, and z, using 204 CE clusters ranging from z = 0.02 to z = 0.25 . Background and foreground galaxies were subtracted from the LF using an annular region around clusters to take large-scale, galaxy-number-count variances into consideration. A LF of each cluster was weighted according to the richness and number of contributing galaxies to construct the composite LF. Taking advantage of accurate photometry of SDSS, we used photometric redshifts to construct composite luminosity functions and thus study a large number of clusters. The robustness of the weighting scheme was tested using a Monte-Carlo simulation. The best-fit Schechter parameters are (M∗,α) = (−21.61±0.26,−1.40±0.11), (−22.01±0.11,−1.00±0.06), (−22.21±0.05,−0.85± 0.03), (−22.31± 0.08,−0.70± 0.05), and (−21.36± 0.06,−0.58± 0.04) in u, g, r , i, and z, respectively. We find that the slope of composite LFs becomes flatter toward a redder color band. Compared with the field LFs of the SDSS, the cluster LFs have brighter characteristic magnitude and flatter slopes in the g, r , i, and z bands. These results are consistent with the hypothesis that the cluster LF has two distinct underlying populations; i.e. the bright end of the LF is dominated by bright early types that follow a Gaussian-like luminosity distribution, while the faintend of the cluster LF is a steep power-law like function dominated by star-forming (bluer) galaxies. We also studied the composite LFs for early-type and late-type galaxies using profile fits, a concentration parameter and u− r color to classify galaxy morphology. A strong dependence of LF on galaxy morphology was found. The faint end slope of the LF is always flatter for early-type galaxies than late-type, regardless of the passband and methodology. These results are consistent with the hypothesis that the cluster regions are dominated by bright elliptical galaxies. This work also provides a good low-redshift benchmark for on-going multi-color photometric studies of high redshift clusters of galaxies using 4–8m class telescopes.

Morphological Butcher-Oemler Effect in the SDSS "Cut and Enhance" Galaxy Cluster Catalog

We investigate the evolution of the fractions of late type cluster galaxies as a function of redshift, using one of the largest, most uniform cluster samples available. The sample consists of 514 clusters of galaxies in the range 0.02<z<0.3 from the Sloan Digital Sky Survey Cut & Enhance galaxy cluster catalog. This catalog was created using a single automated cluster finding algorithm on uniform data from a single telescope, with accurate CCD photometry, thus, minimizing selection biases. We use four independent methods to analyze the evolution of the late type galaxy fraction. Specifically, we select late type galaxies based on: restframe g-r color, u-r color, galaxy profile fitting and concentration index. The first criterion corresponds to the one used in the classical Butcher-Oemler analyses. The last three criteria are more sensitive to the morphological type of the galaxies. In all four cases, we find an increase in the fraction of late type galaxies with increasing redshift, significant at the 99.9% level. The results confirm that cluster galaxies do change colors with redshift (the Butcher-Oemler effect) and, in addition, they change their morphology to later-type toward higher redshift -- indicating a morphological equivalent of the Butcher-Oemler effect. We also find a tendency of richer clusters to have lower fractions of late type galaxies. The trend is consistent with a ram pressure stripping model, where richer clusters have more effective ram pressure due to their higher temperature.