A MORPHOLOGICAL RE-EVALUATION OF GALAXIES IN COMMON FROM THE CATALOG OF ISOLATED GALAXIES AND THE SLOAN DIGITAL SKY SURVEY (DR6)

We analyse the B-R_c colors of galaxies as functions of luminosity and local galaxy density using a large photometric redshift catalog based on the Red-Sequence Cluster Survey. We select two samples of galaxies with a magnitude limit of M_Rc<-18.5 and redshift ranges of 0.2<z< 0.4 and 0.4<z<0.6 containing \~10^5 galaxies each. We model the color distributions of subsamples of galaxies and derive the red galaxy fraction and peak colors of red and blue galaxies as functions of galaxy luminosity and environment. The evolution of these relationships over the redshift range of z~0.5 to z~0.05 is analysed in combination with published results from the Sloan Digital Sky Survey. We find that there is a strong evolution in the restframe peak color of bright blue galaxies in that they become redder with decreasing redshift, while the colors of faint blue galaxies remain approximately constant. This effect supports the ``downsizing'' scenario of star formation in galaxies. While the general dependence of the galaxy color distributions on the environment is small, we find that the change of red galaxy fraction with epoch is a function of the local galaxy density, suggesting that the downsizing effect may operate with different timescales in regions of different galaxy densities.

We investigate the photometric properties of 456 bright galaxies using imaging data recorded during the commissioning phase of the Sloan Digital Sky Survey (SDSS). Morphological classification is carried out by correlating results of several human classifiers. Our purpose is to examine the statistical properties of color indices, scale lengths, and concentration indices as functions of morphology for the SDSS photometric system. We find that u'-g', g'-r', and r'-i' colors of SDSS galaxies match well with those expected from the synthetic calculation of spectroscopic energy distribution of template galaxies and with those transformed from UBVRCIC color data of nearby galaxies. The agreement is somewhat poor, however, for the i'-z' color band, with a discrepancy of 0.1?0.2 mag. With the aid of the relation between surface brightness and radius obtained by Kent in 1985, we estimate the averages of the effective radius of early-type galaxies and the scale length of exponential disks both to be 2.6 kpc for L* galaxies. We find that the half-light radius of galaxies depends slightly on the color bands, consistent with the expected distribution of star-forming regions for late-type galaxies and with the known color gradient for early-type galaxies. We also show that the (inverse) concentration index, defined by the ratio of the half-light Petrosian radius to the 90% light Petrosian radius, correlates tightly with the morphological type; this index allows us to classify galaxies into early (E/S0) and late (spiral and irregular) types, allowing for a 15%?20% contamination from the opposite class compared with eye-classified morphology.

Galaxy spectral evolutionary models are used to compute the following quantities in optical and UV bandpasses: (a) galaxy color, luminosity, two-color diagrams, and surface brightness profiles as functions of redshifty and (b) galaxy counts, and color and redshift distributions as functions of apparent magnitude. These predictions may be used as a guide to prepare and interpret observing runs with the Space Telescope, the Space Schmidt Telescope, and the Starlab Observatory.

Galaxy spectral evolutionary models are used to compute the following quantities in optical and UV bandpasses: (a) galaxy color, luminosity, two-color diagrams, and surface brightness profiles as functions of redshift; and (b) galaxy counts, and color and redshift distributions as functions of apparent magnitude. These predictions may be used as a guide to prepare and interpret observing runs with the Space Telescope, the Space Schmidt Telescope, and the Starlab Observatory.

We examine the relationship between environment and the luminosities, surface brightnesses, colors, and profile shapes of luminous galaxies in the Sloan Digital Sky Survey (SDSS). For the SDSS sample, galaxy color is the galaxy property most predictive of the local environment. Galaxy color and luminosity—measures of the star formation history—jointly comprise the most predictive pair of properties. At fixed luminosity and color, density is not closely related to surface brightness or to Sersic index—measures of galaxy structure. In the text, we discuss what measurable residual relationships exist, generally finding that at red colors and fixed luminosity, the mean density decreases at the highest surface brightnesses and Sersic indices. In general, these results suggest that the structural properties of galaxies are less closely related to galaxy environment than are their masses and star formation histories.

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A new catalog of isolated galaxies from The Sloan Digital Sky Survey (DR5) is presented. 1520 isolated galaxies were found in 1.4 steradians of sky. The selection criteria in this so called UNAM-KIAS catalog was implemented from a variation on the criteria developed by Karachentseva 1973 including full redshift information. Through an image processing pipeline that takes advantage from the high resolution (~ 0.4 ''/pix) and high dynamic range of the SDSS images, a uniform g band morphological classification for all these galaxies is presented. We identify 80% (SaSm) spirals (50% later than Sbc types) on one hand, and a scarce population of early-type E(6.5%) and S0(8%) galaxies amounting to 14.5% on the other hand. This magnitude-limited catalog is ~ 80% complete at 16.5, 15.6, 15.0, 14.6 and 14.4 magnitudes in the ugriz bands respectively. Some representative physical properties including SDSS magnitudes and color distributions, color-color diagrams, absolute magnitude-color, and concentration-color diagrams as a function of morphological type are presented. The UNAM-KIAS Morphological Atlas is also released along with this paper. For each galaxy of type later than Sa, a mosaic is presented that includes: (1) a g-band logarithmic image, (2) a g band filtered-enhanced image where a Gaussian kernel of various sizes was applied and (3) an RGB color image from the SDSS database. For E/S0/Sa galaxies, in addition to the images in (1), (2) and (3), plots of r band surface brightness and geometric profiles (ellipticity, Position Angle PA and A4/B4 coefficients of the Fourier series expansions of deviations of a pure ellipse) are provided...

We present the results of our search for faint Local Group dwarf galaxies in compact high-velocity clouds (HVCs). We used digitized Palomar Observatory Sky Survey (POSS) data to examine 1 deg2 of sky around each of ~250 northern hemisphere HVCs. The POSS images were processed to remove foreground stars and large-scale backgrounds, smoothed to enhance arcminute-sized low surface brightness features, and then compared with the original plates. Using this technique, we located 60 candidate dwarf galaxies in the ~250 deg2 that we surveyed. Follow-up observations of these candidates have revealed several distant clusters of galaxies and a number of possible Galactic cirrus clouds, but no Local Group dwarfs. It appears that many of the low surface brightness features in the sky survey data are plate flaws. The second-generation red POSS plates are sensitive down to surface brightness levels of 25-26 mag arcsec-2, and our follow-up images reached 10 σ limiting magnitudes of R = 21-23 for point sources. Given these limits, all known Local Group galaxies except four of the very diffuse, extended dwarf spheroidals located within 100 kpc of the Milky Way would have been detected had they been in our survey. Therefore, we can rule out the possibility that these HVCs are associated with normal but faint dwarf galaxies. If compact HVCs do contain stars, they must have surface brightnesses 1 mag arcsec-2 fainter than most known Local Group galaxies.

A new approach to estimating photometric redshifts - using Artificial Neural Networks (ANNs) - is investigated. Unlike the standard template-fitting photometric redshift technique, a large spectroscopically-identified training set is required but, where one is available, ANNs produce photometric redshift accuracies at least as good as and often better than the template-fitting method. The Bayesian priors on the underlying redshift distribution are automatically taken into account. Furthermore, inputs other than galaxy colours - such as morphology, angular size and surface brightness - may be easily incorporated, and their utility assessed. Different ANN architectures are tested on a semi-analytic model galaxy catalogue and the results are compared with the template-fitting method. Finally the method is tested on a sample of ~ 20000 galaxies from the Sloan Digital Sky Survey. The r.m.s. redshift error in the range z < 0.35 is ~ 0.021.

We present a catalogue of galaxy photometric redshifts and k-corrections for the Sloan Digital Sky Survey Seven Data Release (SDSS-DR7), available on the World Wide Web. The photometric redshifts were estimated with an artificial neural network using five ugriz bands, concentration indices and Petrosian radii in the g and r bands. We have explored our redshift estimates with different training set concluding that the best choice to improve redshift accuracy comprises the Main Galaxies Sample (MGS), the Luminous Red Galaxies, and galaxies of active galactic nuclei covering the redshift range 0<z<0.3. For the MGS, the photometric redshift estimates agree with the spectroscopic values within rms=0.0227. The derived distribution of photometric redshifts in the range 0<zphot<0.6 agrees well with the model predictions. k-corrections were derived by calibration of the k-correct-v4.2 code results for the MGS with the reference frame (z=0.1) (g-r) colours. We adopt a linear dependence of k corrections on redshift and (g-r) colours that provide suitable distributions of luminosity and colours for galaxies up to redshift zphot=0.6 comparable to the results in the literature. Thus, our k-correction estimate procedure is a powerful, low computational time algorithm capable of reproducing suitable results that can be used for testing galaxy properties at intermediate redshifts using the large SDSS database.

Successful halo-model descriptions of the luminosity dependence of clustering distinguish between the central galaxy in a halo and all the others (satellites). To include colours, we provide a prescription for how the colour–magnitude relation of centrals and satellites depends on halo mass. This follows from two assumptions: (i) the bimodality of the colour distribution at a fixed luminosity is independent of halo mass and (ii) the fraction of satellite galaxies which populate the red sequence increases with luminosity. We show that these two assumptions allow one to build a model of how galaxy clustering depends on colour without any additional free parameters than those required to model the luminosity dependence of galaxy clustering. We then show that the resulting model is in good agreement with the distribution and clustering of colours in the Sloan Digital Sky Survey, both by comparing the predicted correlation functions of red and blue galaxies with measurements and by comparing the predicted colour–mark correlation function with the measured one. Mark correlation functions are powerful tools for identifying and quantifying correlations between galaxy properties and their environments: our results indicate that the correlation between halo mass and environment is the primary driver for correlations between galaxy colours and the environment; additional correlations associated with halo ‘assembly bias’ are relatively small. Our approach shows explicitly how to construct mock catalogues which include both luminosities and colours – thus providing realistic training sets for, e.g., galaxy cluster-finding algorithms. Our prescription is the first step towards incorporating the entire spectral energy distribution into the halo model approach.

The main aim of this paper is to report two new detections of tidal debris in the northern stream of the Sagittarius dwarf galaxy located at 45° and 55° from the center of the galaxy. Our observational approach is based on deep color-magnitude diagrams that provide accurate distances, surface brightness, and the properties of stellar population of the studied region of this tidal stream. The derived distances for these tidal debris wraps are 45 ± 5 and 62 ± 6 kpc, respectively. These detections are also strong observational evidence that the tidal stream discovered by the Sloan Digitized Sky Survey is tidally stripped material from the Sagittarius dwarf and support the idea that the tidal stream completely enwraps the Milky Way in an almost polar orbit. We also confirm these detections by running numerical simulations of the Sagittarius dwarf plus the Milky Way. This model reproduces the present position and velocity of the Sagittarius main body and presents a long tidal stream formed by tidal interaction with the Milky Way potential. The tidal streams of the model traces the last orbit of Sagittarius and confirms our observational detections. This model is also in good agreement with the available observations of the Sagittarius tidal stream. The comparison of our model with the positions and distances of two nonidentified halo overdensities discovered by the Sloan Digitized Sky Survey and the QUEST survey shows that they are actually associated with the trailing arm of the Sagittarius tidal stream. In addition, we identify the proper-motion group discovered by Arnold & Gilmore as a piece of the Sagittarius northern stream. We also present a method for estimating the shape of the Milky Way halo potential using numerical simulations. From our simulations we obtain an oblateness of the Milky Way dark halo potential of 0.85, using the current database of distances and radial velocities of the Sagittarius tidal stream. The color-magnitude diagram of the apocenter of Sagittarius shows that this region of the stream shares the complex star formation history observed in the main body of the galaxy. We present the first evidence for a gradient in the stellar population along the stream, possibly correlated with its different pericenter passages.

B037 is of interest because it is both the most luminous and the most highly reddened cluster known in M31. Deep observations and high spatial resolution images with the Advanced Camera for Surveys on the Hubble Space Telescope (HST) first showed that this cluster is crossed by a dust lane. Photometric data in the F606W and F814W filters obtained in this paper indicate that colors of (F606W — F814W) in the dust lane are redder ∼ 0.4 mag than ones in the other regions of B037. The HST images show that this dust lane seems to be contained in B037, instead of in the M31 disk or the Milky Way. As far as we know, the formation of dust requires gas with a rather high metallicity. However, B037 has a low metallicity of [Fe/H] = −1.07 ± 0.20. So, it seems improbable that the observed dust lane is physically associated with B037. It is clear that the origin of this dust lane is worthy of future study. In addition, based on these images, we present the precise variation of ellipticity and position angle, and of surface brightness profile, and determine the structural parameters of B037 by fitting a single-mass isotropic King model. In the F606W filter, we derive the best-fitting scale radius r0 = 0.56 ± 0.02″ (= 2.16 ± 0.08 pc), a tidal radius rt = 8.6 ± 0.4″ (= 33.1 ± 1.5 pc), and a concentration index c = log(rt/r0) = 1.19±0.02. In the F814W filter, we derive r0 = 0.56 ± 0.01″ (= 2.16 ± 0.04 pc), rt = 8.9 ± 0.3″ (= 34.3 ± 1.2 pc), and c = log(rt/r0) = 1.20 ± 0.01. The extinction-corrected central surface brightness is μ0 = 13.53 ± 0.03 mag arcsec−2 in the F606W filter, and 12.85 ± 0.03 mag arcsec−2 in the F814W filter. We also calculate the half-light radius at rh = 1.05 ± 0.03″ (= 4.04 ± 0.12 pc) in the F606W filter and rh = 1.07 ± 0.01″ (= 4.12 ± 0.04 pc) in the F814W filter. In addition, we derived the complete magnitudes of B037 in the V and I bands by transforming the magnitudes from the ACS system to the standard system, which are in good agreement with previous ground-based broadband photometry studies.

We study the optical colors of 147,920 galaxies brighter than g* = 21, observed in five bands by the Sloan Digital Sky Survey (SDSS) over ~100 deg2 of high Galactic latitude sky along the celestial equator. The distribution of galaxies in the g*-r* versus u*-g* color-color diagram is strongly bimodal, with an optimal color separator of u*-r* = 2.22. We use visual morphology and spectral classification of subsamples of 287 and 500 galaxies, respectively, to show that the two peaks correspond roughly to early- (E, S0, and Sa) and late-type (Sb, Sc, and Irr) galaxies, as expected from their different stellar populations. We also find that the colors of galaxies are correlated with their radial profiles, as measured by the concentration index and by the likelihoods of exponential and de Vaucouleurs' profile fits. While it is well known that late-type galaxies are bluer than early-type galaxies, this is the first detection of a local minimum in their color distribution. In all SDSS bands, the counts versus apparent magnitude relations for the two color types are significantly different and demonstrate that the fraction of blue galaxies increases toward the faint end.

We revisit the shapes of isophotes for elliptical (E) and lenticular (S0) galaxies by studying 847 nearby early-type galaxies selected from the Sloan Digital Sky Survey (SDSS) Data Release 4 (DR4) with velocity dispersions above 200 km s −1 . The IRAF task ELLIPSE was used to derive the deviations of the isophotes from pure ellipses (Fourier coefficients a 3/a and a 4/a), position angles (PAs) and ellipticities as a function of radius. We show the statistical distributions of the a 3/a and a 4/a parameters as a function of velocity dispersion, magnitude and colour. The a 4/a parameter is correlated with the ellipticity and absolute Petrosian magnitude of galaxies. No significant correlation was found between the a 4/a parameter with colour and velocity dispersion. A cross-correlation between the Faint Images of the Radio Sky at Twenty-cm (FIRST) survey and the SDSS data reveals a lack of powerful radio emitters in discy E/S0s, as previously found by Bender et al. We also find that boxy E/S0s favour denser environments while discy E/S0s favour more isolated environments. The median values of changes in the ellipticity and PA between one and one and a half Petrosian half-light radii in the isophotes are about −0.023 and 1. ◦ 61, respectively. The average change in the PA is much larger, about 4. 12, due to an extended tail. The change in ellipticity is weakly correlated with the ellipticity itself, with an increasing ellipticity for galaxies with higher ellipticity as the radius increases. The isophote parameters for the 847 galaxies are available online.