Shiyin Shen

The size distribution of galaxies in the Sloan Digital Sky Survey

We use a complete sample of about 140,000 galaxies from the Sloan Digital Sky Survey (SDSS) to study the size distribution of galaxies and its dependence on their luminosity, stellar mass, and morphological type. The large SDSS database provides statistics of unprecedented accuracy. For each type of galaxy, the size distribution at given luminosity (or stellar mass) is well described by a log-normal function, characterized by its median ¯

Estimating the H I gas fractions of galaxies in the local universe

We use a sample of 800 galaxies with H I mass measurements from the HyperLeda catalogue and optical photometry from the fourth data release of the Sloan Digital Sky Survey (SDSS) to calibrate a new photometric estimator of the H i-to-stellar-mass ratio for nearby galaxies. Our estimator, which is motivated by the Kennicutt-Schmidt star formation law, is log 10 (G H1 /S) = - 1.73238(g - r) + 0.215 182μ i - 4.08451, where μ i is the i-band surface brightness and g - r is the optical colour estimated from the g- and r-band Petrosian apparent magnitudes. This estimator has a scatter of σ = 0.31 dex in log (G HI /S), compared to σ ∼ 0.4 dex for previous estimators that were based on colour alone. We investigate whether the residuals in our estimate of log (G HI /S) depend in a systematic way on a variety of different galaxy properties. We find no effect as a function of stellar mass or 4000 A break strength, but there is a systematic effect as a function of the concentration index of the light. We then apply our estimator to a sample of 10 5 emission-line galaxies in the SDSS Data Release 4 (DR4) and derive an estimate of the H I mass function, which is in excellent agreement with recent results from H I blind surveys. Finally, we re-examine the well-known relation between gas-phase metallicity and stellar mass, and ask whether there is a dependence on H I-to-stellar-mass ratio, as predicted by chemical evolution models. We do find that gas-poor galaxies are more metal rich at fixed stellar mass. We compare our results with the semi-analytic models of De Lucia & Blaizot, which include supernova feedback, as well as the cosmological infall of gas.

Inclination-dependent Luminosity Function of Spiral Galaxies in the Sloan Digital Sky Survey: Implications for Dust Extinction

Using a samples of 61506 spiral galaxies selected from the SDSS DR2, we examine the luminosity function (LF) of spiral galaxies with different inclination angles. We find that the characteristic luminosity of the LF, L � , decreases with increasing inclination, while the faint-end slope, α, depends only weakly on it. The inclination-dependence of the LF is consistent with that expected from a simple model where the optical depth is proportional to the cosine of the inclination angle, and we use a likelihood method to recover both the coefficient in front ofthe cosine, γ, and the LF for galaxies viewed face-on. The value of γ is quite independent of galaxy luminosity in a given band, and the values of γ obtained in this way for the 5 SDSS bands give an extinction curve which is a power law of wavelength (τ ∝ λ n ), with a power index n = 0.96 ± 0.04. Using the dust extinction for galaxies obtained by Kauffmann et al. (2003), we derive an ‘extinction-corrected’ luminosity function

Milky Way vs Andromeda: a tale of two disks

Aims. We study the chemical evolution of the disks of the Milky Way (MW) and of Andromeda (M 31), to identify the common properties and differences between the two major galaxies of the Local Group. Methods. We use a large set of observational data for M 31, including observations of the star formation rate (SFR) and gas profiles, as well as stellar metallicity distributions along its disk. When expressed in terms of the corresponding disk scale lengths, we show that the observed radial profiles of MW and M 31 exhibit interesting similarities, suggesting the possibility of a description within a common framework. Results. We find that the profiles of stars, gas fraction, and metallicity of the two galaxies, as well as most of their global properties, are well described by our model, provided that the star formation efficiency in M 31 disk is twice as high as in the MW. We show that the star formation rate profile of M 31 cannot be described by any form of the Kennicutt-Schmidt law (KS Law) for star formation. We propose that these discrepancies are caused by the fact that M 31 has an active star formation history in the recent past, consistent with the hypotheses of a “head-on” collision with the neighboring galaxy (most probably M 32) about 200 Myr ago. Conclusions. The MW has most probably experienced quiescent secular evolution, making possible a fairly successful description with a simple model. If M 31 is more typical of spiral galaxies, more complex models, involving galaxy interactions, will be required for the description of spirals.

The soft X-ray properties of quasars in the Sloan Digital Sky Survey

We use the ROSAT All-Sky Survey (RASS) to study the soft X-ray properties of a homogeneous sample of 46 420 quasars selected from the third data release of the Sloan Digital Sky Survey (SDSS). Optical luminosities, both at rest frame 2500 angstrom (L-2500) and in [O III] (L-[O III]) span more than three orders of magnitude, while redshifts range over 0.1 < z < 5.4. We detect 3366 quasars directly in the observed 0.1-2.4 keV band. Subsamples of radio-loud quasars (RLQs) and radio-quiet quasars (RQQs) are obtained by cross-matching with the Faint Images of the Radio Sky at Twenty-cm (FIRST) catalogue. We study the distribution of X-ray luminosity as a function of optical luminosity, redshift and radio power using both individual detections and stacks of complete sets of similar quasars. At every optical luminosity and redshift log L-2keV is, to a good approximation, normally distributed with dispersion similar to 0.40, at least brightwards of the median X-ray luminosity. This median X-ray luminosity of quasars is a power law of optical luminosity with index similar to 0.53 for L-2500 and similar to 0.30 for L-[O III]. RLQs are systematically brighter than RQQs by about a factor of 2 at given optical luminosity. The zero-points of these relations increase systematically with redshift, possibly in different ways for RLQs and RQQs. Evolution is particularly strong at low redshift and if the optical luminosity is characterized by L-[O III]. At low redshift and at given L-[O III] the soft X-ray emission from type II active galactic nucleus (AGN) is more than 100 times weaker than that from type I AGN.

THE NEAREST HIGH-VELOCITY STARS REVEALED BY LAMOST DATA RELEASE 1

We report the discovery of 28 candidate high-velocity stars (HVSs) at heliocentric distances of less than 3 kpc, based on the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) Data Release 1. Our sample of HVS candidates covers a much broader color range than the equivalent ranges discussed in previous studies and comprises the first and largest sample of HVSs in the immediate solar neighborhood, at heliocentric distances less than 1-3 kpc. The observed as well as the derived parameters for all candidates are sufficiently accurate to allow us to ascertain their nature as genuine HVSs, of which a subset of 12 objects represents the most promising candidates. Our results also highlight the great potential of discovering statistically large numbers of HVSs of different spectral types in LAMOST survey data. This will ultimately enable us to achieve a better understanding of the nature of Galactic HVSs and their ejection mechanisms, and to constrain the structure of the Galaxy.

THE STATISTICAL NATURE OF THE BRIGHTEST GROUP GALAXIES

We examine the statistical properties of the brightest group galaxies (BGGs) by using a complete spectroscopic sample of groups/clusters of galaxies selected from the Data Release 7 of the Sloan Digital Sky Survey. We test whether BGGs and other bright members of groups are consistent with an ordered population among the total population of group galaxies. We find that the luminosity distributions of BGGs do not follow the predictions from the order statistics (OS). The average luminosities of BGGs are systematically brighter than OS predictions. On the other hand, by properly taking into account the brightening effect of the BGGs, the luminosity distributions of the second brightest galaxies are in excellent agreement with the expectations of OS. The brightening of BGGs relative to the OS expectation is consistent with a scenario that the BGGs on average have overgrown about 20% masses relative to the other member galaxies. The growth (ΔM) is not stochastic but correlated with the magnitude gap (G 1, 2) between the brightest and the second brightest galaxy. The growth (ΔM) is larger for the groups having more prominent BGGs (larger G 1, 2) and averagely contributes about 30% of the final G 1, 2 of the groups of galaxies.

Measuring the X-ray luminosities of SDSS DR7 clusters from ROSAT All Sky Survey

We use ROSAT All Sky Survey broad-band X-ray images and the optical clusters identified from Sloan Digital Sky Survey Data Release 7 to estimate the X-ray luminosities around similar to 65 000 candidate clusters with masses greater than or similar to 10(13) h(-1) M-circle dot based on an optical to X-ray (OTX) code we develop. We obtain a catalogue with X-ray luminosity for each cluster. This catalogue contains 817 clusters (473 at redshift z 3 in X-ray detection. We find about 65 per cent of these X-ray clusters have their most massive member located near the X-ray flux peak; for the rest 35 per cent, the most massive galaxy is separated from the X-ray peak, with the separation following a distribution expected from a Navarro-Frenk-White profile. We investigate a number of correlations between the optical and X-ray properties of these X-ray clusters, and find that the cluster X-ray luminosity is correlated with the stellar mass (luminosity) of the clusters, as well as with the stellar mass (luminosity) of the central galaxy and the mass of the halo, but the scatter in these correlations is large. Comparing the properties of X-ray clusters of similar halo masses but having different X-ray luminosities, we find that massive haloes with masses greater than or similar to 10(14) h(-1) M-circle dot contain a larger fraction of red satellite galaxies when they are brighter in X-ray. An opposite trend is found in central galaxies in relative low-mass haloes with masses less than or similar to 10(14) h(-1) M-circle dot where X-ray brighter clusters have smaller fraction of red central galaxies. Clusters with masses greater than or similar to 10(14) h(-1) M-circle dot that are strong X-ray emitters contain many more low-mass satellite galaxies than weak X-ray emitters. These results are also confirmed by checking X-ray clusters of similar X-ray luminosities but having different characteristic stellar masses. A cluster catalogue containing the optical properties of member galaxies and the X-ray luminosity is available at http://gax.shao.ac.cn/data/Group.html.

THE ORIENTATION OF THE NUCLEAR OBSCURER OF THE ACTIVE GALACTIC NUCLEI

We examine the distribution of axis ratios of a large sample of disk galaxies hosting type 2 active galactic nuclei (AGNs) selected from the Sloan Digital Sky Survey and compare it with a well-defined control sample of non-active galaxies. We find them significantly different, where the type 2 AGNs show both an excess of edge-on objects and deficit of round objects. This systematical bias cannot be explained by a nuclear obscurer oriented randomly with respect to the stellar disk. However, a nuclear obscurer coplanar with the stellar disk also does not fit the data very well. By assuming that the nuclear obscurer having an opening angle of similar to 60 degrees, we find that the observed axis ratio distribution can be nicely reproduced by a mean tilt angle of similar to 30 degrees between the nuclear obscurer and the stellar disk.

Colour–magnitude relations of late-type galaxies

We use a large sample of galaxies drawn from the Sloan Digital Sky Survey (SDSS) and Two-Micron All-Sky Survey (2MASS) to present colour-magnitude relations (CMRs) for late-type galaxies in both optical and optical-infrared (optical-IR) bands. A sample from SDSS Data Release 4 (DR4) is selected to investigate the optical properties. Optical-IR colours are estimated from a position matched sample of DR4 and the 2MASS, in which the photometric aperture mismatch between these two surveys is carefully corrected. It is shown that, after correcting the dust attenuation, the optical colours for faint galaxies (i.e.M-r > -21) have a very weak correlation with the luminosity, whereas the optical colours for bright galaxies (i.e. M-r < -21) are redder than those for more-luminous galaxies. All (optical, optical-IR and IR) colours show similar but stronger correlations with stellar mass than with absolute magnitude. The optical colours correlate more strongly with stellar mass surface density than with stellar mass, whereas optical-IR and IR colours show stronger correlations with stellar mass. By comparing the observed colours of our sample galaxies with the colours predicted by stellar population synthesis model, we find that massive late-type galaxies have older and higher-metallicity stellar population than that of less-massive galaxies. This suggests that CMRs for late-type galaxies are trends defined by the combination of stellar mean age and metallicity. Moreover, our results suggest that the stellar mean metallicity of late-type galaxy is mainly determined by its stellar mass, whereas the star formation history is mainly regulated by the stellar mass surface density.