Galaxies in SDSS and DEEP2: a quiet life on the blue sequence?

In the six billion years between redshifts z=1 and z=0.1, galaxies change due to the aging of their stellar populations, the formation of new stars, and mergers with other galaxies. Here I explore the relative importance of these various effects, finding that while mergers are likely to be important for the red galaxy sequence they are unlikely to affect more than 10% of the blue galaxy sequence. I compare the galaxy population at redshift z=0.1 from the Sloan Digital Sky Survey to that at z=1 from the Deep Extragalactic Evolutionary Probe 2. Galaxies are bluer at z=1: the blue sequence by about 0.3 mag and the red sequence by about 0.1 mag, in redshift z=0.1 (u-g) color. I evaluate the change in color and in the luminosity functions of the two sequences using some simplistic stellar population synthesis models. These models indicate that the luminous end of the red sequence fades less than passive evolution allows by about 0.2 mag. Due to a lack of luminous blue progenitors, ``dry'' mergers betweeen red galaxies then must create the luminous red population at z=0.1, if stellar population models are correct. The blue sequence colors and luminosity function are consistent with a reduction in the star-formation rate since redshift z=1 by a factor of about three, with no change in the number density to within 10%. These results restrict the number of blue galaxies that can fall onto the red sequence by any process, and in particular suggest that if mergers are catastrophic events they must be rare for blue galaxies.Comment: submitted to ApJ, summary and viewgraphs available at http://cosmo.nyu.edu/blanton/deep2sdss

Inside-out growth or inside-out quenching? clues from colour gradients of local galaxies

We constrain the spatial gradient of star formation history within galaxies using the colour gradients in NUV-u and u-i for a local spatially-resolved galaxy sample. By splitting each galaxy into an inner and an outer part, we find that most galaxies show negative gradients in these two colours. We first rule out dust extinction gradient and metallicity gradient as the dominant source for the colour gradient. Then using stellar population models, we explore variations in star formation history to explain the colour gradients. As shown by our earlier work, a two-phase SFH consisting of an early secular evolution (growth) phase and a subsequent rapid evolution (quenching) phase is necessary to explain the observed colour distributions among galaxies. We explore two different inside-out growth models and two different inside-out quenching models by varying parameters of the SFH between inner and outer regions of galaxies. Two of the models can explain the observed range of colour gradients in NUV-u and u-i colours. We further distinguish them using an additional constraint provided by the u-i colour gradient distribution, under the assumption of constant galaxy formation rate and a common SFH followed by most galaxies. We find the best model is an inside-out growth model in which the inner region has a shorter e-folding time scale in the growth phase than the outer region. More spatially resolved ultraviolet (UV) observations are needed to improve the significance of the result.Comment: 11 pages, 7 figures, accepted for publication in MNRA

A Study in Blue: The Baryon Content of Isolated Low Mass Galaxies

We study the baryon content of low mass galaxies selected from the Sloan Digital Sky Survey (SDSS DR8), focusing on galaxies in isolated environments where the complicating physics of galaxy-galaxy interactions are minimized. We measure neutral hydrogen (HI) gas masses and line-widths for 148 isolated galaxies with stellar mass between $10^7$ and $10^{9.5} M_{\odot}$. We compare isolated low mass galaxies to more massive galaxies and galaxies in denser environments by remeasuring HI emission lines from the Arecibo Legacy Fast ALFA (ALFALFA) survey 40% data release. All isolated low mass galaxies either have large atomic gas fractions or large atomic gas fractions cannot be ruled out via their upper limits. We measure a median atomic gas fraction of $f_{\rm gas} = 0.82 \pm 0.13$ for our isolated low mass sample with no systems below 0.30. At all stellar masses, the correlations between galaxy radius, baryonic mass and velocity width are not significantly affected by environment. Finally, we estimate a median baryon to total dynamical mass fraction of $f_{\rm baryon, disk} = 0.15 \pm 0.18$. We also estimate two different median baryon to halo mass fractions using the results of semi-analytic models $(f_{\rm baryon, halo} = 0.04 \pm 0.06)$ and abundance matching $(f_{\rm baryon, halo} = 0.04 \pm 0.02)$. Baryon fractions estimated directly using HI observations appear independent of environment and maximum circular velocity, while baryon fractions estimated using abundance matching show a significant depletion of baryons at low maximum circular velocities.Comment: Re-submitted to ApJ. Updated with referee's comments. 20 pages. Figure 4 and 5 illustrate our key results. Table 1 presents a small sample of isolated galaxies. Table 3 presents scaling relation fit

Stellar Metallicity Gradients in SDSS galaxies

We infer stellar metallicity and abundance ratio gradients for a sample of red galaxies in the Sloan Digital Sky Survey (SDSS) Main galaxy sample. Because this sample does not have multiple spectra at various radii in a single galaxy, we measure these gradients statistically. We separate galaxies into stellar mass bins, stack their spectra in redshift bins, and calculate the measured absorption line indices in projected annuli by differencing spectra in neighboring redshift bins. After determining the line indices, we use stellar population modeling from the EZ\_Ages software to calculate ages, metallicities, and abundance ratios within each annulus. Our data covers the central regions of these galaxies, out to slightly higher than $1 R_{e}$. We find detectable gradients in metallicity and relatively shallow gradients in abundance ratios, similar to results found for direct measurements of individual galaxies. The gradients are only weakly dependent on stellar mass, and this dependence is well-correlated with the change of $R_e$ with mass. Based on this data, we report mean equivalent widths, metallicities, and abundance ratios as a function of mass and velocity dispersion for SDSS early-type galaxies, for fixed apertures of 2.5 kpc and of 0.5 $R_e$.Comment: 19 pages; 8 tables, 12 figures. Submitted to ApJ for publicatio

Astrometry.net: Blind astrometric calibration of arbitrary astronomical images

We have built a reliable and robust system that takes as input an astronomical image, and returns as output the pointing, scale, and orientation of that image (the astrometric calibration or WCS information). The system requires no first guess, and works with the information in the image pixels alone; that is, the problem is a generalization of the "lost in space" problem in which nothing--not even the image scale--is known. After robust source detection is performed in the input image, asterisms (sets of four or five stars) are geometrically hashed and compared to pre-indexed hashes to generate hypotheses about the astrometric calibration. A hypothesis is only accepted as true if it passes a Bayesian decision theory test against a background hypothesis. With indices built from the USNO-B Catalog and designed for uniformity of coverage and redundancy, the success rate is 99.9% for contemporary near-ultraviolet and visual imaging survey data, with no false positives. The failure rate is consistent with the incompleteness of the USNO-B Catalog; augmentation with indices built from the 2MASS Catalog brings the completeness to 100% with no false positives. We are using this system to generate consistent and standards-compliant meta-data for digital and digitized imaging from plate repositories, automated observatories, individual scientific investigators, and hobbyists. This is the first step in a program of making it possible to trust calibration meta-data for astronomical data of arbitrary provenance.Comment: submitted to A