Galaxy Structure as a Driver of the Star Formation Sequence Slope and Scatter

It is well established that (1) star-forming galaxies follow a relation between their star formation rate (SFR) and stellar mass (M$_{\star}$), the "star-formation sequence", and (2) the SFRs of galaxies correlate with their structure, where star-forming galaxies are less concentrated than quiescent galaxies at fixed mass. Here, we consider whether the scatter and slope of the star-formation sequence is correlated with systematic variations in the Sersic indices, $n$, of galaxies across the SFR-M$_{\star}$ plane. We use a mass-complete sample of 23,848 galaxies at $0.5<z<2.5$ selected from the 3D-HST photometric catalogs. Galaxy light profiles parameterized by $n$ are based on Hubble Space Telescope CANDELS near-infrared imaging. We use a single SFR indicator empirically-calibrated from stacks of Spitzer/MIPS 24$\mu$m imaging, adding the unobscured and obscured star formation. We find that the scatter of the star-formation sequence is related in part to galaxy structure; the scatter due to variations in $n$ at fixed mass for star-forming galaxies ranges from 0.14$\pm$0.02 dex at $z\sim2$ to 0.30$\pm$0.04 dex at $z<1$. While the slope of the log(SFR)-log(M$_{\star}$) relation is of order unity for disk-like galaxies, galaxies with $n>2$ (implying more dominant bulges) have significantly lower SFR/M$_{\star}$ than the main ridgeline of the star-formation sequence. These results suggest that bulges in massive $z\sim2$ galaxies are actively building up, where the stars in the central concentration are relatively young. At $z<1$, the presence of older bulges within star-forming galaxies lowers global SFR/M$_{\star}$, decreasing the slope and contributing significantly to the scatter of the star-formation sequence.Comment: Accepted for publication in the Astrophysical Journal Letter

The 3D-HST Survey: <i>Hubble Space Telescope</i> WFC3/G141 Grism Spectra, Redshifts, and Emission Line Measurements for ~ 100,000 Galaxies

We present reduced data and data products from the 3D-HST survey, a 248-orbit $HST$ Treasury program. The survey obtained WFC3 G141 grism spectroscopy in four of the five CANDELS fields: AEGIS, COSMOS, GOODS-S, and UDS, along with WFC3 $H_{140}$ imaging, parallel ACS G800L spectroscopy, and parallel $I_{814}$ imaging. In a previous paper, we presented photometric catalogs in these four fields and in GOODS-N, the fifth CANDELS field. Here we describe and present the WFC3 G141 spectroscopic data, again augmented with data from GO-1600 in GOODS-N (PI: B. Weiner). We developed software to automatically and optimally extract interlaced two-dimensional (2D) and one-dimensional (1D) spectra for all objects in the Skelton et al. (2014) photometric catalogs. The 2D spectra and the multi-band photometry were fit simultaneously to determine redshifts and emission line strengths, taking the morphology of the galaxies explicitly into account. The resulting catalog has redshifts and line strengths (where available) for 22,548 unique objects down to ${{JH}}_{\mathrm{IR}}\leq 24$ (79,609 unique objects down to ${{JH}}_{\mathrm{IR}}\leq 26$). Of these, 5459 galaxies are at $z > 1.5$ and 9621 are at $0.7< z< 1.5$, where Hα falls in the G141 wavelength coverage. The typical redshift error for ${{JH}}_{\mathrm{IR}}\leq 24$ galaxies is ${\sigma }_{z}\approx 0.003\times (1+z)$, i.e., one native WFC3 pixel. The $3\sigma$ limit for emission line fluxes of point sources is $2.1\times {10}^{-17}$ erg $s^{-1} cm^{-2}$. All 2D and 1D spectra, as well as redshifts, line fluxes, and other derived parameters, are publicly available

American Heritage

Bezanson praises the "America Heritage, " calling it one of the happiest ventures in publishing in a long time, because it has no advertising and  historical scholars writing for the general public in an entertaining and even witty style

Go Ahead, Davy Crockett!

Bezanon, who founded an undergraduate major in American Civilzation at Rutgers, investigates the Library's "Crockett's Almanac: Scenes in River Life, Feats on the Lakes, Manners in the Back Woods, Adventures in Texas, &c, &c.," (Philadelphia and New York, 1846), for sources of the Davy Crockett myth

Lincoln and Douglas and the Naughty Nursery Rhyme

This article explores odd bits of Americana ephemera from the 1860 presidential election between Lincoln and Douglas found in the Library

Spatially resolved H&#0945; maps and sizes of 57 strongly star-forming galaxies at z ~ 1 from 3d-hst: evidence for rapid inside-out assembly of disk galaxies

We investigate the buildup of galaxies at z ~ 1 using maps of H&#0945; and stellar continuum emission for a sample of 57 galaxies with rest-frame H&#0945; equivalent widths &gt;100 &#0197; in the 3D-HST grism survey. We find that the H&#0945; emission broadly follows the rest-frame R-band light but that it is typically somewhat more extended and clumpy. We quantify the spatial distribution with the half-light radius. The median H&#0945; effective radius re (H&#0945;) is 4.2 &#0177; 0.1 kpc but the sizes span a large range, from compact objects with re (H&#0945;) ~ 1.0 kpc to extended disks with re (H&#0945;) ~ 15 kpc. Comparing H&#0945; sizes to continuum sizes, we find &lt;re (H&#0945;)/re (R) &gt; =1.3 &#0177; 0.1 for the full sample. That is, star formation, as traced by H&#0945;, typically occurs out to larger radii than the rest-frame R-band stellar continuum; galaxies are growing their radii and building up from the inside out. This effect appears to be somewhat more pronounced for the largest galaxies. Using the measured H&#0945; sizes, we derive star formation rate surface densities, &#0931;SFR. We find that &#0931;SFR ranges from ~0.05 M &#9737; yr&#8211;1 kpc&#8211;2 for the largest galaxies to ~5 M &#9737; yr&#8211;1 kpc&#8211;2 for the smallest galaxies, implying a large range in physical conditions in rapidly star-forming z ~ 1 galaxies. Finally, we infer that all galaxies in the sample have very high gas mass fractions and stellar mass doubling times &lt;500 Myr. Although other explanations are also possible, a straightforward interpretation is that we are simultaneously witnessing the rapid formation of compact bulges and large disks at z ~ 1

STELLAR KINEMATICS OF z

Using stellar kinematics measurements, we investigate the growth of massive, quiescent galaxies from z~2 to today. We present X-Shooter spectra from the UV to NIR and dynamical mass measurements of 5 quiescent massive (>10^11 Msun) galaxies at z~2. This triples the sample of z>1.5 galaxies with well constrained (dsigma <100 km/s) velocity dispersion measurements. From spectral population synthesis modeling we find that these galaxies have stellar ages that range from 0.5-2 Gyr, with no signs of ongoing star formation. We measure velocity dispersions (290-450 km/s) from absorption lines and find that they are 1.6-2.1 times higher than those of galaxies in the SDSS at the same mass. Sizes are measured using GALFIT from HST-WFC3 H160 and UDS K-band images. The dynamical masses correspond well to the SED based stellar masses, with dynamical masses that are ~15% higher. We find that M_*/M_dyn may decrease slightly with time, which could reflect the increase of the dark matter fraction within an increasing effective radius. We combine different stellar kinematic studies from the literature, and examine the structural evolution from z~2 to z~0: we confirm that at fixed dynamical mass, the effective radius increases by a factor of ~2.8, and the velocity dispersion decreases by a factor of ~1.7. The mass density within one effective radius decreases by a factor of ~20, while within a fixed physical radius (1 kpc) it decreases only mildly (~2). When we allow for an evolving mass limit by selecting a population of galaxies at fixed number density, a stronger size growth with time is found (~4), velocity dispersion decreases by a factor of ~1.4, and interestingly, the mass density within 1 kpc is consistent with no evolution. This finding suggests that massive quiescent galaxies at ~2 grow inside-out, consistent with the expectations from minor mergers.Comment: 25 pages, 20 figures. Accepted for publication in the Astrophysical Journa

Observations of Environmental Quenching in Groups in the 11 GYR since <i>z</i> = 2.5: Different Quenching for Central and Satellite Galaxies

We present direct observational evidence for star formation quenching in galaxy groups in the redshift range 0 z z ∼ 0, central galaxies in our sample range in stellar mass from Milky Way/M31 analogs (M★/M⊙ = 6.5×1010) to nearby massive ellipticals (M★/M⊙ = 1.5×1011). Satellite galaxies in the same groups reach masses as low as twice that of the Large Magellanic Cloud (M★/M⊙ = 6.5×109). Using statistical background subtraction, we measure the average rest-frame colors of galaxies in our groups and calculate the evolving quiescent fractions of centrals and satellites over seven redshift bins. Our analysis shows clear evidence for star formation quenching in group halos, with a different quenching onset for centrals and their satellite galaxies. Using halo mass estimates for our central galaxies, we find that star formation shuts off in centrals when typical halo masses reach between 1012 and 1013 M⊙, consistent with predictions from the halo quenching model. In contrast, satellite galaxies in the same groups most likely undergo quenching by environmental processes, whose onset is delayed with respect to their central galaxy. Although star formation is suppressed in all galaxies over time, the processes that govern quenching are different for centrals and satellites. While mass plays an important role in determining the star formation activity of central galaxies, quenching in satellite galaxies is dominated by the environment in which they reside