The Stellar Mass - Halo Mass Relation for Low Mass X-ray Groups at 0.5<z<1 in the CDFS with CSI

Since z~1, the stellar mass density locked in low mass groups and clusters has grown by a factor of ~8. Here we make the first statistical measurements of the stellar mass content of low mass X-ray groups at 0.5<z<1, enabling the calibration of stellar-to-halo mass scales for wide-field optical and infrared surveys. Groups are selected from combined Chandra and XMM-Newton X-ray observations in the Chandra Deep Field South (CDFS). These ultra-deep observations allow us to identify bona fide low mass groups at high redshift and enable measurements of their total halo masses. We compute aggregate stellar masses for these halos using galaxies from the Carnegie-Spitzer-IMACS (CSI) spectroscopic redshift survey. Stars comprise ~3-4% of the total mass of group halos with masses 10^{12.8}<M200/Msun<10^{13.5} (about the mass of Fornax and 1/50th the mass of Virgo). Complementing our sample with higher mass halos at these redshifts, we find that the stellar-to-halo mass ratio decreases toward higher halo masses, consistent with other work in the local and high redshift universe. The observed scatter about the stellar-halo mass relation is ~0.25 dex, which is relatively small and suggests that total group stellar mass can serve as a rough proxy for halo mass. We find no evidence for any significant evolution in the stellar-halo mass relation since z<1. Quantifying the stellar content in groups since this epoch is critical given that hierarchical assembly leads to such halos growing in number density and hosting increasing shares of quiescent galaxies.

Decoding the Star-Forming Main Sequence or: How I Learned to Stop Worrying and Love the Central Limit Theorem

Star-formation rates (SFR) of disk galaxies strongly correlate with stellar mass, with a small dispersion in SSFR at fixed mass, sigma~0.3 dex. With such small scatter this star-formation main sequence (SFMS) has been interpreted as deterministic and fundamental. Here we demonstrate that it is a simple consequence of the central limit theorem. Our derivation begins by approximating in situ stellar mass growth as a stochastic process, much like a random walk (where the expectation of SFR at any time is equal to the SFR at the previous time). We then derive expectation values for median SSFR of star-forming disks and their scatter over time. We generalize the results for stochastic changes in SFR that are not independent of each other but are correlated over time. For unbiased samples of (disk) galaxies, we derive an expectation that <SSFR> should be independent of mass, decline as 1/T, and have a relative scatter that is independent of mass and time. The derived SFMS and its evolution matches published data to z=10 with sufficient accuracy to constrain cosmological parameters. The framework reproduces several important observables, including: the scatter in SSFR at fixed mass; the SFHs of nearby dwarf galaxies and the Milky Way; and the scatter in the Tully-Fisher relation. The evolution of the mass function is less well reproduced and we discuss ways to generalize the framework to include other sources of stellar mass such as mergers. The predicted dispersion in SSFR has consequences for the classification of quiescent galaxies, as such galaxies have heterogeneous formation histories, and many may only be temporarily diminished in their star-formation activity. The implied dispersion in SFHs, and the SFMSs insensitivity to timescales of stochasticity, thus substantially limits the ability to connect massive galaxies to their progenitors over long cosmic baselines. [TRUNC.]

The Mass-Independence of Specific Star Formation Rates in Galactic Disks

The slope of the star formation rate/stellar mass relation (the SFR Main Sequence; ${rm SFR}-M_*$) is not quite unity: specific star formation rates $({rm SFR}/M_*)$ are weakly-but-significantly anti-correlated with $M_*$. Here we demonstrate that this trend may simply reflect the well-known increase in bulge mass-fractions -- portions of a galaxy not forming stars -- with $M_*$. Using a large set of bulge/disk decompositions and SFR estimates derived from the Sloan Digital Sky Survey, we show that re-normalizing SFR by disk stellar mass $({rm sSFR_{rm disk}equiv SFR}/M_{*,{rm disk}})$ reduces the $M_*$-dependence of SF efficiency by $sim0.25$ dex per dex, erasing it entirely in some subsamples. Quantitatively, we find $log {rm sSFR_{disk}}-log M_*$ to have a slope $beta_{rm disk}in[-0.20,0.00]pm0.02$ (depending on SFR estimator and Main Sequence definition) for star-forming galaxies with $M_*geq10^{10}M_{odot}$ and bulge mass-fractions $B/Tlesssim0.6$, generally consistent with a pure-disk control sample ($beta_{rm control}=-0.05pm0.04$). That $langle{rm SFR}/M_{*,{rm disk}}rangle$ is (largely) independent of host mass for star-forming disks has strong implications for aspects of galaxy evolution inferred from any ${rm SFR}-M_*$ relation, including: manifestations of mass quenching (bulge growth), factors shaping the star-forming stellar mass function (uniform $dlog M_*/dt$ for low-mass, disk-dominated galaxies), and diversity in star formation histories (dispersion in ${rm SFR}(M_*,t)$). Our results emphasize the need to treat galaxies as composite systems -- not integrated masses -- in observational and theoretical work.

The Carnegie-Spitzer-IMACS Redshift Survey of Galaxy Evolution since z=1.5: I. Description and Methodology and More!

We describe the Carnegie-Spitzer-IMACS (CSI) Survey, a wide-field, near-IR selected spectrophotometric redshift survey with IMACS on Magellan-Baade. CSI uses a flux-limited sample of galaxies in Spitzer IRAC 3.6micron imaging of SWIRE fields to efficiently trace the stellar mass of average galaxies to z~1.5. This paper provides an overview of the survey selection, observations, and processing of the photometry and spectrophotometry. We also describe the analysis of the data: new methods of fitting synthetic SEDs are used to derive redshifts, stellar masses, emission line luminosities, and coarse information on recent star-formation. Our unique methodology for analyzing low-dispersion spectra taken with multilayer prisms in IMACS, combined with panchromatic photometry from the ultraviolet to the IR, has yielded high quality redshifts for 43,347 galaxies in our first 5.3 sq. degs of the SWIRE XMM-LSS field. A new approach to assessing data quality is also described, and three different approaches are used to estimate our redshift errors, with robust agreement. Over the full range of 3.6micron fluxes of our selection, we find typical redshift uncertainties of sigma_z/(1+z) < 0.015. In comparisons with previously published spectroscopic redshifts we find scatters of sigma_z/(1+z) = 0.011 for galaxies at 0.7< z< 0.9, and sigma_z/(1+z) = 0.014 for galaxies at 0.9< z< 1.2. For galaxies brighter and fainter than i=23 mag, we find sigma_z/(1+z) = 0.008 and sigma_z/(1+z) = 0.022, respectively. Notably, our low-dispersion spectroscopy and analysis yields comparable redshift uncertainties and success rates for both red and blue galaxies, largely eliminating color-based systematics that can seriously bias observed dependencies of galaxy evolution on environment.

The [O III] Nebula of the Merger Remnant NGC 7252: A Likely Faint Ionization Echo

We present images and spectra of a ~10 kpc-sized emission-line nebulosity discovered in the prototypical merger remnant NGC 7252 and dubbed the `[O III] nebula because of its dominant [O III]_5007 line. This nebula seems to yield the first sign of episodic AGN activity still occurring in the remnant, ~220 Myr after the coalescence of two gas-rich galaxies. Its location and kinematics suggest it belongs to a stream of tidal-tail gas falling back into the remnant. Its integrated [O III]_5007 luminosity is 1.4x10^40 erg/s, and its spectrum features some high-excitation lines, including He II_4686. In diagnostic line- ratio diagrams, the nebula lies in the domain of Seyfert galaxies, suggesting that it is photoionized by a source with a power-law spectrum. Yet, a search for AGN activity in NGC 7252 from X-rays to radio wavelengths yields no detection, with the most stringent upper limit set by X-ray observations. The upper luminosity limit of L_{2-10 keV,0} < 5x10^39 erg/s estimated for the nucleus is ~10^3 times lower than the minimum ionizing luminosity of >5x10^42 erg/s necessary to excite the nebula. This large discrepancy suggests that the nebula is a faint ionization echo excited by a mildly active nucleus that has declined by ~3 orders of magnitude over the past 20,000-200,000 years. In many ways this nebula resembles the prototypical `Hannys Voorwerp near IC 2497, but its size is 3x smaller and its [O III] luminosity ~100x lower. We propose that it be classified as an extended emission-line region (EELR). The [O III] nebula is then the lowest-luminosity ionization echo and EELR discovered so far, indicative of recent, probably sputtering AGN activity of Seyfert-like intensity in NGC 7252.

The Carnegie-Spitzer-IMACS Redshift Survey of Galaxy Evolution since z=1.5: I. Description and Methodology

We describe the Carnegie-Spitzer-IMACS (CSI) Survey, a wide-field, near-IR selected spectrophotometric redshift survey with the Inamori Magellan Areal Camera and Spectrograph (IMACS) on Magellan-Baade. By defining a flux-limited sample of galaxies in Spitzer 3.6micron imaging of SWIRE fields, the CSI Survey efficiently traces the stellar mass of average galaxies to z~1.5. This first paper provides an overview of the survey selection, observations, processing of the photometry and spectrophotometry. We also describe the processing of the data: new methods of fitting synthetic templates of spectral energy distributions are used to derive redshifts, stellar masses, emission line luminosities, and coarse information on recent star-formation. Our unique methodology for analyzing low-dispersion spectra taken with multilayer prisms in IMACS, combined with panchromatic photometry from the ultraviolet to the IR, has yielded 37,000 high quality redshifts in our first 5.3 sq.degs of the SWIRE XMM-LSS field. We use three different approaches to estimate our redshift errors and find robust agreement. Over the full range of 3.6micron fluxes of our selection, we find typical uncertainties of sigma_z/(1+z) < 0.015. In comparisons with previously published VVDS redshifts, for example, we find a scatter of sigma_z/(1+z) = 0.012 for galaxies at 0.8< z< 1.2. For galaxies brighter and fainter than i=23 mag, we find sigma_z/(1+z) = 0.009 and sigma_z/(1+z) = 0.025, respectively. Notably, our low-dispersion spectroscopy and analysis yields comparable redshift uncertainties and success rates for both red and blue galaxies, largely eliminating color-based systematics that can seriously bias observed dependencies of galaxy evolution on environment.

The UVJ Selection of Quiescent and Star Forming Galaxies: Separating Early and Late-Type Galaxies and Isolating Edge-on Spirals

We utilize for the first time HST ACS imaging to examine the structural properties of galaxies in the rest-frame U-V versus V-J diagram (i.e., the UVJ diagram) using a sample at 0.6<z<0.9 that reaches a low stellar mass limit (log M/Msun>10.25). The use of the UVJ diagram as a tool to distinguish quiescent galaxies from star forming galaxies (SFGs) is becoming more common due to its ability to separate red quiescent galaxies from reddened SFGs. Quiescent galaxies occupy a small and distinct region of UVJ color space and we find most of them to have concentrated profiles with high Sersic indices (n>2.5) and smooth structure characteristic of early-type systems. SFGs populate a broad, but well-defined sequence of UVJ colors and are comprised of objects with a mix of Sersic indices. Interestingly, most UVJ-selected SFGs with high Sersic indices also display structure due to dust and star formation typical of the n<2.5 SFGs and late-type systems. Finally, we find that the position of a SFG on the sequence of UVJ colors is determined to a large degree by the mass of the galaxy and its inclination. Systems that are closer to edge-on generally display redder colors and lower [OII]3727 luminosity per unit mass as a consequence of the reddening due to dust within the disks. We conclude that the two main features seen in UVJ color space correspond closely to the traditional morphological classes of early and late-type galaxies.

The Star Formation Rate-Density Relation at 0.6<z<0.9 and the Role of Star Forming Galaxies

We study the star formation rates (SFRs) of galaxies as a function of local galaxy density at 0.6<z<0.9. We used a low-dispersion prism in IMACS on the 6.5-m Baade (Magellan I) telescope to obtain spectra and measured redshifts to a precision of sigma_z/(1+z)=1% for galaxies with z<23.3 AB mag. We utilized a stellar mass-limited sample of 977 galaxies above M>1.8x10^{10} Msun to conduct our main analysis. With three different SFR indicators, (1) Spitzer MIPS 24-micron imaging, (2) SED fitting, and (3) [OII]3727 emission, we find the median specific SFR (SSFR) and SFR to decline from the low-density field to the cores of groups and a rich cluster. For the SED and [OII] based SFRs, the decline in SSFR is roughly an order of magnitude while for the MIPS based SFRs, the decline is a factor of ~4. We find approximately the same magnitude of decline in SSFR even after removing the sample of galaxies near the cluster. Galaxies in groups and a cluster at these redshifts therefore have lower star formation (SF) activity than galaxies in the field, as is the case at z~0. We investigated whether the decline in SFR with increasing density is caused by a change in the proportion of quiescent and star forming galaxies (SFGs) or by a decline in the SFRs of SFGs. Using the rest-frame U-V and V-J colors to distinguish quiescent galaxies from SFGs we find the fraction of quiescent galaxies increases from ~32% to 79% from low to high density. In addition, we find the SSFRs of SFGs, selected based on U-V and V-J colors, to decline with increasing density by factors of ~5-6 for the SED and [OII] based SFRs. The MIPS based SSFRs for SFGs decline with a shallower slope. The order of magnitude decline in the SSFR-density relation at 0.6<z<0.9 is therefore driven by both a combination of declining SFRs of SFGs as well as a changing mix of SFGs and quiescent galaxies [ABRIDGED].

M/L and Color Evolution for A Deep Sample of M* Cluster Galaxies at z~1: The Formation Epoch and the Tilt of the Fundamental Plane

We have measured velocity dispersions for a sample of 36 galaxies with J < 21.2 or Mr < -20.6 mag in MS1054-03, a massive cluster of galaxies at z = 0.83. Our data are of uniformly high quality down to our selection limit, our 16-hour exposures typically yielding errors of only delta(dispersion)~10% for L* and fainter galaxies. By combining our measurements with data from the literature, we have 53 cluster galaxies with measured dispersions, and HST/ACS-derived sizes, colors and surface brightnesses. This sample is complete for the typical L* galaxy at z~1, unlike most previous z~1 cluster samples which are complete only for the massive cluster members (>1e11 M_sun). We find no evidence for a change in the tilt of the fundamental plane (FP). Nor do we find evidence for evolution in the slope of the color-dispersion relation and M/L_B-dispersion relations; measuring evolution at a fixed dispersion should minimize the impact of size evolution found in other work. The M/L_B at fixed dispersion evolves by Delta log10 M/L_B=-0.50 +/- 0.03 between z=0.83 and z=0.02 or d(log10 M/L_B)=-0.60 +/- 0.04 dz, and we find Delta (U-V)_z=-0.24 +/- 0.02 mag at fixed dispersion in the rest-frame, matching the expected evolution in M/L_B within 2.25 standard deviations. The implied formation redshift from both the color and M/L_B evolution is z*=2.0 +/- 0.2 +/- 0.3 (sys), during the epoch in which the cosmic star-formation activity peaked, with the systematic uncertainty showing the dependence of z* on the assumptions we make about the stellar populations. The lack of evolution in either the tilt of the FP or in the M/L- and color-dispersion relations imply that the formation epoch depends weakly on mass, ranging from z*=2.3 +1.3 -0.3 at 300 km/s to z*=1.7 +0.3 -0.2 at 160 km/s and implies that the IMF similarly varies slowly with galaxy mass.

The Dependence of Star Formation Rates on Stellar Mass and Environment at z~0.8

We examine the star formation rates (SFRs) of galaxies in a redshift slice encompassing the z=0.834 cluster RX J0152.7-1357. We used a low-dispersion prism in the Inamori Magellan Areal Camera and Spectrograph (IMACS) to identify galaxies with z<23.3 AB mag in diverse environments around the cluster out to projected distances of ~8 Mpc from the cluster center. We utilize a mass-limited sample (M>2x10^{10} M_sun) of 330 galaxies that were imaged by Spitzer MIPS at 24 micron to derive SFRs and study the dependence of specific SFR (SSFR) on stellar mass and environment. We find that the SFR and SSFR show a strong decrease with increasing local density, similar to the relation at z~0. Our result contrasts with other work at z~1 that find the SFR-density trend to reverse for luminosity-limited samples. These other results appear to be driven by star-formation in lower mass systems (M~10^{10} M_sun). Our results imply that the processes that shut down star-formation are present in groups and other dense regions in the field. Our data also suggest that the lower SFRs of galaxies in higher density environments may reflect a change in the ratio of star-forming to non-star-forming galaxies, rather than a change in SFRs. As a consequence, the SFRs of star-forming galaxies, in environments ranging from small groups to clusters, appear to be similar and largely unaffected by the local processes that truncate star-formation at z~0.8.