ABSTRACT Knowledge of galaxy evolution rests on cross-sectional observations of different objects at different times. Understanding of galaxy evolution rests on longitudinal interpretations of how ...these data relate to individual objects moving through time. The connection between the two is often assumed to be clear, but we use a simple "physics-free" model to show that it is not and that exploring its nuances can yield new insights. Comprising nothing more than 2094 loosely constrained lognormal star formation histories (SFHs), the model faithfully reproduces the following data it was not designed to match: stellar mass functions at the slope of the star formation rate/stellar mass relation (the SFR "Main Sequence") at the mean of low-mass galaxies at "fast-" and "slow-track" quenching; downsizing; and a correlation between formation timescale and similar to results from simulations that provides a natural connection to bulge growth. We take these findings-which suggest that quenching is the natural downturn of all SFHs affecting galaxies at rates/times correlated with their densities-to mean that: (1) models in which galaxies are diversified on Hubble timescales by something like initial conditions rival the dominant grow-and-quench framework as good descriptions of the data; or (2) absent spatial information, many metrics of galaxy evolution are too undiscriminating-if not inherently misleading-to confirm a unique explanation. We outline future tests of our model but stress that, even if ultimately incorrect, it illustrates how exploring different paradigms can aid learning and, we hope, more detailed modeling efforts.
We present here a simple model for the star formation history (SFH) of galaxies that is successful in describing both the star formation rate density (SFRD) over cosmic time, as well as the ...distribution of specific star formation rates (sSFRs) of galaxies at the current epoch, and the evolution of this quantity in galaxy populations to a redshift of z = 1. We show first that the cosmic SFRD is remarkably well described by a simple log-normal in time. We next postulate that this functional form for the ensemble is also a reasonable description for the SFHs of individual galaxies. Finally, we show that this model, constrained in detail only at redshifts z < 1, also produces the main sequence of star-formation observed at 1.5 < z < 2.5, again suggesting that the log-normal SFHs are a close approximation to the actual histories of typical galaxies.
We present strong-lensing models as well as mass and magnification maps for the cores of the six Hubble Space Telescope (HST) Frontier Fields galaxy clusters. Our parametric lens models are ...constrained by the locations and redshifts of multiple image systems of lensed background galaxies. We use a combination of photometric redshifts and spectroscopic redshifts of the lensed background sources obtained by us (for A2744 and AS1063), collected from the literature, or kindly provided by the lensing community. Using our results, we (1) compare the derived mass distribution of each cluster to its light distribution, (2) quantify the cumulative magnification power of the HST Frontier Fields clusters, (3) describe how our models can be used to estimate the magnification and image multiplicity of lensed background sources at all redshifts and at any position within the cluster cores, and (4) discuss systematic effects and caveats resulting from our modeling methods. We specifically investigate the effect of the use of spectroscopic and photometric redshift constraints on the uncertainties of the resulting models. We find that the photometric redshift estimates of lensed galaxies are generally in excellent agreement with spectroscopic redshifts, where available. However, the flexibility associated with relaxed redshift priors may cause the complexity of large-scale structure that is needed to account for the lensing signal to be underestimated. Our findings thus underline the importance of spectroscopic arc redshifts, or tight photometric redshift constraints, for high precision lens models. All products from our best-fit lens models (magnification, convergence, shear, deflection field) and model simulations for estimating errors are made available via the Mikulski Archive for Space Telescopes.
Abstract
We present measurements of the surface density of star formation, the star-forming clump luminosity function, and the clump size distribution function, for the lensed galaxy ...SGAS J111020.0+645950.8 at a redshift of
z
= 2.481. The physical size scales that we probe, radii
r
= 30–50 pc, are considerably smaller scales than have yet been studied at these redshifts. The star formation surface density we find within these small clumps is consistent with surface densities measured previously for other lensed galaxies at similar redshift. Twenty-two percent of the rest-frame ultraviolet light in this lensed galaxy arises from small clumps, with
pc. Within the range of overlap, the clump luminosity function measured for this lensed galaxy is remarkably similar to those of
galaxies. In this galaxy, star-forming regions smaller than 100 pc—physical scales not usually resolved at these redshifts by current telescopes—are important locations of star formation in the distant universe. If this galaxy is representative, this may contradict the theoretical picture in which the critical size scale for star formation in the distant universe is of the order of 1 kpc. Instead, our results suggest that current telescopes have not yet resolved the critical size scales of star-forming activity in galaxies over most of cosmic time.
Abstract
We present the discovery of the most distant, dynamically relaxed cool core cluster, SPT-CL J2215−3537 (SPT2215), and its central brightest cluster galaxy (BCG) at
z
= 1.16. Using new X-ray ...observations, we demonstrate that SPT2215 harbors a strong cool core with a central cooling time of 200 Myr (at 10 kpc) and a maximal intracluster medium cooling rate of 1900 ± 400
M
⊙
yr
−1
. This prodigious cooling may be responsible for fueling the extended, star-forming filaments observed in Hubble Space Telescope imaging. Based on new spectrophotometric data, we detect bright O
ii
emission in the BCG, implying an unobscured star formation rate (SFR) of
320
−
140
+
230
M
⊙
yr
−1
. The detection of a weak radio source (2.0 ± 0.8 mJy at 0.8 GHz) suggests ongoing feedback from an active galactic nucleus (AGN), though the implied jet power is less than half the cooling luminosity of the hot gas, consistent with cooling overpowering heating. The extreme cooling and SFR of SPT2215 are rare among known cool core clusters, and it is even more remarkable that we observe these at such high redshift, when most clusters are still dynamically disturbed. The high mass of this cluster, coupled with the fact that it is dynamically relaxed with a highly isolated BCG, suggests that it is an exceptionally rare system that must have formed very rapidly in the early universe. Combined with the high SFR, SPT2215 may be a high-
z
analog of the Phoenix cluster, potentially providing insight into the limits of AGN feedback and star formation in the most massive galaxies.
The slope of the star formation rate/stellar mass relation (the SFR "Main Sequence"; SFR-Mlow *) is not quite unity: specific star formation rates (SFR/Mlow *) are weakly but significantly ...anti-correlated with Mlow *. 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 Mlow *. 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 (sSFR sub(disk) = SFR/M sub(low *,disk)) reduces the Mlow * dependence of SF efficiency by ~0.25 dex per dex, erasing it entirely in some subsamples. Quantitatively, we find log sSFR sub(disk)-log Mlow * to have a slope beta sub(disk) setmembership -0.20, 0.00 + or - 0.02 (depending on the SFR estimator and Main Sequence definition) for star-forming galaxies with Mlow * > or =, slanted 10 super(10) M sub(middot in circle) and bulge mass-fractions B/T <, ~ 0.6, generally consistent with a pure-disk control sample ( beta sub(control) = -0.05 + or - 0.04). That left angle bracketSFR/M sub(low *,disk))right angle bracket is (largely) independent of host mass for star-forming disks has strong implications for aspects of galaxy evolution inferred from any SFR-Mlow * relation, including manifestations of "mass quenching" (bulge growth), factors shaping the star-forming stellar mass function (uniform d log Mlow */dt for low-mass, disk-dominated galaxies), and diversity in star formation histories (dispersion in SFR(Mlow *, t). Our results emphasize the need to treat galaxies as composite systems-not integrated masses-in observational and theoretical work.
We present a detailed analysis of multi-wavelength Hubble Space Telescope/Wide Field Camera 3 (WFC3) imaging and Keck/OSIRIS near-infrared adaptive optics-assisted integral field spectroscopy for a ...highly magnified lensed galaxy at z = 1.70. This young starburst is representative of ultraviolet-selected star-forming galaxies (SFGs) at z ~ 2 and contains multiple individual star-forming regions. Due to the lensing magnification, we can resolve spatial scales down to 100 pc in the source plane of the galaxy. The velocity field shows disturbed kinematics suggestive of an ongoing interaction and there is a clear signature of a tidal tail. We constrain the age, reddening, star formation rate, and stellar mass of the star-forming clumps from spectral energy distribution (SED) modeling of the WFC3 photometry and measure their H alpha luminosity, metallicity, and outflow properties from the OSIRIS data. With strong star-formation-driven outflows in four clumps, RCSGA0327 is the first high-redshift SFG at stellar mass <10 super(10) M sub(middot in circle) with spatially resolved stellar winds. We compare the H alpha luminosities, sizes, and dispersions of the star-forming regions with other high-z clumps as well as local giant H II regions and find no evidence for increased clump star formation surface densities in interacting systems, unlike in the local universe. Spatially resolved SED modeling unveils an established stellar population at the location of the largest clump and a second mass concentration near the edge of the system that is not detected in H alpha emission. This suggests a picture of an equal-mass mixed major merger, which has not triggered a new burst of star formation or caused a tidal tail in the gas-poor component.
The IMACS Cluster Building Survey (ICBS) provides spectra of ~2200 galaxies 0.31 < z < 0.54 in five rich clusters (R <, ~ 5 Mpc) and the field. Infalling, dynamically cold groups with tens of members ...account for approximately half of the supercluster population, contributing to a growth in cluster mass of ~100% by the present day. The ICBS spectra distinguish non-star-forming (PAS) and poststarburst (PSB) from star-forming galaxies-continuously star-forming (CSF) or starbursts (SBH or SBO), identified by anomalously strong HS absorption or OII emission. For the infalling cluster groups and similar field groups, we find a correlation between PAS+PSB fraction and group mass, indicating substantial "preprocessing" through quenching mechanisms that can turn star-forming galaxies into passive galaxies without the unique environment of rich clusters. SBH + SBO starburst galaxies are common, and they maintain an approximately constant ratio (SBH+SBO)/CSF asymptotically = 25% in all environments-from field, to groups, to rich clusters. Similarly, while PSB galaxies strongly favor denser environments, PSB/PAS asymptotically = 10%-20% for all environments. This result, and their timescale tau ~ 500 Myr, indicates that starbursts are not signatures of a quenching mechanism that produces the majority of passive galaxies. We suggest instead that starbursts and poststarbursts signal minor mergers and accretions, in star-forming and passive galaxies, respectively, and that the principal mechanisms for producing passive systems are (1) early major mergers, for elliptical galaxies, and (2) later, less violent processes-such as starvation and tidal stripping, for SO galaxies.
We present optical and near-IR imaging and spectroscopy of SGAS J 105039.6+001730, a strongly lensed galaxy at z = 3.6252 magnified by >30x, and derive its physical properties. We measure a stellar ...mass of log(M sub(*)/M sub(middot in circle)) = 9.5 + or - 0.35, star formation rates from OII 2.2.3727 and H beta of 55 + or - 25 and 84 + or - 24 M sub(middot in circle) yr super(-1), respectively, an electron density of n sub(e) < or =, slant 10 super(3) cm super(-2), an electron temperature of T sub(e) < or =, slant 14,000 K, and a metallicity of 12 + log(O/H) = 8.3 + or - 0.1. The strong C III lambdalambda1907,1909 emission and abundance ratios of C, N, O, and Si are consistent with well-studied starbursts at z ~ 0 with similar metallicities. Strong P Cygni lines and He II lambda1640 emission indicate a significant population of Wolf-Rayet stars, but synthetic spectra of individual populations of young, hot stars do not reproduce the observed integrated P Cygni absorption features. The rest-frame UV spectral features are indicative of a young starburst with high ionization, implying either (1) an ionization parameter significantly higher than suggested by rest-frame optical nebular lines, or (2) differences in one or both of the initial mass function and the properties of ionizing spectra of massive stars. We argue that the observed features are likely the result of a superposition of star forming regions with different physical properties. These results demonstrate the complexity of star formation on scales smaller than individual galaxies, and highlight the importance of systematic effects that result from smearing together the signatures of individual star forming regions within galaxies.
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