Using cosmological hydrodynamic simulations that dynamically incorporate enriched galactic outflows together with analytical modelling, we study the origin of the stellar mass-gas-phase metallicity relation (MZR). We find that metallicities are driven by an equilibrium between the rate of enrichment owing to star formation and the rate of dilution owing to infall of unenriched gas. This equilibrium is in turn governed by the outflow strength. As such, the MZR provides valuable insights and strong constraints on galactic outflow properties across cosmic time. We compare three outflow models: no outflows, a 'constant-wind model that emulates the popular Dekel & Silk scenario, and a 'momentum-driven wind' model that best reproduces z 2 intergalactic medium metallicity data. Only the momentum-driven wind scaling simulation is able to reproduce the observed z ∼ 2 MZR's slope, amplitude, and scatter. In order to understand why, we construct a one-zone chemical evolution model guided by simulations. This model shows that the MZR in our outflow simulations can be understood in terms of three parameters: (i) the equilibrium metallicity Z g,eq = y SFR / ACC (where y = net yield), reflecting the enrichment balance between star formation rate SFR and gas accretion rate ACC ; (ii) the dilution time t d = M g /Ṁ SFR , representing the time-scale for a galaxy to return to Z g,eq after a metallicity-perturbing interaction; and (iii) the blowout mass M blowout , which is the galaxy stellar mass above which winds can escape its halo. Without outflows, galaxy metallicities exceed observations by approximately two to three times, although the slope of the MZR is roughly correct owing to greater star formation efficiencies in larger galaxies. When outflows with mass-loading factor η W are present, galaxies below M blowout obey Z g,eq ≈ y/(1 + η W ), while above M blowout , Z g,eq → y. Our constant-wind model has M blowout ∼ 10 10 M , which yields a sharp upturn in the MZR above this scale and a flat MZR with large scatter below it, in strong disagreement with observations. Our momentum-driven wind model naturally reproduces the observed Z g ∝ M 0.3 * because Z g,eq ∝ η −1 W ∝ M 1/3 * when η W 1 (i.e. at low masses). The flattening of the MZR at M * 10 10.5 M observed by Tremonti et al. is reflective of the mass-scale where η W ∼ 1 rather than a characteristic outflow speed; in fact, the outflow speed plays little role in the MZR except through M blowout . The tight observed MZR scatter is ensured when t d dynamical time, which is only satisfied at all masses in our momentumdriven wind model. We also discuss secondary effects on the MZR, such as baryonic stripping from neighbouring galaxies' outflows.

We study the evolution of galaxy rest-frame ultraviolet (UV) colors in the epoch 4 z 8. We use new wide-field near-infrared data in the Great Observatories Origins Deep Survey -South field from the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey, Hubble Ultra Deep Field 2009 and Early Release Science programs to select galaxies via photometric redshift measurements. Our sample consists of 2812 candidate galaxies at z 3.5, including 113 at z ≃ 7-8. We fit the observed spectral energy distribution (SED) to a suite of synthetic stellar population models, and measure the value of the UV spectral slope (β) from the best-fit model spectrum. We run simulations to show that this measurement technique results in a smaller scatter on β than other methods, as well as a reduced number of galaxies with catastrophically incorrect β measurements (i.e., ∆β > 1). We find that the median value of β evolves significantly from −1.82 +0.00 −0.04 at z = 4, to −2.37 +0.26 −0.06 at z = 7. Additionally, we find that faint galaxies at z = 7 have β = −2.68 +0.39 −0.24 (∼ −2.4 after correcting for observational bias); this is redder than previous claims in the literature, and does not require "exotic" stellar populations (e.g., very-low metallicities or top-heavy initial mass functions) to explain their colors. This evolution can be explained by an increase in dust extinction, from low amounts at z = 7, to A V ∼ 0.5 mag at z = 4. The timescale for this increase is consistent with low-mass AGB stars forming the bulk of the dust. We find no significant (< 2σ) correlation between β and M UV when measuring M UV at a consistent restframe wavelength of 1500 Å. This is particularly true at bright magnitudes, though our results do show evidence for a weak correlation at faint magnitudes when galaxies in the HUDF are considered separately, hinting that dynamic range in sample luminosities may play a role. We do find a strong correlation between β and the stellar mass at all redshifts, in that more massive galaxies exhibit redder colors. The most massive galaxies in our sample have similarly red colors at each redshift, implying that dust can build up quickly in massive galaxies, and that feedback is likely removing dust from low-mass galaxies at z ≥ 7. Thus the stellar-massmetallicity relation, previously observed up to z ∼ 3, may extend out to z = 7 -8.

We use cosmological hydrodynamic simulations to investigate how inflows, star formation and outflows govern the gaseous and metal content of galaxies within a hierarchical structure formation context. In our simulations, galaxy metallicities are established by a balance between inflows and outflows as governed by the mass outflow rate, implying that the mass–metallicity relation reflects how the outflow rate varies with stellar mass. Gas content, meanwhile, is set by a competition between inflow into and gas consumption within the interstellar medium, the latter being governed by the star formation law, while the former is impacted by both wind recycling and preventive feedback. Stochastic variations in the inflow rate move galaxies off the equilibrium mass–metallicity and mass–gas fraction relations in a manner correlated with the star formation rate, and the scatter is set by the time‐scale to re‐equilibrate. The evolution of both relations from z= 3 → 0 is slow, as individual galaxies tend to evolve mostly along the relations. Gas fractions at a given stellar mass slowly decrease with time because the cosmic inflow rate diminishes faster than the consumption rate, while metallicities slowly increase as infalling gas becomes more enriched. Observations from z∼ 3 → 0 are better matched by simulations employing momentum‐driven wind scalings rather than constant wind speeds, but all models predict too low gas fractions at low masses and too high metallicities at high masses. All our models reproduce observed second‐parameter trends of the mass–metallicity relation with the star formation rate and environment, indicating that these are a consequence of equilibrium and not feedback. Overall, the analytical framework of our equilibrium scenario broadly captures the relevant physics establishing the galaxy gas and metal content in simulations, which suggests that the cycle of baryonic inflows and outflows centrally governs the cosmic evolution of these properties in typical star‐forming galaxies.

We discuss the optical and radio properties of ∼30,000 FIRST (radio, 20 cm, sensitive to 1 mJy) sources positionally associated within 1.5 arcsec with an SDSS (optical, sensitive to r * ∼22.2) source in 1230 deg 2 of sky. The matched sample represents ∼30% of the 108,000 FIRST sources and 0.1% of the 2.5 × 10 7 SDSS sources in the studied region. SDSS spectra are available for 4,300 galaxies and 1,154 quasars from the matched sample, and for a control sample of 140,000 galaxies and 20,000 quasars in 1030 deg 2 of sky. This large and unbiased catalog of optical identifications provides much firmer statistical footing for existing results and allows several new findings.The majority (83%) of the FIRST sources identified with an SDSS source brighter than r * =21 are optically resolved; the fraction of resolved objects among the matched sources is a function of the radio flux, increasing from ∼50% at the bright end to ∼90% at the FIRST faint limit. Nearly all optically unresolved radio sources have non-stellar colors indicative of quasars. We estimate an upper limit of ∼5% for the fraction of quasars with broad-band optical colors indistinguishable from those of stars. The distribution of quasars in the radio flux -optical flux plane supports the existence of the "quasar radio-dichotomy"; 8±1% of all quasars with i * <18.5 are radio-loud and this fraction seems independent of redshift and optical luminosity. The radio-loud quasars have a redder median color by 0.08±0.02 mag, and show a 3 times larger fraction of objects with extremely red colors.FIRST galaxies represent 5% of all SDSS galaxies with r * <17.5, and 1% for r * <20, and are dominated by red (u * − r * >2.22) galaxies, especially those with r * >17.5. Magnitude and redshift limited samples show that radio galaxies have a different optical luminosity distribution than non-radio galaxies selected by the same criteria; when galaxies are further separated by their colors, this result remains valid for both blue and red galaxies. For a given optical luminosity and redshift, the observed optical colors of radio-galaxies are indistinguishable from those of all SDSS galaxies selected by identical criteria. The distributions of radio-to-optical flux ratio are similar for blue and red galaxies in redshift-limited samples; this similarity implies that the difference in their luminosity functions, and resulting selection effects, are the dominant cause for the preponderance of red radio galaxies in flux-limited samples. The fraction of radio galaxies whose emission line ratios indicate an AGN (30%) rather than a starburst origin is 6 times larger than the corresponding fraction for all SDSS galaxies (r * <17.5). We confirm that the AGN-to-starburst galaxy number ratio increases with radio flux, and find that radio emission from AGNs is more concentrated than radio emission from starburst galaxies.

We present results from the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CAN-DELS) photometric redshift methods investigation. In this investigation, the results from eleven participants, each using a different combination of photometric redshift code, template spectral energy distributions (SEDs) and priors, are used to examine the properties of photometric redshifts applied to deep fields with broad-band multi-wavelength coverage. The photometry used includes U -band through mid-infrared filters and was derived using the TFIT method. Comparing the results, we find that there is no particular code or set of template SEDs that results in significantly better photometric redshifts compared to others. However, we find codes producing the lowest scatter and outlier fraction utilize a training sample to optimize photometric redshifts by adding zero-point offsets, template adjusting or adding extra smoothing errors. These results therefore stress the importance of the training procedure. We find a strong dependence of the photometric redshift accuracy on the signal-to-noise ratio of the photometry. On the other hand, we find a weak dependence of the photometric redshift scatter with redshift and galaxy color. We find that most photometric redshift codes quote redshift errors (e.g., 68% confidence intervals) that are too small compared to that expected from the spectroscopic control sample. We find that all codes show a statistically significant bias in the photometric redshifts. However, the bias is in all cases smaller than the scatter, the latter therefore dominates the errors. Finally, we find that combining results from multiple codes significantly decreases the photometric redshift scatter and outlier fraction. We discuss different ways of combining data to produce accurate photometric redshifts and error estimates. 1 2 Dahlen et al.

Distant star-forming galaxies show a correlation between their star formation rates (SFR) and stellar masses, and this has deep implications for galaxy formation. Here, we present a study on the evolution of the slope and scatter of the SFR-stellar mass relation for galaxies at 3.5 ≤ z ≤ 6.5 using multiwavelength photometry in GOODS-S from the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS) and Spitzer Extended Deep Survey. We describe an updated, Bayesian spectral-energy distribution fitting method that incorporates effects of nebular line emission, star formation histories that are constant or rising with time, and different dust attenuation prescriptions (starburst and Small Magellanic Cloud). From z=6.5 to z=3.5 star-forming galaxies in CANDELS follow a nearly unevolving correlation between stellar mass and SFR that follows SFR ∼ M a with a= 0.54 ± 0.16 at z ∼ 6 and 0.70 ± 0.21 at z ∼ 4. This evolution requires a star formation history that increases with decreasing redshift (on average, the SFRs of individual galaxies rise with time). The observed scatter in the SFR-stellar mass relation is tight, σ(log SFR/M yr −1 ) < 0.3 − 0.4 dex, for galaxies with log M /M > 9 dex. Assuming that the SFR is tied to the net gas inflow rate (SFR ∼Ṁ gas ), then the scatter in the gas inflow rate is also smaller than 0.3−0.4 dex for star-forming galaxies in these stellar mass and redshift ranges, at least when averaged over the timescale of star formation. We further show that the implied star formation history of objects selected on the basis of their co-moving number densities is consistent with the evolution in the SFR-stellar mass relation.

We discuss measurements of the properties of ∼10,000 asteroids detected in 500 deg 2 of sky in the Sloan Digital Sky Survey (SDSS) commissioning data. The moving objects are detected in the magnitude range 14 < r * < 21.5, with a baseline of ∼5 minutes, resulting in typical velocity errors of ∼3%. Extensive tests show that the sample is at least 98% complete, with the contamination rate of less than 3%.We find that the size distribution of asteroids resembles a broken power-law, independent of the heliocentric distance: D −2.3 for 0.4 km ∼ < D ∼ < 5 km, and D −4 for 5 km ∼ < D ∼ < 40 km. As a consequence of this break, the number of 1 Based on observations obtained with the Sloan Digital Sky Survey.-3asteroids with r * < 21.5 is ten times smaller than predicted by extrapolating the power-law relation observed for brighter asteroids (r * ∼ < 18). The observed counts imply that there are about 530,000 objects with D > 1 km in the asteroid belt, or about four times less than previous estimates. We predict that by its completion SDSS will obtain about 100,000 near simultaneous five-band measurements for a subset drawn from 280,000 asteroids brighter than r * < 21.5 at opposition. Only about a third of these asteroids have been previously observed, and usually in just one band.The distribution of main belt asteroids in the 4-dimensional SDSS color space is bimodal, and the two groups can be associated with S (rocky) and C (carbonaceous) asteroids. A strong bimodality is also seen in the heliocentric distribution of asteroids and suggests the existence of two distinct belts: the inner rocky belt, about 1 AU wide (FWHM) and centered at R ∼2.8 AU, and the outer carbonaceous belt, about 0.5 AU wide and centered at R ∼3.2 AU. The median color of each class becomes bluer by about 0.03 mag AU −1 as the heliocentric distance increases. The observed number ratio of S and C asteroids in a sample with r * < 21.5 is 1.5:1, while in a sample limited by absolute magnitude it changes from 4:1 at 2 AU, to 1:3 at 3.5 AU. In a size-limited sample with D > 1 km, the number ratio of S and C asteroids in the entire main belt is 1:2.3.The colors of Hungarias, Mars crossers, and near-Earth objects, selected by their velocity vectors, are more similar to the C-type than to S-type asteroids, suggesting that they originate in the outer belt. In about 100 deg 2 of sky along the Celestial Equator observed twice two days apart, we find one plausible Kuiper Belt Object (KBO) candidate, in agreement with the expected KBO surface density. The colors of the KBO candidate are significantly redder than the asteroid colors, in agreement with colors of known KBOs. We explore the possibility that SDSS data can be used to search for very red, previously uncatalogued asteroids observed by 2MASS, by extracting objects without SDSS counterparts. We do not find evidence for a significant population of such objects; their contribution is no more than 10% of the asteroid population.

We examine the growth of the stellar content of galaxies from z= 3 → 0 in cosmological hydrodynamic simulations incorporating parametrized galactic outflows. Without outflows, galaxies overproduce stellar masses (M*) and star formation rates (SFRs) compared to observations. Winds introduce a three‐tier form for the galaxy stellar mass and star formation rate functions, where the middle tier depends on the differential (i.e. mass‐dependent) recycling of ejected wind material back into galaxies. A tight M*–SFR relation is a generic outcome of all these simulations and its evolution is well described as being powered by cold accretion, although current observations at z≳ 2 suggest that the star formation in small early galaxies must be highly suppressed. Roughly, one‐third of z= 0 galaxies at masses below M★ are satellites and the star formation in satellites is not much burstier than in centrals. All models fail to suppress the star formation and stellar mass growth in massive galaxies at z≲ 2, indicating the need for an external quenching mechanism such as black hole feedback. All models also fail to produce dwarfs as young and rapidly star forming as observed. An outflow model following scalings expected for momentum‐driven winds broadly matches the observed galaxy evolution around M★ from z= 0 to 3, which is a significant success since these galaxies dominate cosmic star formation, but the failures at higher and lower masses highlight the challenges still faced by this class of models. We argue that central star‐forming galaxies are well described as living in a slowly evolving equilibrium between inflows from gravity and recycled winds, star formation, and strong and ubiquitous outflows that regulate how much inflow forms into stars. Star‐forming galaxy evolution is thus primarily governed by the continual cycling of baryons between galaxies and intergalactic gas.