Galactic winds are a prime suspect for the metal enrichment of the intergalactic medium and may have a strong influence on the chemical evolution of galaxies and the nature of QSO absorption line systems. We use a sample of 1406 galaxy spectra at z ∼ 1.4 from the DEEP2 redshift survey to show that blueshifted Mg II λ λ 2796, 2803Å absorption is ubiquitous in starforming galaxies at this epoch. This is the first detection of frequent outflowing galactic winds at z ∼ 1. The presence and depth of absorption are independent of AGN spectral signatures or galaxy morphology; major mergers are not a prerequisite for driving a galactic wind from massive galaxies. Outflows are found in coadded spectra of galaxies spanning a range of 30× in stellar mass and 10× in star formation rate (SFR), calibrated from K-band and from MIPS IR fluxes. The outflows have column densities of order N H ∼ 10 20 cm −2 and characteristic velocities of ∼ 300 − 500 km/sec, with absorption seen out to 1000 km/sec in the most massive, highest SFR galaxies. The velocities suggest that the outflowing gas can escape into the IGM and that massive galaxies can produce cosmologically and chemically significant outflows. Both the Mg II equivalent width and the outflow velocity are larger for galaxies of higher stellar mass and SFR, with V wind ∼ SFR 0.3 , similar to the scaling in low redshift IR-luminous galaxies. The high frequency of outflows in the star-forming galaxy population at z ∼ 1 indicates that galactic winds occur in the progenitors of massive spirals as well as those of ellipticals. The increase of outflow velocity with mass and SFR constrains theoretical models of galaxy evolution that include feedback from galactic winds, and may favor momentum-driven models for the wind physics.
This paper documents the 16th data release (DR16) from the Sloan Digital Sky Surveys (SDSS), the fourth and penultimate from the fourth phase (SDSS-IV). This is the first release of data from the Southern Hemisphere survey of the Apache Point Observatory Galactic Evolution Experiment 2 (APOGEE-2); new data from APOGEE-2 North are also included. DR16 is also notable as the final data release for the main cosmological program of the Extended Baryon Oscillation Spectroscopic Survey (eBOSS), and all raw and reduced spectra from that project are released here. DR16 also includes all the data from the Time Domain Spectroscopic Survey and new data from the SPectroscopic IDentification of ERosita Survey programs, both of which were co-observed on eBOSS plates. DR16 has no new data from the Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) survey (or the MaNGA Stellar Library “MaStar”). We also preview future SDSS-V operations (due to start in 2020), and summarize plans for the final SDSS-IV data release (DR17).
We analyze Mg ii λλ2796, 2803 and Fe ii λλ2586, 2600 absorption profiles in individual spectra of 105 galaxies at 0.3 < z < 1.4. The galaxies, drawn from redshift surveys of the GOODS fields and the Extended Groth Strip, sample the range in star formation rates (SFRs) occupied by the star-forming sequence with stellar masses log M * /M 9.6 down to SFR 2 M yr −1 at 0.3 < z < 0.7. Using the Doppler shifts of Mg ii and Fe ii absorption as tracers of cool gas kinematics, we detect large-scale winds in 66 ± 5% of the galaxies. Hubble Space Telescope Advanced Camera for Surveys imaging and our spectral analysis indicate that the outflow detection rate depends primarily on galaxy orientation: winds are detected in ∼89% of galaxies having inclinations (i) < 30 • (faceon), while the wind detection rate is ∼45% in objects having i > 50 • (edge-on). Combined with the comparatively weak dependence of wind detection rate on intrinsic galaxy properties, this implies that biconical outflows are ubiquitous in normal, star-forming galaxies at z ∼ 0.5. We find that wind velocity is correlated with galaxy M * at 3.4σ significance, while outflow equivalent width is correlated with SFR at 3.5σ significance, suggesting hosts with higher SFR launch more material and/or generate a larger velocity spread for the absorbing clouds. Assuming the gas is driven into halos with isothermal density profiles, the wind velocities (∼200-400 km s −1 ) permit escape from the halo potentials only for the lowest-M * systems in the sample. However, the gas carries sufficient energy to reach distances 50 kpc, and may therefore be a viable source of material for the massive, cool circumgalactic medium around bright galaxies at z ∼ 0.
We analyze the absorption and emission-line profiles produced by a set of simple, cool gas wind models motivated by galactic-scale outflow observations. We implement monte carlo radiative transfer techniques that track the propagation of scattered and fluorescent photons to generate 1D spectra and 2D spectral images. We focus on the Mg II λλ2796, 2803 doublet and Fe II UV1 multiplet at λ ≈ 2600Å, but the results are applicable to other transitions that trace outflows (e.g. Na I, H I Lyα, Si II). By design, the resonance transitions show blue-shifted absorption but one also predicts strong resonance and fine-structure line-emission at roughly the systemic velocity. This line-emission 'fills-in' the absorption reducing the equivalent width by up to 50%, shift the absorption-lin centroid by tens of km s −1 , and reduce the effective opacity near systemic. Analysis of cool gas outflows that ignores this line-emission may incorrectly infer that the gas is partially covered, measure a significantly lower peak optical depth, and/or conclude that gas at systemic velocity is absent (e.g. an interstellar or slowly infalling component). Because the Fe II lines are connected by opticallythin transitions to fine-structure levels, their profiles more closely reproduce the intrinsic opacity of the wind. Together these results naturally explain the absorption and emission-line characteristics observed for star-forming galaxies at z < 1. We also study a scenario promoted to describe the outflows of z ∼ 3 Lyman break galaxies and find prfiles inconsistent with the observations due to scattered photon emission. Although line-emission complicates the analysis of absorption-line profiles, the surface brightness profiles offer a unique means of assessing the morphology and size of galactic-scale winds. Furthermore, the kinematics and line-ratios offer powerful diagnostics of outflows, motivating deep, spatially-extended spectroscopic observations.
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