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Register a new userNear-ultraviolet Spectroscopy of Star-forming Galaxies from eBOSS: Signatures of Ubiquitous Galactic-scale Outflows
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We present the rest-frame near-ultraviolet (NUV) spectroscopy of star-forming galaxies (SFGs) at 0.6<z<1.2 from the Extended Baryon Oscillation Spectroscopic Survey (eBOSS) in SDSS-IV. One of the eBOSS programs is to obtain 2 arcsec (about 15 kpc) fiber spectra of about 200,000 emission-line galaxies (ELGs) at redshift z>0.6. We use the data from the pilot observations of this program, including 8620 spectra of SFGs at 0.6<z<1.2. The median composite spectra of these SFGs at 2200 Ang < lambda < 4000 Ang feature asymmetric, preferentially blueshifted non-resonant emission, Fe II*, and blueshifted resonant absorption, e.g., Fe II and Mg II, indicating ubiquitous outflows driven by star formation at these redshifts. For the absorption lines, we find a variety of velocity profiles with different degrees of blueshift. Comparing our new observations with the literature, we do not observe the non-resonant emission in the small-aperture (<40 pc) spectra of local star-forming regions with the Hubble Space Telescope, and find the observed line ratios in the SFG spectra to be different from those in the spectra of local star-forming regions, as well as those of quasar absorption-line systems in the same redshift range. We introduce an outflow model that can simultaneously explain the multiple observed properties and suggest that the variety of absorption velocity profiles and the line ratio differences are caused by scattered fluorescent emission filling in on top of the absorption in the large-aperture eBOSS spectra. We develop an observation-driven, model-independent method to correct the emission-infill to reveal the true absorption profiles. Our results show that eBOSS and future dark-energy surveys (e.g., DESI and PFS) will provide rich datasets of NUV spectroscopy for astrophysical applications.
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We perform a joint-analysis of high spatial resolution molecular gas and star-formation rate (SFR) maps in main-sequence star-forming galaxies experiencing galactic-scale outflows of ionised gas. Our aim is to understand the mechanism that determines which galaxies are able to launch these intense winds. We observed CO(1-0) at 1 resolution with ALMA in 16 edge-on galaxies, which also have 2 spatial resolution optical integral field observations from the SAMI Galaxy Survey. Half the galaxies in the sample were previously identified as harbouring intense and large-scale outflows of ionised gas (outflow-types), the rest serve as control galaxies. The dataset is complemented by integrated CO(1-0) observations from the IRAM 30-m telescope to probe the total molecular gas reservoirs. We find that the galaxies powering outflows do not possess significantly different global gas fractions or star-formation efficiencies when compared with a control sample. However, the ALMA maps reveal that the molecular gas in the outflow-type galaxies is distributed more centrally than in the control galaxies. For our outflow-type objects, molecular gas and star-formation is largely confined within their inner effective radius ($rm r_{eff}$), whereas in the control sample the distribution is more diffuse, extending far beyond $rm r_{eff}$. We infer that outflows in normal star-forming galaxies may be caused by dynamical mechanisms that drive molecular gas into their central regions, which can result in locally-enhanced gas surface density and star-formation.
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