Higher resolution optical spectra of $M_{\ast }<10^{10}{M}_{\odot }$ galaxies reveal outflow signatures unresolved by the SDSS

Abstract

Galactic outflows are predicted to be ubiquitous in low-mass galaxies, but observational evidence is lacking. Both a low signal-to-noise and a low spectral resolution can severely hamper the detection of galactic outflows, especially in small galaxies that have intrinsically narrow spectral lines. We obtained new, medium-high resolution (FWHM${}_{\mathrm{inst}}\sim 50-110$~km/s) optical spectra of 52 local star forming galaxies ($0.01<z<0.03$) with stellar masses $10^{8.5}<M_{\ast }/[M_{\odot }]<10^{10}$, using the TNG/DOLORES and NTT/EFOSC2 instruments. Our parent sample consists of SDSS galaxies with available heterodyne single-dish molecular (i.e., CO) line data. The targets of this study are selected among those that, based on the comparison between CO line widths, SDSS spectral resolution, and corresponding SDSS-based H$\alpha$ line widths, have a high chance of being unresolved by SDSS spectroscopy. Our new, spectra reveal overall narrower H$\alpha$ and [OIII]$\lambda 5007$ lines, with signs of asymmetries and broad wings that are absent in the SDSS spectra of the same galaxies. This confirms that SDSS spectroscopy does not resolve the narrow emission lines of low-M${}_{\ast }$ galaxies, which hinders the detection of outflows. We identify outflow signatures in $\sim 30\%$ of our targets based on the H$\alpha$ line spectra. Assuming a typical bi-conical outflow geometry, this detection rate is consistent with theoretical predictions of ubiquitous outflows in the low-mass regime. The outflow incidence is enhanced ($\sim 60\%$) for galaxies with above average star formation rates for the sample (SFR $>10^{-0.74}{\mathrm{M}}_{\odot }/yr$). We estimate ionized gas mass outflow rates ranging from $\sim 0.1-50\times 10^{-3}{\mathrm{M}}_{\odot }/yr$ (mean $\sim 20\times 10^{-3}{\mathrm{M}}_{\odot }/yr$) and corresponding mass loading factors between 0.03 and 0.14 (mean $\sim 0.07$) for the sample.

Accepted for publication in A&A. 21 pages, 16 figures

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