Non-Gaussianity of optical emission lines in SDSS star-forming galaxies and its implications on galactic outflows

Abstract

The shape of emission lines in the optical spectra of star-forming galaxies reveals the kinematics of the diffuse gaseous component. We analyse the shape of prominent emission lines in a sample of $\sim$ 53000 star-forming galaxies from the Sloan Digital Sky Survey, focusing on departures from gaussianity. Departures from a single gaussian profile allow us to probe the motion of gas and to assess the role of outflows. The sample is divided into groups according to their stellar velocity dispersion and star formation rate (SFR). The spectra within each group are stacked to improve the signal-to-noise ratio of the emission lines, to remove individual signatures, and to enhance the effect of SFR on the shapes of the emission lines. The moments of the emission lines, including kurtosis and skewness, are determined. We find that most of the emission lines in strong star-forming systems unequivocally feature negative kurtosis. This signature is present in $\mathrm{H}\unicode{x03B2}$ , $\mathrm{H}\unicode{x03B1}$ , [N ii], and [S ii] in massive galaxies with high SFRs. We attribute it as evidence of radial outflows of ionised gas driven by the star formation of the galaxies. Also, most of the emission lines in low-mass systems with high SFRs feature negative skewness, and we interpret it as evidence of dust obscuration in the galactic disk. These signatures are however absent in the [O iii] line, which is believed to trace a different gas component. The observed trend is significantly stronger in face-on galaxies, indicating that star formation drives the outflows along the galactic rotation axis, presumably the path of least resistance. The data suggest that outflows driven by star formation exert accumulated impacts on the interstellar medium, and the outflow signature is more evident in older galaxies as they have experienced a longer total duration of star formation.

The Role of Active Galactic Nucleus Winds in Galaxy Formation: Connecting AGN Outflows at Low Redshifts to the Formation/Evolution of Their Host Galaxies

“…Further away from the center, the next important structure is the obscuring torus, assumed to form in all AGN, which has a size that could vary between 0.1 and 10 pc. According to the unification scheme, the torus being rich in dust shields the BH at its center, making the radiation field of an AGN anisotropic, and forcing OFs to follow the path of least resistance (Herrera-Camus et al 2020;Alonso-Herrero et al 2021;García-Burillo et al 2021;Rupke et al 2021;Molina et al 2022;Yu et al 2022). Finally, considering the common size of bulges in spiral galaxies (typically 1 kpc; Kormendy & Fisher 2008), the region where we are certain interstellar gas (ISM) is still abundant (remembering that OFs only affect the gas) is the narrow-line region (NLR), an extended region of ionized gas that could reach a few 10 kpc or more (Meena et al 2021).…”