Light breeze in the local Universe

¹ Excellence Cluster Universe, Boltzmannstr. 2, 85748 Garching, Germany
e-mail: alice.concas@tum.de
² Dipartimento di Fisica e Astronomia, Universitá degli Studi di Bologna, V. le Berti Pichat, 6/2 – 40127 Bologna, Italy
³ INAF-Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
⁴ European Southern Observatory, Karl-Schwarzschild-str. 2, 85748 Garching, Germany

Received: 11 August 2016
Accepted: 22 March 2017

Abstract

We analyze a complete spectroscopic sample of galaxies (~600 000) drawn from Sloan Digital Sky Survey (SDSS, DR7) to look for evidence of galactic winds in the local Universe. We focus on the shape of the [OIII]λ5007 emission line as a tracer of ionizing gas outflows. We stack our spectra in a fine grid of star formation rate (SFR) and stellar mass to analyze the dependence of winds on the position of galaxies in the SFR versus mass diagram. We do not find any significant evidence of broad and shifted [OIII]λ5007 emission line which we interpret as no evidence of outflowing ionized gas in the global population. We have also classified these galaxies as star-forming or AGN-dominated according to their position in the standard BPT diagram. We show how the average [OIII]λ5007 profile changes as a function of the nature of the dominant ionizing source. We find that in the star-forming dominated source the oxygen line is symmetric and governed by the gravitational potential well. The AGN or composite AGN\star-formation activity objects, in contrast, display a prominent and asymmetric profile that can be well described by a broad Gaussian component that is blue-shifted from a narrow symmetric core. In particular, we find that the blue wings of the average [OIII]λ5007 profiles are increasingly prominent in the LINERs and Seyfert galaxies. We conclude that, through the identification of strong bulk motion as traced by the warm ionized gas, in the low-redshift Universe, “pure” star-formation activity does not seem capable of driving ionized-gas outflows, while, the presence of optically selected AGN seems to play a primary role. We discuss the implications of these results for the role of the quenching mechanism in the present-day Universe.