Letter to the Editor

The average optical spectra of intense starbursts at z  ~  2: outflows and the pressurization of the ISM^⋆,⋆⋆

¹ GEPI, Observatoire de Paris, CNRS, Université Paris Diderot, 5 place Jules Janssen, 92190 Meudon, France
e-mail: matthew.lehnert@obspm.fr
² Institut d’Astrophysique Spatiale, UMR 8617, CNRS, Université Paris-Sud, Bâtiment 121, 91405 Orsay Cedex, France

Received: 30 June 2011
Accepted: 7 September 2011

Abstract

An important property of star-forming galaxies at z ~ 1−2 is the high local star-formation intensities they maintain over tens of kiloparsecs at levels that are only observed in the nearby Universe in the most powerful nuclear starbursts. To investigate how these high star-formation intensities affect the warm ionized medium, we present an analysis of the average spectra of about 50 such galaxies at z ≈ 1.2−2.6 and of subsamples selected according to their local and global star-formation intensity. Stacking allows us to probe relatively weak lines like [Sii]λλ6716,6731 and [Oi]λ6300, which are tracers of the conditions of the ISM and are undetectable in most individual targets. We find higher gas densities (hence pressures) in intensely star-forming regions compared to fainter diffuse gas and, overall, values that are comparable to starburst regions and the diffuse ISM in nearby galaxies. By modeling the Hα surface brightnesses and [Sii]/Hα line ratios with the Cloudy photoionization code, we find that our galaxies continue trends observed in local galaxies, where gas pressures scale with star-formation intensity. We discuss these results in the context of models of self-regulated star formation, where star formation determines the average thermal and turbulent pressure in the ISM, which in turn determines the rate at which stars can form, finding good agreement with our data. We also confirm the detection of broad, faint lines underlying Hα and [Nii], which have previously been considered evidence of either outflows or active galactic nuclei. Finding that the broad component is only significantly detected in stacks with the highest average local and global star-formation intensities strongly supports the outflow interpretation, and further emphasizes the importance of star-formation feedback and self-regulation in the early Universe.