Letter to the Editor

The effects of star formation on the low-metallicity ISM: NGC 4214 mapped with Herschel/PACS spectroscopy ^*

¹ Laboratoire AIM, CEA/DSM - CNRS - Université Paris Diderot, Irfu/Service d'Astrophysique, CEA Saclay, 91191 Gif-sur-Yvette, France e-mail: diane.cormier@cea.fr
² Max-Planck-Institut fuer extraterrestrische Physik, Postfach 1312, 85741 Garching, Germany

Received: 31 March 2010
Accepted: 29 April 2010

Abstract

We present Herschel/PACS spectroscopic maps of the dwarf galaxy NC4214 observed in 6 far infrared fine-structure lines: [C ii] 158 μm, [O iii] 88 μm, [O i] 63 μm, [O i] 146 μm, [N ii] 122 μm, and [N ii] 205 μm. The maps are sampled to the full telescope spatial resolution and reveal unprecedented detail on ~ 150 pc size scales. We detect [C ii] emission over the whole mapped area, [O iii] being the most luminous FIR line. The ratio of [O iii]/[C ii] peaks at about 2 toward the sites of massive star formation, higher than ratios seen in dusty starburst galaxies. The [C ii]/CO ratios are 20 000 to 70 000 toward the 2 massive clusters, which are at least an order of magnitude larger than spiral or dusty starbursts, and cannot be reconciled with single-slab PDR models. Toward the 2 massive star-forming regions, we find that L_[CII] is 0.5 to 0.8% of the L_TIR. All of the lines together contribute up to 2% of L_TIR. These extreme findings are a consequence of the lower metallicity and young, massive-star formation commonly found in dwarf galaxies. These conditions promote large-scale photodissociation into the molecular reservoir, which is evident in the FIR line ratios. This illustrates the necessity to move to multiphase models applicable to star-forming clusters or galaxies as a whole.