Polychromatic view of intergalactic star formation in NGC 5291

Abstract

Context.Star formation (SF) takes place in unusual places such as way out in the intergalactic medium out of material expelled from parent galaxies.

Aims.Whether SF proceeds in this specific environment in a similar way than in galactic disks is the question we wish to answer. Particularly, we address the reliability of ultraviolet, H$\alpha$ and mid-infrared as tracers of SF in the intergalactic medium.

Methods.We have carried out a multiwavelength analysis of the interacting system NGC 5291 , which is remarkable for its extended HI ring hosting numerous intergalactic HII regions. We combined new ultraviolet (GALEX) observations with archival H$\alpha$, 8 $\mu$m (Spitzer Space Telescope) and HI (VLA B-array) images of the system.

Results.We have found that the morphology of the star forming regions, as traced by the ultraviolet, H$\alpha$, and 8 $\mu$m emission is similar. The 8.0 $\mu$m infrared emission, normalised to emission from dust at 4.5 $\mu$m, which is known to be dominated by PAH bands, is comparable to the integrated emission of dwarf galaxies of the same metallicity and to the emission of individual HII regions in spirals. The 8.0 $\mu$m in the intergalactic environment is therefore an estimator of the star formation rate which is as reliable in that extreme environment as it is for spirals. There is a clear excess of ultraviolet emission compared to individual HII regions in spirals, i.e. the $ [8.0]/[{\rm NUV}]$ and $[{\rm H}\alpha]/[\rm NUV]$ SFR ratios are on average low although there are some large variations from one region to another, which cannot be explained by variations of the metallicity or the dust extinction along the HI structure. Comparing the observed SFR with a model of the evolution of $[{\rm H}\alpha]/[\rm NUV]$ with time favours young, quasi-instantaneous though already fading starbursts. The total star formation rate measured in the intergalactic medium (which accounts for 80% of the total) surrounding NGC 5291 is up to 1.3 $M_{\odot}$ yr-1 - a value typical for spirals - assuming the standard SFR calibrations are valid. The SFR drops by a factor of 2 to 4 in case the star formation is indeed quasi-instantaneous.