High spatial resolution mid-infrared spectroscopy of NGC 5253: The stellar content of the embedded super-star cluster

Abstract

We present the N-band (8-13 micron) spectrum of the hidden compact radio super-star cluster in NGC 5253, C2, obtained with TIMMI2 on the ESO 3.6m telescope. The spectrum is characterised by a rising continuum due to warm dust, a silicate absorption and a strong [SIV] line at 10.5 micron. Weaker lines of [ArIII] at 9.0 micron and [NeII] at 12.8 micron are also present. The continuum can be modeled by an optically thick emission from hot dust emission extinguished by a cold foreground dust screen and a silicate absorption feature with A_sil = 0.73+/-0.05 mag. We show how the spatial scale of the observations greatly determine the mid-IR appearance of NGC 5253 and the important implications that this has on the interpretation of line fluxes in terms of the properties (age, IMF, etc.) of the embedded cluster. We have modeled the observed line fluxes towards C2 using photoionisation models with the most recent spectral energy distributions available to describe the integrated properties of the stellar cluster. Strong constraints on the geometry based on high spatial resolution observations at different wavelengths -- near-IR (HST and Keck), mid-IR (TIMMI2) and radio (VLA) -- allows us to restrain the ionisation parameters to values logU >= -0.5 dex. This constraint on U lead to two possible solutions for the age and upper mass cutoff of C2: 1) a young (< 4 Myr) cluster with a "non-standard" IMF with a low upper mass cutoff Mup < 50 Msun, and 2) a cluster of ~5-6 Myr with a standard high upper mass cutoff (Mup \~ 100 Msun). Arguments in favour and against these two scenarios are presented. The origin of the [OIV] 25.9 micron emission measured by ISO and the possible presence of an intermediate mass black hole inside C2 are also addressed.