A possible far-ultraviolet flux-dependent core mass function in NGC 6357

Abstract

Context. NGC 6357 is a galactic star-forming complex (d ~ 1.7 kpc) composed of several HII regions, a few young stellar clusters, and giant molecular clouds. In particular, the HII regions G353.2+0.9, G353.1+0.6, and G353.2+0.7 are associated with three young clusters; the most prominent of these, Pismis 24, contains some of the most massive stars known. Aims. We aim to derive the properties of the densest compact gas structures (cores) in the region as well as the effects of an intense far-ultraviolet (FUV) radiation field on their global properties. Methods. We mapped the NGC 6357 region at 450 and 850 μm with SCUBA-2 and in the CO(3–2) line with HARP at the JCMT. We also made use of the Herschel Hi-GAL data at 70 and 160 μm. We used the algorithm Gaussclumps to retrieve the compact cores embedded in the diffuse sub-millimetre emission and constructed their spectral energy distribution from 70 to 850 μm, from which we derived mass and temperature. We divided the observed area into an ‘active’ region (i.e. the eastern half, which is exposed to the FUV radiation from the more massive members of the three clusters) and a ‘quiescent’ region (i.e. the western half, which is less affected by FUV radiation). We compared the core mass functions and the temperature distributions in the two areas to look for any differences that could be due to the different levels of FUV radiation. Results. We retrieved 686 dense cores, 411 in the active region and 275 in the quiescent region, with an estimated mass completeness limit of ~5 M⊙. We also attempted to select a sample of pre-stellar cores based on cross-correlation with 70 μm emission and red WISE point sources, which unfortunately is biased due to distance, emission at 70 μm from the dust on the surface of the cores that is heated by the FUV radiation, and saturation in the WISE bands. Most of the cores above the mass completeness limit are likely to be gravitationally bound. The fraction of gas in dense cores is very low, 1.4%. We found a mass-size relation log(M∕M⊙) ~ a × log(D∕arcsec), with a in the range 2.0–2.4, depending on the precise selection of the sample. The temperature distributions in the two sub-regions are clearly different, peaking at ~25 K in the quiescent region and at ~35 K in the active region. The core mass functions are different as well, at a 2σ level, consistent with a Salpeter initial mass function in the quiescent region and flatter than that in the active region. The dense cores lying close to the HII regions are consistent with pre-existing cores being gradually engulfed by a photon dominated region and photoevaporating. A comparison of the obtained distribution of core masses with those derived from simulations of cloud-cloud collisions yields no conclusive evidence of ongoing cloud-cloud collisions. Conclusions. We attribute the different global properties of dense cores in the two sub-regions to the influence of the FUV radiation field.