On the nature of faint mid-infrared sources in M 33

Abstract

Aims. We investigate the nature of 24 μm sources in M 33 that have weak or no associated Hα emission. Both bright evolved stars and embedded star-forming regions are visible as compact infrared sources in the 8 and 24 μm Spitzer maps of M 33 and contribute to the more diffuse and faint emission in these bands. Can we distinguish the two populations? Methods. We carry out deep CO J = 2-1 and J = 1-0 line searches at the location of 18 compact mid-IR sources and two optically selected ones to unveil an ongoing star formation process throughout the disk of M 33. In the absence of high-resolution CO maps we use different assumptions to estimate cloud masses from pointed observations. We also analyze if the spectral energy distribution and mid-IR colors can be used to discriminate between evolved stars and star-forming regions.Results. Molecular emission is detected at the location of 17 sources at the level of 0.3 K km s −1 or higher in at least one of the CO rotational lines. Even though the number of giant molecular clouds drops beyond 4 kpc in M 33, our deep observations reveal that clouds of smaller mass are common out to 6.8 kpc. Estimated cloud masses range between 10 4 and 10 5 M , assuming likely values of the CO-to-H 2 conversion factor and virial equilibrium. Sources that are known to be evolved variable stars show weaker or undetectable CO lines. Evolved stars occupy a well defined region of the IRAC color-color diagrams. Star-forming regions are scattered throughout a larger area, even though the bulk of the distribution has different IRAC colors than evolved variable stars. We estimate that about half of the 24 μm sources without an Hα counterpart are genuine embedded star-forming regions. Sources with faint but compact Hα emission have an incomplete Initial Mass Function (IMF) at the high-mass end and are compatible with a population of young clusters with a stochastically sampled, universal IMF.

The population of young stellar clusters throughout the disk of M 33

“…However, a population of small mass molecular clouds exists at larger distances and it becomes dominant. This is also shown by the positive results of deep CO searches around dim IR sources close to the SF edge (see also Corbelli et al 2011).…”

Star-forming dwarf galaxies in the Virgo cluster: the link between molecular gas, atomic gas, and dust

“…For a uniformly bright disc of radius R u and angular size Ω u = πR 2 u , the main beam temperature to brightness temperature ratio is given by (Heeschen 1961;Corbelli et al 2011)…”

Lessons from the Local Group

“…No clusters can be found below the birthline. Star symbols show the locations of a MIR selected sample of YSC in M33 in better agreement with a stochastic universal IMF sampled IMF (e.g Corbelli et al 2011. for MIR sources embedded in CO clouds which lie along the birthline).…”

“…We know today that spiral arms host the brightest HII regions of M33 because here giant molecular clouds grow by disk gravitational instabilities or collisions and coalescence. However it is become more and more evident that star formation in M33 is present throughout the whole disk in small units Corbelli et al 2011), and there is also evidence of occasional events in the extended warped gaseous disk (Sharma et al 2011;Grossi et al 2011). Thanks to Spitzer satellite 24 μm data it has been possible to selectYoung Stellar Cluster candidates spanning 4 orders of magnitude, in bolometric luminosity, down to 10 37 erg s −1 as faint as embedded B2-type stars.…”