Spitzer observations of star formation in the extreme outer disk of M83 (ngc5236)

Abstract

Spitzer Infrared Array Camera (IRAC) observations of two fields in the extended UV disk (XUV-disk) of M83 have been recently obtained, 3 R HII away from the center of the galaxy (R HII = 6.6 kpc). Galaxy Evolution Explorer (GALEX) UV images have shown the two fields to host in situ recent star formation. The IRAC images are used in conjunction with GALEX data and new HI imaging from The HI Nearby Galaxy Survey (THINGS) to constrain stellar masses and ages of the UV clumps in the fields, and to relate the local recent star formation to the reservoir of available gas. Multi-wavelength photometry in the UV and mid-IR (MIR) bands of 136 UV clumps (spatial resolution >220 pc) identified in the two target fields, together with model fitting of the stellar UV-MIR spectral energy distributions (SEDs), suggests that the clumps cover a range of ages between a few Myr and >1 Gyr with a median value around ≤100 Myr, and have masses in the range 103-3 × 106 M , with a peak . The range of observed ages, for which only a small fraction of the mass in stars appears to have formed in the past 10 Myr, agrees with the dearth of Hα emission observed in these outer fields. At the location of our IRAC fields, the HI map shows localized enhancement and clumping of atomic gas. A comparison of the observed star formation with the gas reservoir shows that the UV clumps follow the Schmidt-Kennicutt scaling law of star formation, and that star formation is occurring in regions with gas densities at approximately (within a factor of a few) the critical density value derived according to the Toomre Q gravitational stability criterion. The significant 8 μm excess in several of the clumps (16% of the total by number accounting for 67% of the 8 μm flux) provides evidence for the existence of dust in these remote fields, in agreement with results for other galaxies. Furthermore, we observe a relatively small excess of emission at 4.5 μm in the clumps (14% 6% by flux), which suggests contribution from hot small grains (1000 K), as already observed in other galaxies. From our data, the outer regions of the M83 galaxy disk show evidence of a time-extended star-formation history over ≲1 Gyr, and of a moderately chemically-evolved interstellar medium, in agreement with recent findings on the metallicity of the outer HII regions of M83. © 2008. The American Astronomical Society. All rights reserved..