On the nature of star formation at large galactic radii

Abstract

We have compared far‐ultraviolet (FUV), near‐ultraviolet (NUV) and Hα measurements for star‐forming regions in 21 galaxies, in order to characterize the properties of their discs at radii beyond the main optical radius (R25). In our representative sample of extended and non‐extended ultraviolet (UV) discs, we find that half of the extended UV discs also exhibit extended Hα emission. We find that extended UV discs fall into two categories: those with a sharp truncation in the Hα disc close to the optical edge (R25), and those with extended emission in Hα as well as in the UV. Although most galaxies with strong Hα truncations near R25 show a significant corresponding falloff in UV emission (a factor of 10–100), the transition tends to be much smoother than in Hα, and significant UV emission often extends well beyond this radius, confirming earlier results by Thilker et al. and others. After correcting for dust attenuation the median fraction of total FUV emission from regions outside of R25 is 1.7 per cent, but it can be as high as 35 per cent in the most extreme cases. The corresponding fractions of Hα emission are approximately half as large on average. This difference reflects both a slightly lower ratio of Hα to UV emission in the H ii regions in the outer discs and a lower fraction of star clusters showing H ii regions. Most H ii regions in the extended disc have fluxes consistent with small numbers of ionizing O‐type stars, and this poor sampling of the upper initial mass function (IMF) in small clusters can probably account for the differences in the emission properties, consistent with earlier conclusions by Zaritsky & Christlein, without needing to invoke a significant change in the stellar IMF itself. Consistent Hα/FUV ratios and brightest H ii region to total Hα fluxes in the inner and extended discs across our whole galaxy sample demonstrate no evidence for a change in the cluster luminosity function or the IMF in the low gas density outer disc.

New Insights on the Formation and Assembly of M83 From Deep Near-Infrared Imaging

“…The slope of the outer disk profile is similar to that determined with an isophote fitting routine, which is expected because the ellipse parameters were chosen at an outer disk isophote. density profile (Crosthwaite et al 2002), the gas velocity field (Rogstad et al 1974, and see Figure 1(f)), and the ionized gas surface density (Martin & Kennicutt 2001;Goddard et al 2010). While the latter of these observations might have pointed to a significant decline in the SFR surface density at this radius, FUV observations show a smooth decline implying that star formation has indeed occurred throughout the outer disk.…”

“…While the latter of these observations might have pointed to a significant decline in the SFR surface density at this radius, FUV observations show a smooth decline implying that star formation has indeed occurred throughout the outer disk. Rather, Goddard et al (2010) have found the ages of star forming regions in the outer disk are typically old enough that the most massive, ionizing stars may have died, thus causing the discrepancy between the Hα and FUV profiles. Therefore, it seems that the outer disk portion of M83 shows signatures of recent gas accretion and star formation that have begun building , and H i (bottom) surface brightness profiles, tracing the total stellar surface density, the recent star formation, and the gas reservoir, respectively.…”

A Pilot Study Using Deep Infrared Imaging to Constrain the Star Formation History of the Xuv Stellar Populations in NGC 4625

“…Four well-known XUV disks have been shown to have discontinuities between their inner and outer disk abundance profiles and an approximately constant abundance profile in the outer disk with values between approximately 0.2 and 0.33 Z (Bresolin et al 2009(Bresolin et al , 2012Gil de Paz et al 2007b;Goddard et al 2010). While the existing gas can be enriched to this level by constant star formation over a period of a few Gyr, the low stellar content in the outer disk appears inconsistent with that level of star formation for an extended time.…”

Can molecular clouds live long?

“…Indeed, the universal IMF looks very attractive, because there are no direct evidences that in normal (not star bursting) galaxies IMF is different even for peripheral regions (Goddard et al 2010;Koda et al 2012). Nevertheless a comparison of IMF of individual young stellar clusters demonstrate that significant IMF variations are real (Dib 2014).…”

“…Another example give LSB galaxies where a steep IMF may explain the combination of metallicity and color indices (Lee et al 2004). The different behaviour of UV and H α radial profiles of spiral galaxies also suggests the variation of IMF along radius (Boissier et al 2007;Krumholz & McKee 2008;Meurer et al 2009), although it also may be explained for the standart IMF (Goddard et al 2010). …”