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Register a new userThe Universal Initial Mass Function In The XUV Disk of M83
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We report deep Subaru Halpha observations of the XUV disk of M83. These new observations enable the first complete census of very young stellar clusters over the entire XUV disk. Combining Subaru and GALEX data with a stellar population synthesis model, we find that (1) the standard, but stochastically-sampled, initial mass function (IMF) is preferred over the truncated IMF, because there are low mass stellar clusters (10^{2-3}Msun) that host massive O-type stars; that (2) the standard Salpeter IMF and a simple aging effect explain the counts of FUV-bright and Halpha-bright clusters with masses >10^3Msun; and that (3) the Halpha to FUV flux ratio over the XUV disk supports the standard IMF. The Subaru Prime Focus Camera (Suprime-Cam) covers a large area even outside the XUV disk -- far beyond the detection limit of the HI gas. This enables us to statistically separate the stellar clusters in the disk from background contamination. The new data, model, and previous spectroscopic studies provide overall consistent results with respect to the internal dust extinction (Av~0.1 mag) and low metallicity (~0.2Zsun) using the dust extinction curve of SMC.
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Using Hubble Space Telescope ACS/WFC data we present the photometry and spatial distribution of resolved stellar populations of four fields within the extended ultraviolet disk (XUV disk) of M83. These observations show a clumpy distribution of main-sequence stars and a mostly smooth distribution of red giant branch stars. We constrain the upper-end of the initial mass function (IMF) in the outer disk using the detected population of main-sequence stars and an assumed constant star formation rate (SFR) over the last 300 Myr. By comparing the observed main-sequence luminosity function to simulations, we determine the best-fitting IMF to have a power law slope $alpha=-2.35 pm 0.3$ and an upper-mass limit $rm M_{u}=25_{-3}^{+17} , M_odot$. This IMF is consistent with the observed H$alpha$ emission, which we use to provide additional constraints on the IMF. We explore the influence of deviations from the constant SFR assumption, finding that our IMF conclusions are robust against all but strong recent variations in SFR, but these are excluded by causality arguments. These results, along with our similar studies of other nearby galaxies, indicate that some XUV disks are deficient in high-mass stars compared to a Kroupa IMF. There are over one hundred galaxies within 5 Mpc, many already observed with HST, thus allowing a more comprehensive investigation of the IMF, and how it varies, using the techniques developed here.
The extended ultraviolet (XUV) disk galaxies are one of the most interesting objects studied in the last few years. The UV emission, revealed by GALEX, extends well beyond the optical disk, after the drop of H$alpha$ emission, the usual tracer of star formation. This shows that sporadic star formation can occur in a large fraction of the HI disk, at radii up to 3 or 4 times the optical radius. In most galaxies, these regions are poor in stars and dominated by under-recycled gas, therefore bear some similarity to early stages of spiral galaxies and high-redshift galaxies. One remarkable example is M83, a nearby galaxy with an extended UV disk reaching 2 times the optical radius. It offers the opportunity to search for the molecular gas and characterise the star formation in outer disk regions, traced by the UV emission. We obtained CO(2-1) observations with ALMA of a small region in a 1.5$times$ 3 rectangle located at $r_{gal}=7.85$ over a bright UV region of M83. There is no CO detection, in spite of the abundance of HI gas, and the presence of young stars traced by their HII regions. Our spatial resolution (17pc x 13pc) was perfectly fitted to detect Giant Molecular Clouds (GMC), but none were detected. The corresponding upper limits occur in an SFR region of the Kennicutt-Schmidt diagram where dense molecular clouds are expected. Stacking our data over HI-rich regions, using the observed HI velocity, we obtain a tentative detection, corresponding to an H$_2$-to-HI mass ratio of $<$ 3 $times$ 10$^{-2}$. A possible explanation is that the expected molecular clouds are CO-dark, because of the strong UV radiation field. The latter preferentially dissociates CO with respect to H$_2$, due to the small size of the star forming clumps in the outer regions of galaxies.
It is well established that the [alpha/Fe] ratios in elliptical galaxies increase with galaxy mass. This relation holds also for early-type dwarf galaxies, although it seems to steepen at low masses. The [alpha/Fe] vs. mass relation can be explained assuming that smaller galaxies form over longer timescales (downsizing), allowing a larger amount of Fe (mostly produced by long-living Type Ia Supernovae) to be released and incorporated into newly forming stars. Another way to obtain the same result is by using a flatter initial mass function (IMF) in large galaxies, increasing in this way the number of Type II Supernovae and therefore the production rate of alpha-elements. The integrated galactic initial mass function (IGIMF) theory predicts that the higher the star formation rate, the flatter the IMF. We have checked, by means of semi-analytical calculations, that the IGIMF theory, combined with the downsizing effect (i.e. the shorter duration of the star formation in larger galaxies), well reproduces the observed [alpha/Fe] vs. mass relation. In particular, we show a steepening of this relation in dwarf galaxies, in accordance with the available observations.
We determine an absolute calibration of the initial mass function (IMF) of early-type galaxies, by studying a sample of 56 gravitational lenses identified by the SLACS Survey. Under the assumption of standard Navarro, Frenk & White dark matter halos, a combination of lensing, dynamical, and stellar population synthesis models is used to disentangle the stellar and dark matter contribution for each lens. We define an IMF mismatch parameter alpha=M*(L+D)/M*(SPS) as the ratio of stellar mass inferred by a joint lensing and dynamical models (M*(L+D)) to the current stellar mass inferred from stellar populations synthesis models (M*(SPS)). We find that a Salpeter IMF provides stellar masses in agreement with those inferred by lensing and dynamical models (<log alpha>=0.00+-0.03+-0.02), while a Chabrier IMF underestimates them (<log alpha>=0.25+-0.03+-0.02). A tentative trend is found, in the sense that alpha appears to increase with galaxy velocity dispersion. Taken at face value, this result would imply a non universal IMF, perhaps dependent on metallicity, age, or abundance ratios of the stellar populations. Alternatively, the observed trend may imply non-universal dark matter halos with inner density slope increasing with velocity dispersion. While the degeneracy between the two interpretations cannot be broken without additional information, the data imply that massive early-type galaxies cannot have both a universal IMF and universal dark matter halos.
The low-mass end of the stellar Initial Mass Function (IMF) is constrained by focusing on the baryon-dominated central regions of strong lensing galaxies. We study in this letter the Einstein Cross (Q2237+0305), a z=0.04 barred galaxy whose bulge acts as lens on a background quasar. The positions of the four quasar images constrain the surface mass density on the lens plane, whereas the surface brightness (H-band NICMOS/HST imaging) along with deep spectroscopy of the lens (VLT/FORS1) allow us to constrain the stellar mass content, for a range of IMFs. We find that a classical single power law (Salpeter IMF) predicts more stellar mass than the observed lensing estimates. This result is confirmed at the 99% confidence level, and is robust to systematic effects due to the choice of population synthesis models, the presence of dust, or the complex disk/bulge population mix. Our non-parametric methodology is more robust than kinematic estimates, as we do not need to make any assumptions about the dynamical state of the galaxy or its decomposition into bulge and disk. Over a range of low-mass power law slopes (with Salpeter being Gamma=+1.35) we find that at a 90% confidence level, slopes with Gamma>0 are ruled out.
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