A Wide-field High Resolution HI Mosaic of Messier 31: I. Opaque Atomic Gas and Star Formation Rate Density

We have undertaken a deep, wide-field HI imaging survey of M31, reaching a maximum resolution of about 50 pc and 2 km/s across a 95x48 kpc region. The HI mass and brightness sensitivity at 100 pc resolution for a 25 km/s wide spectral feature is 1500 M_Sun and 0.28 K. Our study reveals ubiquitous HI self-opacity features, discernible in the first instance as filamentary local minima in images of the peak HI brightness temperature. Local minima are organized into complexes of more than kpc length and are particularly associated with the leading edge of spiral arm features. Just as in the Galaxy, there is only patchy correspondence of self-opaque features with CO(1-0) emission. Localized opacity corrections to the column density exceed an order of magnitude in many cases and add globally to a 30% increase in the atomic gas mass over that inferred from the integrated brightness under the usual assumption of negligible self-opacity. Opaque atomic gas first increases from 20 to 60 K in spin temperature with radius to 12 kpc but then declines again to 20 K beyond 25 kpc. We have extended the resolved star formation law down to physical scales more than an order of magnitude smaller in area and mass than has been possible previously. The relation between total-gas-mass- and star-formation-rate-density is significantly tighter than that with molecular-mass and is fully consistent in both slope and normalization with the power law index of 1.56 found in the molecule-dominated disk of M51 at 500 pc resolution. Below a gas-mass-density of about 5 M_Sun/pc^2, there is a down-turn in star-formation-rate-density which may represent a real local threshold for massive star formation at a cloud mass of about 5x10^4 M_Sun.Comment: Accepted for publication in ApJ, 34 pages, 20 figure

The Ultraviolet View of the Magellanic Clouds from GALEX: A First Look at the LMC Source Catalog

The Galaxy Evolution Exporer (GALEX) has performed unprecedented imaging surveys of the Magellanic Clouds (MC) and their surrounding areas including the Magellanic Bridge (MB) in near-UV (NUV, 1771-2831\AA) and far-UV (FUV, 1344-1786\AA) bands at 5" resolution. Substantially more area was covered in the NUV than FUV, particularly in the bright central regions, because of the GALEX FUV detector failure. The 5$\sigma$ depth of the NUV imaging varies between 20.8 and 22.7 (ABmag). Such imaging provides the first sensitive view of the entire content of hot stars in the Magellanic System, revealing the presence of young populations even in sites with extremely low star-formation rate surface density like the MB, owing to high sensitivity of the UV data to hot stars and the dark sky at these wavelengths. The density of UV sources is quite high in many areas of the LMC and SMC. Crowding limits the quality of source detection and photometry from the standard mission pipeline processing. We performed custom-photometry of the GALEX data in the MC survey region ($<15^{\circ}$ from the LMC, $<10^{\circ}$ from the SMC). After merging multiple detections of sources in overlapping images, the resulting catalog we have produced for the LMC contains nearly 6 million unique NUV point sources within 15$^{\circ}$ and is briefly presented herein. This paper provides a first look at the GALEX MC survey and highlights some of the science investigations that the entire catalog and imaging dataset will make possible.Comment: 16 pages, 8 figures; J. Adv. Space Res. (2013

Star Formation Signatures in Optically Quiescent Early-type Galaxies

In recent years argument has been made that a high fraction of early-type galaxies in the local universe experience low levels (< 1 M_sun/yr) of star formation (SF) that causes strong excess in UV flux, yet leaves the optical colors red. Many of these studies were based on GALEX imaging of SDSS galaxies (z~0.1), and were thus limited by its 5" FWHM. Poor UV resolution left other possibilities for UV excess open, such as the old populations or an AGN. Here we study high-resolution far-ultraviolet HST/ACS images of optically quiescent early-type galaxies with strong UV excess. The new images show that three-quarters of these moderately massive (~5x10^10 M_sun) early-type galaxies shows clear evidence of extended SF, usually in form of wide or concentric UV rings, and in some cases, striking spiral arms. SDSS spectra probably miss these features due to small fiber size. UV-excess early-type galaxies have on average less dust and larger UV sizes (D>40 kpc) than other green-valley galaxies, which argues for an external origin for the gas that is driving the SF. Thus, most of these galaxies appear `rejuvenated' (e.g., through minor gas-rich mergers or IGM accretion). For a smaller subset of the sample, the declining SF (from the original internal gas) cannot be ruled out. SF is rare in very massive early-types (M_* > 10^11 M_sun), a possible consequence of AGN feedback. In addition to extended UV emission, many galaxies show a compact central source, which may be a weak, optically inconspicuous AGN.Comment: Accepted for publication in ApJ Letters. Figures can be printed on B/W printer without loss of information

HIIphot: Automated Photometry of HII Regions Applied to M51

We have developed a robust, automated method, hereafter designated HIIphot, which enables accurate photometric characterization of HII regions while permitting genuine adaptivity to irregular source morphology. HIIphot utilizes object-recognition techniques to make a first guess at the shapes of all sources then allows for departure from such idealized ``seeds'' through an iterative growing procedure. Photometric corrections for spatially coincident diffuse emission are derived from a low-order surface fit to the background after exclusion of all detected sources. We present results for the well-studied, nearby spiral M51 in which 1229 HII regions are detected above the 5-sigma level. A simple, weighted power-law fit to the measured H-alpha luminosity function (HII LF) above log L_H-alpha = 37.6 gives alpha = -1.75+/-0.06, despite a conspicuous break in the HII LF observed near L_H-alpha = 10^38.9. Our best- fit slope is marginally steeper than measured by Rand (1992), perhaps reflecting our increased sensitivity at low luminosities and to notably diffuse objects. HII regions located in interarm gaps are preferentially less luminous than counterparts which constitute M51's grand-design spiral arms and are best fit with a power-law slope of alpha = -1.96+/-0.15. We assign arm/interarm status for HII regions based upon the varying surface brightness of diffuse emission as a function of position throughout the image. Using our measurement of the integrated flux contributed by resolved HII regions in M51, we estimate the diffuse fraction to be approximately 0.45 -- in agreement with the determination of Greenawalt et al. (1998). Automated processing of degraded datasets is undertaken to gauge systematic effects associated with limiting spatial resolution and sensitivity.Comment: 41 pages, 14 figures, Postscript version with high-resolution figures at ftp://ftp.aoc.nrao.edu/staff/dthilker/preprint

Diffuse Far-UV Line Emission from the Low-Redshift Lyman Break Galaxy Analog KISSR242

We present new ultraviolet (UV) observations of the luminous compact blue galaxy KISSR242, obtained with the HST-COS. We identify multiple resolved sub-arcsecond near-UV sources within the COS aperture. The far-UV spectroscopic data show strong outflow absorption lines, consistent with feedback processes related to an episode of massive star-formation. OI, CII, and SiII--SiIV are observed with a mean outflow velocity v_{out} = -60 km/s. We also detect faint fine-structure emission lines of singly ionized silicon for the first time in a low-redshift starburst galaxy. These emissions have been seen previously in deep Lyman break galaxy surveys at z ~ 3. The SiII* lines are at the galaxy rest velocity, and they exhibit a quantitatively different line profile from the absorption features. These lines have a width of ~ 75 km/s, too broad for point-like emission sources such as the HII regions surrounding individual star clusters. The size of the SiII* emitting region is estimated to be ~ 250 pc. We discuss the possibility of this emission arising in overlapping super star cluster HII regions, but find this explanation to be unlikely in light of existing far-UV observations of local star-forming galaxies. We suggest that the observed SiII* emission originates in a diffuse warm halo populated by interstellar gas driven out by intense star-formation and/or accreted during a recent interaction that may be fueling the present starburst episode in KISSR242.Comment: ApJL accepted. 6 pages, 3 figures, 2 table