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Register a new userExtra-Nuclear Starbursts: Young Luminous Hinge Clumps in Interacting Galaxies
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Hinge clumps are luminous knots of star formation near the base of tidal features in some interacting galaxies. We use archival Hubble Space Telescope UV/optical/IR images and Chandra X-ray maps along with GALEX UV, Spitzer IR, and ground-based optical/near-IR images to investigate the star forming properties in a sample of 12 hinge clumps in five interacting galaxies. The most extreme of these hinge clumps have star formation rates of 1 - 9 M(sun)/yr, comparable to or larger than the `overlap region of intense star formation between the two disks of the colliding galaxy system the Antennae. In the HST images, we have found remarkably large (~70 pc) and luminous (M(I) ~ 12.2 to -16.5) sources at the centers of these hinge clumps, sometimes embedded in a linear ridge of fainter star clusters. We have found strong X-ray emission from several of these hinge clumps. In most cases, this emission is well-resolved with Chandra and has a thermal X-ray spectrum, thus it is likely due to hot gas associated with the star formation. The ratio of the extinction-corrected diffuse X-ray luminosity to the mechanical energy rate (the X-ray production efficiency) for the hinge clumps is similar to that in the Antennae galaxies, but higher than those for regions in the normal spiral galaxy NGC 2403. Two of the hinge clumps have point-like X-ray emission much brighter than expected for hot gas; these sources are likely `ultra-luminous X-ray sources (ULXs) due to accretion disks around black holes. The most extreme of these sources, in Arp 240, has a hard X-ray spectrum and an absorbed X-ray luminosity of ~2 X 10^41 erg/s; more than expected by single high mass X-ray binaries (HMXBs), thus it may be either a collection of HMXBs or an intermediate-mass black hole (>=80 M(sun)). [ abridged ]
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We present $HST$ narrow-band near-infrared imaging of Pa$alpha$ and Pa$beta$ emission of 48 local Luminous Infrared Galaxies (LIRGs) from the Great Observatories All-Sky LIRG Survey (GOALS). These data allow us to measure the properties of 810 spatially resolved star-forming regions (59 nuclei and 751 extra-nuclear clumps), and directly compare their properties to those found in both local and high-redshift star-forming galaxies. We find that in LIRGs, the star-forming clumps have radii ranging from $sim90-900$ pc and star formation rates (SFRs) of $sim1times10^{-3}$ to 10 M$_odot$yr$^{-1}$, with median values for extra-nuclear clumps of 170 pc and 0.03 M$_odot$yr$^{-1}$. The detected star-forming clumps are young, with a median stellar age of $8.7$ Myrs, and a median stellar mass of $5times10^{5}$ M$_odot$. The SFRs span the range of those found in normal local star-forming galaxies to those found in high-redshift star-forming galaxies at $rm{z}=1-3$. The luminosity function of the LIRG clumps has a flatter slope than found in lower-luminosity, star-forming galaxies, indicating a relative excess of luminous star-forming clumps. In order to predict the possible range of star-forming histories and gas fractions, we compare the star-forming clumps to those measured in the MassiveFIRE high-resolution cosmological simulation. The star-forming clumps in MassiveFIRE cover the same range of SFRs and sizes found in the local LIRGs and have total gas fractions that extend from 10 to 90%. If local LIRGs are similar to these simulated galaxies, we expect future observations with ALMA will find a large range of gas fractions, and corresponding star formation efficiencies, among the star-forming clumps in LIRGs.
Strong bursts of star formation in galaxies may be triggered either by internal or external mechanisms. We study the distribution and kinematics of the HI gas in the outer regions of 18 nearby starburst dwarf galaxies, that have accurate star-formation histories from HST observations of resolved stellar populations. We find that starburst dwarfs show a variety of HI morphologies, ranging from heavily disturbed HI distributions with major asymmetries, long filaments, and/or HI-stellar offsets, to lopsided HI distributions with minor asymmetries. We quantify the outer HI asymmetry for both our sample and a control sample of typical dwarf irregulars. Starburst dwarfs have more asymmetric outer HI morphologies than typical irregulars, suggesting that some external mechanism triggered the starburst. Moreover, galaxies hosting an old burst (>100 Myr) have more symmetric HI morphologies than galaxies hosting a young one (<100 Myr), indicating that the former ones probably had enough time to regularize their outer HI distribution since the onset of the burst. We also investigate the nearby environment of these starburst dwarfs and find that most of them ($sim$80$%$) have at least one potential perturber at a projected distance <200 kpc. Our results suggest that the starburst is triggered either by past interactions/mergers between gas-rich dwarfs or by direct gas infall from the IGM.
High-resolution radio observations of nearby starburst galaxies have shown that the distribution of their radio emission consists of a compact (<150 pc), high surface brightness, central radio source immersed in a low surface brightness circumnuclear halo. This radio structure is similar to that detected in bright Seyferts galaxies like NGC 7469 or Mrk 331, which display clear circumnuclear rings. While the compact, centrally located radio emission in these starbursts might be generated by a point-like source (AGN), or by the combined effect of multiple radio supernovae and supernova remnants (e.g., the evolved nuclear starburst in Arp~220), it seems well established that the circumnuclear regions of those objects host an ongoing burst of star-formation (e.g., NGC 7469; Colina et al. 2001, Alberdi et al. 2006). Therefore, high-resolution radio observations of Luminous Infra-Red Galaxies (LIRGs) in our local universe are a powerful tool to probe the dominant dust heating mechanism in their nuclear and circumnuclear regions. In this contribution, we show results obtained from VLA-A, MERLIN, and EVN (VLBI) radio observations of the galaxies NGC 7469 (D~70 Mpc) and IRAS 18293-3413 (D ~ 79 Mpc), where two extremely bright radio supernovae have been found. High-resolution studies of these and other LIRGs would allow us to determine the core-collapse supernova rate in them, as well as their star-formation rate.
We report the source size distribution, as measured by ALMA millimetric continuum imaging, of a sample of 13 AzTEC-selected submillimeter galaxies (SMGs) at z_photo ~ 3-6. Their infrared luminosities and star-formation rates (SFR) are L_IR ~ 2-6 x 10^12 L_sun and ~ 200-600 M_sun yr-1, respectively. The size of z ~ 3-6 SMGs ranges from 0.10 to 0.38 with a median of 0.20+0.03-0.05 (FWHM), corresponding to a median circularized effective radius (Rc,e) of 0.67+0.13-0.14 kpc, comparable to the typical size of the stellar component measured in compact quiescent galaxies at z ~ 2 (cQGs) --- R ~ 1 kpc. The median surface SFR density of our z ~ 3-6 SMGs is 100+42-26 M_sun yr-1 kpc-2, comparable to that seen in local merger-driven (U)LIRGsrather than in extended disk galaxies at low and high redshifts. The discovery of compact starbursts in z >~ 3 SMGs strongly supports a massive galaxy formation scenario wherein z ~ 3-6 SMGs evolve into the compact stellar components of z ~ 2 cQGs. These cQGs are then thought to evolve into the most massive ellipticals in the local Universe, mostly via dry mergers. Our results thus suggest that z >~ 3 SMGs are the likely progenitors of massive local ellipticals, via cQGs, meaning that we can now trace the evolutionary path of the most massive galaxies over a period encompassing ~ 90% of the age of the Universe.
We detect and study the properties of faint radio AGN in Luminous Red Galaxies (LRGs). The LRG sample comprises 760,000 objects from a catalog of LRG photometric redshifts constructed from the Sloan Digital Sky Survey (SDSS) imaging data, and 65,000 LRGs from the SDSS spectroscopic sample. These galaxies have typical 1.4 GHz flux densities in the 10s-100s of microJy, with the contribution from a low-luminosity AGN dominating any contribution from star formation. To probe the radio properties of such faint objects, we employ a stacking technique whereby FIRST survey image cutouts at each optical LRG position are sorted by the parameter of interest and median-combined within bins. We find that median radio luminosity scales with optical luminosity (L_opt) as L_1.4 GHz ~ L_opt^(beta), where beta appears to decrease from beta ~ 1 at z = 0.4 to beta ~ 0 at z = 0.7, a result which could be indicative of AGN cosmic downsizing. We also find that the overall LRG population, which is dominated by low-luminosity AGN, experiences significant cosmic evolution between z = 0.2 and z = 0.7. This implies a considerable increase in total AGN heating for these massive ellipticals with redshift. By matching against the FIRST catalog, we investigate the incidence and properties of LRGs associated with double-lobed (FR I/II) radio galaxies. (Abridged)
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