Galaxy formation and evolution

Abstract:

We explore the gas-to-dust mass ratio (M gas/M d) and the CO luminosity-to-M gas conversion factor (αCO) of two well-studied galaxies in the Great Observatories Origins Deep Survey North field that are expected to have different star-forming modes, the starburst GN20 at z = 4.05 and the normal star-forming galaxy BzK-21000 at z = 1.52. Detailed sampling is available for their Rayleigh-Jeans emission via ground-based millimeter (mm) interferometry (1.1-6.6mm) along with Herschel PACS and SPIRE data that probe the peak of their infrared emission. Using the physically motivated Draine & Li models, as well as a modified blackbody function, we measure the dust mass (M dust) of the sources and find (2.0+0.7-0.6 × 109) M ∞ for GN20 and (8.6+0.6-0.9 × 108) M ∞ for BzK-21000. The addition of mm data reduces the uncertainties of the derived M dust by a factor of ∼2, allowing the use of the local M gas/M d versus metallicity relation to place constraints on the αCO values of the two sources. For GN20 we derive a conversion factor of αCO < 1.0 M ∞ pc-2(Kkms-1)-1, consistent with that of local ultra-luminous infrared galaxies, while for BzK-21000 we find a considerably higher value, αCO ∼4.0 M ∞ pc-2(Kkms-1)-1, in agreement with an independent kinematic derivation reported previously. The implied star formation efficiency is ∼25 L ∞/M ∞ for BzK-21000, a factor of ∼5-10 lower than that of GN20. The findings for these two sources support the existence of different disk-like and starburst star formation modes in distant galaxies, although a larger sample is required to draw statistically robust results. © 2011. The American Astronomical Society. All rights reserved.

Abstract:

We present observations of Arp 147, a galaxy system comprising a collisionally created ring galaxy and an early-type galaxy, using the Oxford SWIFT integral field spectrograph (IFS) at the 200-inch Hale telescope. We derive spatially resolved kinematics from the IFS data and use these to study the interaction between the two galaxies. We find the edge-to-edge expansion velocity of the ring is 225 ± 8kms-1, implying an upper limit on the time-scale for the collision of 50Myr. We also calculate that the angle of impact for the collision is between, where 0° would imply a perpendicular collision. The ring galaxy is strongly star forming with the star formation likely to have been triggered by the collision between the two galaxies. We also measure some key physical parameters in an integrated and spatially resolved manner for the ring galaxy. Using the observed B-I colours and the Hα equivalent widths, we conclude that two stellar components (a young and an old population) are required everywhere in the ring to simultaneously match both observed quantities. We are able to constrain the age range, light and mass fractions of the young star formation activity in the ring, finding a modest age range, a light fraction of less than a third, and a negligible (<1 per cent) mass fraction. We postulate that the redder colours observed in the south-east corner of the ring galaxy could correspond to the nuclear bulge of the original disc galaxy from which the ring was created, consistent with the stellar mass in the south-east quadrant being 30-50 per cent of the total. The ring appears to have been a typical disc galaxy prior to the encounter. The ring shows electron densities consistent with typical values for star-forming Hii regions. The eastern half of the ring exhibits a metallicity a factor of ~2 higher than the western half. The ionization parameter, measured across the ring, roughly follows the previously observed trend with metallicity. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.