Seminar
Living on the Edge: Superthin Galaxies and the Cosmic UV Background

Superthin galaxies are bulgeless, late-type spiral galaxies seen edge-on.  HI synthesis observations probe the kinematic structure of their interstellar medium.  Observations of these isolated, quiescent galaxies have reached column densities as low as few x 10¹⁸  atoms . cm^-2 .  The simple structure of the superthins makes them ideal cosmological laboratories (Uson and Matthews 2003). The strength of the cosmic UV background has a strong influence on the formation of structure in the Universe, from the inhibition of the collapse of small haloes to the ionizing escape fraction in galaxies to the global star formation history.  We have used the VIRUS-P integral-field spectrometer on the University of Texas McDonald Observatory 2.7m telescope to observe the edge of the superthin galaxies UGC7321 and UGC1281 in the Hα emission line, limiting the strength of the local UV background below theoretical expectations (Adams et al., 2011).  New,  observations (March 2011) have improved the sensitivity significantly.  The Hα layer shows a peak brightness of  Σ = 1.0 x 10^-19 erg s^-1 cm^-2 arcsec^-2 Å^-1 (~7σ)  for spectra smoothed with a 15″ spatial kernel.  This leads to a measurement of the cosmic UV background induced HI photoionization rate Γ = 2.0 x 10^-14 s^-1 (~7σ, preliminary absolute calibration, Uson et al, BAAS 44, 312-01, 2012).  Contrary to past observational attempts, our measurements covered a large, two-dimensional on-sky area. We reach flux limits that are ~50 times fainter than the sky background with significant smoothing over spatial elements and a sky background model that accounts for variations in the spectral resolution of our instrument.