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, Astron. J. 125, 2455, 2003).   I shall present the results of high-sensitivity VLA observations of UGC7321, IC2233, UGC3697 and UGC10043. Two of these galaxies are rather isolated while the other two are the dominant members of small groups.

We have detected a pattern of "corrugations" in IC 2233, the first such detection in neutral Hydrogen in an external galaxy. UGC 3697, the "Integral Sign" galaxy shows a spectacular S-shaped bending that has been triggered by the passage of an intruder through its disk. UGC 10043, not quite a superthin galaxy, has presented us with an unexpected, rich interaction with a previously unknown companion which is reminiscent of the M51 system (the Whirlpool galaxy) "seen from the side."

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 Halpha emission line, limiting the strength of the local UV background below theoretical expectations (Adams et al., Astrophys. J., 728, 107, 2011).  New, observations (March 2011) have improved the sensitivity significantly.  The Halpha layer shows a peak brightness of 1.0 x 10E-19 erg s-1 cm-2 arcsec-2 A-1 (~7sigma)  for spectra smoothed with a 15¥¥ spatial kernel.  This leads to a measurement of the cosmic UV background induced HI photoionization rate of 2.0 x 10-14 s-1 (~7sigma, 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.

02/02/2017 - 12:00
Prof. Juan Usón