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Date and Time of the Query: 2018-12-13 T10:37:04 PST
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For refcode 2000AJ....120.1306L:
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Copyright by American Astronomical Society. Reproduced by permission
2000AJ....120.1306L THE H II REGIONS OF THE EXTREME OUTER DISK OF NGC 628 MARIO LELIHVRE AND JEAN-RENI ROY Dipartement de Physique and Observatoire du Mont Megantic, Universite Laval, Quebec, QC G1K 7P4, Canada Received 1999 September 22; accepted 2000 June 2 ABSTRACT Employing deep H{alpha} narrowband imagery, we identified and measured the fluxes of 137 small H II regions in the extreme outer disk (R >= R_25_) of the nearly face-on, Sc I galaxy NGC 628. A majority of these H II regions are located in two faint, outer spiral arms visible in H I maps. The faintest H II regions that could be measured have fluxes of only a few times 10^-16^ ergs cm^-2^ s^-1^, which correspond to luminosities of ~10^36^ ergs s^-1^, or a fraction of the Orion Nebula luminosity. The most distant object detected is at R ~ 27 kpc from the galaxy center. The massive star formation rate, as measured from the azimuthally averaged H{alpha} flux, is consistent with a monotonic decrease as far as R ~ 1.3R25 (20 kpc), where there is a sharp drop. The Schmidt law for the whole disk of NGC 628 corresponds to a relatively steep power law with n = 2.9 +/- 0.2, but it "fails" below {SIGMA}_gas_ <= 4 M_sun_ pc^-2^; the index depends strongly on the CO data used. We derived the luminosity function (LF) of 598 H II regions over the whole disk, and we compared the shapes of the cumulative LFs between R >= R_25_ and R < R_25_; the LF of the outer regions is significantly steeper than that of the inner regions. Key words: galaxies: individual (NGC 628)-galaxies: ISM-H II regions-stars: formation
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