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The H [CSC]ii[/CSC] Regions of the Extreme Outer Disk of NGC 628

2000; Institute of Physics; Volume: 120; Issue: 3 Linguagem: Inglês

10.1086/301537

1538-3881

Mario Lelièvre, Jean-René Roy,

Stellar, planetary, and galactic studies

Employing deep Hα narrowband imagery, we identified and measured the fluxes of 137 small H II regions in the extreme outer disk (R ≥ R25) 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 ∼1036 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 Hα 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 Σgas ≤ 4 M⊙ 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 ≥ R25 and R < R25; the LF of the outer regions is significantly steeper than that of the inner regions.