The Interstellar Sight-Line Towards the M33 Galaxy

Abstract

We present medium resolution ultraviolet absorption measurements recorded with the HST-COS and FUSE spectrographs towards the Wolf-Rayet star UIT-236 belonging to the BCLMP 99 HII region in the M33 galaxy. We have detected at least 5 major absorption systems along the 840 kpc sight-line to UIT-236 with average gas cloud LSR velocities of -205, -170, -135, -32, and +3 km s-1. The three most negative velocity cloud components, which we associate with gas in the M33 galaxy, have been detected in the line profiles of CII, CII∗,CIII, NI, NII, OI, OVI, SiII, SiII∗, SiIII, SiIV, PII, SII, ArI, FeII, and FeIII ions. These profiles have been fit with absorption models to determine ion column densities for each cloud component. For the case of highly ionized gas components, the observed column density ratios of OVI/SiIV are most compatible with the predictions of ionization by turbulent mixing layers or in a shock driven medium. A possible formation scenario is that the SiIV is formed within a photo-ionized stellar wind bubble that is being driven into surrounding gas that collisionally produces the observed OVI ions. Using the column density ratios of the CII/CII∗ lines, we have determined electron density values in the range 1.8–6.0 cm-3 for the ionized M33 gas clouds. Such values are similar to those found for other ionized interstellar bubbles such as N51D (in the Large Magellanic Cloud), γ2 Vel, and NGC 2244 (the Rosette Nebula). We have carried out a gas phase element abundance analysis of the three cloud components, indicating that C, N, O, Si, P, Ar, Fe, and Ni are all depleted relative to their solar abundance values. This pattern is confirmed by the observed N(OI)/N(HI) column density ratios which suggest a low metallicity for all three of the neutral gas components. The presence of interstellar shocks and expanding supershells of gas driven by stellar wind outflows from stars like UIT-236 may well account for the element depletion values observed along the M33 sight-line.