The origin of the hot metal-poor gas in NGC1291: Testing the hypothesis of gas dynamics as the cause of the gas heating

摘要

In this paper we test the idea that the low-metallicity hot gas in the centreof NGC 1291 is heated via a dynamical process. In this scenario, the gas fromthe outer gas-rich ring loses energy through bar-driven shocks and falls to thecentre. Heating of the gas to X-ray temperatures comes from the high velocitythat it reaches ($\approx$ 700 \kms) as it falls to the bottom of the potentialwell. This would explain why the stellar metallicity in the bulge region isaround solar while the hot gas metallicity is around 0.1 solar. We carried outan observational test to check this hypothesis by measuring the metallicity ofHII regions in the outer ring to check whether they matched the hot gasmetallicity. For this purpose we obtained medium resolution long slitspectroscopy with FORS1 on the ESO VLT at Paranal and obtained themetallicities using emission line ratio diagnostics. The obtained metallicitiesare compatible with the bulge stellar metallicities but very different from thehot-gas metallicity. However, when comparing the different time-scales, the gasin the ring had time enough to get enriched through stellar processes,therefore we cannot rule out the dynamical mechanism as the heating process ofthe gas. However, the blue colours of the outer ring and the dust structures inthe bar region could suggest that the origin of the X-ray hot gas is due to theinfall of material from further out.