Effects of galactic winds on dIrrs with particular emphasis on NGC1569 and the refill of superbubble cavities
- Author(s)
- Simone Recchi, Gerhard Hensler
- Abstract
NGC1569 is a dwarf irregular galaxy located near the Local Group. Although young stars dominate its light, Hubble Space Telescope observations revealed the presence of older stars (Vallenari & Bomans 1996 A&A, 313, 713; Greggio et al. 1998 ApJ, 504, 725; Angeretti et al. 2005 AJ, 129, 2203). A diffuse X-ray emission (Heckman et al. 1995 ApJ, 448, 98; Ott, Walter & Brinks 2005 MNRAS, 358, 1423) indicates the presence of outflowing hot gas, whose metallicity has also been estimated (Martin, Kobulnicky & Heckman 2002 ApJ, 574, 663) and is larger than the metallicity of the HII regions. The chemical and dynamical evolution of model galaxies resembling NGC1569 has been studied. We have considered either one episode of star formation (SF) of short duration (bursting SF), or more complex behaviours, in which the galaxies have experienced long-lasting episodes of SF in the past, separated by short periods of inactivity (gasping SF). We also have considered the infall of big clouds of gas towards the disk, assuming that the complex of HI clumps and clouds observed by Mušhle et al. (2005, submitted) is the result of a tidally disrupted infailing cloud. Models with a bursting SF are generally unable to account for the chemical and morphological properties of this object, since they either severely underproduce oxygen, or inject too much energy into the system, enough to unbind a too large fraction of the gas initially present in the galaxy. The best way to reproduce the chemical composition of NGC1569 is therefore assuming a gasping regime of SF. Adopting the SF prescriptions derived from the comparison of the color-magnitude diagrams with synthetic ones (Angeretti et al. 2005) we produce results in good agreement with the observations. In the models with gasping SF, the final chemical composition of the galaxy reflects mostly the chemical enrichment from old stellar populations. In fact, if the first episodes of SF are powerful enough to create a galactic wind or to heat up a large fraction of the gas surrounding the star forming region, the metals produced by the last burst of SF are released in a too hot medium to be observed or are directly ejected from the galaxy through the wind. They do not have the chance to pollute the surrounding medium and contribute to the chemical enrichment of the galaxy. Therefore, little or no variations of the chemical composition of the galaxy happen after the last burst. Since most of the metal-poor DGs seem to have similar (N/O) abundance ratios (Izotov & Thuan 1999 ApJ, 511, 639), the gasping regime of SF seems more plausible. The bursting SF regime would in fact produce a larger scatter in this plot. This result depends crucially on the assumption that the quiescence period between the last burst of SF and the previous old episodes is short enough. In this way, SNeIa from the last generations of stars provide enough energy to heat up a significant fraction of gas and sustain the outflow. In our simulations the rate of SNeIa is calculated according to the analytical work of Matteucci & Recchi (2001 ApJ, 558, 351). For a short episode of SF, it peaks at ~ 100 Myr. After this maximum, it declines proportionally to ~ t-1.8. After some time, the energy input from SNeIa drops below the power required to overcome the gravitational potential of the galaxy and some cold gas starts flowing towards the center of the galaxy. At this point the size of the cavity of hot gas is extremely reduced and no significant outflow is present anymore in the galaxy. Any further SF episode would therefore occur in a dense and cold environment and cooling of freshly produced metals would be very likely (as showed by Recchi et al. 2002 A&A, 384, 799). It is therefore interesting to study what is the SF gap required to refill the cavity with cold gas and ensure a fast mixing of metals produced in the last episode of SF. We run two...
- Organisation(s)
- Department of Astrophysics
- Journal
- Astronomische Nachrichten
- Volume
- 326
- Pages
- 508
- ISSN
- 0004-6337
- Publication date
- 2005
- Austrian Fields of Science 2012
- 103003 Astronomy
- Portal url
- https://ucrisportal.univie.ac.at/en/publications/8b659ee1-2997-49b8-b535-048d9ec0e7fe