The X-ray radiation from the majority of galaxies is dominated by bright compact sources (L_X > 10³⁶ erg/s) - accreting neutron stars and black holes in binary stellar systems. In addition, unresolved X-ray emission is present in galaxies of all morphological types. It was revealed earlier by scientists at MPA that a part of this emission is a superposition of a large number of much fainter, L_X ~ 10²⁷-10³⁴ erg/s, compact sources - accreting white dwarfs and stars with active coronae (Research Highlight January 2007, Research Highlight March 2006). For example, collective emission of faint sources explains the most, if not all, of the X-ray emission of the Ridge of our Galaxy. In many galaxies genuine diffuse emission is also present which originates from the hot ionized inter-stellar medium (ISM). The amount and importance of the X-ray emitting gas varies from galaxy to galaxy, generally increasing with its mass. Luminous gas-rich massive ellipticals and nearly gas-free dwarf galaxies represent two opposite ends of the range.

The Andromeda galaxy, our close-by neighbor, gives a unique opportunity to explore a full-size spiral galaxy similar to our own without complications brought about by projection effects and absorption. Its 780 kpc distance and extensive coverage by the two major X-ray observatories - Chandra and XMM-Newton and by the Spitzer Space Telescope made it possible to disentangle different components of X-ray emission and to study them in great detail. The superb angular resolution of Chandra X-ray Observatory permitted to locate and isolate contribution of accreting neutron stars and black holes (Fig.1). After it had been removed, MPA scientists have found a good overall agreement in the morphology and spatial distribution of the remaining unresolved X-ray emission and stellar mass measured through the 3.6 micron near-infrared radiation (Fig.2). Remarkably, the X-ray-to-NIR luminosity ratios are consistent with the Milky Way values. These findings suggest that the bulk of the unresolved X-ray emission in the Andromeda galaxy has the same origin as the Galactic Ridge emission and is produced by a large number of faint compact sources of stellar nature.

This simple picture breaks down in the central ~10-15 arcmin (~2-3 kpc) of the galaxy, where significant excess emission is present in the soft band (Fig.2). The excess emission also reveals itself very prominently as a strong soft component in the X-ray spectrum (Fig.3). The morphology and spectral characteristics suggest that it is associated with warm ionized ISM of the temperature of ~3-4 million K. Interestingly, there is an evidence of non-solar abundance of metals and/or non-equilibrium ionization state of the ISM. The total gas mass is ~few million solar masses with the typical density of ~0.01 particle per cm³. With these parameters, the cooling time of the gas, t_cool ~250 Myrs, is significantly shorter than the life time of the galaxy. As such gas is thermally unstable it can not be in a stationary state in hydrostatic equilibrium in the gravitational potential of the galaxy. On the other hand, the mass loss from evolved stars in the bulge of Andromeda galaxy, ~0.1 M_sun/year, is sufficient to double the gas mass on the time scale of ~35 Myrs, much shorter that the cooling time. This suggests that the gas outflows from the galaxy at the rate, defined by the mass loss rate from the evolved stars, i.e. ~0.1 solar masses per year. The outflow can be powered by type Ia Supernovae which may provide upto ~ 10⁴⁰ erg/s of mechanical energy into the ISM. This is sufficient to heat the gas and to lift it in the gravitational potential of the bulge.

A striking feature of the morphology of the X-ray emitting gas (Fig.4) is that it is elongated along the minor axis, unlike everything else in this galaxy. This suggests that the outflow proceeds predominantly in the direction perpendicular to the disk of the galaxy. The additional asymmetry of the X-ray brightness distribution, in particular the gap seen to the north-west of the nucleus is caused by absorption in spiral arms. The Andromeda galaxy has an inclination angle of 77 degrees with the western side of the galactic disk being closer to us. The ionized gas on this side of the bulge is seen through the spiral arms, therefore the neutral gas and dust in the star forming regions absorb soft X-rays casting a shadow on the ISM emission. This does not happen on the eastern side of the disk, which is located behind the bulge and does not obscure the gas emission. The fact that the spiral arms and 10-kpc star-forming ring do cast a shadow on the ionized gas places a lower limit on its vertical off-plane extent, greater than 2.5 kpc.

Akos Bogdan and Marat Gilfanov

Publications

A.Bogdan & M.Gilfanov, 2008, MNRAS, 388, 56 ADS