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Galactic wind in the bulge of the Andromeda galaxy

Massive elliptical galaxies are known to contain large amounts of ionized gas responsible for their high X-ray luminosity. The gas content of less massive galaxies is still a matter of debate. Scientists at the Max Planck Institute for Astrophysics have found ionized X-ray emitting gas in the bulge of the Andromeda galaxy, the closest giant spiral galaxy and "twin sister" of our own. The gas, which mass does not exceed few million solar masses, has the temperature of 3-4 million K and appears to be outflowing from the galaxy at the rate of ~0.1 solar masses per year. The mass and energy budget of this galactic wind is maintained by the mass loss from evolved stars and by the energy input from type Ia Supernovae.

Fig. 1: Soft X-ray image of the inner bulge (~1.5-2 kpc) of the Andromeda galaxy obtained by Chandra X-ray Observatory. The majority of compact sources are accreting neutron stars and stellar mass black holes in binary systems. Unresolved X-ray emission is also clearly visible.

Fig. 2: The brightness distribution of unresolved X-ray emission in the 0.5 - 2 keV energy band along the major axis of the Andromeda galaxy (compact sources removed). The blue and red symbols show the Chandra and XMM-Newton data, green histogram is the 3.6 micron near-infrared brightness distribution from Spitzer. The grey area indicates the amplitude of systematic uncertainties in the X-ray data related to the imperfectness of the instrumental background subtraction. The good overall agreement between X-ray and near-infrared light distributions is apparent, as well as the presence of an excess in the central ~10 arcmin.

Fig. 3: X-ray spectra of the inner and outer bulge of the Andromeda galaxy and of the nearby gas-poor dwarf elliptical M32. The excess emission in the X-ray brightness distribution in Fig.2 reveals itself as a prominent soft component in the inner bulge spectrum. The spectral shape of the excess emission can be approximately described by the emission of optically thin plasma of the temperature of 3-4 million K.

Fig. 4: Composite image of the Andromeda galaxy based on the multi-wavelength data. The DSS optical image (grey) represents the distribution of the stellar light, while the 24 micron data of Spitzer Space Telescope (red) traces the distribution of cold gas and dust in spiral arms and in the 10-kpc star-forming ring. Shown in purple is the soft X-ray emission from warm ionized gas, the other components of X-ray emission have been removed. A striking feature of the morphology of the X-ray emitting gas is that it is elongated along the minor axis, unlike everything else in this galaxy. The gaps in the surface brightness of the gas emission are caused by absorption by the cold material in spiral arms.

The X-ray radiation from the majority of galaxies is dominated by bright compact sources (LX > 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, LX ~ 1027-1034 erg/s, compact sources - accreting white dwarfs and stars with active coronae (linkPfeil.gifResearch Highlight January 2007, linkPfeil.gifResearch 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, tcool ~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 Msun/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


A.Bogdan & M.Gilfanov, 2008, MNRAS, 388, 56 linkPfeilExtern.gifADS

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