GALAXIES, X-RAY EMISSION

Giuseppina Fabbiano

The study of the x-ray emission of normal galaxies is a very recent part of astronomy. This work has been made possible by the sensitive x-ray imaging observations of the Einstein (HEAO 2) satellite, launched by NASA in November 1978. Before then, with the exclusion of the bright x-ray sources associated with Seyfert nuclei, only four galaxies were known to emit x-rays: the Milky Way, M31 (Andromeda), and the Magellanic Clouds. The Einstein satellite observed over 200 galaxies during its 2 1/2-yr life span. Some were detected with enough detail to allow a study of their x-ray morphology, spectra, and individual sources, and to make comparisons with optical, infrared, and radio data. These observations have shown that normal galaxies of all morphological types are spatially extended sources of x-ray emission with luminosities in the range of 10³⁸-10⁴² erg s^-1. Although this is only a small fraction of the total energy output of a galaxy, x-ray observations are uniquely suited to study phenomena that are otherwise elusive. These include the end products of stellar evolution (supernova remnants and compact remnants, such as neutron stars, white dwarfs, and black holes), the hot component of the interstellar medium, and active nuclear regions.

SPIRAL AND IRREGULAR GALAXIES

Normal stars are responsible for the optical emission of galaxies. However, their integrated x-ray emission is only a small fraction of the x-ray emission of a normal galaxy. Observations of the Milky Way and of the Local Group galaxies suggest that a good fraction of the x-ray emission of late-type (spiral and irregular) galaxies is due to a collection of a relatively small number of individual bright sources, such as close accreting binaries with a compact companion, and supernova remnants, with luminosities ranging from ~ 10³⁵ erg s^-1 up to a few times 10³⁸ erg s^-1.

Only a few very bright individual x-ray sources can be detected in the Einstein images of more distant galaxies, which typically appear as extended x-ray emission regions, because at that distance individual sources could not be resolved with the Einstein instruments. However, we believe that the x-ray emission of these galaxies is due to sources akin to those detected in the Local Group. The x-ray spectra of these galaxies are consistent with the hard spectra expected from binary x-ray sources, and the x-ray luminosities are linearly correlated with the emission in the optical B band, suggesting that the x-ray emission is mostly due to sources constituting a constant fraction of the stellar population.

In the pre-Einstein era, the x-ray sources of the Milky Way were classified as young Population I, or spiral arm, sources with massive early-type star counterparts and Population II, or bulge sources, with low-mass stellar counterparts. The Einstein imaging observations of spiral galaxies have led us to modify this classification and to gain new insight into the evolution of binary x-ray sources. We can now identify a ``spiral arm,'' a ``bulge,'' and a ``disk'' component of the x-ray emitting population.

The presence of spiral arm and bulge (and globular cluster) x-ray sources is immediately demonstrated by the x-ray images of nearby galaxies. Bright point sources are detected in the spiral arms of M31 (see Fig. 1) and M33. In particular, one of the M33 sources has a variable light curve that is similar to those of some massive Galactic x-ray binaries. A bulge component of the x-ray emitting population is also evident in M31, these sources have properties similar to those of low-mass x-ray binaries in the Galaxy. Statistical analyses of the sample of spirals observed with Einstein indicate that x-ray bulge emission is present in all bulge-dominated spirals. Disk x-ray sources are suggested by the close resemblance of the radial profile of the x-ray surface brightness of a few face-on spirals (M83, M51, and NGC 6946) with that of the optical light of their exponential disk. This implies that a good fraction of low-mass x-ray binaries may originate from the evolution of binary systems belonging to the disk stellar population, rather than from dynamical evolution (capture of a low-mass companion by a compact object in a dense environment and/or disruption of globular clusters), as has been suggested to explain galactic low-mass binaries.

Figure 1. The circles show the positions of the x-ray sources of M31, superimposed onto an optical photograph. (Courtesy of L. Van Speybroeck. Reproduced, with permission, from the Annual Review of Astronomy and Astrophysics, 27 © 1989 by Annual Reviews, Inc.) Notice the clustering of sources in the bulge.

Some of the sources detected in spiral galaxies have x-ray luminosities well above the Eddington limit for accretion onto a one-solar-mass compact object, which is ~ 1.3 x 10³⁸ erg s^-1. One of these sources is the supernova SN 1980k detected in NGC 6946 approximately 35 days after maximum light. The variability reported for some bright sources in M101 suggests point-like objects, possibly bright accretion binaries. If these sources are truly single objects, they could indicate the presence of massive black holes in these galaxies. It is, however, possible that the distances of some galaxies might have been overestimated, making these sources appear more luminous that they are in reality.

Another source of x-ray emission that has been searched for in spiral galaxies is diffuse thermal emission from a hot phase of the interstellar medium. Supernovae release ~ 10⁴² erg s^-1 in a galaxy, and it has been suggested that hot gaseous coronae, or galactic fountains, could be produced and should be visible in soft x-rays in the Einstein range. There is evidence of soft thermal diffuse emission both in the galactic plane and in the Large Magellanic Cloud (LMC), and perhaps in M33. However, this type of emission has not been detected in more distant galaxies. The lack of intense diffuse soft x-ray emission could imply that most of the supernova energy is radiated in the unobservable far ultraviolet. The only reported instance of this type of soft x-ray emission in a spiral galaxy is in the edge-on galaxy NGC 4631, where this component could have an x-ray luminosity of 5 x 10³⁹ erg s^-1, which represents ~ 13% of the total emission in the Einstein band.

STARBURST GALAXIES AND NUCLEAR OUTFLOWS

Bluer starburst, often interacting, galaxies tend to have enhanced x-ray emission when compared with galaxies with redder, more normal, colors. The bulk of the x-ray emission of these galaxies can be understood in terms of a number of young supernova remnants and massive x-ray binaries, with X-ray luminosity possibly enhanced by the low metallicity of the accreting gas, similar to those observed in the Magellanic Clouds.

There are galaxies in which the starburst activity is confined to the nuclear regions. Starburst nuclei studied in x-rays include the Milky Way galactic center region, and the nuclei of M82, NGC 253, M83, NGC 6946, IC 342, and NGC 3628. A common characteristic of the emission spectrum of these nuclei is their intense far-infrared emission, indicative of dusty nuclear regions heated by newly formed early-type stars. The x-ray emitting regions are extended (whenever they are observed with high enough spatial resolution) and in M82 there is evidence of a population of bright individual sources. To explain this emission requires, in different cases, different amounts of evolved sources (supernova remnants and x-ray binaries) superimposed on the integrated stellar emission from a young stellar population.

An unexpected result of the Einstein observations of these nuclei has been the discovery of extended emission components, suggestive of gaseous bipolar outflows from the nuclear regions, in the edge-on galaxies M82 and NGC 253. These outflows, if generally associated with violent star formation activity, could be responsible for the formation and enrichment of a large part of the gaseous intracluster medium.

ELLIPTICAL AND SO GALAXIES

A hot gaseous component dominates the x-ray emission of x-ray-luminous early-type galaxies. These galaxies can be ~ 100 times brighter in x-rays than spiral galaxies of similar optical magnitude, where the x-ray emission instead is due to evolved stellar sources. These x-ray bright galaxies also tend to have x-ray spectra different from those of binary sources, and sometimes show distortions of their x-ray surface brightness, relative to the optical images (Fig. 2). The latter suggest that the x-ray emission is not due to the stellar population and is consistent with the interaction of the hot galactic halo with a surrounding cluster gas.

Figure 2. The x-ray map of NGC 4472, an elliptical galaxy in the Virgo cluster, obtained with the imaging proportional counter of the Einstein observatory. Notice the asymmetrical halo. (Reproduced, with permission, from the Annual Review of Astronomy and Astrophysics, 27 © 1989 by Annual Reviews, Inc.)

In an x-ray-luminous galaxy with a large hot gaseous halo, the gas is so dense that it will cool in a time shorter than the galaxy's lifetime and then accrete to the galaxy's core, giving rise to ``cooling flows.'' These cooling flows would have interesting consequences: One could be the formation of new stars from matter detaching from the flows; another could be the accretion of gas into the nucleus and the consequent fueling of nuclear sources. There is some evidence of the latter, in that powerful radio sources, connected with nuclear activity, tend to be found in x-ray-bright, gas-rich galaxies. The presence of a hot gaseous halo is also determinant in the formation of extended radio lobes. In the range of radio core power (i.e., of the intensity of the nuclear source) of the radio galaxy Centaurus A, powerful radio lobes more extended than the optical size of the parent galaxy are only found in relatively gas-poor (i.e., x-ray-dim) galaxies.

One of the potentially very important results of x-ray observations of early-type galaxies is the possibility of measuring their masses. The method generally used employs the equation of hydrostatic equilibrium, in which the gas pressure and the gravitational pull balance. Combining this with the ideal gas law, one obtains

M (< r_gas) = -[(r_gas kT_gas) / (Gµm_H)] [(d log _gas) / (d log r) + (d log T_gas) / (d log r)]

Four quantities must therefore be measured to determine the mass within a certain radius r_gas: the radius itself, the temperature T_gasT at that radius, and the temperature and gas-density (_gas) gradients at that radius. The uncertainty in the mass measurement will reflect the uncertainties in the determination of these quantities. Applied to M87, the dominant galaxy at the center of the Virgo cluster, this method reveals a large amount of dark matter. However, when this method is applied to more normal early-type galaxies, which are more than 100 times less x-ray-luminous that M87, the uncertainties are very large and the presence of large dark halos cannot be demonstrated firmly, although it is suggested in some cases. X-ray measurements with the German satellite ROSAT (launched June 1,1990) and with the future satellites AXAF and XMM will allow accurate mass determinations in the x-ray-bright early-type galaxies. In the case of less x-ray-luminous galaxies it will have to be established first that the x-ray emission is due to a gaseous halo and not to a collection of binary x-ray sources. It will also be important to consider the effect of supernovae on the energy balance of the halo.

GALAXIES AND THE X-RAY BACKGROUND

The extragalactic x-ray background was discovered by Riccardo Giacconi, Herbert Gursky, Frank Paolini, and Bruno Rossi in 1962 in data from the same rocket flight that led to the discovery of the first extrasolar source of x-rays, Sco X-1. Since then a great deal of effort has been spent to determine if this radiation is due to the integrated contributions of different classes of discrete sources or if diffuse processes are responsible for it. The integrated emission of normal galaxies could explain ~ 13% of the 2-keV extragalactic x-ray background. If one includes in this estimate the contribution of low-activity nuclei present in a fraction of these galaxies, the effect of starburst activity, and even more the possibility that these types of activities were more pronounced in the past, this contribution could be significantly larger.

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6. See also Background Radiation, X-Ray; Clusters of Galaxies, X-Ray Observations; Galaxies, Starburst; X-Ray Sources, Galactic Distribution.