Annu. Rev. Astron. Astrophys. 1989. 27: 87-138
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3.1. Widespread Starburst Activity in Peculiar Galaxies

It was mentioned in the previous section that bluer "starburst," often-interacting galaxies tend to have enhanced X-ray emission compared with galaxies having redder, more normal colors (Fabbiano et al. 1982, 1984b, Stewart et al. 1982). The X-ray emission of these galaxies tends to originate from spatially extended regions, excluding a purely nonthermal nuclear origin, and their X-ray spectra exclude on average very soft emission, which suggests that the X-ray emission is not dominated by the thermal emission of a gaseous halo (Fabbiano et al. 1982). There are, however, exceptions to this second statement: The interacting pair NGC 4038 / 9 (the Antennae) possibly has a softer component of the X-ray emission that is of gaseous origin (Fabbiano et al. 1982, Fabbiano & Trinchieri 1983), and a similar component has been suggested in Arp 220 (Eales & Arnaud 1988); gaseous emission has been suggested to occur in ring galaxies (Ghigo et al. 1983), although there is no proof of its existence; and such emission has been reported in spiral-rich compact groups (Bahcall et al. 1984). Detailed observations of nearby galaxies with starburst nuclei show extended gaseous components emanating from the nuclear regions (e.g. Watson et al. 1984, Fabbiano & Trinchieri 1984, Fabbiano 1988b).

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 (Fabbiano et al. 1982, Stewart et al. 1982). Although this explanation is not applicable in general (Fabbiano et al. 1982), the integrated coronal emission of the young stellar population [see Vaiana et al. (1981) for typical values] may dominate in very young starbursts, where the ultraviolet IUE spectra suggest the presence of a very large number of OB stars (Moorwood & Glass 1982, Fabbiano & Panagia 1983). Recently, Ward (1988) has reported a correlation between the Brackett-gamma line emission from a sample of starburst nuclei and their X-ray emission, which is interpreted in terms of a relationship between the number of ionizing photons produced by the OB stars and the associated X-ray binary population, as estimated by Fabbiano et al. (1982). These authors, by comparing the observed radio and X-ray luminosities of their sample of peculiar galaxies with the expected output of a population of supernova remnants and X-ray binaries, also point out that the nonthermal radio emission from supernova remnants is not likely to account for the entire observed radio power, and thus that a different radio emission mechanism is required, as had already been observed by Biermann & Friecke (1977) in their study of Markarian galaxies.

3.2. Starburst Nuclei

There are galaxies in which the starburst activity is confined to the nuclear regions. The first reported instance of X-ray emission from this type of nucleus is that of NGC 7714 by Weedman et al. (1981), who associate this emission with the type of activity discussed above, as opposed to Seyfert-type nonthermal activity. A number of starburst nuclei, often embedded in an otherwise normal spiral galaxy, have been studied in X rays. They include the Galactic center region (Watson et al. 1981), M82 (Van Speybroeck & Bechtold 1981, Watson et al. 1984, Biermann 1984, Kronberg et al. 1985, Schaaf et al. 1989, Fabbiano 1988b), NGC 253 (Van Speybroeck & Bechtold 1981, Fabbiano & Trinchieri 1984, Fabbiano 1988b), M83 (Trinchieri et al. 1985), M51 (this nucleus also contains a Seyfert component, which does not dominate the X-ray emission; Palumbo et al. 1985), NGC 6946 and IC 342 (Fabbiano & Trinchieri 1987), and NGC 3628 (G. Fabbiano et al., in preparation, 1989). A common characteristic of the emission spectra of these nuclei is the intense far-infrared component (see Figure 9 of Fabbiano & Trinchieri 1987, and references therein), indicative of dusty nuclear regions heated by newly formed early-type stars. The X-ray-emitting regions are seen to be extended whenever observed with high-enough spatial resolution, and in M82 there is evidence of a population of bright individual sources (Watson et al. 1984). Typical X-ray luminosities of these nuclear regions are in the 1039 erg s-1 range, except for the Galactic center region, which is ~ 1000 times less luminous. 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. The X-ray spectra of two of these nuclei, NGC 253 and M82 (Fabbiano 1988b, Schaaf et al. 1989), are intrinsically absorbed, consistent with the presence of a dusty emission region. In M82, in particular, it is possible that two different spectral components are present: a softer one, possibly dominated by the newly formed stars and by the interstellar medium shock heated by the frequent supernovae; and a harder one, which could be due to either binary X-ray sources with large intrinsic absorption cutoff or inverse Compton emission resulting from the interaction of the infrared photons in the nucleus with the relativistic electrons responsible for the radio emission. The latter mechanism was invoked by Kruegel et al. (1983) to explain the apparent excess X-ray luminosity of M82 when compared with NGC 253, but it had been dismissed by Watson et al. (1984; see also Fabbiano & Trinchieri 1984) on the basis of a spatial comparison of the X-ray, radio, and far-infrared emission. Schaaf et al. (1989), however, argue that it cannot be excluded that inverse Compton effects are responsible for a sizable fraction of the X-ray emission. It is clear that future observations at higher spectral resolution will be essential for distinguishing thermal and nonthermal emission components.

Line-of-sight extinction affects differently the optical B-band and the X-ray emission in the Einstein band, depending on the "hardness" of the X-ray spectrum. Using this effect, Trinchieri et al. (1985) constrained the maximum allowable extinction in the nuclear region of M83 by comparing the observed X-ray to optical flux ratio with the expected ratio for early-type stars. Any other assumption on the source of X-ray emission would lead to a smaller upper bound on AV. The limits on the extinction thus calculated for M83 and for IC 342 (Fabbiano & Trinchieri 1987) are of order AV < 13 mag, typically smaller than the mid-infrared estimates [35 mag in M83 (Lebofsky & Rieke 1979) and 15 mag in IC 342 (Becklin et al. 1980)]. This result suggests a nonhomogeneous distribution of the dust in the nuclear region, in agreement with the picture of Becklin et al. (1980), based on a similar discrepancy between the 10-µm infrared extinction and that estimated to the stars seen in the near-infrared.

Perhaps the most unexpected result from the Einstein observations of these nuclei has been the discovery of extended emission components, suggestive of gaseous outflows from the nuclear regions, in the edge-on galaxies M82, NGC 253 (Figure 4), and (more recently) NGC 3628. In M82, a correspondence between the region of extended X-ray emission (~ 90" radially from the nucleus) seen in the Einstein high-resolution imager (HRI) and the Halpha filaments was first noticed by Van Speybroeck & Bechtold (1981). Watson et al. (1984) then suggested that this X-ray "halo" is likely to be thermal emission of shock-heated gas escaping the nuclear region. They point out that only 2% of the energy released by supernovae in the nucleus, exploding at a rate of 0.2 yr-1 over a time scale of 107 yr, is needed to heat the gas to X-ray temperatures. In NGC 253 the presence of an extended source, positionally coincident with a region of noncircular motions (Demoulin & Burbidge 1970), was also first reported by Van Speybroeck & Bechtold (1981). Fabbiano & Trinchieri (1984) studied this nuclear region and identified both an emission region associated with the starburst nucleus proper and a "jet-like" feature, or "plume" of emission, extending for ~ 60" along the southern minor axis. They suggested that the latter feature could be due to a bipolar nuclear outflow, similar to the one seen in M82, collimated by the galaxy disk and seen in projection along the minor axis. Fabbiano & Trinchieri further proposed that the northern side of the outflow would not be visible because the soft X-ray photons would be absorbed by the interstellar medium in the disk of NGC 253. The subsequent report of an OH line emission plume from the dusty northern side (Turner 1985) confirms this picture. Optical work on the emission line gas velocity fields in these two galaxies is also in agreement with the proposed gaseous outflows (McCarthy et al. 1987, Bland & Tully 1988). Theoretical models of this phenomenon have been offered by Chevalier & Clegg (1985) and Tomisaka & Ikeuchi (1988). Analyzing the lower resolution, but more sensitive, Einstein Imaging Proportional Counter (IPC) images of these two galaxies, Fabbiano (1988b) found evidence of diffuse X-ray emission at large radii in the northern side of NGC 253, which could be related to the nuclear outflow; in M82, on the other hand, there is clear evidence of an X-ray halo that is elongated along the minor axis and extends as far as ~ 9 kpc from the nucleus (see also Kronberg et al. 1985). This halo is not likely to be bound to the galaxy, and the hot gas may be leaving the system at a rate that could be as high as 0.7 Msun yr-1. These estimates are now uncertain, since neither the gas volume filling factor nor its emission temperature is really known. However, taken at face value, they would imply a maximum lifetime of 7 × 108 yr for the starburst, since the mass present in the nuclear region is ~ 5 × 108 Msun (Rieke et al. 1980), unless either the outflowing gas, cooling at large radii, flows in again in a galactic fountain or fresh gas from the intergalactic medium flows in to fuel the nucleus. Relatively short lifetimes (< 2 × 109 yr) are also suggested by the OH data for the starburst at the nucleus of NGC 253 (Turner 1985).

Figure 4

Figure 4. The Einstein HRI contour map or NGC 253, smoothed with a 7" Gaussian. The insert shows a higher resolution map of the nuclear region. Here the shaded oval represents the nuclear starburst region seen in the radio and infrared. A plume of X-ray emission can be seen extending along the southern minor axis, and this was interpreted as evidence of hot outflowing gas (see Fabbiano & Trinchieri 1984, and references therein).

Fabbiano (1984) remarked that these gaseous outflows should be visible in X rays in many galaxies, since starburst or low-activity nuclei are quite common (Keel 1983). With the present data it is impossible to distinguish them from the underlying disk emission in face-on galaxies such as M83 and M51 (Trinchieri et al. 1985, Palumbo et al. 1985); they should, however, be obvious in edge-on galaxies. Very recently, a reanalysis of the Einstein data of the edge-on galaxy NGC 3628 in different energy bands has shown an elongated soft emission region associated with the nucleus, suggestive of this phenomenon. The presence of a gaseous plume has been confirmed by subsequent optical observations (G. Fabbiano et al., in preparation, 1989). Different authors have pointed out that 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 (Heckman et al. 1987, Fabbiano 1988b). In particular, if a relatively small galaxy like M82 can expel of the order of 1 Msun yr-1, a primordial large elliptical system could expel 1000 times this amount. Therefore, some 1000 such systems in a cluster, undergoing violent star formation over a period of 108 yr, could produce the 1014 Msun of gas that are now found in clusters of galaxies (Jones & Forman 1984). This type of scenario has been modeled by Mathews (1988a).

3.3. Low-Activity Nuclei

The sample of "normal" spiral galaxies observed with the Einstein satellite was selected to exclude known Seyfert nuclei, but some of these galaxies host nuclei with low-level activity not directly related to star formation. These galaxies include M51, whose nuclear region is, however, extended in X rays and therefore dominated by a starburst or by a hot gaseous component (Palumbo et al. 1985); M81, which hosts a small Seyfert nucleus (Peimbert & Torres-Peimbert 1981, Shuder & Osterbrock 1981) detected in X rays as a pointlike source (Elvis & Van Speybroeck 1982, Fabbiano 1988a); and two more galaxies, M33 and NGC 1313, for which the evidence of nuclear activity rests only on the X-ray data (Long et al. 1981b, Markert & Rallis 1983, Gottwald et al. 1987, Fabbiano & Trinchieri 1987, Trinchieri et al. 1988, Peres et al. 1989). A bright nuclear region (LX ~ 1.5 × 1040 erg s-1) was also observed in M100; however, this nucleus appears extended or complex (Palumbo et al. 1981). Other galaxies with relatively low-luminosity nuclei, although more luminous than the ones just mentioned, were surveyed to study their nuclear emission (e.g. Maccacaro & Perola 1981, Maccacaro et al. 1982). In at least one of these, the X-ray emission is extended and therefore not dominated by the nucleus [NGC 1365 (Maccacaro et al. 1982)].

The nucleus of M81 appears as an unresolved source in the Einstein images, with a luminosity LX ~ 1.6 × 1040 erg s-1 (Elvis & Van Speybroeck 1982, Fabbiano 1988b). Variability of this source in the 2-10 keV range over a 5-month time scale has been reported by Schaaf et al. (1989). Fabbiano (1988b) reports that the Einstein IPC spectrum appears to be soft and intrinsically absorbed. This X-ray spectrum is reminiscent of the soft spectral components reported in QSO and bright active nuclei (e.g. Elvis et al. 1985, Arnaud et al. 1985, Pounds et al. 1987, Wilkes & Elvis 1987); an extrapolation of this spectrum to the UV suggests that there is enough photoionizing continuum to excite the optical emission lines, whereas this continuum would be missing for a more conventional (alphaE ~ 0.7) spectral power law (Bruzual et al. 1982). By using an accretion disk model (Bechtold et al. 1987, Czerny & Elvis 1987) to interpret this emission, one can constrain the mass of the central black hole to the range leq 104 - 5 Msun and the accretion rates to values leq 10- (4 - 5) Msun yr-1 (Fabbiano 1988b). This nucleus could therefore be just a low-luminosity specimen of a normal active galactic nucleus. The nucleus of M33 and the unresolved source in NGC 1313 are a factor of 10 less luminous than the nucleus of M81 and have similar spectral characteristics, within the fitting uncertainties (Trinchieri et al. 1988, but see also Gottwald et al. 1987, Fabbiano & Trinchieri 1987). The nucleus of M33 is also variable in X rays (Markert & Rallis 1983), most dramatically in the soft X-ray band (Peres et al. 1989). However, very little or no indication of activity is seen in optical and radio observations of these last two nuclei (von Kapp-her et al. 1978, Glass 1981, Gallagher et al. 1982, O'Connell 1983, Rubin & Ford 1986, J. Gallagher, private communication, 1987). One could speculate (Fabbiano 1988b) that these nuclei might represent a new type of source, possibly the radio-quiet counterpart of sources like the nucleus of 3C 264, which similarly shows no sign of optical activity (Elvis et al. 1981, Fabbiano et al. 1984a). If this is so, it is quite appropriate for these sources to be found in spiral galaxies, in analogy with what is observed in radio-loud and radio-quiet optically active nuclei (Miller 1985).

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