AN X-RAY SURVEY OF LOCAL GROUP GALAXIES
NASA/Goddard Space Flight Center, email@example.com
We have proposed a systematic cataloging of the X-ray source population in Local Group galaxies. Most Local Group galaxies, dominated in number by dwarf spheroidals and irregulars, have never been observed in X-rays. The Local Group dwarfs offer a unique, easily studied sample of the majority of the galaxies in the universe. While their X-ray luminosities are likely to be low, (mostly erg s), it is possible in some of these galaxies to detect individual sources to L as small as erg s. For the gas-rich galaxies, diffuse emission might also be observed. Acquisition of meaningful data from the complete sample of Local Group galaxies requires about 1 million seconds of HRI time.
The Local Group consists of about 30 galaxies within 1 Mpc of the Milky Way (van den Bergh 1994 - see Table 1). Aside form the three prominent spiral galaxies, the Milky Way, M31 and M33, its population consists primarily of low mass irregular and dwarf spheroidal (DSph) galaxies. The majority of these are gas poor. Their properties are summarized in Gallagher & Wyse 1995 (GW95).
Study of the Local Group galaxies has the advantage of proximity, allowing sensitive studies of their composition and dynamics; even detection of DSphs outside the Local Group is difficult. Van den Bergh (1992) has pointed out that as these galaxies have converted virtually all their mass into stars and since they represent a major fraction of the galaxies in the nearby universe, only a very small fraction of galaxies are undergoing star formation at the present epoch. Also, there is strong reason to believe that DSph galaxies contain a very large fraction of their mass as dark matter (GW95). Clearly, if the distribution of matter in the Local Group is typical of that of the nearby universe, then the majority of galaxies are similar to Local Group dwarfs, and thus Local Group dwarfs are important to understand.
Since most Local Group dwarfs are gas poor DSph or irregular galaxies, they consist primarily of old or intermediate age stars, and generally show no current or recent (within 1 Gyr) star formation activity (GW95 and references therein). Thus there is little expectation of finding X-ray bright HMXBs or SNRs. It is also unlikely that these systems will have gravitationally bound X-ray halos; their mass-to-radius ratios are an order of magnitude below that required to confine hot material.
Nevertheless, there are good theoretical reasons to expect X-ray emission from these galaxies. In the few gas rich galaxies, there is the obvious possibility of newly formed massive stars and thus SNRs, HMXBs and hot ISM. But even in the gas poor galaxies there are mechanisms for producing X-ray sources. The stellar populations and metallicities of DSphs have been likened to globular clusters (in fact, the masses of some of the larger globulars are comparable to those of DSphs). As in globular clusters, binary systems could evolve into LMXBs and cataclysmic variables. Also, given our current understanding of super soft sources (Rappaport et al. 1994), it is possible that some of these sources might be found is DSphs. One fundamental difference between DSph galaxies and globular clusters is their central stellar density. This can be two orders of magnitude higher in globular clusters (GW95). It is thought that the high central stellar densities in globular clusters form a nursery for LMXBs and millisecond pulsars. The lower density DSphs offer a contrasting environment, and a comparison of X-ray source populations can test this model. Another possibility is that X-rays are produced by shock heating due to a collision between an outflowing wind and inflowing material ejected during the formative wind phase (Silk et al. 1987).
A few modest attempts have been made to detect X-ray emission from Local Group dwarfs. Most of these have been searches for diffuse emission associated with halos (Thronson et al. 1993; Gizis et al. 1993). For Fornax, Gizis et al. place a stringent upper limit on the X-ray luminosity of a diffuse component at erg s, which corresponds to a mass of solar masses at 10 K. Markert & Donahue (1985) placed a similar upper limit on the total luminosity of NGC 205. Attempts to find counterparts of sources coincident with Local Group dwarfs have yielded decidedly mixed results. Markert & Donahue (1985) showed that one of the two sources detected by the Einstein IPC in the gas rich irregular NGC 6822 is coincident with an H II region and is probably a SNR. They also found two sources coincident with the Ursa Major dwarf with no obvious optical counterparts. M32, the only local group elliptical, contains a central source with L erg s, which could be either a LMXB or a micro-AGN, as well as evidence for two weaker sources (Eskridge et al. 1996). In contrast optical follow-up studies of the many sources detected by the ROSAT PSPC coincident with the gas poor DSphs Fornax, Sculptor and Carina (e.g., 18 in Formax - Gizis et al. 1993) have yielded no associations (Tinney et al. 1996), and lead to sensitive upper limits (typically L erg s) for the maximum luminosity of a source in these galaxies.
Given the fact that extrapolations of the elliptical and spiral X-ray to optical luminosity relations suggest X-ray luminosities between 10 erg s and 10 erg s, it is clear that the statistics of small numbers come into play. As in M32, a single bright LMXB could be the dominant source of X-rays from one of these galaxies. Based on the mixed results from the few that have been studied, it is unfair to classify all dwarf spheroidals and irregulars as inherently uninteresting.
With the above considerations as justification, we have embarked on a systematic survey of the Local Group galaxies using the ROSAT HRI. Our observational goals are: (1) observe each Local Group Galaxy to a ``reasonable'' flux limit (defined by pragmatic limitations imposed by the HRI); (2) create a complete catalog of X-ray sources coincident with these galaxies; and (3) find candidate counterparts via plate searches and new optical observations. A typical observation to reach a sensible minimum source luminosity is 30 to 80 ks.
Because of the intrinsically low X-ray fluxes expected, the Local Group dwarfs, by virtue of their proximity, are the only such galaxies amenable to detailed X-ray studies. Our studies of them can offer keen insight into the largest population of galaxies in the universe. It is important to note that these are the only systems besides our Galaxy and the Magellanic Clouds in which stellar counterparts of X-ray sources can be identified. Also, these are the only extragalactic Population II dominated systems in which source identifications can be performed. Using the observed magnitudes of the counterparts of the Galactic bulge sources, we estimate that unique identifications can be made to distances of 150 kpc (based on the known space density of stars in the DSph galaxies and an HRI error circle of 5''). The stellar counterparts will have optical magnitudes of 22-25.
Eskridge, P.B., White, R.E., III., & Davis, D.S. 1996, ApJL, in press
Gallagher, J.S., III, & Wyse, R.F.G. 1995, preprint
Gizis, J.E., Mould, J.R., & Djorgovski, S. 1993, PASP, 105, 874
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Thronsen, H.A., Hunter, D.A., Bailey, B., Ksir, A., & Hickson, R. 1992, BAAS, 24, 1202
Tinney, C.G., Da Costa, G.S., & Zinnecker, H. 1996, MNRAS, in press
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