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ASCA science highlights

Galaxies (including galactic center)

Like the ASCA results for virtually all classes of object the improvement in resolution, sensitivity and bandpass have made critical differences in the quality of the ASCA data compared to Einstein IPC and ROSAT PSPC results. One major difference for galaxies is that almost all of them are very dim in the X-rays (e.g. there are no galaxies in the HEAO-1 all sky survey) and thus the ASCA results in the E > 2 keV band are breaking new ground. However, because of the low fluxes very long exposures are required to obtain high quality spatially resolved spectra and timing data and, so far, not many objects have been observed for long enough.

The ASCA results on nearby galaxies can be divided into 4 categories : elliptical galaxies, non-nuclear sources, previously unknown nuclear sources and starburst galaxies.

Elliptical Galaxies

Based on Einstein, ROSAT, and BBXRT data it was known that the X-ray emission from bright (M < -22) elliptical galaxies is dominated by emission from hot gas. However, neither the temperature nor the abundance of this gas was known accurately. The ASCA data (Awaki et al. 1994 PASJ 46, L65; Mushotzky et al. 1994 ApJ 436, L79; Loewenstein et al. 1994 ApJ 436, L75; Matsumoto et al. 1995 , Matsushita et al. 1994 ApJ 436, L41) have enabled the accurate determinations of the temperature and abundance of the gas and the discovery of a "hard" X-ray spectral component, presumably the signature of X-ray binaries. In confirmation of the Einstein IPC indications, the lower luminosity ellipticals and SOs tend to have a hard X-ray component giving more than 40% of the total emission, indicating that their total emission is not dominated by hot gas. In one of the major surprises of the ASCA results, the abundances of the hot gas in the luminous ellipticals is sub-solar, not only in Fe but also in Si and S. This has major implications for the number of type I SN in these galaxies and the evolution of the interstellar medium. So far only a few galaxies (NGC4636, NGC5846, and NGC4649: Mushotzky et al. 1994 ApJ 436, L79; Finoguenov et al. 1996 Waseda meeting) have had their temperature and abundance profiles determined. This analysis showed an almost isothermal temperature profile, indicating a very massive dark matter halo, and a very sharp abundance gradient. These observations confirm the presence of large dark matter haloes in isolated elliptical galaxies as first suggested by the Einstein and ROSAT observations (Forman et al. 1985 ApJ 293, 102). More than 6 objects which are both large enough and bright enough for this type of analysis have been observed. Preliminary analysis of ellipticals both in and out of poor clusters (in particular Virgo and Fornax) show very little difference (Matsumoto et al. 1996).

Non-nuclear Sources in Nearby Galaxies

ASCA has the sensitivity to determine broad band X-ray spectra of binary X-ray sources in M31 and M33 (Mitsuda et al. 1994) and, if sufficient exposure is permitted, for the luminous point-like sources seen in many nearby (D < 5 Mpc) galaxies. The results have shown that the mean spectra of the point-like sources can be modeled as due to luminous SNR (in NGC 55 an active star forming galaxy), black hole candidates (mostly in M33) and low mass X-ray binaries (mostly in M31). While the differences from galaxy to galaxy are preliminary this result is unexpected and might provide clues about the origin of X-ray binaries. Observations of IC342 (Okada et al. 1994 New Horizon Meeting) have revealed the presence of strong variability in one of the 3 bright sources, confirming that it is indeed one object and is radiating well above the Eddington limit. In addition all 3 of the point-like sources in this galaxy appear significantly brighter in ASCA than in ROSAT or the Einstein IPC, probably due to the effects of intrinsic absorption in the sources, as is also seen in many galactic binaries. This effect can considerably alter the apparent source luminosity distributions and number counts. We can anticipate similar results for all the nearby galaxies observed with ASCA.

One of the surprises so far has been the detection (along with ROSAT) of very luminous, young (< 30 years old) supernova remnants in several nearby galaxies. The spectra show (Kohmura et al. + refs) that these objects are not line-dominated (as are young SNR like Cas-A and Tycho) and can be well fitted by power laws. However, there was the detection O Fe K line emission from the supernova in M81 and Houck and Bregman (1996 in preparation) have detected Fe K emission from 1986J.

Starbursts

The ASCA spectra (Tsuru et al. 1994, Ptak et al. 1996, Della Cecca et al. 1996) have shown that starburst galaxies have at least 3 spectral components. At E > 2 keV most of the emission comes from a rather compact region, and the spectrum is hard with kT > 6 keV. At E ~ 1 keV strong emission lines from Si, Mg, and S are seen while the Fe L lines are rather weak, indicating either a relative overabundance of the alpha burning products, as expected in a region dominated by massive star formation, or non-ionization equilibrium effects on a very large scale. There is also a softer component (also seen by ROSAT) which, in a few objects, is clearly extended. The apparent oxygen abundance in this component is low. For a few objects (e.g. NGC253) bright point sources are clearly detected at E > 2 keV and their nature is under investigation. The nearest bright star forming regions in the LMC (e.g. 30 Dor) have had very high quality data obtained which will allow detailed comparison with star forming regions in our own galaxy (e.g. Orion, Eta Carina etc.) and with the much fainter starburst galaxies. We can expect substantial progress in the next few years in our understanding of the origin of the hot diffuse emission seen in these objects and its relation to galactic winds, chemical evolution and stellar processes.

The Galactic Center

The ASCA observations of the galactic center were the first made with focussing optics over the the 0.5-10 keV X-ray band (Koyama et al. 1996 PASJ 48, 249). These observations confirm the presence of an extended plasma with K-shell transition lines of highly ionized Si, S, Ar and Fe. A remarkable new discovery is that some of the extended emission shows cold (6.4 keV) iron line fluorescence associated with molecular clouds. This neutral line emission requires illumination by an X-ray source, but there is no obvious source currently active. These results require that an ionizing source was active at the galactic center ~300 yr in the past and is currently dormant. This suggests that our galaxy harbours a supermassive black hole in its nucleus, similar to that thought to exist in Seyfert galaxies.

ASCA science highlights

Last modified: Tuesday, 26-Jun-2001 14:22:33 EDT


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This file was last modified on Tuesday, 26-Jun-2001 14:22:33 EDT

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