ROSAT observations of spiral galaxies

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<ul><li><p>Pergarnon Adv. Space Res. Vol. 16, No. 3, pp. (3)155-(3)158, 1995 </p><p>Copyright 0 1995 COSPAR Printed in Great Britain. All rights reserved. </p><p>0273-I 17719.5 $9.50 + 0.00 0273-1177(95JOOU66-6 </p><p>ROSAT OBSERVATIONS OF SPIRAL GALAXIES </p><p>W. Pietsch </p><p>Man-Plmck-lnstitut fiir Etiraterrestrische Physik, GiessenbachstraJe, D-85740 Garching, Germany </p><p>ABSTRACT </p><p>Results of ROSAT observations of nearby spiral galaxies are discussed. All galaxies show emis- sion from point-like sources (X-ray binaries, super-soft sources, supernova remnants, supernovae short after outburst). For some active galaxies, nuclear point-like sources (starburst or Seyfert like nuclei) and/or extended nuclear emission (hot gas, outflowing from starburst nucleus, or jet) is present. In addition extended emission from million K gas has been detected in disk and/or halo of some galaxies. Thereby, hot halo emission is not only demonstrated in galaxies that are known for activity from observations in other wavelength regimes. Due to the long cooling times of 10 - 10 years the X-ray emitting halo gas may be a fossil sign for shorter-lived activity of a galaxy. The detection of an X-ray halo of the Milky Way may argue for a more active phase of the Galaxy in the past. </p><p>INTRODUCTION </p><p>Einstein satellite observations of galaxies have shown /1,2,3,4/ that normal spiral galaxies are spatially extended sources of X-ray emission with luminosities in the range of 103 erg s-l to a few 1041 erg s-* . The X-ray emission of spiral galaxies is dominated by accreting binaries and supernova remnants (SNRs). This explains that, on average, X-ray spectra of spirals are harder than of elliptical galaxies. X-rays of elliptical galaxies are dominated by the emission of a hot interstellar medium. However, also in some spirals an extended hot gaseous component is present that is manifested in X-rays as a very soft component in the energy spectra or can even be spatially resolved in Einstein satellite images as for the starburst galaxies NGC 253 and M82. </p><p>For several spiral galaxies from Einstein satellite observations point sources radiating well above the Eddington limit for accretion onto a l-Me compact object have been reported. One of them was SN 1980K in NGC 6946. Other sources may be complex emission regions adding several bright sources in volumes with less than kpc diameter or massive black holes. </p><p>A hot gaseous component with temperatures around lo6 K is also detected in the plane and halo of the Milky Way /5,6/. Th is component of the interstellar medium is expected to originate from SNRs in the disk and via galactic fountains also to partly fill the halo of galaxies (e.g. /7,8,9,10,11/). It was proposed that all spirals should, to some amount, show hot gas in the halo. Searches with the Einstein satellite in edge-on galaxies /12/ and the large face-on galaxy Ml01 /13/ could only derive upper limits for such a component. The observations of normal galaxies were however not sensitive enough to decide if hot gas is only present in the halo of starburst galaxies where it had been detected in Einstein satellite observations or if it is just strongly enhanced in these active galaxies. </p><p>(3)155 </p></li><li><p>(3)156 W. Pietsch </p><p>We report on observations with the Rontgen observatory satellite ROSAT /14/ of normal and active nearby spirals. Some of the galaxies have been proposed by the nearby galaxy working group at MPE in collaboration with groups at other institutes. The selection of the program galaxies was due to their extent that allowed spatial resolution by ROSAT and due to low galactic foreground absorption that enabled the detection of soft photons from the target galaxies using the good low energy response of ROSAT . Only part of the data has been finally analyzed and published. </p><p>POINT-LIKE NON-NUCLEAR X-RAY SOURCES </p><p>Typical ROSAT observing times of the sample galaxies are in the range 10 ~ 20 ksec. For galaxies up to 5 Mpc distance this led to X-ray point source detection thresholds of N 1O38 erg s-l and enabled to detect X-ray binaries (persistent or transients) radiating at luminosities below the Ed- dington limit for accretion onto a 1 Mo compact object; for more distant galaxies single point sources had to radiate above this limit to be detected. </p><p>Hundreds of non-nuclear point sources have been newly detected in these observations (e.g. for the LMC /15,16/ or for M31 /17,18/). For M31 source classes are studied and log N - log S cor- relation are compared with similar correlations for the Milky Way. Interesting cases are super-soft sources as a new class of X-ray binaries (e.g. /19/), supernovae short after outburst and historical supernovae. While SN 1987A in the LMC was detected at a very low soft X-ray luminosity of N 1O34 erg s-l not earlier than four years after outburst /20,21/, soft X-rays from SN 19935 in M81 were already reported from the first observation six days after outburst radiating at 3 x 103 erg s-l and only slowly decaying within the following 130 days 1221. The historical SN 19865 in NGC 891 revealed an X-ray luminosity of &gt; 104 erg s-r in the ROSAT PSPC band /23/, and X- ray emission has also been reported from the sites of historical supernovae in Ml01 and NGC 6946 124,251. </p><p>In follow up observations many of the brightest point-like sources turn out to be transients (e.g. /26,27,28/). S o f ar, time variability of only a few point sources in nearby galaxies has been dis- cussed in detail (for a period determination of an eclipsing binary in M33 see /29/). </p><p>EXTENDED EMISSION </p><p>The search for extended X-ray emission in spiral galaxies with ROSAT proved to be very suc- cessful. The good spectral and spatial resolution and the high sensitivity (especially in the 0.1-0.5 keV band) allowed the separation of point sources from diffuse emission that most likely origin- ates from hot thermal plasma (temperatures of few times lo6 K). Diffuse emission was not only discovered in galaxies known to be active. It can be located in the disk (d), halo (h), or in jets or plumes (j). While some of the galaxies do not show any extended emission, for several galaxies more then one of the components, mentioned above, can be separated. Edge-on or inclined viewing of the galaxies allows a direct separation, for galaxies seen face-on only indirect arguments help for the separation. In the following I will shortly report on observations of starburst galaxies, act- ive/interacting galaxies and normal galaxies and indicate, what kinds of diffuse components have been detected. </p><p>The starburst galaxies NGC 253 (d,hj) and M83 (d,h,j?) are bright X-ray sources in the ROSAT band 126,301, P oin source and extended components can be separated. For M83 which is seen t face-on the existence of strong emission in the 0.1-0.4 keV band points at an halo origin of this component. The more structured diffuse component in the harder bands points at hot emission from the disk. In addition there are indications for jet-like emission emanating from the nucleus to the ENE. The soft X-ray contours out of the plane of NGC 253 and the occultation of the soft emission in the NW due to absorbing material in the intervening spiral arms clearly demonstrate </p></li><li><p>ROSAT Observations of Spiral Galaxies (3)157 </p><p>Figure 1: Starburst galaxy NGC 253: Contour plot of ROSAT PSPC soft band image overlaid on an optical image (left); contour plot of the nuclear region of the ROSAT HRI image overlaid on an H, image of H. Schulz (The gray scale makes several cycles from light to dark as the intensity rises.) </p><p>the halo origin of the soft emission. In harder X-rays emission from hot gas in the disk is clearly visible. X-ray emission from a plume of hot gas ejected from the starburst nucleus has already been reported in Einstein satellite observations 1311. This is confined by an H II cone as is demonstrated by the overlay of the ROSAT HRI contours on top of an H, map (fig. 1). </p><p>ROSAT data of several active or interacting nearby galaxies have been investigated. NGC 4258 (d,hj) hosts an Seyfert 1.9 nucleus and shows X-ray emission from point sources, from the anomal- ous arms and from disk and halo 132,331; diffuse X-rays are observed from the inner disk and radio jet of the face-on interacting galaxy M51 (d?,h?j) /27/; in the interacting group containing the edge-on pair NGC 4631/NGC 4656 the galaxy NGC 4631 (h,d?j?) holds an extended soft X-ray halo /34/ while NGC 4656 (j) besides point sources only shows signs of an outflow in the region disturbed by the interaction 1351. </p><p>Diffuse X-ray emission is also seen from normal galaxies like the LMC (d,h?) /26/, the face- on galaxy Ml01 (d,h) /36/ and NGC 4565 (hj?) /35/. Th ere are however many galaxies that do not show strong diffuse emission: SMC 1371, M31 il.81 or NGC 5907 1351. </p><p>CONCLUSIONS </p><p>Diffuse emission from 106K gas has been detected by ROSAT in the halo of several nearby galaxies. They are not restricted to active galaxies but also include galaxies for which no activity was repor- ted before. The gas may have been transported into the halo via galactic fountains or starburst winds during times of starburst or nuclear activity. The hot halo gas has typical cooling times of 10 to 10 years. It may still be detected when the activity has already vanished and therefore represent a sign for fossil activity within the past 10 years. In this contents the question arises if the detection of an X-ray halo of the Milky Way points at a more active phase in the past. </p><p>ACKNOWLEDGEMENTS </p><p>I acknowledge the help of A. Vogler in preparing the figure and H. Schulz for providing the H, </p></li><li><p>(3)158 W. Piemh </p><p>image. 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