Hot Plasma Components

Fabbiano and collaborators have systematically analyzed all Einstein galaxy observations (see [17, 20, 42, 43]). They find normal galaxies of all morphological types as spatially extended sources of X-ray emission with luminosities in the 0.2-3.4keV band in the range of 1038 to 1042 erg s-1. Spiral galaxies only reach a few 1041 ergs-1. On average the X-ray spectra for spiral galaxies are harder than for ellipticals. This is explained by the commonly accepted view that X-rays from elliptical galaxies originate from a hot interstellar medium while the emission of spiral galaxies is dominated by point-like sources with a harder spectrum (XRBs and SNRs). Also nuclear sources may be present and contribute significantly to the total X-ray emission. These sources may either be connected to star forming or Seyfert-like activity. Some spiral galaxies showed in addition to the expected hard component a soft component in their spectra, indicating that an extended gaseous component can also be present in spirals.

Many of these galaxies have been investigated in greater detail in the 0.1-2.4 keV band with ROSAT and are targets of XMM-Newton and Chandra observations. The good spatial and spectral resolution of present X-ray observatories allows us to separate the emission of distinct sources from that arising from surrounding gas within the galaxies. In the interstellar space, stars are born out of the densest regions and massive stars transfer matter back to the ISM via stellar winds and supernova explosions. Therefore, the diffuse component of the X-ray emission from galaxies will help us to better understand the interaction between stars and the ISM as well as the matter cycle within galaxies.

20.4.1 Hot Interstellar Medium and Gaseous Outflows in Spiral and Starburst Galaxies

The existence of a hot gaseous component in the ISM of late type galaxies with temperatures around 106 K was proposed from theoretical considerations. It should originate from SNRs in the galaxies and via fountains also partly fill the halo of galaxies (see e.g. [4,5,10,12,68]). The pre-ROSAT knowledge of the hot ISM was rather sparse. For a review of the local ISM in the Milky Way see [11]. While a hot ISM was detected with Einstein in the LMC and less convincingly in the SMC [77,78], studies of edge-on galaxies [6] and the large face-on galaxy M101 [45] could only derive upper limits for diffuse emission from hot gas. The only Einstein detections of hot ISM in late type galaxies outside the Local Group were reported for the starburst galaxies M82, NGC253, and NGC 3628 [16,18,21,79].

Thanks to the improved sensitivity of ROSAT, X-ray emission from the hot ISM was resolved in many late type galaxies. If fitted with thin thermal plasma models, temperatures were in the million K range. Examples range from the plane and halo of the Milky Way [66] to galaxies at 20Mpc distance and more (see e.g. [13,60]). For M 101 the detected diffuse emission most likely not only originates from the hot ISM in the disk but also from the halo of the galaxy. In addition the ROSAT measurements for the first time showed evidence for shadowing of the soft X-ray background at about 0.25 keV by a M101 spiral arm [67]. Many of the ROSAT results on the hot ISM and gaseous outflows in late type galaxies have been confirmed and investigated in more detail with Chandra Observatory and XMM-Newton (see, e.g., [69, 70, 76]). In the following we will discuss three typical examples for the study of the hot ISM in galaxies with ROSAT from - with increasing distance -the MCs (Fig. 20.8), the prototypical starburst galaxy NGC 253 (Fig. 20.9), and the LINER galaxy NGC 3079 (Fig. 20.11).

Diffuse X-ray emission from the ISM of the LMC was clearly detected in the ROSAT All Sky Survey and in merged images of all pointed observations (see Fig. 20.2). The diffuse X-ray emission of the MCs was systematically studied in all archival pointed ROSAT PSPC observations [64]. Contributions from the X-ray point-like sources in the ROSAT PSPC and HRI catalogues [31, 32,62,63] were

Fig. 20.8 Temperature distribution image of the LMC (left) and SMC (right). Positions of SNRs observed by ROSAT are shown by squares, of XRBs by crossed squares, and for SSSs by double squares. For the LMC the position of the supergiant shells SGS LMC 1-5 are marked as ellipses. Hot ISM with (T = 106-107 K) extends over the whole LMC and SMC [64]

Fig. 20.8 Temperature distribution image of the LMC (left) and SMC (right). Positions of SNRs observed by ROSAT are shown by squares, of XRBs by crossed squares, and for SSSs by double squares. For the LMC the position of the supergiant shells SGS LMC 1-5 are marked as ellipses. Hot ISM with (T = 106-107 K) extends over the whole LMC and SMC [64]

Fig. 20.9 ROSAT PSPC (left [59]) and XMM-Newton EPIC (right [57]) images of the starburst galaxy NGC 253. The images are color coded with red representing lower energy X-ray emission (0.1-0.5 keV) and blue higher energy X-ray emission (1-2 keV). Hard emission (2-10 keV) is shown superimposed in the EPIC image as contours. The ellipse indicates the optical extent of the galaxy. The ROSAT image clearly shows the extended soft emission from the galaxy halo, which is absorbed in the northwest by the ISM of the galaxy disk. The XMM-Newton image shows many point-like sources resolved as well as emission of the hot ISM in the NGC 253 disk. The plume emanating from the nuclear area to the southeast is interpreted as manifestation of an outflow of hot plasma from the starburst nucleus (see factor 3 zoom-in in inset)

Fig. 20.9 ROSAT PSPC (left [59]) and XMM-Newton EPIC (right [57]) images of the starburst galaxy NGC 253. The images are color coded with red representing lower energy X-ray emission (0.1-0.5 keV) and blue higher energy X-ray emission (1-2 keV). Hard emission (2-10 keV) is shown superimposed in the EPIC image as contours. The ellipse indicates the optical extent of the galaxy. The ROSAT image clearly shows the extended soft emission from the galaxy halo, which is absorbed in the northwest by the ISM of the galaxy disk. The XMM-Newton image shows many point-like sources resolved as well as emission of the hot ISM in the NGC 253 disk. The plume emanating from the nuclear area to the southeast is interpreted as manifestation of an outflow of hot plasma from the starburst nucleus (see factor 3 zoom-in in inset)

cut out. The spectral analysis yielded characteristic temperatures of (106-107)K for the hot thin plasma of the ISM, which extends over the whole LMC and SMC (see Fig. 20.8). The total unabsorbed luminosity in the 0.1-2.4 keV band within the observed area amounts to 3.2 x 1038 erg s"1 in the LMC and 1.1 x 1037 erg s"1 in the SMC, respectively. The X-ray luminosity in the LMC is comparable to that of other nearby galaxies with pronounced star formation. In the LMC, hot regions were found especially around the supergiant shell (SGS) LMC 4 and in the field covering SGS LMC 2 and LMC 3. The highest temperatures for the SMC are located in the southwestern part of the galaxy. The diffuse emission is most likely a superposition of the emission from hot gas in the interior of the shells and super-shells as well as from the halo of these galaxies.

As mentioned above NGC 253 was one of the few starburst galaxies in which extended X-ray emission was detected by the Einstein Observatory [16,21]. Because of the low Galactic foreground absorption, its big optical extent, and the edge-on viewing geometry, the prototypical starburst galaxy NGC 253 is ideally suited for a detailed analysis of the X-ray emission from disk and halo. ROSAT PSPC and HRI observations revealed diffuse soft X-ray emission from NGC 253, which contributes 80% to its total X-ray luminosity (5 x 1039 erg s"1, corrected for foreground absorption). The nuclear area, disk, and halo contribution to the luminosity is about equal. The starburst nucleus itself is highly absorbed and not visible in the ROSAT band. The hollow-cone shaped plume traces the outflow of the nuclear starburst and interactions with the ISM to an extent of ~700pc along the SE minor axis. The diffuse emission in the disk follows the spiral arm structure and can be separated in a bright inner and a fainter outer component along the major axis with extents of ±3.4 and ±7.5 kpc, respectively. The coronal halo emission (scale height 1 kpc) is absorbed in the NW by the intervening ISM of the disk. The outer halo can be traced to projected distances from the disk of 9 kpc and shows a horn-like structure with a harder spectrum in the NW halo than in the SE. The emission in the corona and the outer halo is most likely caused by the strong starburst wind. An additional contribution to the coronal emission may come from hot gas fueled by galactic fountains originating in the boiling star-forming disk. A two temperature thermal plasma model (kT = 0.13 and 0.62 keV) or a nonequilibrium cooling model of the halo plasma (see, e.g., [7]) are needed to explain the X-ray spectra (see left panel of Fig. 20.9, [59]).

XMM-Newton EPIC observations of NGC 253 [57] allowed to better constrain the diffuse emission in the nuclear area, plume, and disk. The unresolved emission of the two disk regions can be modeled by two thin thermal plasma components (kT = 0.13 and 0.4keV) plus residual harder emission, with the low temperature component originating from above the disk. The nuclear spectrum can be modeled by a three temperature plasma (0.6, 0.9, and 6 keV) with the higher temperatures increasingly absorbed. The high temperature component most likely originates from the starburst nucleus. The combination of EPIC and RGS also sheds new light on the emission of the complex nuclear region and plume (see right panel of Fig. 20.9 and Fig. 20.10).

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Fig. 20.10 The XMM-Newton EPIC pn spectrum of the nuclear area (left, upper spectrum) clearly shows emission lines indicative of emission from a hot thin plasma and can be compared to the featureless disk-blackbody model which best describes the X-ray spectrum of the black hole XRB NGC 253 X33 (left, spectrum below). The XMM-Newton RGS spectrum of the bright nuclear area of NGC 253 (right) is dominated by bright emission lines [57]

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Fig. 20.10 The XMM-Newton EPIC pn spectrum of the nuclear area (left, upper spectrum) clearly shows emission lines indicative of emission from a hot thin plasma and can be compared to the featureless disk-blackbody model which best describes the X-ray spectrum of the black hole XRB NGC 253 X33 (left, spectrum below). The XMM-Newton RGS spectrum of the bright nuclear area of NGC 253 (right) is dominated by bright emission lines [57]

Fig. 20.11 Left: Contour plot of the broad band PSPC X-ray emission of the region of NGC 3079 and its companions overlaid in white on an reproduction of the POSS red plate. H i-contours are superposed in black. Right: Contour plot of the central emission region of NGC 3079 for ROSAT HRI overlaid on the continuum-subtracted distribution of Ha + [Nil AA6548, 6583 line emission (grey-scale) and the 20 cm continuum distribution (black contours) [58]

Fig. 20.11 Left: Contour plot of the broad band PSPC X-ray emission of the region of NGC 3079 and its companions overlaid in white on an reproduction of the POSS red plate. H i-contours are superposed in black. Right: Contour plot of the central emission region of NGC 3079 for ROSAT HRI overlaid on the continuum-subtracted distribution of Ha + [Nil AA6548, 6583 line emission (grey-scale) and the 20 cm continuum distribution (black contours) [58]

ROSAT PSPC and HRI observations resolved complex emission from the inner 5' around the LINER galaxy NGC 3079 which is seen edge-on. The extended emission in the innermost region is extended and has a luminosity of 1 x 1040ergs-1. It coincides with a super-bubble seen in optical and the east lobe of bipolar radio emission originating from the nucleus. The active nucleus of the galaxy known from radio observations may contribute to the X-ray emission as a point source. In addition there is emission from the disk of the galaxy (7 x 1039ergs-1) that can partly be resolved by the HRI in three point-like sources with luminosities of ~6x 1038 erg s-1 each. The PSPC resolves very soft X-shaped emission from the halo, with LX = 6 x 1039ergs~\ extending to a diameter of 27kpc. The X-ray luminosity of NGC 3079 is higher by a factor of 10 compared to other galaxies of similar optical luminosity. This may be caused by the presence of an AGN rather than by starburst activity (Fig. 20.11, [58]). Chandra observations confirmed the ROSAT findings for the NGC 3079 nuclear super-bubble and large-scale superwind [8,69].

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