Astron. Astrophys. 342, 213-232 (1999)

4. Individual HVC complexes

To investigate whether soft X-ray enhancements are associated with HVCs, we excluded the HVC velocity regime from the velocity range used to determine the absorbing , in particular we integrated over . This exclusion introduces the brightest modelled SXRB intensity just at the positions of the HVCs and thus biases our analysis against detection of soft X-ray enhancements with HVCs, because we evaluate observed minus modelled X-ray intensity distribution only. We now evaluate the solutions of Eq. (2) with determined over the more extreme velocity interval , searching for soft X-ray correlations or anti-correlations with HVCs.

4.1. The HVC complex C

4.1.1. Complex C at lower l and b, and parts of complex D

Kerp et al. (1996) investigated the X-ray intensity distribution towards complex C at , . Here, we discuss parts of complex C, weaker in H I emission, at lower b. Fig. 2a shows the ROSAT PSPC data from the lower-b part of complex C. Panel (b) shows the modelled SXRB intensity distribution, assuming a constant SXRB source intensity across the field. We derived the intensity of the LHB, , and of the distant X-ray source, . Both X-ray images in Fig. 2a and b are scaled similarly. A statistical evaluation of the similarity between the observed and modelled X-ray map gives . This confirms that also statistically the observed and modelled X-ray intensity distributions match. In Fig. 2a we superposed, as contours, the deviations between the observed and the modelled SXRB distributions, starting with the 4- contour level. Dashed contours indicate where the modelled SXRB intensity is brighter than observed; solid contours enclose regions where the modelled SXRB intensity is weaker than observed.

The dashed contours do not enclose the positions of individual HVCs (see Fig. 2d), indicating that we do not detect soft X-ray shadows of HVCs at this significance level. It is more likely that these dashed contour lines of X-ray shadows indicate cloud structure within the LHB. As mentioned in Sect. 3.3.1, the effective photoelectric absorption cross section of the LHB plasma is the largest of all three cross sections: an H I cloud within the LHB will cause a deeper soft X-ray shadow than when the same cloud were located outside the LHB. In consequence, if predicted soft X-ray emission is weaker than observed, one first has to check for the existence of a cloud within the LHB. The dashed contours in Fig. 2a show a patchy distribution; a large area of weaker X-ray emission is located at , . Located close to the dashed contours is an elongated H I filament, part of a much more extended local H I structure (Wennmacher et al. 1998, Kerp & Pietz 1998). An maximum of this structure associated with a filament, denoted as LVC 88+36-2, was studied by Wennmacher et al. (1992). Kerp et al. (1993) detected a strong soft X-ray absorption feature associated with LVC 88+36-2 in pointed ROSAT PSPC data and confirmed that the filament is embedded within the LHB. Thus, the dashed contours indicate, most likely, local maxima of an extended H I structure within the LHB (see Kerp & Pietz 1998).

A second region of low observed SXRB emission, at and , is not associated with a previously identified local H I structure. As mentioned in Sect. 3.4.3, an underestimate on the amount of X-ray absorbing matter is more likely than an overestimate, because H I emission traces neither molecular nor ionized gas. The dashed contours may indicate an additional absorber, either located outside of the LHB (and thus only attenuating the term) or within the local bubble. In the former case, we miss as an absorber; in the latter case, . This difference in absorbing between both model assumptions follows from different amplitudes of the near and distant photoelectric absorption cross sections (see Fig. 1). Consequently, the SXRB minimum is more likely due to a local cloud than to a cloud of higher beyond the local bubble. Hartmann et al. (1998) detected no molecular gas in this direction, although such gas might be anticipated for a cloud outside of the LHB with such a high H I density. A further investigation of the Leiden/Dwingeloo data reveals an H I minimum at , suggesting that some of the local H I may have been ionized and not quantitatively traced by the distribution of . The distance to the absorber thus remains uncertain.

The solid contours in Fig. 2d enclose an HVC catalogued as #182 by Wakker & van Woerden (1991), at , and attributed to HVC complex D. Our analysis suggests an excess soft X-ray emission with a significance level greater than 4. The solid contours also enclose nearby regions of intermediate-velocity gas (, Fig. 2e), implying that IVCs may also be associated with the enhanced X-ray emission. In this particular case, where both HVC and IVC gas appear along the same lines of sight, we cannot determine whether the HVCs or the IVCs are the sources of the excess soft X-ray emission.

Finally, we analyzed the variation of the modelled (Fig. 2b) and observed (Fig. 2a) SXRB emission, as averaged over l and b. We solved the radiation-transfer equation independently for these averaged distributions. Fig. 2f shows the observed and modelled SXRB intensity profiles averaged in l and b. The modelled SXRB intensity profile (solid line) fits the ROSAT observation (dots) well. This shows that the dominant part of the soft X-ray attenuation is traced by H I , and that small-scale as well as large-scale intensity variations of the SXRB can be explained by photoelectric absorption. This result justifies again our assumption that = const. and = const. (see Sect. 3.3) across each field.

4.1.2. Complex C at higher l and b

Fig. 4a shows the ROSAT keV map of complex C between , . The field also includes much of HVC complex A as well as the high-velocity filament which connects HVC complex C with A (Wakker & van Woerden 1991). The map covers such a large range in l and b that the distribution varies appreciably across the field. This yields the opportunity to study the variation of the SXRB source intensity distribution with galactic latitude. In the upper left of Fig. 4a, strong soft X-ray attenuation by the neutral matter associated with the North Celestial Pole Loop (Meyerdierks et al. 1991) is visible (see also Fig. 4c, , ). Significant amounts of molecular material are found near this structure (Heithausen et al. 1993), for instance in the Polaris Flare (, ; Heithausen & Thaddeus 1990). Towards the Polaris Flare, the Leiden/Dwingeloo data show a maximum of (Fig. 4c). The Lockman et al. (1986) area of minimum (, ) is located at the other end of the field. The data show a ratio in the absorbing column densities.

Fig. 4a-f. Soft X-ray background towards the higher-l and -b end of HVC complex C, (see Sect. 4.1.2). a  The SXRB intensities observed in the ROSAT keV band. b  The SXRB map modelled according to Eq. (2) using Leiden/Dwingeloo data and assuming in addition to the local X-ray radiation of , also assumed constant across the field. Dark colours denote low X-ray intensities ; bright colours denote high intensities . c  The distribution across the field within the range . d  Greyscale: the distribution in the HVC regime, . The contours are described below. e  Greyscale: the distribution in the IVC regime, . The contours are described below. f  The intensity profiles averaged in l and b from the maps in panel a , dots with error-bars, and b , solid lines. Superposed as contours are the intensity deviations between the observed a and modelled b SXRB maps. The contours proceed from the 5- level in steps of 2 . Solid contours in a , d , and e mark areas of excess X-ray emission; dashed contours mark areas of weaker X-ray emission than expected from the map in b . The angular resolution of the images is .

We evaluated the X-ray source terms using all three methods described in Sect. 3.4.4 and found and . The -test of the observed and modelled X-ray map indicates that . The differences between the observed and modelled map are significant. Most probably, the structure of the interstellar medium covered by the field of interest is much too inhomogenious to be fitted by our simple approach. However, Fig. 4f shows that the modelling of the X-ray data fits the overall SXRB intensity distribution well, especially if we take the bright X-ray enhancements around into consideration. However,to distinguish between excess emission areas and large scale intensity variations of and , we restrict our interpretation of the X-ray deviations to high galactic latitudes and to peak deviations more significant than . In Fig. 4a, most of the contours are oriented parallel to . Nearby are the main parts of HVC complex C (see Fig. 4d) and the Lockman et al. window, which is enclosed by dashed contours. In this region, our value is lower by a factor of two than the value given by Snowden et al. (1994b, 1998), while is higher by about the same factor. To investigate this discrepancy (see Freyberg 1997), we extracted the Lockman Window data from our map and evaluated the radiation transfer equation in this area once again, restricting our analysis to a region of in extent in both l and b. We derived and , applying only the first method described in Sect. 3.4.4. Using the second and third methods, we find values which, although in closer agreement with Snowden et al. (1994b), do not fit the averaged l and b intensity profiles. Moreover, if we extrapolate the values to the data shown in Fig. 4a, we fail to reproduce the observations. In contrast, the first-method values do fit the Lockman Window region in the averaged l and b profiles (see Fig. 4f). This shows that a solution of the radiation-transfer equation demands determination of over areas large enough not to be biased by local events.

The solid contours roughly trace some of the brighter parts of HVC complex C, suggesting that these bright HVCs, in addition to other parts of complex C (Kerp et al. 1996), are associated with excess soft X-ray emission. The Pietz et al. (1996) H I "velocity-bridges" suggest the interaction of some HVC matter with the conventional-velocity regime. The velocity bridges VB 112 + 48 and VB 115 + 47, both at the high-b end of complex C, are enclosed by 5- contours. VB 112 + 57.5 is an area of soft X-ray radiation enhanced to the 11- level. VB 111 + 35 and VB 133 + 55 were not detected as enhancements in the keV ROSAT data. If we add the four velocity bridges already found to be X-ray bright by Kerp et al. (1996) using similar methods, we find that 7 of 11 bridges are located close to soft X-ray enhancements. The velocity bridges span the range of conventional velocities to those of the HVCs, and thus their association with enhanced X-ray emission does not, in itself, distinguish between an HVC or an IVC connection (see Fig. 4e, and Sect. 5.2).

The suggestion that velocity bridges are associated with a distortion of the velocity field due to an HVC, requires disproving that the bridges are different from normal IVC structures. The large distance to the HVCs (several kpc for the nearest, Wakker & van Woerden 1997, and possibly hundreds of kpc for many HVCs, Blitz et al. 1998) make it unlikely that HVCs are physically linked to those IVCs which are carriers of dust. Much of the intermediate-velocity H I is associated with dust cirri (Deul & Burton 1990), and is therefore likely to be rather local, rarely extending to z-heights of more than 150 pc. The velocity regime of cirrus-carrying IVCs, however, is frequently trespassed upon by HVCs: the crossing of the Magellanic Stream from positive to negative velocities is a case in point. HVC gas trespassing on lower velocities will have a different chemical composition from the dust-carrying IVCs. In Sect. 5.2 we will discuss this point for HVC complex C in more detail.

In the special case of a wide extent in galactic latitude, it is interesting to study the observed and modelled SXRB intensity variations against l and b, as shown in Fig. 4f. Again, the solid line marks the modelled intensity profile based only on H I data. The b-variations show quantitative agreement between observationed and modelled values, deviating only close to , at the location of the North Celestial Pole Loop (region of highest opacity), and above . These deviations are significant: the error-bars correspond to the 3- level. Most likely, we observe an intensity variation of proportional to increasing b. Fig. 4f shows that the X-ray intensity variation is correctly predicted by the modelled SXRB intensity distribution, but starting at , the modelled SXRB intensity deviates increasingly from the observed one. Towards these high-b regions we may predominantly observe the local interstellar medium, and consequently a larger extent of the local X-ray emitting region. Finally, we note that the modelled SXRB longitude profile closely matches the observed one for . This position coincides with the border of the X-ray enhancements associated with HVC complex C.

4.2. The HVC complex GCN

The mean towards the galactic center HVC complex GCN , is significantly higher than towards the other regions discussed here. The field displays the complex H I column density structure within the range (Fig. 5c). Solving of the radiation-transfer equation gives and . The test of the observed and modelled data gives . Fig. 5 shows the ROSAT data (panel a) our solution of Eq. (2) (panel b) using the Leiden/Dwingeloo H I data. This field shows well-defined large-scale X-ray intensity gradients in the ROSAT data which are reproduced by our solution of Eq. (2), confirming that the intensity variations are dominated by photoelectric absorption effects.

Fig. 5a-f. Maps of the X-ray and H I sky towards HVC complex GCN (see Sect. 4.2). a  ROSAT keV SXRB distribution ( and ). b  Modelled SXRB image derived from the H I data, assuming both a constant distant X-ray background and a constant local X-ray source . Solid contours indicate excess X-ray emission; dashed contours indicate an emission deficiency. The contours proceed from the 4- level in 2- steps . c   distribution of the soft X-ray absorbing ISM ( colour coded within the range . d  Greyscale: the HVC distribution ( and ). The contours are described in b . e  IVC distribution ( and ). The contours are described in b . f  SXRB intensity profiles, averaged in l and b across the map (panel a : dots and error bars; panel b : solid lines). The images in a and b are scaled identically and have an angular resolution of . The dot in a marks the position of Mrk 509 where Sembach et al. (1995) detected highly-ionized high-velocity gas in HST absorption-line measurements, at a location coinciding with excess soft X-ray emission.

The distant X-ray intensity is quite high, and (within the uncertainties) equal to the intensity value of the lower end of HVC complex C (Sect. 4.1.1). Furthermore, the intensity of both areas agree. We note that both ROSAT areas cover a comparable range in galactic coordinates, but refer to opposite galactic hemispheres, which suggests that closer to the inner Galaxy, the northern and southern galactic sky have approximately the same SXRB distant source intensity, and that is not patchy across the individual fields. Thus, we can consider as constant towards the same galactic longitude range in both galactic hemispheres.

The bright X-ray area, localed near in the center of Fig. 5a, shows excess soft X-ray emission enclosed by solid contours. The galaxy Mrk 509 is marked by the dot. Sembach et al. (1995) used HST absorption-line measurements to detect highly-ionized high-velocity gas belonging to HVC complex GCN. They attribute the source of the ionization to photoionization. Our ROSAT data suggest, additionally, the presence of collisionally ionized gas along the line of sight towards Mrk 509. Sembach et al. may have detected the cooler portion of the collisionally ionized plasma. Figure 5d shows the distribution of the GCN clouds across the field. They are patchily distributed and have only low column densities of . Very close by, some filaments are found which belong to the HVC complex GCP; we can not distinguish whether the excess emission originates in GCN or in GCP. Following Sembach et al. (1995), we attribute the excess emission to complex GCN. Thus, in contrast to HVC complex C, where we found a close positional correlation between neutral HVC gas and the X-ray bright areas, the GCN complex allows no straightforward interpretation. Blitz et al. (1998) include complex GCN amongst those suggested to be at large, extragalactic distances. If this is true, one must consider the physical circumstances which would allow the presence of collisionally- and photoionized gas associated with this complex.

Fig. 5f shows the l and b profiles of the GCN maps. Again, the modelled SXRB intensity distribution fits the observation, confirming that the areas of excess soft X-ray radiation are well determined by the methods applied.

4.3. The HVC complex WA

HVC complex WA (Wannier et al. 1972; see also Wakker & van Woerden 1991), roughly confined to the region , , displays the positive velocities characteristic of most HVCs in this general region of the sky. The radial velocity is, of course, only one component of the velocity vector; the positive radial velocity does not rule out, by itself, that the HVC could be colliding with the galactic disk. Regarding the X-ray radiation transfer, it is interesting that the HVC complex WA is located opposite the direction of HVC complex C, but also in the northern sky.

Fig. 6 shows the ROSAT keV map (panel a) and the modelled SXRB intensity (panel b). We derive an intensity of for the LHB, and for the distant X-rays. These LHB and distant X-ray count rates mark the extreme intensity values found in our sample of HVC complexes. Because of the short ROSAT integration times towards complex WA, the observed and modelled SXRB maps show some deviations. The -test of the maps reveal that . Statistically, the modelled SXRB map fits the observed one well. However, only a few 4- contours are present in Fig. 6a.

Fig. 6a-f. Maps of the X-ray and H I sky towards HVC complex WA (see Sect. 4.3). a  Observed keV SXRB distribution ( and ). b  Modelled SXRB image derived from the H I data, assuming a constant intensity distribution across the field of both X-ray source terms, and . Images a and b are scaled identically; the angular resolution of the maps is . Solid contours indicate excess X-ray emission; dashed contours, a lack of emission. The contours proceed from the 4- level in steps of 2, where . c  Distribution of in the range with . d  Greyscale: the distribution of in the appropriate positive-velocity HVC range ( with ). The contours are described in b . e  Greyscale: the distribution of in the IVC range ( with ). The contours are described in b . f  SXRB intensity averaged over l and b. The dots and error bars refer to the observed map in a ; the solid line represents the model in b .

The SXRB radiation is locally weaker near , (4). Either we miss additional matter attenuating the soft X-rays but not traced by , or, more likely, the photoelectric absorption cross section is locally larger due to a cloud within the LHB plasma. At , H I maps in the Leiden/Dwingeloo atlas show a local deficiency of neutral gas, correlated with the contours given in Fig. 6.

Toward the general direction of HVC complex WA we identified soft X-ray enhancements with known HVCs (Fig. 6d). The 4- contour centered , is positionally associated with the HVC catalogued as #66 by Wakker & van Woerden (1991). The solid contour near , , lies in between the HVCs catalogued as #176 and #162. (These HVCs are within our WA field but Wakker & van Woerden (1991) did not assign them to complex WA.) Because of the limited quality of the ROSAT data and the possibility of residual systematic uncertainties, we do not claim a firm detection of excess X-ray emission from the WA HVCs. Fig. 6d shows the distribution of the HVCs towards the field. The range of the HVCs displayed is only . Fig. 6f shows the averaged SXRB intensity profiles derived from the observed and modelled keV SXRB data; within the uncertainties of the data, the modelled SXRB variation fits the observational data well.

© European Southern Observatory (ESO) 1999

Online publication: December 22, 1998
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