Intensity of the 1/4 keV Emission from the Local Galactic Halo

M. Juda
Harvard-Smithsonian Center for Astrophysics

Abstract

In the simplest model, the diffuse 1/4 keV background at high galactic latitude is a combination of emission from three components: the local interstellar medium (LISM), the galactic halo, and the extragalactic background. Separation of the LISM contribution from the distant ones can be accomplished via the detection of intensity variations in the 1/4 keV background caused by absorption by intervening neutral material. ROSAT PSPC observations in the direction of high galactic latitude enhancements in IRAS 100 mm emission, identified by Désert, Bazell, & Boulanger (1988, ApJ, 334, 815) as likely molecular cirrus clouds, provide good candidates for seeing these shadows. For several such fields at both high northern and southern galactic latitudes, I have generated maps of the intensity in the 1/4 keV band following the procedure suggested by Snowden et al. (1994, ApJ, 424, 714) and fit for an anticorrelation with the 100 mm intensity. The derived average brightness of the galactic halo in both the north and the south is ~ 1.0×10-3 counts s-1 arcmin -2. Assuming coronal equilibrium at 106 K and solar abundances, this intensity would be produced by an emission measure of ~ 5.3×10-3 cm-6 pc. Viewed face-on this emission measure results in a 1/4 keV band luminosity per unit area of ~ 1.8×1037 ergs s-1 kpc -2.

1  Introduction

The existence of a hot galactic halo was suggested as a mechanism to confine clouds at high galactic scale height[16] long before the capability to directly observe it existed. With the first observation of the 1/4 keV background[2] questions arose as to how much of the observed intensity might be due to a galactic halo; various combinations of local, distant, and distributed emission have been proposed. Observations of edge-on galaxies[3,10,19] have demonstrated that some have extended hot halos; while observations of face-on spiral galaxies[5] provide upper limits to emission from their halos.

Prior to the launch of ROSAT the simplest explanation[11] that fit the available data for our galaxy was that the 1/4 keV background was predominantly local in origin (i.e. that it arose in a local, hot, low-density cavity, in front of most of the neutral material - the Local Bubble). Observations with the PSPC on ROSAT have provided the first definitive evidence for emission from outside the local interstellar medium[4,12,14,18]. The detection of shadows in the 1/4 keV background allows us to divide this background into its individual components.

2  The Clouds

Ideal targets for shadowing observations with the ROSAT PSPC should have low total column density ( £ 2×1020 cm-2) but have significant variation in column density on angular scales of ~ 1° so that the PSPC can image any shadow. Désert, Bazell, & Boulanger (1988 hereafter DBB) compiled a list of clouds based on excess IRAS 100 micron emission relative to HI column density, which provides an ideal starting point to search for appropriate shadowing candidates. Additional cloud candidates can be found in other papers that have used the IRAS 100 micron data to find candidates for molecular gas at high latitude[7]. I have searched the ROSAT public archive for PSPC observations in the direction of clouds from the DBB survey with |b|  > 60°.

Tables 1 and 2 list clouds for which suitable PSPC observations were found; the fields are labeled with their number from DBB where applicable. Also given are the galactic longitude and latitude of the center of the observed field, the HI column density in this direction, the average IRAS 100 micron intensity and its range, and the name of the ROSAT observation. Not all of the clouds meet the ideal candidate criteria listed above; some have fairly large total HI column density. Figure 1 shows the ROSAT all-sky survey maps of the 1/4 keV background toward the galactic poles[15]; the locations of the observed fields are indicated.

Table 1: Northern Galactic Pole Fields
DBB Cloud l b < NH > a < I100 > b I100 range NHoffset Ravgc Shadow depthc Field
8 5.82 72.75 2.03 0.87  0.0 - 2.7  1.00 1023 600 Mrk 463
12 15.07 69.11 2.49 0.90  0.0 - 2.5  1.44  998 400 alpha bootis
104 83.82 66.36 1.15 -0.28 -0.7 - 0.4  1.47 1447 100 1411+442
136 105.99 74.29 1.70 0.44 -0.5 - 2.0  1.19 1034 230 NGC 5055
136 108.16 71.47 1.28 0.44 -0.2 - 2.6  1.26  995 200 G107+71
190 142.84 84.22 1.22 0.17 -0.3 - 1.2  1.01 1032   0 NGC 4631
198 140.34 84.70 1.22 0.13 -0.3 - 0.8  1.07 1061   0 NGC 4656
202 145.55 64.98 1.86 0.49 -0.1 - 3.0  1.28  789 240 1150+497
202 140.84 61.39 1.32 0.44 -0.2 - 1.8  0.79  862 300 PHAD
207 150.05 67.76 1.70 0.41 -0.3 - 2.3  1.22  895 250 Mrk 42
261 188.87 82.05 1.75 0.77  0.2 - 2.2  0.85 1068 100 B2 1225+317
306 210.94 63.39 2.02 1.75  0.6 - 7.2 -0.04  992 300 G211+63
356 240.97 65.94 2.80 2.36  1.8 - 3.2  0.03  942   0 PG 1121+145
363 242.99 77.17 2.13 1.85  1.1 - 3.5 -0.05 1136 100 N79-299A
382 255.55 66.54 3.37 3.66  2.1 - 8.6 -0.94  997 250 11395+1033
440 305.51 78.57 1.91 1.26  0.7 - 2.9  0.42 1215 200 3C277.2
504 350.80 65.49 1.99 1.00  0.3 - 2.3  0.82 1196 200 E1401+098

Table 2: Southern Galactic Pole Fields
DBB Cloud l b < NH > a < I100 > b I100 range NHoffset Ravgc Shadow depth Field
5 4.67 -64.11 1.33 0.39 -0.2 - 1.4  0.87  987   0 IC 1459
17 8.90 -81.24 1.33 0.12 -0.4 - 1.4  1.19  844 230 A2744
17 19.53 -80.99 1.33 0.15 -0.4 - 1.1  1.16 1575 500 ESO 409 G-25
26 25.19 -75.87 1.55 0.50  0.0 - 1.6  0.96  968 210 A4038
33 26.22 -67.21 2.45 1.83  0.7 - 4.7  0.30  743 180 G026-67
99 85.82 -85.86 1.40 0.62  0.1 - 4.9  0.67  652   0 HR 173
168 122.32 -72.35 3.71 2.45  1.8 - 4.4  0.83  481  50 PKS0048-097
188 138.00 -65.72 4.06 4.12  2.7 - 8.5 -0.79  499 100 G138-66
222 147.06 -76.66 1.90 0.73  0.2 - 1.6  1.03  634   0 Mrk 1152
222 151.82 -75.04 1.78 0.91  0.4 - 1.7  0.71  666   0 RX J0120.0 135
232 162.49 -72.24 2.07 0.77  0.3 - 1.6  1.16  649   0 Abell 222
246 167.76 -57.98 2.43 1.72  1.2 - 2.7  0.41  472  90 o Ceti
266 160.81 -65.89 2.17 1.32  0.7 - 2.3  0.62  593  40 ARP 318
192.10 -68.10 2.76 1.72  0.6 - 5.7  0.73  540  80 G192-67
324 225.31 -66.33 1.92 0.79 -0.2 - 7.5  0.99  936 300 G225-66
328 220.03 -77.36 1.51 0.25 -0.2 - 1.7  1.21  694 150 RX J1048.4-2758
335 229.01 -63.97 2.05 0.62  0.0 - 2.0  1.33  907   0 IC 1860
355 237.28 -65.65 2.78 0.74  0.1 - 3.6  1.92  876 200 FORNAX
378 234.16 -88.56 1.88 0.68  0.1 - 2.4  1.08  601 140 GSGP4
417 283.17 -78.27 1.75 0.27 -0.3 - 2.2  1.44  760 150 NGC 424
489 359.66 -86.88 1.54 0.39 -0.2 - 1.4  1.08  808   0 SCULPTOR C,D

a HI in units of 1020 cm-2 from Stark et al. 1992
b Average IRAS 100 mm intensity in units of MJ  sr-1
c 1/4 keV band ate in units of 10-6 counts s-1 arcmin-2

Target Locations
Figure 1: ROSAT all-sky survey images of the 1/4 keV band for the north and south galactic poles. The directions of the fields toward the clouds considered in this poster are indicated.

3  ROSAT Data Reduction

Observations of extended emission with the ROSAT PSPC require special procedures to remove background contamination and to properly determine the exposure across the detector. I follow the procedures described in Snowden et al. (1994a). The 1/4 keV band I used consists of the R1L and R2 pulse height bands[13]. Large variations in the background level for some of the observations required the elimination of large fractions of the total observed time from analysis. Maps of the counts, exposure, and modeled background were made with ~ 15 arcsec pixels for each of the fields. Point sources were searched for by sliding a circular source region and concentric annular background region over the maps; the diameters of the circle and annulus were adjusted to account for the variation in the telescope point spread function with off-axis angle. Areas where counts within the source region were inconsistent with the expected background at the 3-s level were masked out during subsequent analysis as sources. Regions in which the exposure was less than 10% of the average exposure over the central 40 arcmin diameter of the PSPC were also excluded from analysis.

Tables 1 and 2 include the average 1/4 keV intensity and an estimate of the depth of any shadow that may be present for each of the observed fields. Several fields do not show any evidence for a shadow. Figure 2 shows one of the better examples of shadowed 1/4 keV emission (cloud 306 or G211+63). Both the 1/4 keV and the 100 micron maps have been smoothed with a gaussian with a ~ 1 arcmin sigma for display purposes. The color image is the 1/4 keV intensity; the contours are 100 micron intensity.

1/4 keV image of G211+63 field with IRAS 100 micron contours
Figure 2: Map of the 1/4 keV emission toward one of the better examples of shadowing, cloud 306 (G211+63); color indicates brightness (blue - faint / red - bright). Contours show IRAS 100 micron emission and are spaced every 0.5 MJy sr-1.

4  Comparison to IRAS 100 mm emission

Photoelectric absorption by neutral gas associated with the dust responsible for the IRAS 100  mm emission causes an anticorrelation between the 1/4 keV and 100 mm intensities. Photoelectric absorption in the 1/4 keV band is predominantly due to H atoms (from both HI and H2) and HeI in the neutral gas. A simple model for the expected 1/4 keV band counting rates is shown schematically in figure 3. The model has two x-ray emitting components: a local uniform component across the target field and partially absorbed emission from a uniform distant component located beyond all of the absorbing gas.

Absorption Geometry
Figure 3: Cartoon of the absorption geometry.

In order to obtain reasonable statistical uncertainties on the 1/4 keV band rates, the counts, exposure, and background maps were binned to ~ 8 arcmin cells and rates for each cell were calculated. The average 100 mm intensity for each cell was also calculated. The observed rates for each field were fit with a model of the form:

ROBSi = RL+RDexp(-sNH NHi),
(1)
where ROBSi is the observed rate in the ith cell, RL is the rate due to the local emitting region, RD is the unabsorbed rate of the distant emitting region, sNH is the column density dependent photoelectric absorption cross section, and the column density in the ith cell is given by:
NHi = aI100mi+NHoffset
(2)
where I100mi is the average 100 mm intensity in the cell, a = 1.176×1020 cm-2/(MJy sr-1) and NHoffset is determined from a comparison of the average IRAS 100 micron for the field and the HI column density from Stark et al. (1992)[17]. Values for NHoffset are listed in tables 1 and 2. The absorption cross section as a function of column density was calculated for a 106 K thermal equilibrium plasma. Only RL and RD were allowed to vary during the fits. As an example, figure 4 displays the results of the fit to cloud 306 (G211+36).

anticorr.gif
anticorr_chisq.gif
Figure 4: Example of the fit to the absorption of the 1/4 keV emission by neutral material traced by 100 micron emission. The field shown in this fit is the same one displayed in figure 2.

5  Results & Discussion

Tables 3 and 4 list the results of the fits described in the previous section. In many of the fields the absence of a strong shadow result in an upper limit on the brightness of the component beyond absorbing material or strongly coupled derived brightnesses of the two x-ray emitting components. A comparison of the average rates with the fit local rates indicates that most of the observed intensity has an origin in front of the absorbing material (the Local Bubble). To truly say that the fit values RD reflect a halo component we must: know that the absorbing material causing the shadow is within the ``disk'' of the galaxy and assess the contribution from any extragalactic component.

Table 3: Northern Field Fits
Rates in units of 10-6 counts s-1 arcmin-2
DBB Cloud l b RL RD c2/n Field
8 5.82 72.75 580±20 2918±140 322/172 Mrk 463
12 15.07 69.11 643±20 3613±180 362/168 alpha bootis
104 83.82 66.36 946±55 1731±200 580/149 1411+442
136 105.99 74.29 754±50 1030±190 248/173 NGC 5055
136 108.16 71.47 718±33 1422±180 271/174 G107+71
190 142.84 84.22 817±115 727±425 216/169 NGC 4631
198 140.34 84.70 959±70 308±270 307/163 NGC 4656
202 145.55 64.98 578±40 1299±250 223/173 1150+497
202 140.84 61.39 574±40 1040±160 215/170 PHAD
207 150.05 67.76 710±40 928±210 242/172 Mrk 42
261 188.87 82.05 808±45 1592±390 237/161 B2 1225+317
306 210.94 63.39 809±14 952±80 239/178 G211+63
356 240.97 65.94 843±60 999±675 274/177 PG 1121+145
363 242.99 77.17 1028±40 853±320 247/171 N79-299A
382 255.55 66.54 893±20 1347±300 212/179 11395+1033
440 305.51 78.57 963±35 1660±220 372/174 3C277.2
504 350.80 65.49 984±60 1318±410 328/174 E1401+098

Table 4: Southern Field Fits
Rates in units of 10-6 counts s-1 arcmin-2
DBB Cloud l b RL RD c2/n Field
5 4.67 -64.11 932±46 213±190 354/166 IC 1459
17 8.90 -81.24 621±45 806±180 289/161 A2744
17 19.53 -80.99 774±115 3199±480 292/172 ESO 409 G-25
26 25.19 -75.87 744±100 981±490 186/161 A4038
33 26.22 -67.21 621±15 998±120 221/171 G026-67
99 85.82 -85.86 622±65 £290 244/174 HR 173
168 122.32 -72.35 442±34 £1550 162/175 PKS0048-097
188 138.00 -65.72 461±15 998±380 196/175 G138-66
222 147.06 -76.66 589±60 £620 191/175 Mrk 1152
222 151.82 -75.04 654±60 £360 217/161 RX J0120.0 135
232 162.49 -72.24 606±60 £775 265/169 Abell 222
246 167.76 -57.98 340±50 1182±470 277/169 o Ceti
266 160.81 -65.89 529±45 £750 231/173 ARP 318
192.10 -68.10 466±12 617±125 366/176 G192-67
324 225.31 -66.33 727±20 1001±105 258/175 G225-66
328 220.03 -77.36 423±70 1236±350 212/171 RX J1048.4-2758
335 229.01 -63.97 858±40 £575 257/171 IC 1860
355 237.28 -65.65 661±35 2493±450 196/176 FORNAX
378 234.16 -88.56 417±15 1134±90 423/159 GSGP4
417 283.17 -78.27 550±50 1139±300 214/170 NGC 424
489 359.66 -86.88 744±50 £480 246/171 SCULPTOR C,D

The distances to these clouds are not known. Interstellar absorption line measurements toward several stars at a variety of distances in the direction of each of the clouds could be used limit the distance to the cloud; without such distance limits we must resort to other determinations. Most of the clouds in this data set for which velocities have been determined are at low velocity. Statistical arguments have been made[8] that assign these clouds to the edges of the Local Bubble.

The extragalactic intensity at 1/4 keV is unknown but observations searching for shadows caused by external galaxies[1,6,9] have provided both upper ( 62 keV cm-2 s-1 sr-1 keV-1) and lower ( 32 keV cm-2 s-1 sr-1 keV-1) limits on the extragalactic 1/4 keV surface brightness. If we use the average of these two values as an estimate of the surface brightness of the extragalactic component and convert to rates, the extragalactic contribution is ~ 580×10-6 counts s-1 arcmin-2. This value includes a correction for absorption by neutral material within the extended warm, partially ionized gas traced by Ha emission; this correction was not included in the fits that I performed and so for comparison the above rate should be reduced to ~ 390×10-6 counts s-1 arcmin-2.

The variation in the values of RD suggest that the region causing the halo emission does not extend far from the disk of the galaxy. It may be reasonable to assume that much of the hot halo gas lies closer than the gas responsible for the Ha emission. If this is the case the rates for the halo emission do not require a correction for absorption by that extended, warm material.

In the north the average value of all the points after removing a contribution from extragalactic emission is ~ 1000×10-6 counts s-1 arcmin-2. A plasma in collisional equilibrium at 106 K will produce this rate at an emission measure of ~ 5.3×10-3 cm-6 pc. While it is unlikely that the emission is in collisional equilibrium, this model for the emission provides a convenient reference for comparison with other results. A 106 K collisional plasma has a power per unit emission integral of 1.13×10-22 erg cm3 s-1 so an emission measure of 5.3×10-3 cm-6 pc implies an average surface brightness of the local galactic halo of ~ 1.8×1037 ergs s-1 kpc-2. Two of the lines of sight (clouds 8 and 12) are in the direction of the North Polar Spur (NPS), a region of enhanced emission associated with the Sco-Cen OB association. This nearby ``bubble'' is large enough that it may have broken out into the lower portion of the galactic halo and could be a source of hot gas into the halo. If the two points toward the NPS are ignored the average drops to ~ 760×10-6 counts s-1 arcmin-2.

In the south the average value of the points that are not just upper limits is ~ 925×10-6 counts s-1 arcmin-2, after removing a contribution from the extragalactic emission. This value is about the same as in the north but since the upper limits are not included and most of the upper limits are less than this value even before accounting for an extragalactic contribution it is clear the local halo in the south is not as bright as in the north. Calculating an average in the south using all of the points and using half the value of the upper limits as a value for RD results in a halo rate of ~ 500×10-6 counts s-1 arcmin-2.

References

[1]
Barber, C. R., & Warwick, R. S. 1994, MNRAS, 267, 270.
[2]
Bowyer, C. S., Field, G. B., & Mack, J. E. 1968, Nature, 217, 32.
[3]
Bregman, J. N. & Pildis, R. A. 1994, ApJ, 420, 570.
[4]
Burrows, D. N. & Mendenhall, J. A. 1991, Nature, 351, 629.
[5]
Cui, W., Sanders, W. T., McCammon, D., Snowden, S. L., & Womble, D. S. 1996, ApJ, 468, 102.
[6]
Cui, W., Sanders, W. T., McCammon, D., Snowden, S. L., & Womble, D. S. 1996, ApJ, 468, 117.
[7]
Heiles, C., Reach, W. T., & Koo, B.-C. 1988, ApJ, 332, 313.
[8]
Magnani, L., Hartmann, D., & Speck, B. G. 1996, ApJS, 106, 447.
[9]
McCammon, D. & Sanders, W. T. 1990, ARA&A, 28, 657.
[10]
Pietsch, W., Vogler, A., Kahabka, P., Jain, A. & Klein, U. 1994, A&A, 284, 386.
[11]
Snowden, S. L., Cox, D. P., McCammon, D., & Sanders, W. T. 1990, ApJ, 354, 211.
[12]
Snowden, S. L., Mebold, U., Hirth, W., Herbstmeier, U., & Schmitt, J. H. M. M. 1991, Science, 252, 1529.
[13]
Snowden, S. L., McCammon, D. Burrows, D. N., & Mendenhall, J. A. 1994a, ApJ, 424, 714.
[14]
Snowden, S. L., Hasinger, G., Jahoda, K., Lockman, F. J., McCammon, D., & Sanders, W. T. 1994b, ApJ, 430, 601.
[15]
Snowden, S. L., Egger, R., Freyberg, M. J., McCammon, D., Plucinsky, P. P., Sanders, W. T., Schmitt, J. H. M. M., Truemper, J., Voges, W. 1997, ApJ 485, 125.
[16]
Spitzer, L. 1956, ApJ, 124, 20.
[17]
Stark, A. A., Gammie, C. F., Wilson, R. W., Bally, J., Linke, R. A., Heiles, C., & Hurwitz, M. 1992, ApJS, 78, 77.
[18]
Wang, Q. D., & Yu, K. C. 1995, AJ, 109, 698.
[19]
Wang, Q. D., Waterbos, R. A. M., Streakley, M. F., Norman, C. A. & Braun, R. 1995, ApJ, 439, 176.
Acknowledgments
This research made significant use of the SkyView program developed under the auspices of the High Energy Astrophysics Science Archive Research Center (HEASARC) at the GSFC Laboratory for High Energy Astrophysics.


File translated from TEX by TTH, version 2.01.
On 14 Apr 1999, 16:34.