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Haj&la, OKiK CotOH , TUcis is* puAcT^i To occcaw. (—- TUa_ HeVc^-GV^VU^ CO\f\ at the UR des an distri- ^dium, 5 basic those atures solar *ate is stend- •out I , e.g., )f ion- log Ih. [R] '-, e.g., le av- 0.00 0.25 0.50 0.75 e-half 1.00 1.25 > 1.50 FIG. 1. Composite all-sky velocity-integrated Ha map. Data along from VTSS, SHASSA, and the WHAM northern sky survey (DM (WHAM-NSS) have been combined to produce these very i the deep (EM 2=1 cm_6pc) emission maps. The two Aitoff- irived Hammer projections are centered at (top) €=0° and (bottom) (Ha) €=180°. Areas covered by the imaging surveys have arc- minute resolution while those only surveyed by WHAM have ^-free one-degree resolution. Emission is predominantly from the .on of Galaxy, although the imaging surveys can contain other bright, indi- extended sources from the local universe (|uLsrI^500 kms"1; n av- LMC, SMC, M31, etc.). Adapted from Finkbeincr, 2003. ing a fg6.ti.jpg (JPEG Image, 1216x1793 pixels) - Scaled... http://www.iop.Org/EJ/article/0067-0049/149/2/405... Log Intensity [Royleighs] lofl 02/19/09 13:40 980 Haffner etal.: The warm ionized medium inspiral galaxies NGC 157 Q 3B.5 > log L r- . NGC 157 Q 38.5 < log L < 39.6 O 37.5 < log L < 38.5 »* * Q2, o log I < 37.5 . t' k*M * • 9Hi * •> .•A • • •> V. %* I H*-km* v • %\ • • •• i , a«i>o . — II FIG. 9. Steps in the quantification of the total Ho- luminosity from the DIG for the representative disk galaxy NGC 157. (a) Continuum subtracted Ha image, (b) Schematic form of H II region catalog, giving position and key to the Ha luminosity of each region, (c)Diffuse Ha map after subtracting off the catalogued H II regions.The brightest H II regions are indicated by circles, (d) Measurement of upper limit for DIG. H II regions are blanked off, then each is assigned a local value of DIG surface brightness. Ellipse shows limit of integrated DIG flux measured. From Zurita et al., 2000. then adding a contribution from the areas of the H II We can see that the DIG emits around half of the regions, assuming this is proportional to their projected total Ha output, that there are systematic modulations areas times their local DIG surface brightness; or (c), as of this tendency with radius, and that there is a slight in (a) but making the contribution proportional to the tendency for the fraction to increase with radius. We area of the H II regions times the mean DIG surface also note that the projected area of the disk occupied by brightness outside the regions across the disk of the gal the DIG is of order 80(±10)% for all objects shown in axy. Modes (a), (b), and (c) give, respectively, lower lim Fig. 11. In a separate study of over 100 galaxies, Oey et its, upper limits, and approximate estimates for the total DIG emission from the observed galaxies. In Fig. 11 we al. (2007) found that the amount of DIG Ha to total Ha show the results of this method of estimating the DIG, from a galaxy ranged from 20% to nearly 100% with a for six galaxies, shown as the ratio of the DIG Ha lumi- mean near 60%. Voges (2006) presented results suggest vosfcj to \Yve total luminosity for the galaxy plotted in ing an inverse correlation between the DIG Ha fraction terms of galactocentric radius. and the star formation rate per unit area. Rev. Mod. Phys., Vol. 81, No. 3, July-September2009 982 Haffner et al.: The warm ionized medium in spiral galaxies observed DIG surface brightness is not uniform on large scales, as would be expected since the initial model as sumes a uniform slab structure for the HI involved in converting the Lyman continuum photons to Ha. The missing structural parameter can be supplied using the observed HI column density, as shown in Fig. 13, where in zones of low H I column density the ratio of observed to predicted DIG surface brightness is reduced. Maps of these two quantities give excellent coincidence of fea tures, and go a step further in showing that the principal DIG ionization sources must be the O stars in luminous H II regions. Modeling the effect of clustered superno- vae on the distribution of the H I, Clarke and Oey (2002) also found that the resulting dumpiness of the medium had a significant effect on the escape fraction of the ion FIG. 11. Radial variation of the ratio of integrated DIG lumi nosity in Ha to the total luminosity for six spiral galaxies izing radiation. Zurita et al. (2002) carried out a number (within 0.15R/R2s crowding precludes accurate estimates). A of different modeling tests of the basic hypothesis, vary canonical value for the integrated ratio of 2=50% is found, with ing the law relating the escape fraction of ionizing pho a total fractional area subtended by the DIG of —80%. From tons with the luminosity of an H II region, and varying Zurita et al, 2000. the mean absorption coefficient of the inhomogeneous i neutral fraction of the DIG. However, they concluded that it was not possible without H I data of improved cause of the availability of a VLA H I map of reasonable angular resolution to go further in testing different pho resolution, as explained shortly. In Fig. 12, we show a ton escape laws, or to estimate what fraction of the DIG comparison between the observed surface brightness ionization could be due to mechanisms other than that distribution in the DIG and one of the simplest models tested. used. In this model, 30% of emitted Lyman continuum photons escape from each H II region, and propagate through the DIG isotropically. C. Line ratio studies The predicted Ha surface brightness is derived by summing the contributions to the ionizing radiation field There have been relatively few quantitative spectro from each of the H II regions. We can see that the result scopic studies of the DIG in face-on galaxies. A pioneer is remarkably similar globally to the observed distribu ing study of [N II] and Ha across the face of NGC 1068 tion, and is itself a fair verification of the initial hypoth was carried out by Bland-Hawthorn et al. (1991), who esis. However, a more quantitative look at the compari found very high [N ll]/Ha ratios and discussed possible son shows that the ratio between the predicted and causes for this high excitation. Hoopes and Walterbos NGC 157-Constant escape fraction (30%) NGC 157—Oeprojected Ho image ^••fito^ > VK& C Q - *T. "; ° v^cjja .• ^^mff^^m^' —~73hRftJrT Jmh "t^ggggTW' '"'" • - 3 kpc '" 3 kpc -...*«*. -y - i i FIG. 12. Comparison of a DIG model withobservations. (Left) Modeled surface brightness in Ha of the DIG in the diskof NGC 157 assuming 30% of Lye photons escape from each H II region, and a simple propagation law through a (macroscopically) uniform slab model for the disk. The result is a projection in the plane of the predicted 3D Ha column density. (Right) Deprojected image of the galaxy with H II regions masked, and a cutoff of 73 Xcos i pccm-6 applied to limit the H II region contamination of the DIG. This shows the basic similarity between these models and the observed DIG, though further refine ments are important (see Fig. 13). From Zurita et al., 2002. Rev. Mod. Phys., Vol. 81, No. 3, July-September 2009 fg9.1i.gif (GIF Image, 1305x1336 pixels) - Scaled ... http://www.iop.Org/EJ/article/0004-63 7X/5 23/1/223 x (A 0.5 // J'-v.v..V'--'.:<:.T-"fi-.-: V. 0.0 hi • 0.0 0.5 1.0 1.5 [N il]/Hcx lofl 02/19/09 13:43 REVIEWS OF MODERN PHYSICS, VOLUME 81, JULY-SEPTEMBER 2009 The warm Ionized medium in spiral galaxies L. M. Haffner Departmentof Astronomy, University of Wisconsin-Madison, Madison, Wisconsin USA, 53706 R.-J. Dettmar Astronomisches Institut, Ruhr-University Bochum, Germany J.
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