Friday 2/23/2007H II Nuclei in Nearby GalaxiesWendy Hawley’s Journal Club1H II Nuclei in Nearby Galaxies“Properties of H II Regions in the Centers of Nearby Galaxies” (Ho, L.C.,Filippenko, A.V., & Sargent, W.L.W. 1997, ApJ 487, 579)"Dynamical Evidence a for Massive, Young Globular Cluster in NGC1569" (Ho, L.C. & Filippenko, A.V. 1996, ApJ 466, L83)Wendy HawleyJournal Club 2/23/07Friday 2/23/2007H II Nuclei in Nearby GalaxiesWendy Hawley’s Journal Club2Outline• H II Regions in Nearby Galaxies– Motivation– The New Survey– Basic Properties of Nuclear H II Regions– Discussion– Summary• Evidence for a massive, young GC in NGC 1569Friday 2/23/2007H II Nuclei in Nearby GalaxiesWendy Hawley’s Journal Club3Motivation• Understanding H II regions can lead to anunderstanding of other problems in astrophysics• H II nuclei serve as probes of massive star formation• More feeble in output than starburst nuclei, but thereare a large # of nearby H II nuclei• Comparison with disk H II regions gives insight• This paper uses a new survey to study H II nucleiFriday 2/23/2007H II Nuclei in Nearby GalaxiesWendy Hawley’s Journal Club4The New Survey• optical spectroscopic survey of bright nearby galacticnuclei (Ho, Filippenko & Sargent 1995)• used Hale 5m at Palomar to obtain high S/N, moderate-resolution (100-200 km/s) spectra• galaxies sample: all galaxies in the Revised Shapley-AmesCatalog of Bright Galaxies with δ>0 degrees and BT≤12.5mag• nearly statistically complete and contains 486 galaxiesover a range of all morphological types• because of the size of sample and selection criteria, thesample is a fair representation of local (z~0) high surfacebrightness galaxiesFriday 2/23/2007H II Nuclei in Nearby GalaxiesWendy Hawley’s Journal Club5H II nuclei detected• dominant ionization mechanism for each nucleus determined bylooking at intensity ratios for prominent optical emission lines• H II nuclei, like disk H II regions, are defined byphotoionization by UV radiation from young, massive stars asprimary source of ionization• from 4200-6900 Å, strongest emission lines from hydrogenrecombination and [O III] λλ4959, 5007• strength of [O III] lines relative to hydrogen depends onexcitation• H II nuclei distinguished from AGNs by comparitively weaklow-ionization transitions of [N II] λλ6548, 6583, [S II] λλ6716, 6731, and [O I] λλ6300, 6363• 206 total H II nuclei detected, ~80% observed underphotometric conditionsFriday 2/23/2007H II Nuclei in Nearby GalaxiesWendy Hawley’s Journal Club6Uncertainties• data contains limited spatial information• Pogge (1989) found that H II nuclei line emission patternsare complicated and extended• spectra extracted from a fixed aperture, therefore physicalsize varies depending on distance• uncertainty for extended line-emitting material: flux willbe underestimated• uncertainty for compact emission: flux may beoverestimated due to integration over several discrete andpossibly overlapping regions• sample is large, so the individual fluctuations shouldaverage outFriday 2/23/2007H II Nuclei in Nearby GalaxiesWendy Hawley’s Journal Club7Comparison Data• comparison to starburst nuclei:– 62 objects in French (1980), Balzano (1983), andVeilleux & Osterbrock (1987) for forbidden-lineintensities and 98 objects in Balzano (1983) for Hαluminosities• comparison to disk H II regions:– line-intensity ratios for ~200 regions in disks fromMcCall et al. (1985), Ryder (1995). line measurementsfor dwarf irregulars from French (1980) and Dinerstein& Shields (1986). Hα luminosities for 95 disk H IIregions from Kennicutt (1988)Friday 2/23/2007H II Nuclei in Nearby GalaxiesWendy Hawley’s Journal Club8Basic Properties of H II Nuclei• H II nuclei 42% of all galaxies with BT<12.5 mag• Late-types contain H II nuclei more frequentlythan early-types (Sc-Im: up to 80%, Sb: 51%, Sa:22%, S0: 8%, none in ellipticals)• galaxies with H II nuclei are less luminous thanthose with AGN• early refers to S0-Sbc, late refers to Sc-I0, and sixpeculiar galaxies omittedFriday 2/23/2007H II Nuclei in Nearby GalaxiesWendy Hawley’s Journal Club9FIG 1LuminosityDistributionearlylateLess luminous,but overlappingwith earlyFriday 2/23/2007H II Nuclei in Nearby GalaxiesWendy Hawley’s Journal Club10Barred Statistics• 50-60% of disk galaxies in survey are barred(about the same as general galaxy population)• barred systems should have enhanced starformation at centers• evidence found for increased star formation rate inbarred sample• bars exert greatest impact on star formation inearly-type nuclei and minor effects in late-typesFriday 2/23/2007H II Nuclei in Nearby GalaxiesWendy Hawley’s Journal Club11FIG 2Hα luminosityDistributionFriday 2/23/2007H II Nuclei in Nearby GalaxiesWendy Hawley’s Journal Club12Early- and Late-type difference• from Kolmogorov-Smirnov (K-S) test, the probability theearly and late types are drawn from the same population isonly PKS=1.9 x 10-5• The difference in luminosities cannot be due only to thedifference in median distance (21.5 Mpc for early and 16.9for late)• If surface brightness of Hα emission constant in centralregions of both groups, it would only account for a 70%difference• the real reason: enhancement of central star formation frombar-driven gas inflow, which affects only resulting early-type systemsFriday 2/23/2007H II Nuclei in Nearby GalaxiesWendy Hawley’s Journal Club13FIG 3Balmer Equiv.WidthsFriday 2/23/2007H II Nuclei in Nearby GalaxiesWendy Hawley’s Journal Club14FIG 4ReddeningFriday 2/23/2007H II Nuclei in Nearby GalaxiesWendy Hawley’s Journal Club15Reddening Uncertainty• K-S test shows difference is significant between disk andearly-type (PKS=0.0022), but insignificant between diskand late-type (PKS=0.29)• uncertainty: unknown source of systematic error maycause higher value of reddening for early-type galaxies• although this affects quantitative results, the conclusionsshould be the same• potentially worse uncertainty is the lack of knowledgeabout total extinction in galactic nuclei• method: using Balmer decrement to estimate extinctionassumes uniform foreground dust screen, which isespecially an oversimplification in nuclear regions• measure free-free continuum in radio wavelengths wouldbe better, but hasn’t been applied to nucleiFriday 2/23/2007H II Nuclei in Nearby GalaxiesWendy Hawley’s Journal Club16Electron Density