The initial cluster mass function (ICMF) has been studied in disk galaxies and the Magellanic clo... more The initial cluster mass function (ICMF) has been studied in disk galaxies and the Magellanic clouds using the luminosity function of clusters found in those galaxies. In order to better gauge the universality of the ICMF we have expanded the survey to include stellar clusters in 19 irregular galaxies in a volume-limited survey using data from the Sloan Digital Sky
The natal environments of massive star clusters in starburst galaxies have not been studied well ... more The natal environments of massive star clusters in starburst galaxies have not been studied well even though such information would be vital to explain the contribution (or lack thereof) of these systems to the feedback mechanisms of star formation in galaxies. Fourteen embedded star clusters in M33 were first detected in radio imagery and are thought to be extremely young
HST/WFPC2 imagery and STIS spectra of the circumstellar shell NGC 6888 are presented. The nebula ... more HST/WFPC2 imagery and STIS spectra of the circumstellar shell NGC 6888 are presented. The nebula was formed by the collision of the stellar wind of the WN6 star HD 192163 with material shed during its preceding red supergiant phase. The resulting nebula is an 8 pc x 5 pc photoionized shell enveloped in a shock-heated skin of emission powered by
We present $Spitzer$ IRAC and MIPS imaging of four low surface brightness (LSB) galaxies: Malin 1... more We present $Spitzer$ IRAC and MIPS imaging of four low surface brightness (LSB) galaxies: Malin 1, UGC 6614, UGC 6879, UGC 9024. Our study shows that the gigantic disks such as Malin 1 and UGC 9024 appear just like a point source in the IRAC and MIPS images. Their morphological features suggest that IR emission is concentrated in central few
Proceedings of The International Astronomical Union, 1999
We report the results of longslit spectroscopy of the wind-blown bubble and photoevaporating knot... more We report the results of longslit spectroscopy of the wind-blown bubble and photoevaporating knots around the O6.5iiif star BD+ 60(deg) 2522 made with the Space Telescope Imaging Spectrograph. The Of star is the primary ionizing source for the H ii region NGC 7635, located in the Perseus Arm. The spectra were taken through a 0.2'' x 52'' slit with low
We present the results of our examination of newly acquired images obtained with WFPC-2 of the Bu... more We present the results of our examination of newly acquired images obtained with WFPC-2 of the Bubble Nebula, NGC 7635. These images were taken through narrow-band filters centered on important nebular diagnostic lines, including H alpha, H beta, [O III] at 500.7 nm, [N II] at 658.4 nm, [S II] at 671.7+673.1 nm and the line-free continuum filter F547M. The
Wind-blown bubbles are formed by the interaction of stellar ionizing radiation and winds on the c... more Wind-blown bubbles are formed by the interaction of stellar ionizing radiation and winds on the circumstellar material. The ram pressure of the confined wind sweeps up the ambient enivronment into a shell made visible by virtue of its ionization state, maintained by shock-heating and the UV flux of the star. Depending on the mass-loss history of the central star, this
We noticed an error in the coefficient of equation (3); the correct equation is((taucool)/yr)~5.7... more We noticed an error in the coefficient of equation (3); the correct equation is((taucool)/yr)~5.7×107(nu/GHz)-1/2(B/muG)1/2((UB+Urad)/(10-12 ergs cm-3))-1. (1)This did not impact the results or conclusions made in the rest of the text. All calculations using this equation were in fact correct.
With the goal of investigating the degree to which the mid-infrared emission traces the star form... more With the goal of investigating the degree to which the mid-infrared emission traces the star formation rate (SFR), we analyze Spitzer 8 µm and 24 µm data of star-forming regions in a sample of 33 nearby galaxies with available HST/NICMOS images in the Paα (λ1.8756 µm) emission line. The galaxies are drawn from the SINGS sample, and cover a range of morphologies and a factor ∼10 in oxygen abundance. Published data on local low-metallicity starburst galaxies and Luminous Infrared Galaxies are also included in the analysis. Both the stellar-continuum-subtracted 8 µm emission and the 24 µm emission correlate with the extinction-corrected Paα line emission, although neither relationship is linear. Simple models of stellar populations and dust extinction and emission are able to reproduce the observed non-linear trend of the 24 µm emission versus number of ionizing photons, including the modest deficiency of 24 µm emission in the low metallicity regions, which results from a combination of decreasing dust opacity and dust temperature at low luminosities. Conversely, the trend of the 8 µm emission as a function of the number of ionizing photons is not well reproduced by the same models. The 8 µm emission is contributed, in larger measure than the 24 µm emission, by dust heated by non-ionizing stellar populations, in addition to the ionizing ones, in agreement with previous findings. Two SFR calibrations, one using the 24 µm emission and the other using a combination of the 24 µm and Hα luminosities (Kennicutt et al. 2007a), are presented. No calibration is presented for the 8 µm emission, because of its significant dependence on both metallicity and environment. The calibrations presented here should be directly applicable to systems dominated by on-going star formation.
One possible way for spiral galaxies to internally evolve would be for gas to flow to the center ... more One possible way for spiral galaxies to internally evolve would be for gas to flow to the center and form stars in a central disk ( pseudo-bulge). If the inflow rate is faster than the rate of star formation, a central concentration of gas will form. In this paper we present radial profiles of stellar and 8 m emission from polycyclic aromatic hydrocarbons (PAHs) for 11 spiral galaxies to investigate whether the interstellar medium in these galaxies contains a central concentration above that expected from the exponential disk. In general, we find that the two-dimensional CO and PAH emission morphologies are similar, and that they exhibit similar radial profiles. We find that in 6 of the 11 galaxies there is a central excess in the 8 m and CO emission above the inward extrapolation of an exponential disk. In particular, all four barred galaxies in the sample have strong central excesses in both 8 m and CO emission. These correlations suggest that the excess seen in the CO profiles is, in general, not simply due to a radial increase in the CO emissivity. In the inner disk, the ratio of the stellar to the 8 m radial surface brightness is similar for 9 of the11 galaxies, suggesting a physical connection between the average stellar surface brightness and the average gas surface brightness at a given radius. We also find that the ratio of the CO to 8 m PAH surface brightness is consistent over the sample, implying that the 8 m PAH surface brightness can be used as an approximate tracer of the interstellar medium.
Using data obtained for twelve galaxies as part of the Spitzer Infrared Nearby Galaxies Survey (S... more Using data obtained for twelve galaxies as part of the Spitzer Infrared Nearby Galaxies Survey (SINGS) and the Westerbork Synthesis Radio Telescope (WSRT)-SINGS radio continuum survey, we study how star formation activity affects the far-infrared (FIR)-radio correlation within galaxies by testing a phenomenological model which describes the radio image as a smeared version of the FIR image. The physical basis of this description is that cosmic-ray (CR) electrons will diffuse measurably farther than the mean free path of dust-heating photons before decaying by synchrotron radiation. This description works well in general. Galaxies with higher infrared surface brightnesses have best-fit smoothing scale-lengths of a few hundred parsecs, substantially shorter than those for lower surface brightness galaxies. We interpret this result to suggest that galaxies with higher disk averaged star formation rates have had a recent episode of enhanced star formation and are characterized by a higher fraction of young CR electrons that have traveled only a few hundred parsecs from their acceleration sites in supernova remnants compared to galaxies with lower star formation activity.
The published optical fluxes included two corrections from an air mass of unity to an air mass of... more The published optical fluxes included two corrections from an air mass of unity to an air mass of zero. The updated Table 2 below provides the optical fluxes after a single correction from an air mass of unity to an air mass of zero. Note that the supplemental RC3 BV data use the NED conversion from magnitudes to janskys.
The ultraviolet-to-radio continuum spectral energy distributions are presented for all 75 galaxie... more The ultraviolet-to-radio continuum spectral energy distributions are presented for all 75 galaxies in the Spitzer Infrared Nearby Galaxies Survey (SINGS). A principal component analysis of the sample shows that most of the sample's -2spectral variations stem from two underlying components, one representative of a galaxy with a low infrared-to-ultraviolet ratio and one representative of a galaxy with a high infrared-to-ultraviolet ratio. The influence of several parameters on the infrared-to-ultraviolet ratio is studied (e.g., optical morphology, disk inclination, far-infrared color, ultraviolet spectral slope, and star formation history). Consistent with our understanding of normal star-forming galaxies, the SINGS sample of galaxies in comparison to more actively star-forming galaxies exhibits a larger dispersion in the infrared-to-ultraviolet versus ultraviolet spectral slope correlation. Early type galaxies, exhibiting low star formation rates and high optical surface brightnesses, have the most discrepant infrared-to-ultraviolet correlation. These results suggest that the star formation history may be the dominant regulator of the broadband spectral variations between galaxies. Finally, a new discovery shows that the 24 µm morphology can be a useful tool for parametrizing the global dust temperature and ultraviolet extinction in nearby galaxies. The dust emission in dwarf/irregular galaxies is clumpy and warm accompanied by low ultraviolet extinction, while in spiral galaxies there is typically a much larger diffuse component of cooler dust and average ultraviolet extinction. For galaxies with nuclear 24 µm emission, the dust temperature and ultraviolet extinction are relatively high compared to disk galaxies.
Featured in Los Angeles Times "Illuminating a black hole" December 9, 2008 Reinhard Gen... more Featured in Los Angeles Times "Illuminating a black hole" December 9, 2008 Reinhard Genzel, leader of the European team based at the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, said researchers had the best evidence yet that "supermassive ...
Mid-infrared diagnostics are presented for a large portion of the Spitzer Infrared Nearby Galaxie... more Mid-infrared diagnostics are presented for a large portion of the Spitzer Infrared Nearby Galaxies Survey (SINGS) sample plus archival data from the Infrared Space Observatory and the Spitzer Space Telescope. The SINGS dataset includes low-and high-resolution spectral maps and broadband imaging in the infrared for over 160 nuclear and extranuclear regions within 75 nearby galaxies spanning a wide range of morphologies, metallicities, luminosities, and star formation rates. Our main result is that these mid-infrared diagnostics effectively constrain a target's dominant power source. The combination of a high ionization line index and PAH strength serves as an efficient discriminant between AGN and star-forming nuclei, confirming progress made with ISO spectroscopy on starbursting and ultraluminous infrared galaxies. The sensitivity of Spitzer -2allows us to probe fainter nuclear and star-forming regions within galaxy disks. We find that both star-forming nuclei and extranuclear regions stand apart from nuclei that are powered by Seyfert or LINER activity. In fact, we identify areas within four diagnostic diagrams containing >90% Seyfert/LINER nuclei or >90% H II regions/H II nuclei. We also find that, compared to starbursting nuclei, extranuclear regions typically separate even further from AGN, especially for low-metallicity extranuclear environments. In addition, instead of the traditional mid-infrared approach to differentiating between AGN and star-forming sources that utilizes relatively weak high-ionization lines, we show that strong low-ionization cooling lines of X-ray dominated regions like [Si II] 34.82 µm can alternatively be used as excellent discrimants. Finally, the typical target in this sample shows relatively modest interstellar electron density (∼ 400 cm −3 ) and obscuration (A V ∼ 1.0 mag for a foreground screen), consistent with a lack of dense clumps of highly obscured gas and dust residing in the emitting regions.
High resolution mid-infrared spectra are presented for 155 nuclear and extranuclear regions from ... more High resolution mid-infrared spectra are presented for 155 nuclear and extranuclear regions from the Spitzer Infrared Nearby Galaxies Survey (SINGS). The fluxes for nine atomic forbidden and three molecular hydrogen mid-infrared emission lines are also provided, along with upper limits in key lines for infrared-faint targets. The SINGS sample shows a wide range in the ratio of [S III] 18.71 µm/[S III] 33.48 µm, but the average ratio of the ensemble indicates a typical interstellar electron density of 300-400 cm −3 on ∼23 ′′ ×15 ′′ scales and 500-600 cm −3 using ∼11 ′′ ×9 ′′ apertures, independent of whether the region probed is a star-forming nuclear, a star-forming extranuclear, or an AGN environment. Evidence is provided that variations in gas-phase metallicity play an important role in driving variations in radiation field hardness, as indicated -2by [Ne III] 15.56 µm/[Ne II] 12.81 µm, for regions powered by star formation. Conversely, the radiation hardness for galaxy nuclei powered by accretion around a massive black hole is independent of metal abundance. Furthermore, for metal-rich environments AGN are distinguishable from star-forming regions by significantly larger [Ne III] 15.56 µm/[Ne II] 12.81 µm ratios. Finally, [Fe II] 25.99 µm/[Ne II] 12.81 µm versus [Si II] 34.82 µm/[S III] 33.48 µm also provides an empirical method for discerning AGN from normal star-forming sources. However, similar to [Ne III] 15.56 µm/[Ne II] 12.81 µm, these mid-infrared line ratios lose their AGN/star-formation diagnostic powers for very low metallicity star-forming systems with hard radiation fields.
We present new images (groundbased optical and mid-infrared (MIR) from the Spitzer Space Telescop... more We present new images (groundbased optical and mid-infrared (MIR) from the Spitzer Space Telescope) and spectra (from Spitzer) of the archetypal starburst galaxy M 82. The Spitzer data show that the MIR emission extends at least 6 kpc along the minor axis of the galaxy. We use the optical and infrared data to demonstrate that the extended emission is dominated by emission from dust. The colors of the MIR emission and the spectra indicate that there is a strong component of aromatic feature emission (the MIR features commonly attributed to polycyclic aromatic hydrocarbons). The dust continuum and aromatic feature emission are both strong in the well-known superwind region of this galaxy; clearly the carrier of the aromatic features can survive in close proximity to the wind, far from the plane of the galaxy. We also see significant emission by dust well outside the superwind region, providing the clearest picture to date of the dust distribution in the halo of this galaxy.
The quantity of dust in a spiral disk can be estimated using the dust's typical emission or the e... more The quantity of dust in a spiral disk can be estimated using the dust's typical emission or the extinction of a known source. In this paper, we compare two techniques, one based on emission and one on absorption, applied on sections of fourteen disk galaxies. The two measurements reflect, respectively the average and apparent optical depth of a disk section. Hence, they depend differently on the average number and optical depth of ISM structures in the disk.
The initial cluster mass function (ICMF) has been studied in disk galaxies and the Magellanic clo... more The initial cluster mass function (ICMF) has been studied in disk galaxies and the Magellanic clouds using the luminosity function of clusters found in those galaxies. In order to better gauge the universality of the ICMF we have expanded the survey to include stellar clusters in 19 irregular galaxies in a volume-limited survey using data from the Sloan Digital Sky
The natal environments of massive star clusters in starburst galaxies have not been studied well ... more The natal environments of massive star clusters in starburst galaxies have not been studied well even though such information would be vital to explain the contribution (or lack thereof) of these systems to the feedback mechanisms of star formation in galaxies. Fourteen embedded star clusters in M33 were first detected in radio imagery and are thought to be extremely young
HST/WFPC2 imagery and STIS spectra of the circumstellar shell NGC 6888 are presented. The nebula ... more HST/WFPC2 imagery and STIS spectra of the circumstellar shell NGC 6888 are presented. The nebula was formed by the collision of the stellar wind of the WN6 star HD 192163 with material shed during its preceding red supergiant phase. The resulting nebula is an 8 pc x 5 pc photoionized shell enveloped in a shock-heated skin of emission powered by
We present $Spitzer$ IRAC and MIPS imaging of four low surface brightness (LSB) galaxies: Malin 1... more We present $Spitzer$ IRAC and MIPS imaging of four low surface brightness (LSB) galaxies: Malin 1, UGC 6614, UGC 6879, UGC 9024. Our study shows that the gigantic disks such as Malin 1 and UGC 9024 appear just like a point source in the IRAC and MIPS images. Their morphological features suggest that IR emission is concentrated in central few
Proceedings of The International Astronomical Union, 1999
We report the results of longslit spectroscopy of the wind-blown bubble and photoevaporating knot... more We report the results of longslit spectroscopy of the wind-blown bubble and photoevaporating knots around the O6.5iiif star BD+ 60(deg) 2522 made with the Space Telescope Imaging Spectrograph. The Of star is the primary ionizing source for the H ii region NGC 7635, located in the Perseus Arm. The spectra were taken through a 0.2'' x 52'' slit with low
We present the results of our examination of newly acquired images obtained with WFPC-2 of the Bu... more We present the results of our examination of newly acquired images obtained with WFPC-2 of the Bubble Nebula, NGC 7635. These images were taken through narrow-band filters centered on important nebular diagnostic lines, including H alpha, H beta, [O III] at 500.7 nm, [N II] at 658.4 nm, [S II] at 671.7+673.1 nm and the line-free continuum filter F547M. The
Wind-blown bubbles are formed by the interaction of stellar ionizing radiation and winds on the c... more Wind-blown bubbles are formed by the interaction of stellar ionizing radiation and winds on the circumstellar material. The ram pressure of the confined wind sweeps up the ambient enivronment into a shell made visible by virtue of its ionization state, maintained by shock-heating and the UV flux of the star. Depending on the mass-loss history of the central star, this
We noticed an error in the coefficient of equation (3); the correct equation is((taucool)/yr)~5.7... more We noticed an error in the coefficient of equation (3); the correct equation is((taucool)/yr)~5.7×107(nu/GHz)-1/2(B/muG)1/2((UB+Urad)/(10-12 ergs cm-3))-1. (1)This did not impact the results or conclusions made in the rest of the text. All calculations using this equation were in fact correct.
With the goal of investigating the degree to which the mid-infrared emission traces the star form... more With the goal of investigating the degree to which the mid-infrared emission traces the star formation rate (SFR), we analyze Spitzer 8 µm and 24 µm data of star-forming regions in a sample of 33 nearby galaxies with available HST/NICMOS images in the Paα (λ1.8756 µm) emission line. The galaxies are drawn from the SINGS sample, and cover a range of morphologies and a factor ∼10 in oxygen abundance. Published data on local low-metallicity starburst galaxies and Luminous Infrared Galaxies are also included in the analysis. Both the stellar-continuum-subtracted 8 µm emission and the 24 µm emission correlate with the extinction-corrected Paα line emission, although neither relationship is linear. Simple models of stellar populations and dust extinction and emission are able to reproduce the observed non-linear trend of the 24 µm emission versus number of ionizing photons, including the modest deficiency of 24 µm emission in the low metallicity regions, which results from a combination of decreasing dust opacity and dust temperature at low luminosities. Conversely, the trend of the 8 µm emission as a function of the number of ionizing photons is not well reproduced by the same models. The 8 µm emission is contributed, in larger measure than the 24 µm emission, by dust heated by non-ionizing stellar populations, in addition to the ionizing ones, in agreement with previous findings. Two SFR calibrations, one using the 24 µm emission and the other using a combination of the 24 µm and Hα luminosities (Kennicutt et al. 2007a), are presented. No calibration is presented for the 8 µm emission, because of its significant dependence on both metallicity and environment. The calibrations presented here should be directly applicable to systems dominated by on-going star formation.
One possible way for spiral galaxies to internally evolve would be for gas to flow to the center ... more One possible way for spiral galaxies to internally evolve would be for gas to flow to the center and form stars in a central disk ( pseudo-bulge). If the inflow rate is faster than the rate of star formation, a central concentration of gas will form. In this paper we present radial profiles of stellar and 8 m emission from polycyclic aromatic hydrocarbons (PAHs) for 11 spiral galaxies to investigate whether the interstellar medium in these galaxies contains a central concentration above that expected from the exponential disk. In general, we find that the two-dimensional CO and PAH emission morphologies are similar, and that they exhibit similar radial profiles. We find that in 6 of the 11 galaxies there is a central excess in the 8 m and CO emission above the inward extrapolation of an exponential disk. In particular, all four barred galaxies in the sample have strong central excesses in both 8 m and CO emission. These correlations suggest that the excess seen in the CO profiles is, in general, not simply due to a radial increase in the CO emissivity. In the inner disk, the ratio of the stellar to the 8 m radial surface brightness is similar for 9 of the11 galaxies, suggesting a physical connection between the average stellar surface brightness and the average gas surface brightness at a given radius. We also find that the ratio of the CO to 8 m PAH surface brightness is consistent over the sample, implying that the 8 m PAH surface brightness can be used as an approximate tracer of the interstellar medium.
Using data obtained for twelve galaxies as part of the Spitzer Infrared Nearby Galaxies Survey (S... more Using data obtained for twelve galaxies as part of the Spitzer Infrared Nearby Galaxies Survey (SINGS) and the Westerbork Synthesis Radio Telescope (WSRT)-SINGS radio continuum survey, we study how star formation activity affects the far-infrared (FIR)-radio correlation within galaxies by testing a phenomenological model which describes the radio image as a smeared version of the FIR image. The physical basis of this description is that cosmic-ray (CR) electrons will diffuse measurably farther than the mean free path of dust-heating photons before decaying by synchrotron radiation. This description works well in general. Galaxies with higher infrared surface brightnesses have best-fit smoothing scale-lengths of a few hundred parsecs, substantially shorter than those for lower surface brightness galaxies. We interpret this result to suggest that galaxies with higher disk averaged star formation rates have had a recent episode of enhanced star formation and are characterized by a higher fraction of young CR electrons that have traveled only a few hundred parsecs from their acceleration sites in supernova remnants compared to galaxies with lower star formation activity.
The published optical fluxes included two corrections from an air mass of unity to an air mass of... more The published optical fluxes included two corrections from an air mass of unity to an air mass of zero. The updated Table 2 below provides the optical fluxes after a single correction from an air mass of unity to an air mass of zero. Note that the supplemental RC3 BV data use the NED conversion from magnitudes to janskys.
The ultraviolet-to-radio continuum spectral energy distributions are presented for all 75 galaxie... more The ultraviolet-to-radio continuum spectral energy distributions are presented for all 75 galaxies in the Spitzer Infrared Nearby Galaxies Survey (SINGS). A principal component analysis of the sample shows that most of the sample's -2spectral variations stem from two underlying components, one representative of a galaxy with a low infrared-to-ultraviolet ratio and one representative of a galaxy with a high infrared-to-ultraviolet ratio. The influence of several parameters on the infrared-to-ultraviolet ratio is studied (e.g., optical morphology, disk inclination, far-infrared color, ultraviolet spectral slope, and star formation history). Consistent with our understanding of normal star-forming galaxies, the SINGS sample of galaxies in comparison to more actively star-forming galaxies exhibits a larger dispersion in the infrared-to-ultraviolet versus ultraviolet spectral slope correlation. Early type galaxies, exhibiting low star formation rates and high optical surface brightnesses, have the most discrepant infrared-to-ultraviolet correlation. These results suggest that the star formation history may be the dominant regulator of the broadband spectral variations between galaxies. Finally, a new discovery shows that the 24 µm morphology can be a useful tool for parametrizing the global dust temperature and ultraviolet extinction in nearby galaxies. The dust emission in dwarf/irregular galaxies is clumpy and warm accompanied by low ultraviolet extinction, while in spiral galaxies there is typically a much larger diffuse component of cooler dust and average ultraviolet extinction. For galaxies with nuclear 24 µm emission, the dust temperature and ultraviolet extinction are relatively high compared to disk galaxies.
Featured in Los Angeles Times "Illuminating a black hole" December 9, 2008 Reinhard Gen... more Featured in Los Angeles Times "Illuminating a black hole" December 9, 2008 Reinhard Genzel, leader of the European team based at the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, said researchers had the best evidence yet that "supermassive ...
Mid-infrared diagnostics are presented for a large portion of the Spitzer Infrared Nearby Galaxie... more Mid-infrared diagnostics are presented for a large portion of the Spitzer Infrared Nearby Galaxies Survey (SINGS) sample plus archival data from the Infrared Space Observatory and the Spitzer Space Telescope. The SINGS dataset includes low-and high-resolution spectral maps and broadband imaging in the infrared for over 160 nuclear and extranuclear regions within 75 nearby galaxies spanning a wide range of morphologies, metallicities, luminosities, and star formation rates. Our main result is that these mid-infrared diagnostics effectively constrain a target's dominant power source. The combination of a high ionization line index and PAH strength serves as an efficient discriminant between AGN and star-forming nuclei, confirming progress made with ISO spectroscopy on starbursting and ultraluminous infrared galaxies. The sensitivity of Spitzer -2allows us to probe fainter nuclear and star-forming regions within galaxy disks. We find that both star-forming nuclei and extranuclear regions stand apart from nuclei that are powered by Seyfert or LINER activity. In fact, we identify areas within four diagnostic diagrams containing >90% Seyfert/LINER nuclei or >90% H II regions/H II nuclei. We also find that, compared to starbursting nuclei, extranuclear regions typically separate even further from AGN, especially for low-metallicity extranuclear environments. In addition, instead of the traditional mid-infrared approach to differentiating between AGN and star-forming sources that utilizes relatively weak high-ionization lines, we show that strong low-ionization cooling lines of X-ray dominated regions like [Si II] 34.82 µm can alternatively be used as excellent discrimants. Finally, the typical target in this sample shows relatively modest interstellar electron density (∼ 400 cm −3 ) and obscuration (A V ∼ 1.0 mag for a foreground screen), consistent with a lack of dense clumps of highly obscured gas and dust residing in the emitting regions.
High resolution mid-infrared spectra are presented for 155 nuclear and extranuclear regions from ... more High resolution mid-infrared spectra are presented for 155 nuclear and extranuclear regions from the Spitzer Infrared Nearby Galaxies Survey (SINGS). The fluxes for nine atomic forbidden and three molecular hydrogen mid-infrared emission lines are also provided, along with upper limits in key lines for infrared-faint targets. The SINGS sample shows a wide range in the ratio of [S III] 18.71 µm/[S III] 33.48 µm, but the average ratio of the ensemble indicates a typical interstellar electron density of 300-400 cm −3 on ∼23 ′′ ×15 ′′ scales and 500-600 cm −3 using ∼11 ′′ ×9 ′′ apertures, independent of whether the region probed is a star-forming nuclear, a star-forming extranuclear, or an AGN environment. Evidence is provided that variations in gas-phase metallicity play an important role in driving variations in radiation field hardness, as indicated -2by [Ne III] 15.56 µm/[Ne II] 12.81 µm, for regions powered by star formation. Conversely, the radiation hardness for galaxy nuclei powered by accretion around a massive black hole is independent of metal abundance. Furthermore, for metal-rich environments AGN are distinguishable from star-forming regions by significantly larger [Ne III] 15.56 µm/[Ne II] 12.81 µm ratios. Finally, [Fe II] 25.99 µm/[Ne II] 12.81 µm versus [Si II] 34.82 µm/[S III] 33.48 µm also provides an empirical method for discerning AGN from normal star-forming sources. However, similar to [Ne III] 15.56 µm/[Ne II] 12.81 µm, these mid-infrared line ratios lose their AGN/star-formation diagnostic powers for very low metallicity star-forming systems with hard radiation fields.
We present new images (groundbased optical and mid-infrared (MIR) from the Spitzer Space Telescop... more We present new images (groundbased optical and mid-infrared (MIR) from the Spitzer Space Telescope) and spectra (from Spitzer) of the archetypal starburst galaxy M 82. The Spitzer data show that the MIR emission extends at least 6 kpc along the minor axis of the galaxy. We use the optical and infrared data to demonstrate that the extended emission is dominated by emission from dust. The colors of the MIR emission and the spectra indicate that there is a strong component of aromatic feature emission (the MIR features commonly attributed to polycyclic aromatic hydrocarbons). The dust continuum and aromatic feature emission are both strong in the well-known superwind region of this galaxy; clearly the carrier of the aromatic features can survive in close proximity to the wind, far from the plane of the galaxy. We also see significant emission by dust well outside the superwind region, providing the clearest picture to date of the dust distribution in the halo of this galaxy.
The quantity of dust in a spiral disk can be estimated using the dust's typical emission or the e... more The quantity of dust in a spiral disk can be estimated using the dust's typical emission or the extinction of a known source. In this paper, we compare two techniques, one based on emission and one on absorption, applied on sections of fourteen disk galaxies. The two measurements reflect, respectively the average and apparent optical depth of a disk section. Hence, they depend differently on the average number and optical depth of ISM structures in the disk.
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Papers by B. Buckalew