Publications (26)60.81 Total impact
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Article: Radial Distribution of Stars, Gas, and Dust in Sings Galaxies. II. Derived Dust Properties
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ABSTRACT: We present a detailed analysis of the radial distribution of dust properties in the SINGS sample, performed on a set of ultraviolet (UV), infrared (IR), and H I surface brightness profiles, combined with published molecular gas profiles and metallicity gradients. The internal extinction, derived from the total-IR (TIR)-to-far-UV (FUV) luminosity ratio, decreases with radius, and is larger in Sb-Sbc galaxies. The TIR-to-FUV ratio correlates with the UV spectral slope β, following a sequence shifted to redder UV colors with respect to that of starbursts. The star formation history (SFH) is identified as the main driver of this departure. Both L TIR/L FUV and β correlate well with metallicity, especially in moderately face-on galaxies. The relation shifts to redder colors with increased scatter in more edge-on objects. By applying physical dust models to our radial spectral energy distributions, we have derived radial profiles of the total dust mass surface density, the fraction of the total dust mass contributed by polycyclic aromatic hydrocarbons (PAHs), and the intensity of the radiation field heating the grains. The dust profiles are exponential, their radial scale length being constant from Sb to Sd galaxies (only ~10% larger than the stellar scale length). Many S0/a-Sab galaxies have central depressions in their dust radial distributions. The PAH abundance increases with metallicity for 12 + log(O/H) < 9, and at larger metallicities the trend flattens and even reverses, with the SFH being a plausible underlying driver for this behavior. The dust-to-gas ratio is also well correlated with metallicity and therefore decreases with galactocentric radius. Although most of the total emitted IR power (especially in the outer regions of disks) is contributed by dust grains heated by diffuse starlight with a similar intensity as the local Milky Way radiation field, a small amount of the dust mass (~1%) is required to be exposed to very intense starlight in order to reproduce the observed fluxes at 24 μm, accounting for ~10% of the total integrated IR power.The Astrophysical Journal 08/2009; 701(2):1965. · 6.02 Impact Factor -
Article: Spatially Resolved Ultraviolet, Hα, Infrared, and Radio Star Formation in M81
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ABSTRACT: We present Multiband Imaging Photometer for Spitzer (MIPS) observations of M81 at 24, 70, and 160 μm. The grand design nature of M81 is clearly seen, showing two well-resolved spiral arms containing numerous bright star-forming regions. The MIPS images reveal a significant amount of cold dust associated with the spiral arms. We investigate the variation of the ultraviolet (UV), Hα, and infrared (IR) luminosities and star formation rate (SFR) indicators across the face of M81 using the MIPS images and archival UV and Hα images. For regions in M81, we find that UV and Hα SFRs (uncorrected for dust attenuation) are always lower than the IR SFR. The cause of this behavior is dust attenuation and/or using SFR calibrations appropriate for entire galaxies, not regions in galaxies. The characteristics of the dust attenuation for the regions indicate the dust grains and/or geometry are different from those in starburst galaxies. The behavior of the infrared-radio correlation in M81 is seen to vary from the global average, with variations correlated with the morphology of M81.The Astrophysical Journal Supplement Series 12/2008; 154(1):215. · 13.46 Impact Factor -
Article: Star Formation in NGC 5194 (M51a): The Panchromatic View from GALEX to Spitzer
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ABSTRACT: Far-ultraviolet to far-infrared images of the nearby galaxy NGC 5194 (M51a), from a combination of space-based (Spitzer, GALEX, and Hubble Space Telescope) and ground-based data, are used to investigate local and global star formation and the impact of dust extinction. The Spitzer data provide unprecedented spatial detail in the infrared, down to sizes ~500 pc at the distance of NGC 5194. The multiwavelength set is used to trace the relatively young stellar populations, the ionized gas, and the dust absorption and emission in H II-emitting knots, over 3 orders of magnitude in wavelength range. As is common in spiral galaxies, dust extinction is high in the center of the galaxy (AV ~ 3.5 mag), but its mean value decreases steadily as a function of galactocentric distance, as derived from both gas emission and stellar continuum properties. In the IR/UV-UV color plane, the NGC 5194 H II knots show the same trend observed for normal star-forming galaxies, having a much larger dispersion (~1 dex peak to peak) than starburst galaxies. We identify the dispersion as due to the UV emission predominantly tracing the evolved, nonionizing stellar population, up to ages ~50-100 Myr. While in starbursts the UV light traces the current star formation rate (SFR), in NGC 5194 it traces a combination of current and recent past SFRs. Possibly, mechanical feedback from supernovae is less effective at removing dust and gas from the star formation volume in normal star-forming galaxies than in starbursts because of the typically lower SFR densities in the former. The application of the starburst opacity curve for recovering the intrinsic UV emission (and deriving SFRs) in local and distant galaxies appears therefore appropriate only for SFR densities 1 M☉ yr-1 kpc-2. Unlike the UV emission, the monochromatic 24 μm luminosity is an accurate local SFR tracer for the H II knots in NGC 5194, with a peak-to-peak dispersion of less than a factor of 3 relative to hydrogen emission line tracers; this suggests that the 24 μm emission carriers are mainly heated by the young, ionizing stars. However, preliminary results show that the ratio of the 24 μm emission to the SFR varies by a factor of a few from galaxy to galaxy; this variation needs to be understood and carefully quantified before the 24 μm luminosity can be used as an SFR tracer for galaxy populations. While also correlated with star formation, the 8 μm emission is not directly proportional to the number of ionizing photons; it is overluminous, by up to a factor of ~2, relative to the galaxy's average in weakly ionized regions and is underluminous, by up to a factor of ~3, in strongly ionized regions. This confirms earlier suggestions that the carriers of the 8 μm emission are heated by more than one mechanism.The Astrophysical Journal 12/2008; 633(2):871. · 6.02 Impact Factor -
Article: Warm Molecular Hydrogen in the Spitzer SINGS Galaxy Sample
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ABSTRACT: Results on the properties of warm molecular hydrogen in 57 normal galaxies are derived from measurements of H2 rotational transitions, obtained as part of SINGS. This study extends previous extragalactic surveys of emission lines of H2 to fainter and more common systems (LFIR = 107-6 × 1010 L☉). The 17 μm S(1) transition is securely detected in the nuclear regions of 86% of galaxies with stellar masses above 109.5 M☉. The derived column densities of warm H2 (T ≥ 100 K), although averaged over kiloparsec-scale areas, are commensurate with values observed in resolved photodissociation regions. They amount to between 1% and >30% of the total H2. The power emitted in the three lowest energy transitions is on average 30% of the power of the bright [Si II] cooling line (34.8 μm) and about 4 × 10-4 of the total infrared power for star-forming galaxies, which is consistent with excitation in PDRs. The fact that the H2 line intensities scale tightly with the aromatic band emission, even though the average radiation field intensity varies by a factor of 10, can also be understood if both tracers originate predominantly in PDRs, either dense or diffuse. Many of the 25 LINER/Seyfert targets strongly depart from the rest of the sample, in having warmer excited H2 and excess H2 rotational power with respect to the dust emission. We propose a threshold in H2-to-aromatic band power ratios, allowing the identification of low-luminosity AGNs by an excess H2 excitation. A dominant contribution from shock heating is favored in these objects. Finally, we detect in nearly half the star-forming targets nonequilibrium ortho-to-para ratios, consistent with the effects of FUV pumping combined with incomplete ortho-para thermalization, or possibly nonequilibrium photodissociation fronts.The Astrophysical Journal 12/2008; 669(2):959. · 6.02 Impact Factor -
Article: Mid-Infrared IRS Spectroscopy of NGC 7331: A First Look at the Spitzer Infrared Nearby Galaxies Survey (SINGS) Legacy
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ABSTRACT: The nearby spiral galaxy NGC 7331 was spectrally mapped from 5 to 38 μm using all modules of Spitzer's Infrared Spectrograph (IRS). A strong new dust emission feature, presumed due to polycyclic aromatic hydrocarbons, was discovered at 17.1 μm. The feature's intensity is nearly half that of the ubiquitous 11.3 μm band. The 7-14 μm spectral maps revealed significant variation in the 7.7 and 11.3 μm PAH features between the stellar ring and nucleus. Weak [O IV] 25.9 μm line emission was found to be centrally concentrated in the nucleus, with an observed strength of over 10% of the combined neon line flux, indicating an AGN or unusually active massive star photoionization. Two [S III] lines fix the characteristic electron density in the H II regions at ne 200 cm-3. Three detected H2 rotational lines, tracing warm molecular gas, together with the observed IR continuum, are difficult to match with standard photodissociation region (PDR) models. Either additional PDR heating or shocks are required to simultaneously match lines and continuum.The Astrophysical Journal Supplement Series 12/2008; 154(1):199. · 13.46 Impact Factor -
Article: The Calibration of Mid-Infrared Star Formation Rate Indicators
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ABSTRACT: With the goal of investigating the degree to which the MIR emission traces the SFR, we analyze Spitzer 8 and 24 μm data of star-forming regions in a sample of 33 nearby galaxies with available HST NICMOS images in the Paα (1.8756 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 LIRGs 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 nonlinear 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 nonionizing 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 and coworkers), 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 ongoing star formation.The Astrophysical Journal 12/2008; 666(2):870. · 6.02 Impact Factor -
Article: The Star Formation Efficiency in Nearby Galaxies: Measuring Where Gas Forms Stars Effectively
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ABSTRACT: We measure the star formation efficiency (SFE), the star formation rate per unit gas, in 23 nearby galaxies and compare it to expectations from proposed star formation laws and thresholds. We use HI maps from THINGS and derive H2 maps from HERACLES and BIMA SONG CO. We estimate the star formation rate by combining GALEX FUV maps and SINGS 24 micron maps, infer stellar surface density profiles from SINGS 3.6 micron data, and use kinematics from THINGS. We measure the SFE as a function of: the free-fall and orbital timescales; midplane gas pressure; stability of the gas disk to collapse (including the effects of stars); the ability of perturbations to grow despite shear; and the ability of a cold phase to form. In spirals, the SFE of H2 alone is nearly constant at 5.25 +/- 2.5 x 10^(-10) yr^(-1) (equivalent to an H2 depletion time of 1.9x10^9 yr) as a function of all of these variables at our 800 pc resolution. Where the ISM is mostly HI, on the other hand, the SFE decreases with increasing radius in both spiral and dwarf galaxies, a decline reasonably described by an exponential with scale length 0.2-0.25 r_25. We interpret this decline as a strong dependence of GMC formation on environment. The ratio of H2 to HI appears to be a smooth function of radius, stellar surface density, and pressure spanning from the H2-dominated to HI-dominated ISM. The radial decline in SFE is too steep to be reproduced only by increases in the free-fall time or orbital time. Thresholds for large-scale instability suggest that our disks are stable or marginally stable and do not show a clear link to the declining SFE. We suggest that ISM physics below the scales that we observe - phase balance in the HI, H2 formation and destruction, and stellar feedback - governs the formation of GMCs from HI.10/2008; -
Article: THINGS: The HI Nearby Galaxy Survey
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ABSTRACT: We present "The HI Nearby Galaxy Survey (THINGS)", a high spectral (<=5.2 km/s) and spatial (~6") resolution survey of HI emission in 34 nearby galaxies obtained using the NRAO Very Large Array (VLA). The overarching scientific goal of THINGS is to investigate fundamental characteristics of the interstellar medium (ISM) related to galaxy morphology, star formation and mass distribution across the Hubble sequence. Unique characteristics of the THINGS database are the homogeneous sensitivity as well as spatial and velocity resolution of the HI data which is at the limit of what can be achieved with the VLA for a significant number of galaxies. A sample of 34 objects at distances 2<D<15Mpc (resulting in linear resolutions of ~100 to 500pc) are targeted in THINGS, covering a wide range of star formation rates (10^-3 to 6 M_sun/yr), total HI masses M_HI (0.01 to 14x10^9 M_sun), absolute luminosities M_B (-11.5 to -21.7 mag) and metallicities (7.5 to 9.2 in units of 12+log[O/H]). We describe the setup of the VLA observations, the data reduction procedures and the creation of the final THINGS data products. We present an atlas of the integrated HI maps, the velocity fields, the second moment (velocity dispersion) maps and individual channel maps of each THINGS galaxy. The THINGS data products are made publicly available through a dedicated webpage. Accompanying THINGS papers address issues such as the small-scale structure of the ISM, the (dark) matter distribution in THINGS galaxies, and the processes leading to star formation.10/2008; -
Article: The relations among 8, 24 and 160 μm dust emission within nearby spiral galaxies
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ABSTRACT: We investigate the relations among the stellar continuum-subtracted 8 μm polycyclic aromatic hydrocarbon (PAH 8 μ m) emission, 24 μm hot dust emission and 160 μm cold dust emission in 15 nearby face-on spiral galaxies in the Spitzer Infrared Nearby Galaxies Survey sample. The relation between PAH 8 and 24 μm emission measured in ∼2 kpc regions is found to exhibit a significant amount of scatter, and strong spatial variations are observed in the (PAH 8 μ m)/24 μm surface brightness ratio. In particular, the (PAH 8 μ m)/24 μm surface brightness ratio is observed to be high in the diffuse interstellar medium and low in bright star-forming regions and other locations with high 24 μm surface brightness. PAH 8 μ m emission is found to be well-correlated with 160 μm emission on spatial scales of ∼2 kpc, and the (PAH 8 μ m)/160 μm surface brightness ratio is generally observed to increase as the 160 μm surface brightness increases. These results suggest that the PAHs are associated with the diffuse, cold dust that produces most of the 160 μm emission in these galaxies, and the variations in the (PAH 8 μ m)/160 μm ratio may generally be indicative of either the intensity or the spectrum of the interstellar radiation field that is heating both the PAHs and the diffuse interstellar dust.Monthly Notices of the Royal Astronomical Society 09/2008; 389(2):629 - 650. · 4.90 Impact Factor -
Article: Tracing spiral density waves in M81
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ABSTRACT: We use SPITZER IRAC 3.6 and 4.5micron near infrared data from the Spitzer Infrared Nearby Galaxies Survey (SINGS), optical B, V and I and 2MASS Ks band data to produce mass surface density maps of M81. The IRAC 3.6 and 4.5micron data, whilst dominated by emission from old stellar populations, is corrected for small-scale contamination by young stars and PAH emission. The I band data are used to produce a mass surface density map by a B-V colour-correction, following the method of Bell and de Jong. We fit a bulge and exponential disc to each mass map, and subtract these components to reveal the non-axisymmetric mass surface density. From the residual mass maps we are able to extract the amplitude and phase of the density wave, using azimuthal profiles. The response of the gas is observed via dust emission in the 8micron IRAC band, allowing a comparison between the phase of the stellar density wave and gas shock. The relationship between this angular offset and radius suggests that the spiral structure is reasonably long lived and allows the position of corotation to be determined. Comment: 15 pages, 17 figures, accepted for publication in MNRAS04/2008; -
Article: Variations in 24‐μm morphologies among galaxies in the Spitzer Infrared Nearby Galaxies Survey: new insights into the Hubble sequence
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ABSTRACT: To study the distribution of star formation and dust emission within nearby galaxies, we measured five morphological parameters in the 3.6- and 24-μm wavebands for 65 galaxies in the Spitzer Infrared Nearby Galaxies Survey (SINGS) and eight galaxies that were serendipitously observed by SINGS. The morphological parameters demonstrate strong variations along the Hubble sequence, including statistically significant differences between S0/a-Sab and Sc-Sd galaxies. Early-type galaxies are generally found to be compact, centralized, symmetric sources in the 24-μm band, while late-type galaxies are generally found to be extended, asymmetric sources. These results suggest that the processes that increase the real or apparent sizes of galaxies' bulges also lead to more centralized 24-μm dust emission. Several phenomena, such as strong nuclear star formation, Seyfert activity, or outer ring structures, may cause galaxies to deviate from the general morphological trends observed at 24 μm. We also note that the 24-μm morphologies of Sdm-Im galaxies are quite varied, with some objects appearing very compact and symmetric but others appearing diffuse and asymmetric. These variations reflect the wide variation in star formation in irregular galaxies as observed at other wavelengths. The variations in the 24-μm morphological parameters across the Hubble sequence mirror many of the morphological trends seen in other tracers of the ISM and in stellar emission. However, the 24-μm morphological parameters for the galaxies in this sample do not match the morphological parameters measured in the stellar wavebands. This implies that the distribution of dust emission is related to but not equivalent to the distribution of stellar emission.Monthly Notices of the Royal Astronomical Society 09/2007; 380(4):1313 - 1334. · 4.90 Impact Factor -
Article: An Initial Look at the Far Infrared-Radio Correlation within Nearby Star-forming Galaxies using the Spitzer Space Telescope
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ABSTRACT: (Abridged) We present an initial look at the far infrared-radio correlation within the star-forming disks of four nearby, nearly face-on galaxies (NGC 2403, NGC 3031, NGC 5194, and NGC 6946). Using Spitzer MIPS imaging and WSRT radio continuum data, observed as part of the Spitzer Infrared Nearby Galaxies Survey (SINGS), we are able to probe variations in the logarithmic 24mu/22cm (q_24) and 70mu/22cm (q_70) surface brightness ratios across each disk at sub-kpc scales. We find general trends of decreasing q_24 and q_70 with declining surface brightness and with increasing radius. The residual dispersion around the trend of q_24 and q_70 versus surface brightness is smaller than the residual dispersion around the trend of q_24 and q_70 versus radius, on average by ~0.1 dex, indicating that the distribution of star formation sites is more important in determining the infrared/radio disk appearance than the exponential profiles of disks. We have also performed preliminary phenomenological modeling of cosmic ray electron (CRe^-) diffusion using an image-smearing technique, and find that smoothing the infrared maps improves their correlation with the radio maps. Exponential kernels tend to work better than Gaussian kernels which suggests that additional processes besides simple random-walk diffusion in three dimensions must affect the evolution of CRe^-s. The best fit smoothing kernels for the two less active star-forming galaxies (NGC 2403 and NGC 3031) have much larger scale-lengths than those of the more active star-forming galaxies (NGC 5194 and NGC 6946). This difference may be due to the relative deficit of recent CRe^- injection into the interstellar medium (ISM) for the galaxies having largely quiescent disks.11/2005; -
Article: Star Formation in NGC5194 (M51a): The Panchromatic View from GALEX to Spitzer
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ABSTRACT: (Abridged) Far ultraviolet to far infrared images of the nearby galaxy NGC5194, from Spitzer, GALEX, Hubble Space Telescope and ground--based data, are used to investigate local and global star formation, and the impact of dust extinction in HII-emitting knots. In the IR/UV-UV color plane, the NGC5194 HII knots show the same trend observed for normal star-forming galaxies, having a much larger dispersion than starburst galaxies. We identify the dispersion as due to the UV emission predominantly tracing the evolved, non-ionizing stellar population, up to ages 50-100 Myr. While in starbursts the UV light traces the current SFR, in NGC5194 it traces a combination of current and recent-past SFR. Unlike the UV emission, the monochromatic 24 micron luminosity is an accurate local SFR tracer for the HII knots in NGC5194; this suggests that the 24 micron emission carriers are mainly heated by the young, ionizing stars. However, preliminary results show that the ratio of the 24 micron emission to the SFR varies by a factor of a few from galaxy to galaxy. While also correlated with star formation, the 8 micron emission is not directly proportional to the number of ionizing photons. This confirms earlier suggestions that the carriers of the 8 micron emission are heated by more than one mechanism.08/2005; -
Article: Recent imaging results from SINGS
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ABSTRACT: The Spitzer Infrared Nearby Galaxy Survey (SINGS) is a comprehensive infrared imaging and spectroscopic survey of 75 nearby galaxies. One of the primary goals of SINGS is to compare star-formation rates derived from far-infrared emission with rates derived from other generally accepted star-formation indicators, specifically ultraviolet emission, Hα emission, and radio synchrotron emission. A related secondary goal is the search for other emission mechanisms, such as mid-infrared PAH emission, that may correlate with these star-formation indicators. Further study of these star-formation tracers will give us better understanding of the physical processes that link them all together. We present recent results from SINGS observations of NGC 7331 and M81 that relate to this work, and we also show some of the IRAC and MIPS images recently obtained by SINGS.04/2005; 329:5P. -
Article: First Results from THINGS: The HI Nearby Galaxy Survey
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ABSTRACT: We describe The HI Nearby Galaxy Survey (THINGS), the largest program ever undertaken at the VLA to perform 21-cm HI observations of the highest quality (~7'', <= 5 km/s resolution) of nearby galaxies. The goal of THINGS is to investigate key characteristics related to galaxy morphology, star formation and mass distribution across the Hubble sequence. A sample of 34 objects with distances between 3 and 10 Mpc will be observed, covering a wide range of evolutionary stages and properties. Data from THINGS will complement SINGS, the Spitzer Infrared Nearby Galaxy Survey. For the THINGS sample, high-quality observations at comparable resolution will thus be available from the X-ray regime through to the radio part of the spectrum. THINGS data can be used to investigate issues such as the small-scale structure of the ISM, its three-dimensional structure, the (dark) matter distribution and processes leading to star formation. To demonstrate the quality of the THINGS data products, we present some prelimary HI maps here of four galaxies from the THINGS sample.08/2004; -
Article: Mid-Infrared IRS Spectroscopy of NGC 7331: A First Look at the SINGS Legacy
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ABSTRACT: The nearby spiral galaxy NGC 7331 was spectrally mapped from 5-38um using all modules of Spitzer's IRS spectrograph. A strong new dust emission feature, presumed due to PAHs, was discovered at 17.1um. The feature's intensity is nearly half that of the ubiquitous 11.3um band. The 7-14um spectral maps revealed significant variation in the 7.7 and 11.3um PAH features between the stellar ring and nucleus. Weak [OIV] 25.9um line emission was found to be centrally concentrated in the nucleus, with an observed strength over 10% of the combined neon line flux, indicating an AGN or unusually active massive star photo-ionization. Two [SIII] lines fix the characteristic electron density in the HII regions at n_e < ~200 cm^-3. Three detected H_2 rotational lines, tracing warm molecular gas, together with the observed IR continuum, are difficult to match with standard PDR models. Either additional PDR heating or shocks are required to simultaneously match lines and continuum.07/2004; -
Article: IRAC Observations of M81
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ABSTRACT: IRAC images of M81 show three distinct morphological constituents: a smooth distribution of evolved stars with bulge, disk, and spiral arm components; a clumpy distribution of dust emission tracing the spiral arms; and a pointlike nuclear source. The bulge stellar colors are consistent with M-type giants, and the disk colors are consistent with a slightly younger population. The dust emission generally follows the blue and ultraviolet emission, but there are large areas that have dust emission without ultraviolet and smaller areas with ultraviolet but little dust emission. The former are presumably caused by extinction, and the latter may be due to cavities in the gas and dust created by supernova explosions. The nucleus appears fainter at 8 um than expected from ground-based 10 um observations made four years ago.07/2004; -
Article: The Density and Temperature of Molecular Clouds in M33
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ABSTRACT: We have observed the $^{12}$CO J=2-1, J=3-2, and $^{13}$CO J=2-1 lines in a sample of seven giant molecular clouds in the Local Group spiral galaxy M33. The $^{12}$CO/$^{13}$CO J=2-1 line ratio is constant across the entire sample, while the observed $^{12}$CO J=3-2/J=2-1 line ratio has a weak dependence on the star formation environment of the cloud, with large changes in the line ratio seen only for clouds in the immediate vicinity of an extremely luminous HII region. A large velocity gradient analysis indicates that clouds without HII regions have temperatures of 10-20 K, clouds with HII regions have temperatures of 15-100 K, and the cloud in the giant HII region has a temperature of at least 100 K. Interestingly, the giant HII region appears capable of raising the kinetic temperature of the molecular gas only for clouds that are quite nearby ($< 100$ pc). The continuous change of physical conditions across the observed range of star formation environments suggests that the unusual physical conditions in the cloud in the giant HII region are due to post-star formation changes in the molecular gas, rather than intrinsic properties of the gas related to the formation of the giant HII region. Comment: 14 pages, aastex, 4 postscript figures; accepted for publication in ApJ; also available at http://www.physics.mcmaster.ca/Wilson_Preprints01/1997; -
Article: Spatial Variation of (12) CO Line Ratios in M33
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ABSTRACT: Three molecular cloud regions in M33 have been mapped in the (12) CO J=2-1 line. Incorporating previously taken J=1-0 data, (12) CO line ratios were calculated in these regions to determine whether the line ratios, and hence the physical conditions, vary from one region to another within the galaxy. By studying a nearby galaxy such as M33, we can measure spatial variation of this ratio on scales of 100-200 pc. Two regions are centered on molecular clouds in the inner disk, MC19 and MC32. The third is centered on the giant HII region NGC 595, in a region where the star formation rates and efficiencies are higher than those of the inner disk by an order of magnitude. Despite this range of conditions, there is no significant difference between the ratios found for gas in the direction of NGC 595 and those found for the inner disk regions. On average in all three regions, the ratio of J=2-1 to J=1-0 integrated intensities is about 0.70, with one position in the MC32 region where R is greater than 1. This result suggests that density has a stronger influence on the value of the ratio than temperature. This idea is supported by the fact that the position of high ratio value does not correspond with any HII regions as seen in Hα . In order to fit the line ratios to physical parameters, both LTE and LVG (large velocity gradient) approximations are used.11/1992; 24:1222. -
Article: The star formation law on sub-kpc resolution in THINGS
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ABSTRACT: We present results from a comprehensive analysis of the relation between HI, H_2, total gas (HI+H_2) and star formation rate surface density (SigmaHI, SigmaH2, Sigmagas and SigmaSFR respectively) in 18 nearby galaxies (7 spirals and 11 HI-dominated dwarfs) at 750 pc resolution. We use high resolution HI data from THINGS, CO data from IRAM and BIMA, 24 mum data from the Spitzer Space Telescope, and UV data from GALEX. We find that a Schmidt-type power law with index N=1.0 ± 0.2 relates SigmaSFR and SigmaH2 across our sample of spiral galaxies, i.e. that H_2 forms stars at a constant efficiency in spirals with an average molecular gas depletion time of ÃÅ 2 \cdot 10^9 years. We interpret the linear relation and constant depletion time as evidence that stars are forming in GMCs with approximately uniform properties and that SigmaH2 may be more a measure of the filling fraction of giant molecular clouds than changing conditions in the molecular gas. We find no correlation between SigmaHI and SigmaSFR. The dwarf galaxies in our sample resemble the outer disks of spirals in SigmaSFR-Sigmagas space. This is likely due to the similarity of both environments (low,density, low,metallicity, HI,dominated). We furthermore find a sharp saturation of SigmaHI at ÃÅ9 Mo pc-2 in both the spiral and the dwarf galaxies. For the spirals, we observe gas in excess of this cutoff to be molecular.Sheth, K. ; Noriega-Crespo, A. ; Ingalls, J.; Paladini, R.: The Evolving ISM in the Milky Way and Nearby Galaxies, 12 (2009).
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Institutions
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2007–2009
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Bucknell University
- Department of Physics and Astronomy
Lewisburg, PA, USA
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2008
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Space Telescope Science Institute
Baltimore, MD, USA
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1992
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McMaster University
Hamilton, Ontario, Canada
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