Publications (18)45.71 Total impact
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Article: Resolving the Far-IR Line Deficit: Photoelectric Heating and Far-IR Line Cooling in NGC 1097 and NGC 4559
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ABSTRACT: The physical state of interstellar gas and dust is dependent on the processes which heat and cool this medium. To probe heating and cooling of the interstellar medium over a large range of infrared surface brightness, on sub-kiloparsec scales, we employ line maps of [C II] 158 μm, [O I] 63 μm, and [N II] 122 μm in NGC 1097 and NGC 4559, obtained with the Photodetector Array Camera & Spectrometer on board Herschel. We matched new observations to existing Spitzer Infrared Spectrograph data that trace the total emission of polycyclic aromatic hydrocarbons (PAHs). We confirm at small scales in these galaxies that the canonical measure of photoelectric heating efficiency, ([C II] + [O I])/TIR, decreases as the far-infrared (far-IR) color, νf ν(70 μm) νf ν(100 μm), increases. In contrast, the ratio of far-IR cooling to total PAH emission, ([C II] + [O I])/PAH, is a near constant ~6% over a wide range of far-IR color, 0.5 < νf ν(70 μm) νf ν(100 μm) 0.95. In the warmest regions, where νf ν(70 μm) νf ν(100 μm) 0.95, the ratio ([C II] + [O I])/PAH drops rapidly to 4%. We derived representative values of the local ultraviolet radiation density, G 0, and the gas density, n H, by comparing our observations to models of photodissociation regions. The ratio G 0/n H, derived from fine-structure lines, is found to correlate with the mean dust-weighted starlight intensity, U, derived from models of the IR spectral energy distribution. Emission from regions that exhibit a line deficit is characterized by an intense radiation field, indicating that small grains are susceptible to ionization effects. We note that there is a shift in the 7.7/11.3 μm PAH ratio in regions that exhibit a deficit in ([C II] + [O I])/PAH, suggesting that small grains are ionized in these environments.The Astrophysical Journal 02/2012; 747(1):81. · 6.02 Impact Factor -
Article: Spitzer IRS Spectral Mapping of the Toomre Sequence: Spatial Variations of PAH, Gas, and Dust Properties in Nearby Major Mergers
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ABSTRACT: We have mapped the key mid-IR diagnostics in eight major merger systems of the Toomre Sequence (NGC4676, NGC7592, NGC6621, NGC2623, NGC6240, NGC520, NGC3921, and NGC7252) using the Spitzer Infrared Spectrograph (IRS). With these maps, we explore the variation of the ionized-gas, PAH, and warm-gas (H_2) properties across the sequence and within the galaxies. While the global PAH interband strength and ionized gas flux ratios ([Ne III]/[Ne II]) are similar to those of normal star forming galaxies, the distribution of the spatially resolved PAH and fine structure line flux ratios is significant different from one system to the other. Rather than a constant H_2/PAH flux ratio, we find that the relation between the H_2 and PAH fluxes is characterized by a power law with a roughly constant exponent (0.61+/-0.05) over all merger components and spatial scales. While following the same power law on local scales, three galaxies have a factor of ten larger integrated (i.e. global) H_2/PAH flux ratio than the rest of the sample, even larger than what it is in most nearby AGNs. These findings suggest a common dominant excitation mechanism for H_2 emission over a large range of global H_2/PAH flux ratios in major mergers. Early merger systems show a different distribution between the cold (CO J=1-0) and warm (H_2) molecular gas component, which is likely due to the merger interaction. Strong evidence for buried star formation in the overlap region of the merging galaxies is found in two merger systems (NGC6621 and NGC7592) as seen in the PAH, [Ne II], [Ne III], and warm gas line emission, but with no apparent corresponding CO (J=1-0) emission. Our findings also demonstrate that the variations of the physical conditions within a merger are much larger than any systematic trends along the Toomre Sequence.10/2011; -
Article: The Displaced Dusty ISM of NGC 3077: Tidal Stripping in the M 81 Triplet
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ABSTRACT: We present the detection of extended (~30 kpc^2) dust emission in the tidal \hi\ arm near NGC 3077 (member of the M\,81 triplet) using SPIRE on board Herschel. Dust emission in the tidal arm is typically detected where the \hi\ column densities are >10^21 cm^-2. The SPIRE band ratios show that the dust in the tidal arm is significantly colder (~13 K) than in NGC 3077 itself (~31 K), consistent with the lower radiation field in the tidal arm. The total dust mass in the tidal arm is ~1.8 x 10^6 M_sun (assuming beta=2), i.e. substantially larger than the dust mass associated with NGC 3077 (~2 x 10^5 M_sun). Where dust is detected, the dust-to-gas ratio is 6+/-3 x 10^-3, consistent within the uncertainties with what is found in NGC 3077 and nearby spiral galaxies with Galactic metallicities. The faint HII regions in the tidal arm can not be responsible for the detected enriched material and are not the main source of the dust heating in the tidal arm. We conclude that the interstellar medium (atomic HI, molecules and dust) in this tidal feature was pre-enriched and stripped off NGC 3077 during its recent interaction (~3 x 10^8 yr ago) with M 82 and M 81. This implies that interaction can efficiently remove heavy elements and enriched material (dust, molecular gas) from galaxies. As interactions were more frequent at large lookback times, it is conceivable that they could substantially contribute (along with galactic outflows) to the enrichment of the intergalactic medium.01/2011; -
Article: A Spitzer Unbiased Ultradeep Spectroscopic Survey
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ABSTRACT: We carried out an unbiased, spectroscopic survey using the low-resolution module of the infrared spectrograph (IRS) onboard Spitzer targeting two 2.6 square arcminute regions in the GOODS-North field. The IRS was used in a spectral mapping mode with 5 hr of effective integration time per pixel. One region was covered between 14 and 21 μm and the other between 20 and 35 μm. We extracted spectra for 45 sources. About 84% of the sources have reported detections by GOODS at 24 μm, with a median f ν(24 μm) ~ 100 μJy. All but one source are detected in all four IRAC bands, 3.6 to 8 μm. We use a new cross-correlation technique to measure redshifts and estimate IRS spectral types; this was successful for ~60% of the spectra. Fourteen sources show significant polycyclic aromatic hydrocarbon emission, four mostly SiO absorption, eight present mixed spectral signatures (low PAH and/or SiO) and two show a single line in emission. For the remaining 17, no spectral features were detected. Redshifts range from z ~ 0.2 to z ~ 2.2, with a median of 1. IR luminosities are roughly estimated from 24 μm flux densities, and have median values of 2.2 × 1011 L ☉ and 7.5 × 1011 L ☉ at z ~ 1 and z ~ 2, respectively. This sample has fewer active galactic nuclei than previous faint samples observed with the IRS, which we attribute to the fainter luminosities reached here.The Astrophysical Journal 10/2009; 705(1):68. · 6.02 Impact Factor -
Article: Spitzer-IRS Study of the Antennae Galaxies NGC 4038/39
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ABSTRACT: Using the Infrared Spectrograph on the Spitzer Space Telescope, we observed the Antennae galaxies obtaining spectral maps of the entire central region and high signal-to-noise 5-38 μm spectra of the two galactic nuclei and six infrared-luminous regions. The total infrared luminosity of our six IR peaks plus the two nuclei is L IR = 3.8 × 1010 L ☉, with their derived star formation rates ranging between 0.2 and 2 M ☉ yr–1, with a total of 6.6 M ☉ yr–1. None of the typical mid-IR tracers of active galactic nucleus activity is detected in either nucleus of the system, excluding the presence of a dust-enshrouded accretion disk. The hardest and most luminous radiation originates from two compact clusters in the southern part of the overlap region, which also have the highest dust temperatures. Polycyclic aromatic hydrocarbon (PAH) emission and other tracers of softer radiation are spatially extended throughout and beyond the overlap region, but regions with a harder and more intense radiation field show a reduced PAH strength. The strong H2 emission is rather confined around the nucleus of NGC 4039, where shocks appear to be the dominant excitation mechanism, and the southern part of the overlap region, where it traces the most recent starburst activity. The luminosity ratio between the warm molecular gas (traced by the H2 lines) and the total far-IR emission is ~ 1.6 × 10–4, similar to that found in many starbursts and ultraluminous infrared galaxies. The total mass of warm H2 in the Antennae is 2.5 × 107 M ☉, with a fraction of warm to total H2 gas mass of about 0.35%. The average warm H2 temperature is 302 ± 26 K and appears anticorrelated with the radiation field hardness, possibly due to an evolution of the photodissociation region (PDR) morphology. The previously reported tight correlation between the H2 and PAH emission was not found but higher total PAH emission to continuum ratios were found in PDRs with warmer gas.The Astrophysical Journal 06/2009; 699(2):1982. · 6.02 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: Gaps in the Cloud Cover? Comparing Extinction Measures in Spiral Disks
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ABSTRACT: Dust in galaxies can be mapped either by the FIR/submillimeter emission, the optical or infrared reddening of starlight, or the extinction of a known background source. We compare two dust extinction measurements for a set of 15 sections in 13 nearby galaxies to determine the scale of the dusty interstellar medium (ISM) responsible for disk opacity: one using stellar reddening and the other a known background source. In our earlier papers, we presented extinction measurements of 29 galaxies, based on calibrated counts of distant background objects identified though foreground disks in Hubble Space Telescope WFPC2 images. For the 13 galaxies that overlap with the Spitzer Infrared Nearby Galaxies Survey, we now compare these results with those obtained from an I - L color map. Our goal is to determine whether or not a detected distant galaxy indicates a gap in the dusty ISM, and hence to better understand the nature and geometry of the disk extinction. We find that distant galaxies are predominantly in low-extinction sections marked by the color maps, indicating that their number depends both on the cloud cover of Spitzer-resolved dust structures, mostly the spiral arms, and a diffuse, unresolved underlying disk. We note that our infrared color map [E(I - L)] underestimates the overall dust presence in these disks severely because it implicitly assumes the presence of a dust screen in front of the stellar distribution.The Astronomical Journal 12/2007; 134(4):1655. · 4.03 Impact Factor -
Article: Spatially Resolved Spitzer-IRS Spectroscopy of the Central Region of M82
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ABSTRACT: We present high spatial resolution (~ 35 parsec) 5-38 um spectra of the central region of M82, taken with the Spitzer Infrared Spectrograph. From these spectra we determined the fluxes and equivalent widths of key diagnostic features, such as the [NeII]12.8um, [NeIII]15.5um, and H_2 S(1)17.03um lines, and the broad mid-IR polycyclic aromatic hydrocarbon (PAH) emission features in six representative regions and analysed the spatial distribution of these lines and their ratios across the central region. We find a good correlation of the dust extinction with the CO 1-0 emission. The PAH emission follows closely the ionization structure along the galactic disk. The observed variations of the diagnostic PAH ratios across M82 can be explained by extinction effects, within systematic uncertainties. The 16-18um PAH complex is very prominent, and its equivalent width is enhanced outwards from the galactic plane. We interpret this as a consequence of the variation of the UV radiation field. The EWs of the 11.3um PAH feature and the H_2 S(1) line correlate closely, and we conclude that shocks in the outflow regions have no measurable influence on the H_2 emission. The [NeIII]/[NeII] ratio is on average low at ~0.18, and shows little variations across the plane, indicating that the dominant stellar population is evolved (5 - 6 Myr) and well distributed. There is a slight increase of the ratio with distance from the galactic plane of M82 which we attribute to a decrease in gas density. Our observations indicate that the star formation rate has decreased significantly in the last 5 Myr. The quantities of dust and molecular gas in the central area of the galaxy argue against starvation and for negative feedback processes, observable through the strong extra-planar outflows. Comment: 15 pages, 12 figures, 3 tables, ApJ, emulateapj11/2007; -
Article: Dust and Atomic Gas in Dwarf Irregular Galaxies of the M81 Group: The SINGS and THINGS view
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ABSTRACT: [abridged] We present observations of the dust and atomic gas phase in seven dwarf irregular galaxies of the M81 group from the SINGS and THINGS surveys. The Spitzer observations provide a first glimpse of the nature of the non-atomic ISM in these metal-poor (Z~0.1 Z_sun), quiescent (SFR~0.001-0.1 M_sun/yr) dwarf galaxies. Dust emission is detected in five out of the seven targets. Most detected dust emission is restricted to HI column densities >1x10^21 cm^-2. Spitzer spectroscopy of two regions in the brightest galaxies (IC 2574 and Holmberg II) show distinctly different spectral shapes. The spectrum of IC 2574 shows aromatic features that are less luminous (relative to the FIR luminosity) compared to an average SINGS spiral galaxy by a factor of \~7 . The aromatic features in Holmberg~II (which has only a slightly lower gas-phase metallicity) are fainter than in IC 2574 by an order of magnitude. This result emphazises that the strength of the aromatic features is not a simple linear function of metallicity. We estimate dust masses of ~10^4-10^6 M_sun for the M81 dwarf galaxies, resulting in an average dust--to--gas ratio (M_dust/M_HI) of ~3x10^-4 (1.5x10^-3 if only the HI that is associated with dust emission is considered); this is an order of magnitude lower than the typical value derived for the SINGS spirals. The dwarf galaxies are underluminous per unit star formation rate at 70um as compared to the more massive galaxies in SINGS by a factor of ~2. However, the average 70um/160um ratio in the sample dwarf galaxies is higher than what is found in the other galaxies of the SINGS sample. This can be explained by a combination of a lower dust content in conjunction with a higher dust temperature in the dwarfs.02/2007; -
Article: The Displaced Dusty Interstellar Medium of NGC 3077: Tidal Stripping in the M 81 Triplet
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ABSTRACT: We present the detection of extended (∼30 kpc 2) dust emission in the tidal H i arm near NGC 3077 (member of the M 81 triplet) using SPIRE on board Herschel. Dust emission in the tidal arm is typically detected where the H i column densities are >10 21 cm −2 . The SPIRE band ratios show that the dust in the tidal arm is significantly colder (∼13 K) than in NGC 3077 itself (∼31 K), consistent with the lower radiation field in the tidal arm. The total dust mass in the tidal arm is ∼1.8 × 10 6 M (assuming β = 2), i.e., substantially larger than the dust mass associated with NGC 3077 (∼2 × 10 5 M). Where dust is detected, the dust-to-gas ratio is (6 ± 3) × 10 −3 , consistent within the uncertainties with what is found in NGC 3077 and nearby spiral galaxies with Galactic metallicities. The faint H ii regions in the tidal arm cannot be responsible for the detected enriched material and are not the main source of the dust heating in the tidal arm. We conclude that the interstellar medium (atomic H i, molecules, and dust) in this tidal feature was pre-enriched and stripped off NGC 3077 during its recent interaction (∼3 × 10 8 yr ago) with M 82 and M 81. This implies that interaction can efficiently remove heavy elements and enriched material (dust and molecular gas) from galaxies. As interactions were more frequent at large look-back times, it is conceivable that they could substantially contribute (along with galactic outflows) to the enrichment of the intergalactic medium.The Astrophysical Journal Letters 08/2006; 726:11-6. · 5.53 Impact Factor -
Article: The opaque nascent starburst in NGC 1377: Spitzer SINGS observations
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ABSTRACT: We analyze extensive data on NGC1377 from the Spitzer Infrared Nearby Galaxies Survey (SINGS). Within the category of nascent starbursts, that we previously selected by their infrared to radio continuum ratios in large excess of the average and their hot dust, NGC1377 has the largest infrared excess yet measured. Optical imaging reveals a morphological distortion suggestive of a recent accretion event. Infrared spectroscopy reveals a compact and opaque source dominated by a hot, self-absorbed continuum (tau ~ 20 in the 10 micron silicate band). We provide physical evidence against non-stellar activity being the heating source. HII regions are detected through the single [NeII] line, probing <1% of the ionizing radiation. Not only is the optical depth very high, but >85% of ionizing photons are suppressed by dust. The only other detected emission features are molecular hydrogen lines, arguably excited mainly by shocks, besides photodissociation regions, and weak aromatic bands. The new observations support our interpretation in terms of an extremely young starburst (<1 Myr). More generally, galaxies deficient in radio synchrotron are likely observed within a few Myr of the onset of a starburst and after a long quiescence, prior to the replenishment of the ISM with cosmic rays. The similar infrared-radio properties of NGC1377 and some infrared-luminous galaxies suggest that NGC1377 constitutes an archetype to better understand starburst evolution. Although rare locally because observed in a brief evolutionary stage, nascent starbursts may represent a non-negligible fraction of merger-induced starbursts dominating deep infrared counts. Since they differ dramatically from usual starburst templates, they bear important consequences for the interpretation of deep surveys.The Astrophysical Journal 05/2006; · 6.02 Impact Factor -
Article: Spatially Resolved Spitzer Spectroscopy of the Starburst Nucleus in NGC 5253
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ABSTRACT: We present new Spitzer Space Telescope data on the nearby, low-metallicity starburst galaxy NGC 5253, from the Infrared Array Camera IRAC and the Infrared Spectrograph IRS. The mid-IR luminosity profile of NGC 5253 is clearly dominated by an unresolved cluster near the center, which outshines the rest of the galaxy at longer wavelengths. We find that the [NeIII]/[NeII] ratio decreases from $\sim8.5$ at the center to $\sim2.5$ at a distance of $\sim250$ pc. The [SIV]/[SIII] follows the [NeIII]/[NeII] ratio remarkably well, being about 4-5 times lower at all distances. Our spectra reveal for the first time PAH emission feature at 11.3$\mu$m and its equivalent width increases significantly with distance from the center. The good anti-correlation between the PAH strength and the product between hardness and luminosity of the UV radiation field suggests photo-destruction of the PAH molecules in the central region. The high-excitation [OIV]25.91$\mu$m line was detected at $0.42\times10^{-20}$W cm$^{-2}$. Our results demonstrate the importance of spatially resolved mid-IR spectroscopy. Comment: 5 pages, 4 figures, accepted for publication in ApJL04/2006; -
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: S4MC observations of dust in the Small Magellanic Cloud Supernova remnant 1E 0102.2-7219
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ABSTRACT: The quantity and composition of dust produced in core-collapse supernovae is a matter of much debate. Observations of dust around quasars at high redshift have been explained via models invoking efficient dust production, with typical yields of 0.1-1 msol\ of dust per supernova. Observations of supernovae and their remnants, however, have yet to provide evidence for efficient dust production. Observations by Stanimirovic et al. (2005) of the young, oxygen rich, supernova remnant 1E 0102.2-7219 in the Small Magellanic Cloud placed an upper limit on the dust produced in the supernova of 8Ã 10-4 msol, far less than the amount predicted by models. We present 5-35 micron IRS spectral mapping observations of SNR E 0102 obtained as part of the Spitzer Spectroscopic Survey of the Small Magellanic Cloud (S^4MC). The spectral information allows us to quantify the contribution of the [O IV] emission line at 25.8 micron to the 24 micron MIPS flux--a major source of uncertainty for Stanimirovic et al.--to be ÃÅ 10 % of the total 24 micron flux. From fits to the spectral continuum we find on the order of 5Ã 10-4 msol\ of sim 110 K dust in the remnant. We compare the spatial distribution of the hot dust continuum with that of to the line emission and infer the physical conditions in the remnant. We also discuss the composition of the hot dust as revealed by the infrared spectrum.Sheth, K.; Noriega-Crespo, A.; Ingalls, J. ; Paladini, R.: The Evolving ISM in the Milky Way and Nearby Galaxies, 58 (2009). -
Article: Mapping far-IR emission from the central kiloparsec of NGC 1097
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ABSTRACT: Using photometry of NGC 1097 from the Herschel PACS (Photodetector Array Camera and Spectrometer) instrument, we study the resolved properties of thermal dust continuum emission from a circumnuclear starburst ring with a radius ~900 pc. These observations are the first to resolve the structure of a circumnuclear ring at wavelengths that probe the peak (i.e. û ~ 100 üm) of the dust spectral energy distribution. The ring dominates the far-infrared (far-IR) emission from the galaxy - the high angular resolution of PACS allows us to isolate the ring's contribution and we find it is responsible for 75, 60 and 55% of the total flux of NGC 1097 at 70, 100 and 160 üm, respectively. We compare the far-IR structure of the ring to what is seen at other wavelengths and identify a sequence of far-IR bright knots that correspond to those seen in radio and mid-IR images. The mid- and far-IR band ratios in the ring vary by less than ñ20% azimuthally, indicating modest variation in the radiation field heating the dust on ~600 pc scales. We explore various explanations for the azimuthal uniformity in the far-IR colors of the ring including a lack of well-defined age gradients in the young stellar cluster population, a dominant contribution to the far-IR emission from dust heated by older (>10 Myr) stars and/or a quick smoothing of local enhancements in dust temperature due to the short orbital period of the ring. Finally, we improve previous limits on the far-IR flux from the inner ~600 pc of NGC 1097 by an order of magnitude, providing a better estimate of the total bolometric emission arising from the active galactic nucleus and its associated central starburst. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA. -
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 Msolar yr-1 kpc-2. Unlike the UV emission, the monochromatic 24 mum 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 mum emission carriers are mainly heated by the young, ionizing stars. However, preliminary results show that the ratio of the 24 mum 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 mum luminosity can be used as an SFR tracer for galaxy populations. While also correlated with star formation, the 8 mum 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 mum emission are heated by more than one mechanism.The Astrophysical Journal, v.633, 871-893 (2005). -
Article: Spitzer-IRS Study of the Antennae Galaxies NGC 4038/39
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ABSTRACT: Using the Infrared Spectrograph on the Spitzer Space Telescope, we observed the Antennae galaxies obtaining spectral maps of the entire central region and high signal-to-noise 5-38 μm spectra of the two galactic nuclei and six infrared-luminous regions. The total infrared luminosity of our six IR peaks plus the two nuclei is L_(IR) = 3.8 × 10^(10)L☉, with their derived star formation rates ranging between 0.2 and 2 M☉ yr^(–1), with a total of 6.6 M☉ yr^(–1). None of the typical mid-IR tracers of active galactic nucleus activity is detected in either nucleus of the system, excluding the presence of a dust-enshrouded accretion disk. The hardest and most luminous radiation originates from two compact clusters in the southern part of the overlap region, which also have the highest dust temperatures. Polycyclic aromatic hydrocarbon (PAH) emission and other tracers of softer radiation are spatially extended throughout and beyond the overlap region, but regions with a harder and more intense radiation field show a reduced PAH strength. The strong H_2 emission is rather confined around the nucleus of NGC 4039, where shocks appear to be the dominant excitation mechanism, and the southern part of the overlap region, where it traces the most recent starburst activity. The luminosity ratio between the warm molecular gas (traced by the H_2 lines) and the total far-IR emission is ~ 1.6 × 10^(–4), similar to that found in many starbursts and ultraluminous infrared galaxies. The total mass of warm H_2 in the Antennae is 2.5 × 10^7 M☉ , with a fraction of warm to total H_2 gas mass of about 0.35%. The average warm H_2 temperature is 302 ± 26 K and appears anticorrelated with the radiation field hardness, possibly due to an evolution of the photodissociation region (PDR) morphology. The previously reported tight correlation between the H_2 and PAH emission was not found but higher total PAH emission to continuum ratios were found in PDRs with warmer gas.
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Institutions
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2006–2012
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University of Toledo
- Department of Physics and Astronomy
Toledo, OH, USA -
Cornell University
New York City, NY, USA
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2007–2009
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The University of Arizona
Tucson, AZ, USA
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