J. A. Baldwin

Michigan State University, East Lansing, Michigan, United States

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Publications (125)432.12 Total impact

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    Neelam Dhanda Batra, Jack A. Baldwin
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    ABSTRACT: We measured the metallicity Z in the broad emission line regions (BELRs) of 43 SDSS quasars with the strongest N IV] and N III] emission lines. These N-Loud QSOs have unusually low black hole masses. We used the intensity ratio of N lines to collisionally-excited emission lines of other heavy elements to find metallicities in their BELR regions. We found that 7 of the 8 line-intensity ratios that we employed give roughly consistent metallicities as measured, but that for each individual QSO their differences from the mean of all metallicity measurements depends on the ionization potential of the ions that form the emission lines. After correcting for this effect, the different line-intensity ratios give metallicities that generally agree to within the 0.24 dex uncertainty in the measurements of the line-intensity ratios. The metallicities are very high, with mean log Z for the whole sample of 5.5 Z_sun and a maximum of 18 Z_sun. Our results argue against the possibility that the strong N lines represent an overabundance only of N but not of all heavy elements. They are compatible with either (1) the BELR gas has been chemically enriched by the general stellar population in the central bulge of the host galaxy but the Locally Optimally-emitting Cloud model used in the analysis needs some fine tuning, or (2) that instead this gas has been enriched by intense star formation on the very local scale of the active nucleus that has resulted in an abundance gradient within the BELR.
    01/2014; 439(1).
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    ABSTRACT: High ionization star forming galaxies are easily identified with strong emission line techniques such as the BPT diagram, but for ionization levels below log([O III]/Hβ) ~ 0.3 they become confused with low-ionization AGN, making their physical interpretation difficult. Mean field independent component analysis (MFICA) is a novel approach to processing emission line spectra that can disentangle the AGN and starlight contributions to emission lines allowing the properties of pure AGN and pure starburst galaxies to be interpreted over a wide range of ionization. We applied MFICA to large sample of low-z SDSS galaxies and created subsamples of pure star forming galaxies resulting in a sequence of varying ionization. We used a locally optimally emitting (LOC) cloud model to fit emission line ratios that constrain the excitation mechanism, spectral energy distribution, abundances and physical conditions. Preliminary results in fitting diagrams that constrain the excitation mechanism indicate that the variation of starburst galaxies is due to a change in the radial distribution of clouds, rather than differences in metallicity, ionization parameter or spectral energy distribution. This confirms that MFICA is a powerful tool to assess differences in emission line properties solely due to starbursts. We briefly discuss future work that will decipher other properties in star forming galaxies.
    01/2014;
  • Xiang Wang, G. J. Ferland, J. A. Baldwin
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    ABSTRACT: Infrared observations have discovered a variety of objects, including filaments in the Crab Nebula and cool-core clusters of galaxies, where H2 the 2.121 μm line is far stronger than nearby H I lines. A variety of processes could be responsible for this emission. Although many complete shock or PDR calculations of H2 emission have been published, we know of no simple calculation that shows the emission spectrum and level populations of thermally excited low-density H2. We present a range of purely thermal collisional simulations, corresponding to constant gas kinetic temperature but different densities. We consider the cases of predominantly atomic and molecular regions. The resulting excitation diagrams show that excitation temperatures are sometimes smaller than the gas kinetic temperature except at densities where the level populations have gone to LTE. Considering only commonly observed IR lines, the population distribution at low densities for the v=0 manifold could be misinterpreted as thermal but at too low a kinetic temperature. The excitation diagrams at low densities have a curvature for the v=0 manifold but not the v=1, 2 manifolds. This mimics the classic signature of continuum fluorescent excitation of the molecule. The excitation diagrams are discontinuous between different vibrational energy levels at low density and this could be used as a density diagnostic. Differences between the low-density thermal and the fluorescent or shocked cases could be observed if enough levels are detected.
    01/2014;
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    ABSTRACT: The metallicity of active galactic nuclei (AGNs), which can be measured by emission line ratios in their broad and narrow line regions (BLRs and NLRs), provides invaluable information about the physical connection between the different components of AGNs. From the archival databases of the International Ultraviolet Explorer, the Hubble Space Telescope and the Sloan Digital Sky Survey, we have assembled the largest sample available of AGNs which have adequate spectra in both the optical and ultraviolet bands to measure the narrow line ratio [N II]/H{\alpha} and also, in the same objects, the broad-line N V/C IV ratio. These permit the measurement of the metallicities in the NLRs and BLRs in the same objects. We find that neither the BLR nor the NLR metallicity correlate with black hole masses or Eddington ratios, but there is a strong correlation between NLR and BLR metallicities. This metallicity correlation implies that outflows from BLRs carry metal-rich gas to NLRs at characteristic radial distances of ~ 1.0 kiloparsec. This chemical connection provides evidence for a kinetic feedback of the outflows to their hosts. Metals transported into the NLR enhance the cooling of the ISM in this region, leading to local star formation after the AGNs turn to narrow line LINERs. This post-AGN star formation is predicted to be observable as an excess continuum emission from the host galaxies in the near infrared and ultraviolet, which needs to be further explored.
    Monthly Notices of the Royal Astronomical Society 12/2013; · 5.52 Impact Factor
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    ABSTRACT: We investigate the physical cause of the great range in the ionization level seen in the spectra of narrow lined active galactic nuclei (AGN). Mean field independent component analysis identifies examples of individual SDSS galaxies whose spectra are not dominated by emission due to star formation (SF), which we designate as AGN. We assembled high S/N ratio composite spectra of a sequence of these AGN defined by the ionization level of their narrow-line regions (NLR), extending down to very low-ionization cases. We used a local optimally emitting cloud (LOC) model to fit emission-line ratios in this AGN sequence. These included the weak lines that can be measured only in the co-added spectra, providing consistency checks on strong line diagnostics. After integrating over a wide range of radii and densities our models indicate that the radial extent of the NLR is the major parameter in determining the position of high to moderate ionization AGN along our sequence, providing a physical interpretation for their systematic variation. Higher ionization AGN contain optimally emitting clouds that are more concentrated towards the central continuum source than in lower ionization AGN. Our LOC models indicate that for the objects that lie on our AGN sequence, the ionizing luminosity is anticorrelated with the NLR ionization level, and hence anticorrelated with the radial concentration and physical extent of the NLR. A possible interpretation that deserves further exploration is that the ionization sequence might be an age sequence where low ionization objects are older and have systematically cleared out their central regions by radiation pressure. We consider that our AGN sequence instead represents a mixing curve of SF and AGN spectra, but argue that while many galaxies do have this type of composite spectra, our AGN sequence appears to be a special set of objects with negligible SF excitation.
    Monthly Notices of the Royal Astronomical Society 10/2013; 437(3). · 5.52 Impact Factor
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    ABSTRACT: We present a range of steady-state photoionization simulations, corresponding to different assumed shell geometries and compositions, of the unseen postulated rapidly expanding outer shell to the Crab Nebula. The properties of the shell are constrained by the mass that must lie within it, and by limits to the intensities of hydrogen recombination lines. In all cases the photoionization models predict very strong emission from high ionization lines that will not be emitted by the Crab's filaments, alleviating problems with detecting these lines in the presence of light scattered from brighter parts of the Crab. The NIR [Ne VI] $\lambda$7.652 $\mu$m line is a particularly good case; it should be dramatically brighter than the optical lines commonly used in searches. The C IV $\lambda1549\AA$ doublet is predicted to be the strongest absorption line from the shell, which is in agreement with HST observations. We show that the cooling timescale for the outer shell is much longer than the age of the Crab, due to the low density. This means that the temperature of the shell will actually "remember" its initial conditions. However, the recombination time is much shorter than the age of the Crab, so the predicted level of ionization should approximate the real ionization. In any case, it is clear that IR observations present the best opportunity to detect the outer shell and so guide future models that will constrain early events in the original explosion.
    The Astrophysical Journal 07/2013; 774(2). · 6.73 Impact Factor
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    ABSTRACT: We present an analysis of the optical spectra of narrow emission-line galaxies, based on mean field independent component analysis (MFICA). Samples of galaxies were drawn from the Sloan Digital Sky Survey (SDSS) and used to generate compact sets of `continuum' and `emission-line' component spectra. These components can be linearly combined to reconstruct the observed spectra of a wider sample of galaxies. Only 10 components - five continuum and five emission line - are required to produce accurate reconstructions of essentially all narrow emission-line galaxies; the median absolute deviations of the reconstructed emission-line fluxes, given the signal-to-noise ratio (S/N) of the observed spectra, are 1.2-1.8 sigma for the strong lines. After applying the MFICA components to a large sample of SDSS galaxies we identify the regions of parameter space that correspond to pure star formation and pure active galactic nucleus (AGN) emission-line spectra, and produce high S/N reconstructions of these spectra. The physical properties of the pure star formation and pure AGN spectra are investigated by means of a series of photoionization models, exploiting the faint emission lines that can be measured in the reconstructions. We are able to recreate the emission line strengths of the most extreme AGN case by assuming the central engine illuminates a large number of individual clouds with radial distance and density distributions, f(r) ~ r^gamma and g(n) ~ n^beta, respectively. The best fit is obtained with gamma = -0.75 and beta = -1.4. From the reconstructed star formation spectra we are able to estimate the starburst ages. These preliminary investigations serve to demonstrate the success of the MFICA-based technique in identifying distinct emission sources, and its potential as a tool for the detailed analysis of the physical properties of galaxies in large-scale surveys.
    Monthly Notices of the Royal Astronomical Society 01/2013; 430(4). · 5.52 Impact Factor
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    ABSTRACT: In our previous work, we mapped out the Crab Nebula in the NIR H2 2.12 um line and found 55 “knots” of molecular hydrogen that correlate with low ionization optical emission. Using K-band spectra, we determined that the H2 emission has a high excitation temperature, ~3000 K, close to the dissociation temperature of H2. Knot 51 is a bright, spatially isolated knot of molecular hydrogen located near the tip of a long tendril of filamentary gas in the Crab Nebula and presents the best opportunity to match observations and simulations due to its small radial velocity and minimal background/foreground contamination. We present the results of modeling Knot 51 using the plasma simulation code Cloudy to simultaneously reproduce the observed optical emission for ionized and neutral gas, as well as the bright H2 emission, within the volume of the knot. We find that models including only the Crab's synchrotron continuum are ruled out due to their failure to reproduce the thermal H2 spectrum. Instead, our best fitting models have extra heating in the core, possibly due to ionizing particles or shocks, and reproduce the observed spectrum within only a trace amount of H2. Furthermore, in this environment, H2 forms primarily through associative detachment, rather than grain catalysis.
    01/2013;
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    ABSTRACT: Understanding how molecules and dust might have formed within a rapidly expanding young supernova remnant is important because of the obvious application to vigorous supernova activity at very high redshift. In previous papers, we found that the H2 emission is often quite strong, correlates with optical low-ionization emission lines, and has a surprisingly high excitation temperature. Here we study Knot 51, a representative, bright example, for which we have available long slit optical and NIR spectra covering emission lines from ionized, neutral, and molecular gas, as well as HST visible and SOAR Telescope NIR narrow-band images. We present a series of CLOUDY simulations to probe the excitation mechanisms, formation processes and dust content in environments that can produce the observed H2 emission. We do not try for an exact match between model and observations given Knot 51's ambiguous geometry. Rather, we aim to explain how the bright H2 emission lines can be formed from within the volume of Knot 51 that also produces the observed optical emission from ionized and neutral gas. Our models that are powered only by the Crab's synchrotron radiation are ruled out because they cannot reproduce the strong, thermal H2 emission. The simulations that come closest to fitting the observations have the core of Knot 51 almost entirely atomic with the H2 emission coming from just a trace molecular component, and in which there is extra heating. In this unusual environment, H2 forms primarily by associative detachment rather than grain catalysis. In this picture, the 55 H2-emitting cores that we have previously catalogued in the Crab have a total mass of about 0.1 M_sun, which is about 5% of the total mass of the system of filaments. We also explore the effect of varying the dust abundance. We discuss possible future observations that could further elucidate the nature of these H2 knots.
    Monthly Notices of the Royal Astronomical Society 12/2012; · 5.52 Impact Factor
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    ABSTRACT: We report on a spectral principal component analysis (SPCA) of a sample of 816 quasars, selected to have small Fe II velocity shifts with spectral coverage in the rest wavelength range 3500--5500 \AA. The sample is explicitly designed to mitigate spurious effects on SPCA induced by Fe II velocity shifts. We improve the algorithm of SPCA in the literature and introduce a new quantity, \emph{the fractional-contribution spectrum}, that effectively identifies the emission features encoded in each eigenspectrum. The first eigenspectrum clearly records the power-law continuum and very broad Balmer emission lines. Narrow emission lines dominate the second eigenspectrum. The third eigenspectrum represents the Fe II emission and a component of the Balmer lines with kinematically similar intermediate velocity widths. Correlations between the weights of the eigenspectra and parametric measurements of line strength and continuum slope confirm the above interpretation for the eigenspectra. Monte Carlo simulations demonstrate the validity of our method to recognize cross talk in SPCA and firmly rule out a single-component model for broad Hbeta. We also present the results of SPCA for four other samples that contain quasars in bins of larger Fe II velocity shift; similar eigenspectra are obtained. We propose that the Hbeta-emitting region has two kinematically distinct components: one with very large velocities whose strength correlates with the continuum shape, and another with more modest, intermediate velocities that is closely coupled to the gas that gives rise to Fe II emission.
    The Astrophysical Journal 10/2012; 760(2). · 6.73 Impact Factor
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    ABSTRACT: (abridged) We study the consequence of star formation (SF) in an self-gravity dominated accretion disk in quasars. The warm skins of the SF disk are governed by the radiation from the inner part of the accretion disk to form Compton atmosphere (CAS). The CAS are undergoing four phases to form broad line regions. Phase I is the duration of pure accumulation supplied by the SF disk. During phase II clouds begin to form due to line cooling and sink to the SF disk. Phase III is a period of preventing clouds from sinking to the SF disk through dynamic interaction between clouds and the CAS. Finally, phase IV is an inevitable collapse of the entire CAS through line cooling. This CAS evolution drives the episodic appearance of BLRs. Geometry and dynamics of BLRs can be self-consistently derived from the thermal instability of the CAS during phases II and III by linear analysis. The metallicity gradient of SF disk gives rise to different properties of clouds from outer to inner part of BLRs. We find that clouds have column density N_H < 10^22cm^{-2} in the metal-rich regions whereas they have N_H > 10^22 cm^{-2} in the metal-poor regions. The metal-rich clouds compose the high ionization line (HIL) regions whereas the metal-poor clouds are in low ionization line (LIL) regions. Metal-rich clouds in HIL regions will be blown away by radiation pressure, forming the observed outflows. The LIL regions are episodic due to the mass cycle of clouds with the CAS in response to continuous injection by the SF disk, giving rise to different types of AGNs. Based on SDSS quasar spectra, we identify a spectral sequence in light of emission line equivalent width from Phase I to IV. A key phase in the episodic appearance of the BLRs is bright type II AGNs with no or only weak BLRs. We discuss observational implications and tests of the theoretical predictions of this model.
    The Astrophysical Journal 01/2012; 746(2). · 6.73 Impact Factor
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    ABSTRACT: We are using a combination of archival NASA ADAP data and new ground-based observations to study molecular cores in the Crab Nebula's filaments. This component of the Crab has largely been ignored since the pioneering discovery, by Graham, Wright & Longmore (1990), of H2 emission at two locations. Many of the absorbing dust blobs long known to exist in the Crab also are likely to be associated with these molecular cores. Some important open questions are: How much mass is in the molecular component? What excites the H2 emission? How were the molecules formed? What is the nature of the dust? Where/when/how was the dust formed? Are these structures similar to cool-core cluster filaments, which also emit strongly in H2? Our new ground-based NIR images and spectra show many additional H2 knots scattered along the Crab's filaments. We are currently analyzing the large grid of Spitzer GTO long-slit and high-resolution IRS spectra of the Crab, which include many of the molecular cores. We have supplemented them with a grid of long-slit optical spectra at the exact same positions, giving spectral coverage over a wide range of emission lines from ionized, neutral and atomic gas. We are adding CO observations at selected points, to further constrain the properties of the molecular cores. HST [O III] and [S II] images show the structure of individual molecular knots at 1017 cm resolution. We also are carrying out a new survey of dust features using archival HST continuum images, to better determine how dust fits into the picture. Spitzer and Herschel images provide a lower-resolution look covering the dust thermal emission peak. Our aim is to assemble the necessary pan-chromatic data set and then construct accurate plasma simulation models that will allow us to answer the questions posed above.
    01/2012;
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    ABSTRACT: We used K-band spectra to measure the H2 excitation temperatures in six molecular knots associated with the filaments in the Crab Nebula. The temperatures are quite high - in the range T ~ 2000-3000K, just below the H2 dissociation temperature. This is the temperature range over which the H2 1-0 S(1) line at 2.121\mum has its maximum emissivity per unit mass, so there may be many additional H2 cores with lower temperatures that are too faint to detect. We also measured the electron density in adjacent ionized gas, which on the assumption of gas pressure balance indicates densities in the molecular region n_mol ~ 20,000 H baryons cm-3, although this really is just a lower limit since the H2 gas may be confined by other means. The excited region may be just a thin skin on a much more extensive blob of molecular gas that does not have the correct temperature and density to be as easily detectable. At the opposite extreme, the observed knots could consist of a fine mist of molecular gas in which we are detecting essentially all of the H2. Future CO observations could distinguish between these two cases. The Crab filaments serve as the nearby laboratories for understanding the very much larger filamentary structures that have formed in the intracluster medium of cool-core galaxy clusters.
    Monthly Notices of the Royal Astronomical Society 12/2011; 421(1). · 5.52 Impact Factor
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    ABSTRACT: It has been suggested that the high metallicity generally observed in active galactic nuclei (AGNs) and quasars originates from ongoing star formation in the self-gravitating part of accretion disks around supermassive black holes (SMBHs). We designate this region as the star-forming (SF) disk, in which metals are produced from supernova explosions (SNexp) while at the same time inflows are driven by SNexp-excited turbulent viscosity to accrete onto the SMBHs. In this paper, an equation of metallicity governed by SNexp and radial advection is established to describe the metal distribution and evolution in the SF disk. We find that the metal abundance is enriched at different rates at different positions in the disk, and that a metallicity gradient is set up that evolves for steady-state AGNs. Metallicity as an integrated physical parameter can be used as a probe of the SF disk age during one episode of SMBH activity. In the SF disk, evaporation of molecular clouds heated by SNexp blast waves unavoidably forms hot gas. This heating is eventually balanced by the cooling of the hot gas, but we show that the hot gas will escape from the SF disk before being cooled, and diffuse into the broad-line regions (BLRs) forming with a typical rate of ~1 M ☉ yr–1. The diffusion of hot gas from an SF disk depends on ongoing star formation, leading to the metallicity gradients in BLR observed in AGNs. We discuss this and other observable consequences of this scenario.
    The Astrophysical Journal 08/2011; 739(1):3. · 6.73 Impact Factor
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    ABSTRACT: We have carried out a near-infrared, narrow-band imaging survey of the Crab Nebula, in the H2 2.12 micron and Br-gamma 2.17 micron lines, using the Spartan Infrared camera on the SOAR Telescope. Over a 2.8' x 5.1' area that encompasses about 2/3 of the full visible extent of the Crab, we detect 55 knots that emit strongly in the H2 line. We catalog the observed properties of these knots. We show that they are in or next to the filaments that are seen in optical-passband emission lines. Comparison to HST [S II] and [O III] images shows that the H2 knots are strongly associated with compact regions of low-ionization gas. We also find evidence of many additional, fainter H2 features, both discrete knots and long streamers following gas that emits strongly in [S II]. A pixel-by-pixel analysis shows that about 6 percent of the Crab's projected surface area has significant H2 emission that correlates with [S II] emission. We measured radial velocities of the [S II] lambda6716 emission lines from 47 of the cataloged knots and find that most are on the far (receding) side of the nebula. We also detect Br-gamma emission. It is right at the limit of our survey, and our Br-gamma filter cuts off part of the expected velocity range. But clearly the Br-gamma emission has a quite different morphology than the H2 knots, following the long linear filaments that are seen in H-alpha and in [O III] optical emission lines.
    The Astrophysical Journal Supplement Series 03/2011; 194(2). · 16.24 Impact Factor
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    ABSTRACT: We use our new optical-imaging and spectrophotometric survey of key diagnostic emission lines in 30 Doradus, together with CLOUDY photoionization models, to study the physical conditions and ionization mechanisms along over 4000 individual lines of sight at points spread across the face of the extended nebula, out to a projected radius 75 pc from R136 at the center of the ionizing cluster NGC 2070. We focus on the physical conditions, geometry and importance of radiation pressure on a point-by-point basis, with the aim of setting observational constraints on important feedback processes. We find that the dynamics and large scale structure of 30 Dor are set by a confined system of X-ray bubbles in rough pressure equilibrium with each other and with the confining molecular gas. Although the warm (10,000K) gas is photoionized by the massive young stars in NGC 2070, the radiation pressure does not currently play a major role in shaping the overall structure. The completeness of our survey also allows us to create a composite spectrum of 30 Doradus, simulating the observable spectrum of a spatially-unresolved, distant giant extragalactic H II region. We find that the highly simplified models used in the "strong line" abundance technique do in fact reproduce our observed lines strengths and deduced chemical abundances, in spite of the more than one order of magnitude range in the ionization parameter and density of the actual gas in 30 Dor.
    The Astrophysical Journal 01/2011; · 6.73 Impact Factor
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    ABSTRACT: The observed phenomena of star formation (SF) and active galactic nuclei (AGN) have long been thought to be intimately linked, but to date the details of their connection have remained elusive. In recent years the Sloan Digital Sky Survey (SDSS) has provided very large samples of high quality galaxy spectra, allowing progress to be made in statistical analyses of the AGN/SF link. However, such analyses currently rely on measurements of particular emission line ratios and the Baldwin, Phillips & Terlevich (BPT) diagrams to classify objects as SF-dominated, AGN-dominated, or "composite". Such classifications do not produce complete samples of objects with SF or AGN, and are not sensitive to the low-level activity that may shed important light on the relationship between the different sources. I will present a new technique, based on mean field independent component analysis, which allows the fractional contribution of AGN and SF to each object to be quantified. In doing so, we can classify objects based on direct measurements of their compositions, rather than the indirect BPT-based methods. We are able to disentangle the AGN and SF contributions to the "composite" region, that encompasses a large number of the objects observed by the SDSS. Additionally, low-level SF can be identified in AGN-dominated galaxies, and low-level AGN in SF-dominated galaxies. The technique can be applied to all galaxy spectra in the SDSS with the required spectral coverage, and readily extended to encompass other spectroscopic surveys. Together, these developments allow a new level of precision in the study of the link between AGN and SF. JTA acknowledges the support of an STFC PhD studentship.
    01/2011;
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    ABSTRACT: In 2008 February, we obtained a grid of long-slit spectra of 30 Dor using the RC spectrograph on the 4m Blanco telescope at Cerro Tololo Inter-American Observatory (CTIO). The spectrograph slit was 5 arcmin in length, and was positioned at a total of 37 different locations spanning the nebula (Figure 1(b)). Table 2 lists for each slit position the identifying position number, the R.A. and decl. of the slit center in J2000 coordinates, the P.A., and the total exposure time. (2 data files).
    VizieR Online Data Catalog. 11/2010;
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    ABSTRACT: We have completed a new optical imaging and spectrophotometric survey of a 140 × 80 pc2 region of 30 Doradus centered on R136, covering key optical diagnostic emission lines including Hα, Hβ, Hγ, [O III] λλ4363, 4959, 5007, [N II] λλ6548, 6584, [S II] λλ6717, 6731 [S III] λ6312, and in some locations [S III] λ9069. We present maps of fluxes and intensity ratios for these lines, and catalogs of isolated ionizing stars, elephant-trunk pillars, and edge-on ionization fronts. The final science-quality spectroscopic data products are available to the public. Our analysis of the new data finds that, while stellar winds and supernovae undoubtedly produce shocks and are responsible for shaping the nebula, there are no global spectral signatures to indicate that shocks are currently an important source of ionization. We conclude that the considerable region covered by our survey is well described by photoionization from the central cluster where the ionizing continuum is dominated by the most massive O stars. We show that if 30 Dor were viewed at a cosmological distance, its integrated light would be dominated by its extensive regions of lower surface brightness rather than by the bright, eye-catching arcs.
    The Astrophysical Journal Supplement Series 10/2010; 191(1):160. · 16.24 Impact Factor
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    E. D. Loh, J. A. Baldwin, G. J. Ferland
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    ABSTRACT: In a sub-arcsec near-infrared survey of the Crab Nebula using the new Spartan Infrared Camera, we have found several knots with high surface brightness in the H_2 2.12 micron line and a very large H_2 2.12 micron to Br-gamma ratio. The brightest of these knots has an intensity ratio I(H_2 2.12 micron)/I(Br-gamma) = 18+/-9, which we show sets a lower limit on the ratio of masses in the molecular and recombination (i.e. ionized) zones M_mol / M_rec >/- 0.9, and a total molecular mass within this single knot M_mol >/- 5E-5 M_sun. We argue that the knot discussed here probably is able to emit so strongly in the 2.12 micron line because its physical conditions are better tuned for such emission than is the case in other filaments. It is unclear whether this knot has an unusually large M_mol / M_rec ratio, or if many other Crab filaments also have similar amounts of molecular gas which is not emitting because the physical conditions are not so well tuned. Comment: Accepted for publication in ApJ Letters
    The Astrophysical Journal Letters 05/2010; · 6.35 Impact Factor

Publication Stats

299 Citations
432.12 Total Impact Points

Institutions

  • 2001–2014
    • Michigan State University
      • Department of Physics and Astronomy
      East Lansing, Michigan, United States
  • 2006
    • Rice University
      • Department of Physics and Astronomy
      Houston, Texas, United States
  • 2003
    • Space Telescope Science Institute
      Baltimore, Maryland, United States
  • 1993
    • University of Kentucky
      • Department of Physics & Astronomy
      Lexington, Kentucky, United States
  • 1991
    • The Ohio State University
      Columbus, Ohio, United States