J. A. Baldwin

Michigan State University, East Lansing, Michigan, United States

Are you J. A. Baldwin?

Claim your profile

Publications (124)467.49 Total impact

  • [Show abstract] [Hide abstract]
    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.
  • Xiang Wang · G. J. Ferland · J. A. Baldwin
    [Show abstract] [Hide abstract]
    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.
  • Source
    [Show abstract] [Hide abstract]
    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). DOI:10.1088/0004-637X/774/2/112 · 6.28 Impact Factor
  • [Show abstract] [Hide abstract]
    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.
  • Source
    [Show abstract] [Hide abstract]
    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; DOI:10.1093/mnras/sts695 · 5.23 Impact Factor
  • [Show abstract] [Hide abstract]
    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.
  • Source
    [Show abstract] [Hide abstract]
    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). DOI:10.1111/j.1365-2966.2011.20353.x · 5.23 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    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). DOI:10.1088/0067-0049/194/2/30 · 14.14 Impact Factor
  • E. W. Pellegrini · J. A. Baldwin · G. J. Ferland
    [Show abstract] [Hide abstract]
    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).
  • Source
    E. D. Loh · J. A. Baldwin · G. J. Ferland
    [Show abstract] [Hide abstract]
    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; DOI:10.1088/2041-8205/716/1/L9 · 5.60 Impact Factor
  • Joshua Niedzielski · J. A. Baldwin · E. Pellegrini
    [Show abstract] [Hide abstract]
    ABSTRACT: An edge-on ionization front in the Galactic H II region M20 was observed on two nights with the SOAR telescope. Grids of long-slit spectra were taken that include the Halpha, Hbeta and [S II] 6717, 6731 emission lines, which were used to determine the reddening correction, Halpha surface brightness and electron density at thousands of points across the ionization front. The electron density is found to increase sharply to about 1000 cm-3 within the region of high Halpha surface brightness. In addition, the [N II] 6548, 6583, and 5755 emission lines were used to determine the electron temperature across the ionization front. Our results are compared to those of Lefloch et al (2002) who used far-infrared and radio spectra.
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The Orion Bar is one of the nearest and best-studied photodissociation or photon-dominated regions (PDRs). Observations reveal the presence of H2 lines from vibrationally or rotationally excited upper levels that suggest warm gas temperatures (400-700 K). However, standard models of PDRs are unable to reproduce such warm rotational temperatures. In this paper, we attempt to explain these observations with new comprehensive models which extend from the H+ region through the Bar and include the magnetic field in the equation of state. We adopt the model parameters from our previous paper which successfully reproduced a wide variety of spectral observations across the Bar. In this model, the local cosmic ray density is enhanced above the galactic background, as is the magnetic field, and which increases the cosmic ray heating elevating the temperature in the molecular region. The pressure is further enhanced above the gas pressure in the H+ region by the momentum transferred from the absorbed starlight. Here, we investigate whether the observed H2 lines can be reproduced with standard assumptions concerning the grain photoelectric emission. We also explore the effects due to the inclusion of recently computed H2 + H2, H2 + H, and H2 + He collisional rate coefficients.
    The Astrophysical Journal 07/2009; 701(1):677. DOI:10.1088/0004-637X/701/1/677 · 6.28 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Previous work has shown the Orion Bar to be an interface between ionized and molecular gas, viewed roughly edge-on, which is excited by the light from the Trapezium cluster. Much of the emission from any star-forming region will originate from such interfaces, so the Bar serves as a foundation test of any emission model. Here we combine X-ray, optical, infrared (IR), and radio data sets to derive emission spectra along the transition from H+ to H0 to H2 regions. We then reproduce the spectra of these layers with a simulation that simultaneously accounts for the detailed microphysics of the gas, the grains, and molecules, especially H2 and CO. The magnetic field, observed to be the dominant pressure in another region of the Orion Nebula, is treated as a free parameter, along with the density of cosmic rays. Our model successfully accounts for the optical, IR, and radio observations across the Bar by including a significant magnetic pressure and also heating by an excess density of cosmic rays, which we suggest is due to cosmic rays being trapped in the compressed magnetic field. In the Orion Bar, as we had previously found in M17, momentum carried by radiation and winds from the newly formed stars pushes back and compresses the surrounding gas. There is a rough balance between outward momentum in starlight and the total pressure in atomic and molecular gas surrounding the H+ region. If the gas starts out with a weak magnetic field, the starlight from a newly formed cluster will push back the gas and compress the gas, magnetic field, and cosmic rays until magnetic pressure becomes an important factor.
    The Astrophysical Journal 03/2009; 693(1):285-302. DOI:10.1088/0004-637X/693/1/285 · 6.28 Impact Factor
  • Eric W. Pellegrini · J. A. Baldwin · G. J. Ferland
    [Show abstract] [Hide abstract]
    ABSTRACT: Newly formed hot stars clearly interact with and shape the surrounding molecular gas from which they form, but there are several possible feedback processes: supernovae, stellar winds, thermal gas pressure and/or radiation pressure. Our aim is to calibrate the important physical processes of larger star forming regions at distances too great to be spatially resolved using the Orion nebula with one O star, M17 with about 10, up to NGC 3603 and 30 Doradus with up to 100 O stars. Here we present a new survey of optical spectra over much of NGC 3603 and 30 Doradus covering the Halpha, Hbeta, [O III] and [S II] emission lines. A subsample of positions was also observed with long-slit K-Band spectroscopy. In addition to the spectra, we have obtained high resolution, narrow-band images of NGC 3603 and 30 Doradus covering 8'x10' and 12'x13' respectively, in Halpha, [SII] and [OIII] plus continuum passbands. These fill in the gaps between the spectra. These images are up to 10 times higher resolution than the Magellanic Cloud Emission Line Survey and cover a field of view 12 times larger than HST. By combining X-ray, optical, IR and radio observations we constrain our self-consistent models through the H+ , H0 and molecular regions, allowing us to study the physical nature of the interaction of stars and their parent molecular cloud. The optical spectra fill in gaps to the available data set including a large scale mapping of the electron density measured from the [SII] emission which is a critical constraint in the construction of simulated lines of sight using the plasma physics code CLOUDY. The measurements together with the models allow us to compare the relative importance of gas pressure, radiation pressure, stellar winds and the hot X-ray emitting component resulting from massive star formation.
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: This paper addresses the question, what does the spectrum of a typical quasar reveal about the velocity structure within its broad emission line region clouds? Turbulent (i.e., nonthermal) broadening of spectral lines can be due to macroturbulence or microturbulence. Microturbulence affects line formation and the emitted spectrum and may be required to account for the observed smoothness of the line profiles. The velocity field is crucial since it addresses the fundamental nature of the individual clouds and the global structure of the active galactic nuclei (AGNs) environment. For example, stellar winds or magnetically confined blobs might be highly microturbulent, requiring only a few internally broadened clouds to account for the observed smooth line profiles in AGNs. On the other hand, clouds in pressure confinement would have only thermal line widths, requiring many clouds moving in a large-scale velocity field to achieve the same effect. There are almost no previous studies of the effects of microturbulence, even though the observation that AGN lines are very smooth seems to require additional line broadening mechanisms. We present a broad range of photoionization calculations in which the microturbulence is varied between 0 km s-1 (thermal broadening only) and 104 km s-1, an upper limit set by the observed line width. In general, the line spectrum grows stronger relative to the continuum as turbulence increases. This is because lines more easily escape due to diminished line optical depth and permitted lines are selectively strengthened by continuum pumping. Comparisons with observations reveal two cases. The predicted relative intensities of the majority of the strong lines in typical objects do not depend strongly on the microturbulent field. A turbulence of ~103 km s-1 does not violate observations, but is not required either. However, in the sharp-lined quasars, some lines require a turbulence of the same order as the observed line width to reproduce the spectrum.
    The Astrophysical Journal 12/2008; 542(2):644. DOI:10.1086/317051 · 6.28 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We present high-resolution spectrophotometric observations of the Orion Nebula, made with the Cassegrain echelle spectrograph on the Blanco 4 m telescope at Cerro Tololo Inter-American Observatory (CTIO). The resolution and signal-to-noise ratio make it possible to identify 444 emission lines in the 3498-7468 Å range, down to 104 times fainter than Hβ. We present a detailed atlas of these emission lines along with an analysis of the associated errors. This data set is used to study the velocity field in the Orion Nebula. The forbidden lines split into two distinct groups. The low-ionization group has ions with an ionization potential less than 20 eV. Lines of these ions, [O I], [N I], [Ni II], and [Fe II], have recession velocities, relative to the hydrogen lines, of +10 to +15 km s-1. There is a sharp change to the second, high-ionization group, which includes lines of ions with ionization potentials larger than 20 eV, namely, [S II], [O II], [N II], and [Fe III]. These lines have velocities around +3 km s-1, with a slight trend of decreasing velocity with the increasing ionization potential. This is consistent with previously proposed dynamical models in which lines of ions with different ionization potentials originate at different distances from the ionizing stars. Significant acceleration appears to take place across the narrow region where Fe2+ exists. Across this region the gas receives an acceleration of ~ 2.5 × 10-5 cm s-2. This provides a constraint on hydrodynamical models. We set a limit He II 4686/Hβ < 7 × 10-5, which in turn sets a limit to the intensity of the ionizing continuum at energies higher than 54 eV. Modern stellar atmospheres predict a continuum that is far stronger than is present in the region near θ1 Ori C.
    The Astrophysical Journal Supplement Series 12/2008; 129(1):229. DOI:10.1086/313416 · 14.14 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We present follow-up optical g', r', and i' imaging and spectroscopy of serendipitous X-ray sources detected in six archival Chandra images included in the Chandra Multiwavelength Project (ChaMP). Of the 486 X-ray sources detected between 3 × 10-16 and 2 × 10-13 (with a median flux of 3 × 10-15) ergs cm-2 s-1, we find optical counterparts for 377 (78%), or 335 (68%) counting only unique counterparts. We present spectroscopic classifications for 125 objects, representing 75% of sources with r* < 21 optical counterparts (63% to r* = 22). Of all classified objects, 63 (50%) are broad-line active galactic nuclei (AGNs), which tend to be blue in (g*-r*) colors. X-ray information efficiently segregates these quasars from stars, which otherwise strongly overlap in these SDSS colors until z > 3.5. We identify 28 sources (22%) as galaxies that show narrow emission lines, while 22 (18%) are absorption line galaxies. Eight galaxies lacking broad-line emission have X-ray luminosities that require they host an AGN (logLX > 43). Half of these have hard X-ray emission suggesting that high gas columns obscure both the X-ray continuum and the broad emission line regions. We find objects in our sample that show signs of X-ray or optical absorption, or both, but with no strong evidence that these properties are coupled. ChaMP's deep X-ray and optical imaging enable multiband selection of small and/or high-redshift groups and clusters. In these six fields we have discovered three new clusters of galaxies, two with z > 0.4, and one with photometric evidence for a similar redshift.
    The Astrophysical Journal Supplement Series 12/2008; 150(1):43. DOI:10.1086/379818 · 14.14 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We present a search for spatial and redshift correlations in a 2 Å resolution spectroscopic survey of the Lyα forest at 2.15 < z < 3.37 toward 10 QSOs concentrated within a 1° diameter field. We find a signal at 2.7 σ significance for correlations of the Lyα absorption-line wavelengths between different lines of sight over the whole redshift range. The significance rises to 3.2 σ if we restrict the redshift range to 2.60 < z < 3.37, and to 4.0 σ if we further restrict the sample to lines with rest equivalent widths 0.1 ≤ W0 < 0.9 Å. We conclude that a significant fraction of the Lyα forest arises in structures whose correlation length extends at least over 30' (~26 h-1 comoving Mpc at z = 2.6 for H0 ≡ 100 h km s-1 Mpc-1, Ω = 1.0, Λ = 0). We have also calculated the three-dimensional two-point correlation function for Lyα absorbers; we do not detect any significant signal in the data. However, we note that line blending prevents us from detecting the signal produced by a 100% overdensity of Lyα absorbers in simulated data. We find that the Lyα forest redshift distribution provides a more sensitive test for such clustering than the three-dimensional two-point correlation function.
    The Astrophysical Journal 12/2008; 532(1):77. DOI:10.1086/308526 · 6.28 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The narrow-line region (NLR) of the Seyfert 2 galaxy NGC 3393 is dominated by a symmetric structure which appears as S-shaped arms in Hubble Space Telescope (HST) images. These arms, which occupy the central few arcseconds of the nucleus, border a linear, triple-lobed radio source. We use HST imaging and spectra, ground-based optical images, long-slit spectra, Fabry-Perot imaging spectroscopy, and VLA radio data to perform a detailed investigation of the kinematics and ionization of the line-emitting gas in NGC 3393 and of its relationship with the relativistic gas responsible for the radio emission. The excitation map [O III] λ5007/(Hα + [N II] λλ6548, 6584) shows a biconical structure, consistent with the anisotropic nuclear ionizing radiation expected in the unified scheme. Extrapolation to ionizing frequencies of our upper limit to the 2100 Å flux of the nuclear source provides a factor ≥3 × 104 too few ionizing photons to account for the recombination line emission, which also suggests that the nuclear ionizing source radiates anisotropically. However, the kinetic energy of the outflow is sufficient to power the line emission via photoionizing shocks, and a tentative detection of extended UV emission is consistent with this model. Furthermore, the broad component of the emission lines has a similar orientation and spatial extent as the triple radio source. Nevertheless, other tests are inconsistent with the photoionizing shock model—there is no correlation between local velocity dispersion, surface brightness, and excitation, and the gaseous abundances of [Ca II], Al II], and Mg II are much lower than expected if these species have been liberated into the gas phase through grain destruction by shocks. We conclude that the radio lobes appear to have created denser regions of gas on their leading edges, thus forming the S-shaped arms, but that the ionization is most likely due to photoionization by an obscured central source.
    The Astrophysical Journal Supplement Series 12/2008; 129(2):517. DOI:10.1086/313422 · 14.14 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We used a very large set of models of broad emission line region (BELR) clouds in active galactic nuclei to investigate the formation of the observed Fe II emission lines. We show that photoionized BELR clouds cannot produce both the observed shape and observed equivalent width of the 2200-2800 Å Fe II UV bump unless there is considerable velocity structure corresponding to a microturbulent velocity parameter vturb ≥ 100 km s-1 for the locally optimally emitting cloud models used here. This could be either microturbulence in gas that is confined by some phenomenon such as MHD waves or a velocity shear such as in the various models of winds flowing off the surfaces of accretion disks. The alternative way that we can find to simultaneously match both the observed shape and equivalent width of the Fe II UV bump is for the Fe II emission to be the result of collisional excitation in a warm, dense gas. Such gas would emit very few lines other than Fe II. However, since the collisionally excited gas would constitute yet another component in an already complicated picture of the BELR, we prefer the model involving turbulence. In either model, the strength of Fe II emission relative to the emission lines of other ions such as Mg II depends as much on other parameters (either vturb or the surface area of the collisionally excited gas) as it does on the iron abundance. Therefore, the measurement of the iron abundance from the Fe II emission in quasars becomes a more difficult problem.
    The Astrophysical Journal 12/2008; 615(2):610. DOI:10.1086/424683 · 6.28 Impact Factor

Publication Stats

2k Citations
467.49 Total Impact Points

Institutions

  • 2001–2013
    • 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
  • 1982–1996
    • University of Cambridge
      • Institute of Astronomy
      Cambridge, England, United Kingdom
  • 1995
    • NASA
      Вашингтон, West Virginia, United States
  • 1993–1994
    • University of Texas at Austin
      Austin, Texas, United States
    • The University of Manchester
      Manchester, England, United Kingdom
  • 1980
    • University of California, Santa Cruz
      Santa Cruz, California, United States