Physics Commons^™

Nitrogen-To-Oxygen Abundance Ratio Variation In Quiescent Galaxies, Renbin Yan

Physics and Astronomy Faculty Publications

For the first time, we establish a gas-phase abundance pattern calibration for quiescent galaxies using optical emission lines. Quiescent galaxies have warm ionized gas showing line ratios similar to low-ionization nuclear emission line regions. The ionization mechanism for the gas is still an unsettled puzzle. Despite the uncertainty in the ionization mechanism, we argue that we can still infer certain gas-phase abundance pattern from first principles. We show that the relative trend in N/O abundance can still be reliably measured based on [N II] λλ6548,6583/[O II] λλ3726,3729 and a direct measurement of the electron temperature. We construct a composite direct …

Molpop-Cep: An Exact, Fast Code For Multi-Level Systems, Andrés Asensio Ramos, Moshe Elitzur

Physics and Astronomy Faculty Publications

We present MOLPOP-CEP, a universal line transfer code that allows the exact calculation of multi-level line emission from a slab with variable physical conditions for any arbitrary atom or molecule for which atomic data exist. The code includes error control to achieve any desired level of accuracy, providing full confidence in its results. Publicly available, MOLPOP-CEP employs our recently developed coupled escape probability (CEP) technique, whose performance exceeds other exact methods by orders of magnitude. The program also offers the option of an approximate solution with different variants of the familiar escape probability method. As an illustration of the MOLPOP-CEP …

Sdss Iv Manga – Ssfr Profiles And The Slow Quenching Of Discs In Green Valley Galaxies, Francesco Belfiore, Roberto Maiolino, Kevin Bundy, Karen Masters, Matthew Bershady, Grecco A. Oyarzún, Lihwai Lin, Mariana Cano-Diaz, David Wake, Ashley Spindler, Daniel Thomas, Joel R. Brownstein, Niv Drory, Renbin Yan

Physics and Astronomy Faculty Publications

We study radial profiles in H α equivalent width and specific star formation rate (sSFR) derived from spatially resolved SDSS-IV MaNGA spectroscopy to gain insight on the physical mechanisms that suppress star formation and determine a galaxy’s location in the SFR-M_⋆ diagram. Even within the star-forming ‘main sequence’, the measured sSFR decreases with stellar mass, in both an integrated and spatially resolved sense. Flat sSFR radial profiles are observed for log(M_⋆/M_⊙) < 10.5, while star-forming galaxies of higher mass show a significant decrease in sSFR in the central regions, a likely consequence of both larger bulges and an inside-out growth history. Our primary focus is the green valley, constituted by galaxies lying below the star formation main sequence, but not fully passive. In the green valley we find sSFR profiles that are suppressed with respect to star-forming galaxies of the same mass at all galactocentric distances out to 2 effective radii. The responsible quenching mechanism therefore appears to affect the entire galaxy, not simply an …

The [Cii] 158ΜM Line Emission In High-Redshift Galaxies, Guilaine Lagache, Morgane Cousin, Marios Chatzikos

Physics and Astronomy Faculty Publications

Gas is a crucial component of galaxies, providing the fuel to form stars, and it is impossible to understand the evolution of galaxies without knowing their gas properties. The [CII] fine structure transition at 158 μm is the dominant cooling line of cool interstellar gas, and is the brightest of emission lines from star forming galaxies from FIR through metre wavelengths, almost unaffected by attenuation. With the advent of ALMA and NOEMA, capable of detecting [CII]-line emission in high-redshift galaxies, there has been a growing interest in using the [CII] line as a probe of the physical conditions of …

The Validity Of 21 Cm Spin Temperature As A Kinetic Temperature Indicator In Atomic And Molecular Gas, Gargi Shaw, Gary J. Ferland, I. Hubeny

Physics and Astronomy Faculty Publications

The gas kinetic temperature (T_K) of various interstellar environments is often inferred from observations that can deduce level populations of atoms, ions, or molecules using spectral line observations; H I 21 cm is perhaps the most widely used, and has a long history. Usually the H I 21 cm line is assumed to be in thermal equilibrium and the populations are given by the Boltzmann distribution. A variety of processes, many involving Lyα, can affect the 21 cm line. Here we show how this is treated in the spectral simulation code Cloudy, and present numerical …

Atomic Data Revisions For Transitions Relevant To Observations Of Interstellar, Circumgalactic, And Intergalactic Matter, Frances H. Cashman, Varsha P. Kulkarni, Romas Kisielius, Gary J. Ferland, Pavel Bogdanovich

Physics and Astronomy Faculty Publications

Measurements of element abundances in galaxies from astrophysical spectroscopy depend sensitively on the atomic data used. With the goal of making the latest atomic data accessible to the community, we present a compilation of selected atomic data for resonant absorption lines at wavelengths longward of 911.753 Å (the H I Lyman limit), for key heavy elements (heavier than atomic number 5) of astrophysical interest. In particular, we focus on the transitions of those ions that have been observed in the Milky Way interstellar medium (ISM), the circumgalactic medium (CGM) of the Milky Way and/or other galaxies, and the intergalactic medium …

Which Stars Are Ionizing The Orion Nebula?, C. R. O'Dell, W. Kollatschny, Gary J. Ferland

Physics and Astronomy Faculty Publications

The common assumption that θ¹ Ori C is the dominant ionizing source for the Orion Nebula is critically examined. This assumption underlies much of the existing analysis of the nebula. In this paper we establish through comparison of the relative strengths of emission lines with expectations from Cloudy models and through the direction of the bright edges of proplyds that θ² Ori A, which lies beyond the Bright Bar, also plays an important role. θ¹ Ori C does dominate ionization in the inner part of the Orion Nebula, but outside of the Bright Bar as far as …

Sdss-Iv Manga – The Spatially Resolved Transition From Star Formation To Quiescence, Francesco Belfiore, Roberto Maiolino, Claudia Maraston, Eric Emsellem, Matthew A. Bershady, Karen L. Masters, Dmitry Bizyaev, Médéric Boquien, Joel R. Brownstein, Kevin Bundy, Aleksandar M. Diamond-Stanic, Niv Drory, Timothy M. Heckman, David R. Law, Olena Malanushenko, Audrey Oravetz, Kaike Pan, Alexandre Roman-Lopes, Daniel Thomas, Anne-Marie Weijmans, Kyle B. Westfall, Renbin Yan

Physics and Astronomy Faculty Publications

Using spatially resolved spectroscopy from SDSS-IV MaNGA we have demonstrated that low ionization emission-line regions (LIERs) in local galaxies result from photoionization by hot evolved stars, not active galactic nuclei, hence tracing galactic region hosting old stellar population where, despite the presence of ionized gas, star formation is no longer occurring. LIERs are ubiquitous in both quiescent galaxies and in the central regions of galaxies where star formation takes place at larger radii. We refer to these two classes of galaxies as extended LIER (eLIER) and central LIER (cLIER) galaxies, respectively. cLIERs are late-type galaxies primarily spread across the green …

Orion's Veil: Magnetic Field Strengths And Other Properties Of A Pdr In Front Of The Trapezium Cluster, Thomas H. Troland, W. M. Goss, C. L. Brogan, R. M. Crutcher, D. A. Roberts

Physics and Astronomy Faculty Publications

We present an analysis of physical conditions in the Orion Veil, an atomic photon-dominated region (PDR) that lies just in front (≈2 pc) of the Trapezium stars of Orion. This region offers an unusual opportunity to study the properties of PDRs, including the magnetic field. We have obtained 21 cm H i and 18 cm (1665 and 1667 MHz) OH Zeeman effect data that yield images of the line-of-sight magnetic field strength B _los in atomic and molecular regions of the Veil. We find B _los ≈ −50 to −75 μG in the atomic gas across much of the …

Sdss Iv Manga – Spatially Resolved Diagnostic Diagrams: A Proof That Many Galaxies Are Liers, Francesco Belfiore, Roberto Maiolino, Claudia Maraston, Eric Emsellem, Matthew A. Bershady, Karen L. Masters, Renbin Yan, Dmitry Bizyaev, Médéric Boquien, Joel R. Brownstein, Kevin Bundy, Niv Drory, Timothy M. Heckman, David R. Law, Alexandre Roman-Lopes, Kaike Pan, Letizia Stanghellini, Daniel Thomas, Anne-Marie Weijmans, Kyle B. Westfall

Physics and Astronomy Faculty Publications

We study the spatially resolved excitation properties of the ionized gas in a sample of 646 galaxies using integral field spectroscopy data from the Sloan Digital Sky Survey IV Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) programme. Making use of Baldwin–Philips–Terlevich diagnostic diagrams we demonstrate the ubiquitous presence of extended (kpc scale) low-ionization emission-line regions (LIERs) in both star-forming and quiescent galaxies. In star-forming galaxies LIER emission can be associated with diffuse ionized gas, most evident as extraplanar emission in edge-on systems. In addition, we identify two main classes of galaxies displaying LIER emission: ‘central LIER’ (cLIER) galaxies, where …

Stellar Mass–Gas-Phase Metallicity Relation At 0.5 ≤ Z ≤ 0.7: A Power Law With Increasing Scatter Toward The Low-Mass Regime, Yicheng Guo, David C. Koo, Yu Lu, John C. Forbes, Marc Rafelski, Jonathan R. Trump, Ricardo Amorín, Guillermo Barro, Romeel Davé, S. M. Faber, Nimish P. Hathi, Hassen Yesuf, Michael C. Cooper, Avishai Dekel, Puragra Guhathakurta, Evan N. Kirby, Anton M. Koekemoer, Pablo G. Pérez-González, Lihwai Lin, Jeffery A. Newman, Joel R. Primack, David J. Rosario, Christopher N. A. Willmer, Renbin Yan

Physics and Astronomy Faculty Publications

We present the stellar mass (M_*)–gas-phase metallicity relation (MZR) and its scatter at intermediate redshifts (0.5 ≤ z ≤ 0.7) for for 1381 field galaxies collected from deep spectroscopic surveys. The star formation rate (SFR) and color at a given M_* of this magnitude-limited (R ≲ 24 AB) sample are representative of normal star-forming galaxies. For masses below 10⁹ M_☉ our sample of 237 galaxies is ~10 times larger than those in previous studies beyond the local universe. This huge gain in sample size enables superior constraints on the MZR and its scatter …

H, He-Like Recombination Spectra – I. L-Changing Collisions For Hydrogen, Francisco Guzmán, N. R. Badnell, R. J. R. Williams, P. A. M. Van Hoof, Marios Chatzikos, Gary J. Ferland

Physics and Astronomy Faculty Publications

Hydrogen and helium emission lines in nebulae form by radiative recombination. This is a simple process which, in principle, can be described to very high precision. Ratios of He I and H I emission lines can be used to measure the He⁺/H⁺ abundance ratio to the same precision as the recombination rate coefficients. This paper investigates the controversy over the correct theory to describe dipole l-changing collisions (nl → nl′ = l ± 1) between energy-degenerate states within an n-shell. The work of Pengelly & Seaton has, for half-a-century, been considered the definitive …

Interpreting The Ionization Sequence In Star-Forming Galaxy Emission-Line Spectra, Chris T. Richardson, James T. Allen, Jack A. Baldwin, Paul C. Hewett, Gary J. Ferland, Anthony Crider, Helen Meskhidze

Physics and Astronomy Faculty Publications

High-ionization star-forming (SF) galaxies are easily identified with strong emission-line techniques such as the BPT diagram, and form an obvious ionization sequence on such diagrams. We use a locally optimally emitting cloud model to fit emission-line ratios that constrain the excitation mechanism, spectral energy distribution, abundances and physical conditions along the star formation ionization sequence. Our analysis takes advantage of the identification of a sample of pure SF galaxies, to define the ionization sequence, via mean field independent component analysis. Previous work has suggested that the major parameter controlling the ionization level in SF galaxies is the metallicity. Here we …

Orion's Veil. Iv. H2 Excitation And Geometry, N. P. Abel, Gary J. Ferland, C. R. O'Dell, Thomas H. Troland

Physics and Astronomy Faculty Publications

The foreground Veil of material that lies in front of the Orion Nebula is the best studied sample of the interstellar medium because we know where it is located, how it is illuminated, and the balance of thermal and magnetic energy. In this work, we present high-resolution STIS observations toward the Trapezium, with the goal of better understanding the chemistry and geometry of the two primary Veil layers, along with ionized gas along the line of sight. The most complete characterization of the rotational/vibrational column densities of H₂ in the almost purely atomic components of the Veil are presented, …

The Jcmt Nearby Galaxies Legacy Survey – X. Environmental Effects On The Molecular Gas And Star Formation Properties Of Spiral Galaxies, Angus Mok, C. D. Wilson, J. Golding, B. E. Warren, F. P. Israel, S. Serjeant, J. H. Knapen, José R. Sánchez-Gallego, P. Barmby, G. J. Bendo, E. Rosolowsky, P. Van Der Werf

Physics and Astronomy Faculty Publications

We present a study of the molecular gas properties in a sample of 98 H I – flux selected spiral galaxies within ∼25 Mpc, using the CO J = 3 − 2 line observed with the James Clerk Maxwell Telescope. We use the technique of survival analysis to incorporate galaxies with CO upper limits into our results. Comparing the group and Virgo samples, we find a larger mean H₂ mass in the Virgo galaxies, despite their lower mean H I mass. This leads to a significantly higher H₂ to H I ratio for Virgo galaxies. Combining our data …

The Inferred Evolution Of The Cold Gas Properties Of Candels Galaxies At 0.5 < Z < 3.0, G. Popping, K. Caputi, S. C. Trager, Rachel S. Somerville, Avishai Dekel, Susan A. Kassin, Dale D. Kocevski, Anton M. Koekemoer, Sandra M. Faber, Henry C. Ferguson, A. Galametz, Norman A. Grogin, Yicheng Guo, Y. Lu, Arjen Van Der Wel, Benjamin J. Weiner

Physics and Astronomy Faculty Publications

We derive the total cold gas, atomic hydrogen, and molecular gas masses of approximately 24 000 galaxies covering four decades in stellar mass at redshifts 0.5 < z < 3.0, taken from the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey survey. Our inferences are based on the inversion of a molecular hydrogen based star formation law, coupled with a prescription to separate atomic and molecular gas. We find that: (1) there is an increasing trend between the inferred cold gas (H i and H₂), H i, and H₂ mass and the stellar mass of galaxies down to stellar masses of 10⁸ M_⊙ already in place at z = 3; (2) the molecular fractions of cold gas increase with increasing stellar mass and look-back time; (3) there is hardly any evolution in the mean H i content of galaxies at fixed stellar mass; (4) the cold gas fraction and relative amount of molecular …

Collisional Excitation Of [C Ii], [O I] And Co In Massive Galaxies, R. E. A. Canning, Gary J. Ferland, A. C. Fabian, R. M. Johnstone, P. A. M. Van Hoof, R. L. Porter, N. Werner, R. J. R. Williams

Physics and Astronomy Faculty Publications

Many massive galaxies at the centres of relaxed galaxy clusters and groups have vast reservoirs of warm (∼10 000 K) and cold (≲100 K) gas. In many such low-redshift systems this gas is lifted into the hot interstellar medium in filamentary structures, which are long lived and are typically not forming stars. Two important questions are how far do these reservoirs cool and if cold gas is abundant what is the cause of the low star formation efficiency? Heating and excitation of the filaments from collisions and mixing of hot particles in the surrounding X-ray gas describes well the optical …

The Nature And Frequency Of Outflows From Stars In The Central Orion Nebula Cluster, C. R. O'Dell, Gary J. Ferland, W. J. Henney, M. Peimbert, Ma. T. García-Díaz, Robert H. Rubin

Physics and Astronomy Faculty Publications

Recent Hubble Space Telescope images have allowed the determination with unprecedented accuracy of motions and changes of shocks within the inner Orion Nebula. These originate from collimated outflows from very young stars, some within the ionized portion of the nebula and others within the host molecular cloud. We have doubled the number of Herbig–Haro objects known within the inner Orion Nebula. We find that the best-known Herbig–Haro shocks originate from relatively few stars, with the optically visible X-ray source COUP 666 driving many of them. While some isolated shocks are driven by single collimated outflows, many groups of shocks are …

Herschel Dust Emission As A Probe Of Starless Cores Mass: Mcld 123.5+24.9 Of The Polaris Flare, Gururaj Anil Wagle, Thomas H. Troland, Gary J. Ferland, Nicholas P. Abel

Physics and Astronomy Faculty Publications

We present newly processed archival Herschel images of molecular cloud MCLD 123.5+24.9 in the Polaris Flare. This cloud contains five starless cores. Using the spectral synthesis code Cloudy, we explore uncertainties in the derivation of column densities, and hence masses of molecular cores from Herschel data. We first consider several detailed grain models that predict far-infrared grain opacities. Opacities predicted by the models differ by more than a factor of two, leading to uncertainties in derived column densities by the same factor. Then we consider uncertainties associated with the modified blackbody fitting process used by observers to estimate column densities. …

Metal-Poor, Strongly Star-Forming Galaxies In The Deep2 Survey: The Relationship Between Stellar Mass, Temperature-Based Metallicity, And Star Formation Rate, Chun Ly, Jane R. Rigby, Michael Cooper, Renbin Yan

Physics and Astronomy Faculty Publications

We report on the discovery of 28 z ≈ 0.8 metal-poor galaxies in DEEP2.

For the remainder of the abstract, please download this article or visit https://doi.org/10.1088/0004-637X/805/1/45.

Accurate Determination Of The Free–Free Gaunt Factor – Ii. Relativistic Gaunt Factors, P. A. M. Van Hoof, Gary J. Ferland, R. J. R. Williams, K. Volk, Marios Chatzikos, M. Lykins, R. L. Porter

Physics and Astronomy Faculty Publications

When modelling an ionized plasma, all spectral synthesis codes need the thermally averaged free–free Gaunt factor defined over a very wide range of parameter space in order to produce an accurate prediction for the spectrum. Until now no data set exists that would meet these needs completely. We have therefore produced a table of relativistic Gaunt factors over a much wider range of parameter space than has ever been produced before. We present tables of the thermally averaged Gaunt factor covering the range ¹⁰log γ² = −6 to 10 and ¹⁰log u = −16 to 13 for …

Bootstrapping Dielectronic Recombination From Second-Row Elements And The Orion Nebula, N. R. Badnell, Gary J. Ferland, T. W. Gorczyca, D. Nikolić, Gururaj Anil Wagle

Physics and Astronomy Faculty Publications

Dielectronic recombination (DR) is the dominant recombination process for most heavy elements in photoionized clouds. Accurate DR rates for a species can be predicted when the positions of autoionizing states are known. Unfortunately such data are not available for most third- and higher-row elements. This introduces an uncertainty that is especially acute for photoionized clouds, where the low temperatures mean that DR occurs energetically through very low-lying autoionizing states. This paper discusses S²⁺ → S⁺ DR, the process that is largely responsible for establishing the [S iii]/[S ii] ratio in nebulae. We derive an empirical rate coefficient using …

Atomic Data For Zn Ɪɪ: Improving Spectral Diagnostics Of Chemical Evolution In High-Redshift Galaxies, Romas Kisielius, Varsha P. Kulkarni, Gary J. Ferland, Pavel Bogdanovich, Debopam Som, Matt L. Lykins

Physics and Astronomy Faculty Publications

Damped Lyα (DLA) and sub-DLA absorbers in quasar spectra provide the most sensitive tools for measuring the element abundances of distant galaxies. The estimation of abundances from absorption lines depends sensitively on the accuracy of the atomic data used. We have started a project to produce new atomic spectroscopic parameters for optical and UV spectral lines using state-of-the-art computer codes employing a very broad configuration interaction (CI) basis. Here we report our results for Zn ii, an ion used widely in studies of the interstellar medium (ISM) as well as DLAs and sub-DLAs. We report new calculations of many …

P-Manga Galaxies: Emission-Lines Properties – Gas Ionization And Chemical Abundances From Prototype Observations, F. Belfiore, R. Maiolino, K. Bundy, D. Thomas, C. Maraston, D. Wilkinson, S. F. Sánchez, M. Bershady, G. A. Blanc, M. Bothwell, S. L. Cales, L. Coccato, N. Drory, E. Emsellem, H. Fu, J. Gelfand, D. Law, K. Masters, J. Parejko, C. Tremonti, D. Wake, A. Weijmans, Renbin Yan, T. Xiao, K. Zhang, T. Zheng, K. Bizyaev, K. Kinemuchi, D. Oravetz, A. Simmons

Physics and Astronomy Faculty Publications

MaNGA (Mapping Nearby Galaxies at Apache Point Observatory) is a 6-yr Sloan Digital Sky Survey (SDSS-IV) survey that will obtain spatially resolved spectroscopy from 3600 to 10 300 Å for a representative sample of over 10 000 nearby galaxies. In this paper, we present the analysis of nebular emission-line properties using observations of 14 galaxies obtained with P-MaNGA, a prototype of the MaNGA instrument. By using spatially resolved diagnostic diagrams, we find extended star formation in galaxies that are centrally dominated by Seyfert/LINER-like emission, which illustrates that galaxy characterizations based on single fibre spectra are necessarily incomplete. We observe extended …

Accurate Determination Of The Free–Free Gaunt Factor – I. Non-Relativistic Gaunt Factors, P. A. M. Van Hoof, R. J. R. Williams, K. Vollk, Marios Chatzikos, Gary J. Ferland, M. Lykins, R. L. Porter, Ye Wang

Physics and Astronomy Faculty Publications

Modern spectral synthesis codes need the thermally averaged free–free Gaunt factor defined over a very wide range of parameter space in order to produce an accurate prediction for the spectrum emitted by an ionized plasma. Until now no set of data exists that would meet this need in a fully satisfactory way. We have therefore undertaken to produce a table of very accurate non-relativistic Gaunt factors over a much wider range of parameters than has ever been produced before. We first produced a table of non-averaged Gaunt factors, covering the parameter space ¹⁰log ϵ_i = −20 to +10 …

On The Observability Of Optically Thin Coronal Hyperfine Structure Lines, Marios Chatzikos, Gary J. Ferland, R. J. R. Williams, A. C. Fabian

Physics and Astronomy Faculty Publications

We present CLOUDY calculations for the intensity of coronal hyperfine lines in various environments. We model indirect collisional and radiative transitions, and quantify the collisionally excited line emissivity in the density-temperature phase space. As an observational aid, we also express the emissivity in units of that in the 0.4-0.7 keV band. For most hyperfine lines, knowledge of the X-ray surface brightness and the plasma temperature is sufficient for rough estimates. We find that the radiation fields of both Perseus A and Virgo A can enhance the populations of highly ionized species within 1 kpc. They can also enhance line emissivity …

Ion-By-Ion Cooling Efficiencies, Orly Gnat, Gary J. Ferland

Physics and Astronomy Faculty Publications

We present ion-by-ion cooling efficiencies for low-density gas. We use Cloudy (version 10.00) to estimate the cooling efficiencies for each ion of the first 30 elements (H-Zn) individually. We present results for gas temperatures between 10⁴ and 10⁸ K, assuming low densities and optically thin conditions. When nonequilibrium ionization plays a significant role the ionization states deviate from those that obtain in collisional ionization equilibrium (CIE), and the local cooling efficiency at any given temperature depends on specific nonequilibrium ion fractions. The results presented here allow for an efficient estimate of the total cooling efficiency for any ionic …

Structure And Feedback In 30 Doradus. Ii. Structure And Chemical Abundances, E. W. Pellegrini, J. A. Baldwin, Gary J. Ferland

Physics and Astronomy Faculty Publications

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 …

A Survey Of Molecular Hydrogen In The Crab Nebula, E. D. Loh, J. A. Baldwin, Z. K. Curtis, Gary J. Ferland, C. R. O'Dell, A. C. Fabian, Philippe Salomé

Physics and Astronomy Faculty Publications

We have carried out a near-infrared, narrowband imaging survey of the Crab Nebula, in the H₂ 2.12 μm and Brγ 2.17 μm lines, using the Spartan Infrared camera on the SOAR Telescope. Over a 2.8 × 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 Hubble Space Telescope [S …

Herschel Images Of Ngc 6720: H2 Formation On Dust Grain, P. A. M. Van Hoof, G. C. Van De Steene, M. J. Barlow, K. M. Exter, B. Sibthorpe, T. Ueta, V. Peris, M. A. T. Groenewegen, J. A. D. L. Blommaert, M. Cohen, W. De Meester, Gary J. Ferland, W. K. Gear, H. L. Gomez, P. C. Hargrave, E. Huygen, R. J. Ivison, C. Jean, S. J. Leeks, T. L. Lim, G. Olofsson, E. T. Polehampton, S. Regibo, P. Royer, B. M. Swinyard, B. Vandenbussche, H. Van Winckel, C. Waelkens, H. J. Walker, R. Wesson

Physics and Astronomy Faculty Publications

Herschel PACS and SPIRE images have been obtained of NGC 6720 (the Ring nebula). This is an evolved planetary nebula with a central star that is currently on the cooling track, due to which the outer parts of the nebula are recombining. From the PACS and SPIRE images we conclude that there is a striking resemblance between the dust distribution and the H₂ emission, which appears to be observational evidence that H₂ forms on grain surfaces. We have developed a photoionization model of the nebula with the Cloudy code which we used to determine the physical conditions of …