158 micron forbidden C II mapping of NGC 6946 - Probing the atomic medium

Madden, S. C.; Geis, N.; Genzel, R.; Herrmann, F.; Jackson, James M.; Poglitsch, A.; Stacey, G. J.; Townes, C. H.

doi:10.1086/172539

Cited by 113 publications

(149 citation statements)

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“…(3) is L COS ≈ 8 × 10 −28 erg s −1 (H −1 ). This value is significantly lower than in the diffuse medium of our galaxy (≈10 −26 −10 −25 erg s −1 (H −1 ); Lehner et al 2004) or in other nearby galaxies (e.g., Madden et al 1993), but it is close to the average cooling rate in DLAs (≈10 −27 erg s −1 (H −1 ); Wolfe et al 2003a). Assuming that the photoelectric effect on dust is the dominant gas heating mechanism (see Sect.…”

Section: Cooling Rate and Sfrcontrasting

confidence: 57%

Chemical enrichment and physical conditions in I Zw 18

Lebouteiller

Heap

Hubený

et al. 2013

A&A

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Context. Low-metallicity star-forming dwarf galaxies are prime targets to understand the chemical enrichment of the interstellar medium. The H  region contains the bulk of the mass in blue compact dwarfs, and it provides important constraints on the dispersal and mixing of heavy elements released by successive star-formation episodes. The metallicity of the H  region is also a critical parameter to investigate the future star-formation history, as metals provide most of the gas cooling that will facilitate and sustain star formation. Aims. Our primary objective is to study the enrichment of the H  region and the interplay between star-formation history and metallicity evolution. Our secondary objective is to constrain the spatial-and time-scales over which the H  and H  regions are enriched, and the mass range of stars responsible for the heavy element production. Finally, we aim to examine the gas heating and cooling mechanisms in the H  region. Methods. We observed the most metal-poor star-forming galaxy in the Local Universe, I Zw 18, with the Cosmic Origin Spectrograph onboard Hubble. The abundances in the neutral gas are derived from far-ultraviolet absorption-lines (H , C , C *, N , O , ...) and are compared to the abundances in the H  region. Models are constructed to calculate the ionization structure and the thermal processes. We investigate the gas cooling in the H  region through physical diagnostics drawn from the fine-structure level of C + . Results. We find that H  region abundances are lower by a factor of ∼2 as compared to the H  region. There is no differential depletion on dust between the H  and H  region. Using sulfur as a metallicity tracer, we calculate a metallicity of 1/46 Z (vs. 1/31 Z in the H  region). From the study of the C/O, [O/Fe], and N/O abundance ratios, we propose that C, N, O, and Fe are mainly produced in massive stars. We argue that the H  envelope may contain pockets of pristine gas with a metallicity essentially null. Finally, we derive the physical conditions in the H  region by investigating the C * absorption line. The cooling rate derived from C * is consistent with collisions with H 0 atoms in the diffuse neutral gas. We calculate the star-formation rate from the C * cooling rate assuming that photoelectric effect on dust is the dominant gas heating mechanism. Our determination is in good agreement with the values in the literature if we assume a low dust-to-gas ratio (∼2000 times lower than the Milky Way value).

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Section: Cooling Rate and Sfrcontrasting

confidence: 57%

Chemical enrichment and physical conditions in I Zw 18

Lebouteiller

Heap

Hubený

et al. 2013

A&A

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“…Much of the HI 21 cm emission from the inner disks of galaxies may therefore be the result of the photodissociation of molecular clouds by the UV starlight of nearby O and B stars, a conclusion in accord with those of Allen et al (1986Allen et al ( , 1997 and Tilanus and Allen (1989) based on high-spatial-resolution maps of HI in M83, M81, and M51. Madden et al (1993) have mapped NGC 6946 in [CII] 158 m (Fig. 42) and concluded that as much as 70-80 % of the observed HI, especially in the inner regions, may be photodissociated H 2 .…”

Section: B Molecular Cloudsmentioning

confidence: 99%

Photodissociation regions in the interstellar medium of galaxies

Hollenbach

Tielens²

1999

Rev. Mod. Phys.

737

712

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The interstellar medium of galaxies is the reservoir out of which stars are born and into which stars inject newly created elements as they age. The physical properties of the interstellar medium are governed in part by the radiation emitted by these stars. Far-ultraviolet (6 eVϽh Ͻ13.6 eV) photons from massive stars dominate the heating and influence the chemistry of the neutral atomic gas and much of the molecular gas in galaxies. Predominantly neutral regions of the interstellar medium in which the heating and chemistry are regulated by far ultraviolet photons are termed PhotoDissociation Regions (PDRs). These regions are the origin of most of the non-stellar infrared (IR) and the millimeter and submillimeter CO emission from galaxies. The importance of PDRs has become increasingly apparent with advances in IR and submillimeter astronomy. The IR emission from PDRs includes fine structure lines of C, C ϩ , and O; rovibrational lines of H 2 ; rotational lines of CO; broad mid-IR features of polycyclic aromatic hydrocarbons; and a luminous underlying IR continuum from interstellar dust. The transition of H to H 2 and C ϩ to CO occurs within PDRs. Comparison of observations with theoretical models of PDRs enables one to determine the density and temperature structure, the elemental abundances, the level of ionization, and the radiation field. PDR models have been applied to interstellar clouds near massive stars, planetary nebulae, red giant outflows, photoevaporating planetary disks around newly formed stars, diffuse clouds, the neutral intercloud medium, and molecular clouds in the interstellar radiation field-in summary, much of the interstellar medium in galaxies. Theoretical PDR models explain the observed correlations of the [CII] 158 m with the CO Jϭ1 -0 emission, the CO Jϭ1 -0 luminosity with the interstellar molecular mass, and the [CII] 158 m plus [OI] 63 m luminosity with the IR continuum luminosity. On a more global scale, PDR models predict the existence of two stable neutral phases of the interstellar medium, elucidate the formation and destruction of star-forming molecular clouds, and suggest radiation-induced feedback mechanisms that may regulate star formation rates and the column density of gas through giant molecular clouds. [S0034-6861(99)01001-6] CONTENTS

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“…The 158 lm line results from transitions between the 2 P 3/2 and 2 P 1/2 fine-structure states in the ground 2s 2 2p term of C + . Most of the emission from the Galaxy and other nearby spirals arises in the diffuse CNM gas rather than from star-forming regions in spiral arms, or photodissociation regions on the surfaces of molecular clouds (e.g., Madden et al 1993). The last point is especially relevant for DLAs where molecules are rarely detected (Lu et al 1997;Petitjean et al 2000).…”

Section: The Ideamentioning

confidence: 99%

Cii* Absorption in Damped Lyα Systems. I. Star Formation Rates in a Two‐Phase Medium

Wolfe

Prochaska

Gawiser

2003

ApJ

160

261

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We describe a technique that for the first time measures star formation rates (SFRs) in damped Ly systems (DLAs) directly. We assume that massive stars form in DLAs and that the far-ultraviolet (FUV) radiation they emit heats the gas by the grain photoelectric mechanism. We infer the heating rate by equating it to the cooling rate measured by the strength of C ii* 1335.7 absorption. Since the heating rate is proportional to the product of the dust-to-gas ratio, the grain photoelectric heating efficiency, and the SFR per unit area, _ Ã Ã , we can deduce _ Ã Ã for DLAs in which the cooling rate and dust-to-gas ratio have been measured. We consider models in which the dust consists of carbonaceous grains and silicate grains. We present twophase models in which the cold neutral medium (CNM) and warm neutral medium (WNM) are in pressure equilibrium. In the CNM model the line of sight passes through CNM and WNM gas, while in the WNM model the line of sight passes only through WNM gas. Since the grain photoelectric heating efficiency is at least an order of magnitude higher in the CNM than in the WNM, most of the C ii* absorption arises in the CNM in the CNM model. We use the measured C ii* absorption lines to derive _ Ã Ã for a sample of %30 DLAs in which has been inferred from element depletion patterns. We show that the inferred _ Ã Ã corresponds to an average over the star-forming volume of the DLA rather than to local star formation along the line of sight. We obtain the average _ Ã Ã and show that _ Ã Ã ¼ 10 À2:2 M yr À1 kpc À2 for the CNM solution and _ Ã Ã ¼ 10 À1:3 M yr À1 kpc À2 for the WNM solution. Interestingly, the SFR per unit area in the CNM solution is similar to that measured in the Milky Way interstellar medium.

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158 micron forbidden C II mapping of NGC 6946 - Probing the atomic medium

Cited by 113 publications

References 0 publications

Chemical enrichment and physical conditions in I Zw 18

Chemical enrichment and physical conditions in I Zw 18

Photodissociation regions in the interstellar medium of galaxies

Cii* Absorption in Damped Lyα Systems. I. Star Formation Rates in a Two‐Phase Medium

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