An Interferometer Investigation of the 21-CENTIMETER Hydrogen-Line Absorption.

Clark, B. G.

doi:10.1086/148426

Cited by 152 publications

(54 citation statements)

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“…Early observations of the HI 21cm emission/absorption spectra in our galaxy showed that the atomic gas itself has multiple phases (Clark 1965;Radhakrishnan & Goss 1972), a clumpy dense phase with low spin temperature and a more diffuse widely distributed phase with a high spin temperature. Theoretically it has been established that in steady state (i.e.…”

Section: Introductionmentioning

confidence: 81%

Cold H i in faint dwarf galaxies

Patra

Chengalur

Караченцев

et al. 2015

Mon. Not. R. Astron. Soc.

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We present the results of a study of the amount and distribution of cold atomic gas, as well its correlation with recent star formation in a sample of extremely faint dwarf irregular galaxies. Our sample is drawn from the Faint Irregular Galaxy GMRT Survey (FIGGS) and its extension, FIGGS2. We use two different methods to identify cold atomic gas. In the first method, line-of-sight HI spectra were decomposed into multiple Gaussian components and narrow Gaussian components were identified as cold HI . In the second method, the brightness temperature (T B ) is used as a tracer of cold HI . We find that the amount of cold gas identified using the T B method is significantly larger than the amount of gas identified using Gaussian decomposition. We also find that a large fraction of the cold gas identified using the T B method is spatially coincident with regions of recent star formation, although the converse is not true. That is only a small fraction of the regions with recent star formation are also covered by cold gas. For regions where the star formation and the cold gas overlap, we study the relationship between the star formation rate density and the cold HI column density. We find that the star formation rate density has a power law dependence on the HI column density, but that the slope of this power law is significantly flatter than that of the canonical Kennicutt-Schmidt relation.

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Section: Introductionmentioning

confidence: 81%

Cold H i in faint dwarf galaxies

Patra

Chengalur

Караченцев

et al. 2015

Mon. Not. R. Astron. Soc.

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“…It is well known that the neutral atomic ISM has several phases that coexist side by side with very different properties (e.g., Clark 1965;Wolfire et al 1995Wolfire et al , 2003Heiles & Troland 2003b). The main constituents are the cold neutral medium (CNM) and the warm neutral medium (WNM) with spin temperatures on the order of <100 K (Strasser et al 2007) and ∼10 4 K (Murray et al 2014;Roy et al 2013a), respectively.…”

Section: Phases Of the Neutral Atomic Hydrogenmentioning

confidence: 99%

THOR: The H i, OH, Recombination line survey of the Milky Way

Bihr

Beuther

Ott

et al. 2015

A&A

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To study the atomic, molecular, and ionized emission of giant molecular clouds (GMCs) in the Milky Way, we initiated a large program with the Karl G. Jansky Very Large Array (VLA): "THOR: The H , OH, Recombination line survey of the Milky Way". We map the 21 cm H  line, 4 OH lines, up to 19 Hα recombination lines and the continuum from 1 to 2 GHz of a significant fraction of the Milky Way (l = 15• −67• , |b| ≤ 1 • ) at an angular resolution of ∼20 . Starting in 2012, as a pilot study we mapped 4 square degrees of the GMC associated with the W43 star formation complex. The rest of the THOR survey area was observed during 2013 and 2014. In this paper, we focus on the H  emission from the W43 GMC complex. Classically, the H  21 cm line is treated as optically thin with properties such as the column density calculated under this assumption.This approach might yield reasonable results for regions of low-mass star formation, however, it is not sufficient to describe GMCs. We analyzed strong continuum sources to measure the optical depth along the line of sight, and thus correct the H  21 cm emission for optical depth effects and weak diffuse continuum emission. Hence, we are able to measure the H  mass of this region more accurately and our analysis reveals a lower limit for the H  mass of M = 6.6 −1.8 × 10 6 M (v LSR = 60−120 km s −1 ), which is a factor of 2.4 larger than the mass estimated with the assumption of optically thin emission. The H  column densities are as high as N H  ∼ 150 M pc −2 ≈ 1.9 × 10 22 cm −2 , which is an order of magnitude higher than for low-mass star formation regions. This result challenges theoretical models that predict a threshold for the H  column density of ∼10 M pc −2 , at which the formation of molecular hydrogen should set in. By assuming an elliptical layered structure for W43, we estimate the particle density profile. For the atomic gas particle density, we find a linear decrease toward the center of W43 with values decreasing from n H  = 20 cm −3 near the cloud edge to almost 0 cm −3 at its center. On the other hand, the molecular hydrogen, traced via dust observations with the Herschel Space Observatory, shows an exponential increase toward the center with densities increasing to n H 2 > 200 cm −3 , averaged over a region of ∼10 pc. While atomic and molecular hydrogen are well mixed at the cloud edge, the center of the cloud is strongly dominated by H 2 emission. We do not identify a sharp transition between hydrogen in atomic and molecular form. Our results, which challenge current theoretical models, are an important characterization of the atomic to molecular hydrogen transition in an extreme environment.

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“…Most of it will be in the form of neutral atomic hydrogen (H i) whose hyperfine transition can be detected at the 21 cm wavelength. Since the early work of Clark (1965), the neutral atomic ISM has been known to exist in two phases, known as the cold neutral medium (CNM) and the warm neutral medium (WNM). Clark (1965) first suggested that H i seen in emission and absorption had different line widths because it arose from two distinct components of the ISM.…”

Section: Introductionmentioning

confidence: 99%

The Shapes of the H I Velocity Profiles of the Things Galaxies

Ianjamasimanana¹,

Blok²,

Walter³

et al. 2012

The Astronomical Journal

117

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We analyze the shapes of the H i velocity profiles of The H i Nearby Galaxy Survey to study the phase structure of the neutral interstellar medium and its relation to global galaxy properties. We use a method analogous to the stacking method sometimes used in high-redshift H i observations to construct high-signal-to-noise (S/N) profiles. We call these high-S/N profiles super profiles. We analyze and discuss possible systematics that may change the observed shapes of the super profiles. After quantifying these effects and selecting a subsample of unaffected galaxies, we find that the super profiles are best described by a narrow and a broad Gaussian component, which are evidence of the presence of the cold neutral medium and the warm neutral medium. The velocity dispersion of the narrow component ranges from ∼3.4 to ∼8.6 km s −1 with an average of 6.5 ± 1.5 km s −1 , whereas that of the broad component ranges from ∼10.1 to ∼24.3 km s −1 with an average of 16.8 ± 4.3 km s −1 . We find that the super profile parameters correlate with star formation indicators such as metallicity, far-UV−near-UV colors, and Hα luminosities. The flux ratio between the narrow and broad components tends to be highest for high-metallicity, high-star-formation-rate galaxies. We show that the narrow component identified in the super profiles is associated with the presence of star formation, and possibly with molecular hydrogen.

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An Interferometer Investigation of the 21-CENTIMETER Hydrogen-Line Absorption.

Cited by 152 publications

References 0 publications

Cold H i in faint dwarf galaxies

Cold H i in faint dwarf galaxies

THOR: The H i, OH, Recombination line survey of the Milky Way

The Shapes of the H I Velocity Profiles of the Things Galaxies

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