EXCITATION TEMPERATURE OF THE WARM NEUTRAL MEDIUM AS A NEW PROBE OF THE Lyα RADIATION FIELD

Murray, Claire E.; Lindner, Robert R.; Stanimirović, Snežana; Goss, W. M.; Heiles, Carl; Dickey, John M.; Pingel, N. M.; Lawrence, Allen; Jencson, J.; Babler, B.; Hennebelle, P.

doi:10.1088/2041-8205/781/2/l41

Cited by 36 publications

(50 citation statements)

References 32 publications

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“…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. Furthermore, their density differs by two order of magnitude (CNM: n H ∼ 50 cm −3 , WNM: n H ∼ 0.5 cm −3 , e.g., Stahler et al 2005).…”

Section: Phases Of the Neutral Atomic Hydrogenmentioning

confidence: 99%

“…Furthermore, their density differs by two order of magnitude (CNM: n H ∼ 50 cm −3 , WNM: n H ∼ 0.5 cm −3 , e.g., Stahler et al 2005). Because of the different spin temperature and density of the CNM and WNM, their corresponding optical depths are significantly different with typical values of τ WNM ∼ 10 −3 −10 −4 (Murray et al 2014) and τ CNM 0.1 (Strasser & Taylor 2004). This is important for our interpretation.…”

Section: Phases Of the Neutral Atomic Hydrogenmentioning

confidence: 99%

“…If we assume a CNM cloud with varying T spin (CNM) ∼ 20−80 K and varying optical depth surrounded by a WNM with T spin (WNM) ∼ 7000 K (Murray et al 2014) and optical depth τ(WNM) ∼ 5 × 10 −3 , we can calculate the column density of each component separately (CNM and WNM). Furthermore, we can calculate the brightness temperature that we would observe and apply our correction method described in Sect.…”

Section: Phases Of the Neutral Atomic Hydrogenmentioning

confidence: 99%

“…In contrast, the 21 cm spin-flip transition of hydrogen offers a well-known method to measure the atomic gas content. Even A&A 580, A112 (2015) though the 21 cm line is well studied (e.g., Radhakrishnan et al 1972;Gibson et al 2000Gibson et al , 2005aTaylor et al 2003;Heiles & Troland 2003a,b;Strasser & Taylor 2004;Goldsmith & Li 2005;Stil et al 2006;Kalberla & Kerp 2009;McClure-Griffiths et al 2012;Roy et al 2013a;Fukui et al 2015;Murray et al 2014;Motte et al 2014), it is difficult to disentangle the different contributions of the cold and warm H  in emission and absorption for different spin temperatures and optical depths. In addition, radio continuum emission at the frequency of the H  emission line can also suppress the intensity of the observed H  emission, an effect that is especially significant for the Galactic plane.…”

Section: Introductionmentioning

confidence: 99%

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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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Section: Phases Of the Neutral Atomic Hydrogenmentioning

confidence: 99%

Section: Phases Of the Neutral Atomic Hydrogenmentioning

confidence: 99%

Section: Phases Of the Neutral Atomic Hydrogenmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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

Bihr

Beuther

Ott

et al. 2015

A&A

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“…AGD allows for the interpretation of large volumes of spectral data and for the ability to objectively compare observations to numerical simulations in a statistically robust way. While the development of AGD was motivated by radio astrophysics, specifically the 21 cm SPectral line Observations of Neutral Gas with the EVLA 10 (21-SPONGE) survey (Murray et al 2014(Murray et al , 2015, the algorithm can be used to search for one-dimensional Gaussian (or any other single-peaked spectral profile)-shaped components in any data set.…”

Section: Introduction: 21 CM Gaussian Fitsmentioning

confidence: 99%

Autonomous Gaussian Decomposition

Lindner¹,

Vera-Ciro²,

Murray³

et al. 2015

Self Cite

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We present a new algorithm, named Autonomous Gaussian Decomposition (AGD), for automatically decomposing spectra into Gaussian components. AGD uses derivative spectroscopy and machine learning to provide optimized guesses for the number of Gaussian components in the data, and also their locations, widths, and amplitudes. We test AGD and find that it produces results comparable to human-derived solutions on 21 cm absorption spectra from the 21 cm SPectral line Observations of Neutral Gas with the EVLA (21-SPONGE) survey. We use AGD with Monte Carlo methods to derive the H I line completeness as a function of peak optical depth and velocity width for the 21-SPONGE data, and also show that the results of AGD are stable against varying observational noise intensity. The autonomy and computational efficiency of the method over traditional manual Gaussian fits allow for truly unbiased comparisons between observations and simulations, and for the ability to scale up and interpret the very large data volumes from the upcoming Square Kilometer Array and pathfinder telescopes.

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Probing the gas content of radio galaxies through H I absorption stacking

Geréb

Morganti

Oosterloo

2014

A&A

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Using the Westerbork Synthesis Radio Telescope, we carried out shallow H i absorption observations of a flux-selected (S 1.4 GHz > 50 mJy) sample of 93 radio active galactic nuclei (AGN), which have available SDSS (Sloan Digital Sky Survey) redshifts between 0.02 < z < 0.23. Our main goal is to study the gas properties of radio sources down to S 1.4 GHz flux densities not systematically explored before using, for the first time, stacking of absorption spectra of extragalactic H i. Despite the shallow observations, we obtained a direct detection rate of ∼29%, comparable with deeper studies of radio galaxies. Furthermore, detections are found at every S 1.4 GHz flux level, showing that H i absorption detections are not biased toward brighter sources. The stacked profiles of detections and non-detections reveal a clear dichotomy in the presence of H i, with the 27 detections showing an average peak τ = 0.02 corresponding to N(H i) ∼ (7.4 ± 0.2) × 10 18 (T spin /c f ) cm −2 , while the 66 non-detections remain undetected upon stacking with a peak optical depth upper limit τ < 0.002 corresponding to N(H i) < (2.26 ± 0.06) × 10 17 (T spin /c f ) cm −2 (using a FWHM of 62 km s −1 , derived from the mean width of the detections). Separating the sample into compact and extended radio sources increases the detection rate, optical depth, and FWHM for the compact sample. The dichotomy for the stacked profiles of detections and non-detections still holds between these two groups of objects. We argue that orientation effects connected to a disk-like distribution of the H i can be partly responsible for the dichotomy that we see in our sample. However, orientation effects alone cannot explain all the observational results, and some of our galaxies must be genuinely depleted of cold gas. A fraction of the compact sources in the sample are confirmed by previous studies as likely young radio sources (compact steep spectrum and gigahertz peaked spectrum sources). These show an even higher detection rate of 55%. Along with their high integrated optical depth and wider profile, this reinforces the idea that young radio AGN are embedded in a medium that is rich in atomic gas. Part of our motivation is to probe for the presence of faint H i outflows at low optical depth using stacking. However, the stacked profiles do not reveal any significant blueshifted wing. We are currently collecting more data to investigate the presence of outflows. The results presented in this paper are particularly relevant for future surveys in two ways. The lack of bias toward bright sources is encouraging for the search for H i in sources with even lower radio fluxes planned by such surveys. The results also represent a reference point when searching for H i absorption at higher redshifts.

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EXCITATION TEMPERATURE OF THE WARM NEUTRAL MEDIUM AS A NEW PROBE OF THE Lyα RADIATION FIELD

Cited by 36 publications

References 32 publications

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

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

Autonomous Gaussian Decomposition

Probing the gas content of radio galaxies through H I absorption stacking

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