skip to main content
US FlagAn official website of the United States government
dot gov icon
Official websites use .gov
A .gov website belongs to an official government organization in the United States.
https lock icon
Secure .gov websites use HTTPS
A lock ( lock ) or https:// means you've safely connected to the .gov website. Share sensitive information only on official, secure websites.

Explore Research Products in the PAR
It may take a few hours for recently added research products to appear in PAR search results.


Title: Revisiting the Vertical Distribution of H i Absorbing Clouds in the Solar Neighborhood
The vertical distribution of cold neutral hydrogen (Hi) clouds is a constraint on models of the structure, dynamics, and hydrostatic balance of the interstellar medium. In 1978, Crovisier pioneered a method to infer the vertical distribution of Hiabsorbing clouds in the solar neighborhood. Using data from the Nançay 21 cm absorption survey, Crovisier determined the mean vertical displacement of cold Hiclouds, 〈∣z∣〉. We revisit that author’s analysis and explore the consequences of truncating the Hiabsorption sample in Galactic latitude. For any nonzero latitude limit, we find that the quantity inferred by Crovisier is not the mean vertical displacement but rather a ratio involving higher moments of the vertical distribution. The resultant distribution scale heights are thus ∼1.5 to ∼3 times smaller than previously determined. In light of this discovery, we develop a Bayesian Monte Carlo Markov Chain method to infer the vertical distribution of Hiabsorbing clouds. We fit our model to the original Nançay data and find a vertical distribution moment ratio 〈∣z∣3〉/〈∣z∣2〉 = 97 ± 15 pc, which corresponds to a Gaussian scale heightσz= 61 ± 9 pc, an exponential scale heightλz= 32 ± 5 pc, and a rectangular half-widthWz,1/2= 129 ± 20 pc. Consistent with recent simulations, the vertical scale height of cold Hiclouds appears to remain constant between the inner Galaxy and the Galactocentric distance of the solar neighborhood. Local fluctuations might explain the large-scale height observed at the same Galactocentric distance on the far side of the Galaxy.  more » « less
Award ID(s):
2202340 2303902
PAR ID:
10515857
Author(s) / Creator(s):
; ;
Publisher / Repository:
The Astrophysical Journal
Date Published:
Journal Name:
The Astrophysical Journal
Volume:
966
Issue:
2
ISSN:
0004-637X
Page Range / eLocation ID:
206
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. ; ; (, Bulletin American Astronomical Society)
    The three-dimensional distribution of neutral hydrogen in the Milky Way disk is a key constraint on models of Galactic spiral structure, galaxy evolution, and star formation. In particular, the vertical distributions of the different phases of hydrogen (ionized, warm neutral, cold neutral, and molecular) inform our understanding of the evolution of gas between these phases. Although the scale height of the HI emission disk has been well-characterized across the Galaxy, the vertical distribution of the cold HI component is significantly more challenging to constrain due to the sensitive absorption observations required to characterize this phase. Almost four decades ago, Crovisier (1987) pioneered a kinematic method to estimate the vertical distribution of cold HI clouds in the solar neighborhood using the latest results from the Nancay 21-cm absorption survey. This method was subsequently used in other studies to constrain the vertical distribution of neutral and molecular clouds. We have discovered an error in Crovisier's method that can lead to a factor of two inaccuracy in the inferred scale height. We will discuss the mistake and, using the original Nancay data and a corrected method based on Crovisier's technique, demonstrate the magnitude of the error in the inferred scale height of the local cold HI disk. Furthermore, we will introduce a new Monte Carlo Markov Chain method to infer the vertical distribution of HI absorbing clouds with fewer assumptions and better accuracy. This method will be used with the latest HI absorption data from the Galactic ASKAP HI survey of the Milky Way disk to provide an unprecedented view of the 3D distribution of the cold neutral medium in the solar neighborhood. 
    more » « less
  2. ; ; ; ; ; ; ; ; ; ; et al (, The Astrophysical Journal)
    Abstract Measuring the properties of the cold neutral medium (CNM) in low-metallicity galaxies provides insights into heating and cooling mechanisms in early Universe-like environments. We report detections of two localized atomic neutral hydrogen (Hi) absorption features in NGC 6822, a low-metallicity (0.2Z⊙) dwarf galaxy in the Local Group. These are the first unambiguous CNM detections in a low-metallicity dwarf galaxy outside the Magellanic Clouds. The Local GroupL-band Survey (LGLBS) enabled these detections, due to its high spatial (15 pc for Hiemission) and spectral (0.4 km s−1) resolution. We introduce LGLBS and describe a custom pipeline for searching for Hiabsorption at high angular resolution and extracting associated Hiemission. A detailed Gaussian decomposition and radiative transfer analysis of the NGC 6822 detections reveals five CNM components, with key properties: a mean spin temperature of 32 ± 6 K, a mean CNM column density of 3.1 × 1020cm−2, and CNM mass fractions of 0.33 and 0.12 for the two sightlines. Stacking nondetections does not reveal low-level signals below our median optical depth sensitivity of 0.05. One detection intercepts a star-forming region, with the Hiabsorption profile encompassing the CO (2−1) emission, indicating coincident molecular gas and a depression in high-resolution Hiemission. We also analyze a nearby sightline with deep, narrow Hiself-absorption dips, where the background warm neutral medium is attenuated by intervening CNM. The association of CNM, CO, and Hαemissions suggests a close link between the colder, denser Hiphase and star formation in NGC 6822. 
    more » « less
  3. ; ; (, The Astrophysical Journal)
    The cold neutral medium (CNM) is where neutral atomic hydrogen (H I) is converted into molecular clouds, so the structure and kinematics of the CNM are key drivers of galaxy evolution. Here we provide new constraints on the vertical distribution of the CNM using the recently developed kinematic_scaleheight software package and a large catalog of sensitive H I absorption observations. We estimate the thickness of the CNM in the solar neighborhood to be σ z ∼ 50–90 pc, assuming a Gaussian vertical distribution. This is a factor of ∼2 smaller than typically assumed, indicating that the thickness of the CNM in the solar neighborhood is similar to that found in the inner Galaxy, consistent with recent simulation results. If we consider only structures with H I optical depths τ > 0.1 or column densities N(H I) > 1019.5 cm‑2, which recent work suggests are thresholds for molecule formation, we find σ z ∼ 50 pc. Meanwhile, for structures with τ < 0.1 or column densities N(H I) < 1019.5 cm‑2, we find σ z ∼ 120 pc. These thicknesses are similar to those derived for the thin- and thick-disk molecular cloud populations traced by CO emission, possibly suggesting that cold H I and CO are well mixed. Approximately 20% of CNM structures are identified as outliers, with kinematics that are not well explained by Galactic rotation. We show that some of these CNM structures—perhaps representing intermediate-velocity clouds—are associated with the Local Bubble wall. We compare our results to recent observations and simulations, and we discuss their implications for the multiphase structure of the Milky Way's interstellar medium. 
    more » « less
  4. ; ; ; ; (, The Astrophysical Journal)
    Abstract We present a survey undertaken with the Atacama Large Millimeter/submillimeter Array (ALMA) to study the galaxies associated with a representative sample of 16 damped Lyαabsorbers (DLAs) atz ≈ 4.1–4.5, using the [Cii] 158μm ([Cii]) line. We detect seven [Cii]-emitting galaxies in the fields of five DLAs, all of which have absorption metallicity [M/H] > −1.5. We find that the detectability of these Hi-selected galaxies with ALMA is a strong function of DLA metallicity, with a detection rate of 7 1 − 20 + 11 % for DLAs with [M/H] > −1.5 and 0+18% for DLAs with [M/H] < −1.5. The identified DLA galaxies have far-IR properties similar to those of typical star-forming galaxies atz ∼ 4, with estimated obscured star formation rates ranging from ≲6M⊙yr−1to 110M⊙yr−1. High-metallicity DLAs therefore provide an efficient way to identify and study samples of high-redshift, star-forming galaxies, without preselecting the galaxies by their emission properties. The agreement between the velocities of the metal absorption lines of the DLA and the [Cii] emission line of the DLA galaxy indicates that the metals within the DLA originated in the galaxy. With observed impact parameters between 14 and 59 kpc, this indicates that star-forming galaxies atz ∼ 4 have a substantial reservoir of dense, cold, neutral gas within their circumgalactic medium that has been enriched with metals from the galaxy. 
    more » « less
  5. ; ; ; ; ; ; ; ; ; ; et al (, The Astrophysical Journal)
    Abstract We investigate the conditions for the Hi-to-H2transition in the solar neighborhood by analyzing Hiemission and absorption measurements toward 58 Galactic lines of sight (LOSs) along with12CO(1–0) (CO) and dust data. Based on the accurate column densities of the cold and warm neutral medium (CNM and WNM), we first perform a decomposition of gas into atomic and molecular phases, and show that the observed LOSs are mostly Hi-dominated. In addition, we find that the CO-dark H2, not the optically thick Hi, is a major ingredient of the dark gas in the solar neighborhood. To examine the conditions for the formation of CO-bright molecular gas, we analyze the kinematic association between Hiand CO, and find that the CNM is kinematically more closely associated with CO than the WNM. When CNM components within CO line widths are isolated, we find the following characteristics: spin temperature < 200 K, peak optical depth > 0.1, CNM fraction of ∼0.6, andV-band dust extinction > 0.5 mag. These results suggest that CO-bright molecular gas preferentially forms in environments with high column densities where the CNM becomes colder and more abundant. Finally, we confront the observed CNM properties with the steady-state H2formation model of Sternberg et al. and infer that the CNM must be clumpy with a small volume filling factor. Another possibility would be that missing processes in the model, such as cosmic-rays and gas dynamics, play an important role in the Hi-to-H2transition. 
    more » « less
  • Citation Formats
  • MLA
  • APA
  • Chicago
  • BibTeX
  • Text Encoded Conversion
  • XML
  • Save / Share this Record
  • Send to Email