A Neutral Hydrogen Self-Absorption Cloud in the SGPS
Anne Green2003
Sign up for access to the world's latest research
checkGet notified about relevant papers
checkSave papers to use in your research
checkJoin the discussion with peers
checkTrack your impact
Abstract
Using data from the Southern Galactic Plane Survey (SGPS) we analyze an HI self-absorption cloud centered on l = 318.0 ◦ , b =-0.5 ◦ , and velocity, v =-1.1 km s −1. The cloud was observed with the Australia Telescope Compact Array (ATCA) and the Parkes Radio Telescope, and is at a near kinematic distance of � 400 pc with derived dimensions of � 5 × 11 pc. We apply two different methods to find the optical depth and spin temperature. In both methods we find upper limit spin temperatures ranging from 20 K to 25 K and lower limit optical depths ∼ 1. We look into the nature of the HI emission and find that 60-70 % originates behind the cloud. We analyze a second cloud at the same velocity centered on l = 319 ◦ and b = 0.4 ◦ with an upper limit spin temperature of 20 K and a lower limit optical depth of 1.6. The similarities in spin temperature, optical depth, velocity, and spatial location are evidence the clouds are associated, possibly as one large cloud consisting of smaller clumps ...
Related papers
H I Emission and Absorption in the Southern Galactic Plane Survey
Publications of the Astronomical Society of Australia, 2001
We present preliminary results from the Southern Galactic Plane Survey (SGPS) Test Region and Parkes data. As part of the pilot project for the Southern Galactic Plane Survey, observations of a Test Region (325·5° ≤l ≤ 333·5°; −0·5° ≤ b ≤ 3·5°) were completed in December 1998. Single-dish observations of the full survey region (253° ≤ l ≤ 358 ° |b| ≤ 1°) with the Parkes Radio Telescope were completed in March 2000. We present a sample of SGPS H I data, with particular attention to the smallest-and largest-scale structures seen in absorption and emission, respectively. On the large scale, we detect many prominent H I shells. On the small scale, we note extremely compact, cold clouds seen in H I self-absorption. We explore how these two classes of objects probe opposite ends of the H I spatial power spectrum.
“Missing Link” Clouds in the Southern Galactic Plane Survey
The Astrophysical Journal, 2005
We present an automated routine to search for HI self-absorption features within the Southern Galactic Plane Survey (SGPS). The data were taken with the Australia Telescope Compact Array (ATCA) and the Parkes Radio Telescope and encompass 3 • × 105 • of sky in the Galactic plane. We apply our routine to this entire region and derive spin temperatures and column densities for 70 of the larger HISA complexes, finding spin temperatures ranging from 6-41 K with HI number densities of a few cm −3. These 'missing link' clouds fill in the spin temperature and density gaps between dense molecular clouds and diffuse atomic clouds. We compare the HI emission with 12 CO emission and find that ∼60% of detected HI self-absorption is correlated in space and in velocity with a
Emission and Absorption in the Southern Galactic Plane Survey ∗
2000
We present preliminary results from the Southern Galactic Plane Survey (SGPS) Test Region and Parkes data. As part of the pilot project for the Southern Galactic Plane Survey, observations of a Test Region (325. ◦5 ≤ l ≤ 333. ◦5; −0. ◦5 ≤ b ≤ 3. ◦5) were completed in December 1998. Single dish observations of the full survey region (253◦ ≤ l ≤ 358◦; |b| ≤ 1◦) with the Parkes Radio Telescope were completed in March 2000. We present a sample of SGPS H i data with particular attention to the smallest and largest scale structures seen in absorption and emission, respectively. On the large scale, we detect many prominent H i shells. On the small scale, we note extremely compact, cold clouds seen in H i self-absorption. We explore how these two classes of objects probe opposite ends of the H i spatial power spectrum.
G28.17+0.05: An Unusual Giant H i Cloud in the Inner Galaxy
The Astrophysical Journal, 2001
New 21 cm H I observations have revealed a giant H I cloud in the Galactic plane that has unusual properties. It is quite well defined, about 150 pc in diameter at a distance of 5 kpc, and contains as much as 10 5 M ⊙ of atomic hydrogen. The outer parts of the cloud appear in H I emission above the H I background, while the central regions show H I self-absorption. Models which reproduce the observations have a core with a temperature 40 K and an outer envelope as much as an order of magnitude hotter. The cold core is elongated along the Galactic plane, whereas the overall outline of the cloud is approximately spherical. The warm and cold parts of the H I cloud have a similar, and relatively large, line width ∼ 7 km s −1 . The cloud core is a source of weak, anomalouslyexcited 1720 MHz OH emission, also with a relatively large line width, which delineates the region of H I self-absorption but is slightly blue-shifted in velocity. The intensity of the 1720 MHz OH emission is correlated with N H derived from models of the cold core. There is 12 CO emission associated with the cloud core. Most of the cloud mass is in molecules, and the total mass is > 2 × 10 5 M ⊙ . In the cold core the H I mass fraction may be ∼ 10%. The cloud has only a few sites of current star formation. There may be ∼ 100 more objects like this in the inner Galaxy; every line of sight through the Galactic plane within 50 • of the Galactic center probably intersects at least one. We suggest that G28.17+0.05 is a cloud being observed as it enters a spiral arm and that it is in the transition from the atomic to the molecular state.
2 Fitting Together the HI Absorption and Emission in the SGPS
2016
In this paper we study 21-cm absorption spectra and the corresponding emission spectra toward bright continuum sources in the test region (326 • < l < 333 •) of the Southern Galactic Plane Survey. This survey combines the high resolution of the Australia Telescope Compact Array with the full brightness temperature information of the Parkes single dish telescope. In particular, we focus on the abundance and temperature of the cool atomic clouds in the inner galaxy. The resulting mean opacity of the HI, < κ >, is measured as a function of Galactic radius; it increases going in from the solar circle, to a peak in the molecular ring of about four times its local value. This suggests that the cool phase is more abundant there, and colder, than it is locally. The distribution of cool phase temperatures is derived in three different ways. The naive, "spin temperature" technique overestimates the cloud temperatures, as expected. Using two alternative approaches we get good agreement on a histogram of the cloud temperatures, T cool , corrected for blending with warm phase gas. The median temperature is about 65 K, but there is a long tail reaching down to temperatures below 20 K. Clouds with temperatures below 40 K are common, though not as common as warmer clouds (40 to 100 K).
Tracing the Formation of Molecular Clouds in a Low-metallicity Galaxy: An H i Narrow Self-absorption Survey of the Large Magellanic Cloud
The Astrophysical Journal, 2019
Cold atomic hydrogen clouds are the precursors of molecular clouds. Due to self-absorption, the opacity of cold atomic hydrogen may be high, and this gas may constitute an important mass component of the interstellar medium. Atomic hydrogen gas can be cooled to temperatures much lower than found in the cold neutral medium through collisions with molecular hydrogen. In this paper, we search for H I narrow self-absorption (HINSA) features in the Large Magellanic Cloud (LMC) as an indicator of such cold H I clouds, and use the results to quantify atomic masses and atomic-to-molecular gas ratio. Our search for HINSA features was conducted toward molecular clouds in the LMC using the ATCA+Parkes H I survey and the MAGMA CO survey. HINSA features are prevalent in the surveyed sightlines. This is the first detection of HINSA in an external galaxy. The HINSA-H I/H 2 ratio in the LMC varies from 0.5×10 −3 to 3.4×10 −3 (68% interval), with a mean value of (1.31±0.03)×10 −3 , after correcting for the effect of foreground H I gas. This is similar to the Milky Way value and indicates that similar fractions of cold gas exist in the LMC and the Milky Way, despite their differing metallicities, dust content and radiation fields. The low ratio also confirms that, as with the Milky Way, the formation timescale of molecular clouds is short. The ratio shows no radial gradient, unlike the case for stellar metallicity. No correlation is found between our results and those from previous H I absorption studies of the LMC.
HINSA as a tool for studying dark clouds and star formation
Traditionally it has been difficult to obtain estimates of the HI content of molecular clouds due to the sheer complexity of the galactic background HI emission. However, with the use of the recently discovered HINSA (HI Narrow Self-Absorption) features we are for the first time able to make direct measurements of the HI column density in cold molecular clouds with high extinction. This allows us to study a variety of properties in these clouds including the molecular to atomic hydrogen ratio. Measurements and understanding of this ratio can give us estimates of the chemical ages of these clouds, in turn providing us with constraints on star formation. More specifically we are able to place some constraints on the timescale over which a molecular cloud collapses from a diffuse (A v < 1) to a compact star-forming state. Such constraints would have considerable impact on several disputed areas of star formation theory including the role of magnetic fields and ambipolar diffusion. With new observations at the Green Bank Telescope we have greatly increased the amount of available HINSA data previously obtained using the Arecibo telescope, and though our analysis is still very much preliminary, we are beginning to see that HINSA and its correlations with molecular, IR, and optical data may prove to be an even more useful tool in studying dark molecular clouds and other objects than previously anticipated.
The structure of galactic HI in directions of low total column density
The Astrophysical Journal, 1986
Compact H i Clouds at High Forbidden Velocities in the Inner Galaxy
The Astrophysical Journal, 2006
The VLA Galactic Plane Survey (VGPS) of the first Galactic quadrant was searched for H I emission with velocities well above the maximum velocity allowed by Galactic rotation. A sample of 17 small fast-moving clouds was identified. The distribution of the ensemble of clouds in longitude and velocity indicates that the clouds are part of the Galactic disk, despite their large forbidden velocity. The median angular diameter of the clouds detected in the VGPS is 3. ′ 4. These clouds would not be noticed in previous low resolution surveys because of strong beam dilution. Assuming each cloud is located at the tangent point, a median cloud has a diameter of 10 pc, H I mass of 60 M ⊙ , and a velocity more than 25 km s −1 beyond the local terminal velocity derived from 12 CO observations. Three clouds
Extra-planar HI in the Inner Milky Way
Arxiv preprint astro-ph/0410161, 2004
The extra-planar H I in the inner parts of the Milky Way has been discovered to contain numerous cloud-like structures when observed in the 21cm line with the Green Bank Telescope. These halo clouds have motions consistent with Galactic rotation and do not seem to be related to the classic high-velocity clouds. They are found to distances >1 kpc from the plane and can contain hundreds of M ⊙ of H I. Spectra of many of the halo clouds show evidence of coexisting cool and warm H I phases. A preliminary high-resolution study of one of the clouds suggests that it consists of a diffuse envelope and a few dense cores, with a peak N H I reaching 4 × 10 20 cm −2 . The clouds are often organized into larger structures, one example of which was discovered near ℓ = 35 • rising higher than 2 kpc above the Galactic plane. New observations should answer some fundamental questions about the nature of these clouds.
Related topics
Related papers
Fitting Together the HI Absorption and Emission in the Southern Galactic Plane Survey
2008
ABSTRACT In this paper we study 21 cm absorption spectra and the corresponding emission spectra toward bright continuum sources in the test region (326 &j0;< l< 333 &j0;) of the Southern Galactic Plane Survey. This survey combines the high resolution of the Australia Telescope Compact Array with the full brightness temperature information of the Parkes single-dish telescope. In particular, we focus on the abundance and temperature of the cool atomic clouds in the inner Galaxy.
Low surface brightness Hα observations of local intergalactic hydrogen clouds
The Astrophysical Journal …, 1995
We present upper limits on the local ionizing background based on a search for extended H␣ emission from three nearby intergalactic H I clouds: the Leo Ring (M96 group), both the NE and SW lobes of the Haynes-Giovanelli Virgo Cloud (H I 1225ϩ01), and the H I tidal tails associated with the NGC 4631/4656 group. These clouds were chosen to have 21 cm emission that is extended (10 -100 kpc) and distant from any associated galaxy. Deep, wide-field CCD images were acquired through narrow-(131 Å) and broadband R filters with the Burrell Schmidt telescope on Kitt Peak. We set a 95% confidence upper limit on the H␣ surface brightness for the areas of the clouds detected in H I of 1.6 ϫ 10 Ϫ19 ergs s Ϫ1 cm Ϫ2 arcsec Ϫ2 in Leo and of 3.7 ϫ 10 Ϫ19 ergs s Ϫ1 cm Ϫ2 arcsec Ϫ2 in Virgo. We limit the local ionizing background to ⌽ 0 Ͻ 5.0 ϫ 10 3 photons s Ϫ1 cm Ϫ2 sr Ϫ1 (95%) at the location of the Leo Ring cloud and ⌽ 0 Ͻ 1.1 ϫ 10 4 photons s Ϫ1 cm Ϫ2 sr Ϫ1 at the Virgo cloud, assuming spherical clouds. (Limits are a factor of 2 higher for a thin face-on slab.) The limits correspond to J 0 Ͻ 3.3 ϫ 10 Ϫ23 and 7.6 ϫ 10 Ϫ23 ergs s Ϫ1 cm Ϫ2 sr Ϫ1 Hz Ϫ1 for a Ϫ1/ 2 spectrum (1.6 times higher for a Ϫ1.4 spectrum) between 1 and 4 ryd. Such low limits suggest that quasar light, and not galactic light, dominates the ionizing background at low redshift. The H␣ limit on the Leo cloud is significantly below a previously reported detection. In the field of the edge-on galaxies NGC 4631 and NGC 4656, we detect H␣ from ionized gas extending nearly 16 kpc above N4631, which could have been blown out by starburst activity in the plane, and a low surface brightness companion or stellar tidal tail. This companion lies between N4631's H I tidal tails and may have played a role in creating the H I tidal tails, or it may represent star formation within the tidal tails. We also report the tentative detection of an ultrafaint ''sheet'' of H␣ emission extending from NGC 4631 to NGC 4656.
Atomic and Molecular Interstellar Absorption Lines toward the High Galactic Latitude Stars HD 141569 and HD 157841 at Ultra–High Resolution
The Astrophysical Journal, 1998
We present ultra-high resolution (0.32 km s -1 ) spectra obtained with the 3.9m Anglo-Australian Telescope (AAT) and Ultra-High-Resolution Facility (UHRF), of interstellar Na i D 1 , Na i D 2 , Ca ii K, K i and CH absorption toward two high galactic latitude stars HD 141569 and HD 157841. We have compared our data with 21-cm observations obtained from the Leiden/Dwingeloo H i survey. We derive the velocity structure, column densities of the clouds represented by the various components and identify the clouds with ISM structures seen in the region at other wavelengths. We further derive abundances, linear depletions and H 2 fractional abundances for these clouds, wherever possible. Both stars are located in regions of IRAS 100µm emission associated with high galactic latitude molecular clouds (HLCs) : HD 141569 lies, in projection, close to MBM 37 and the Lynds dark cloud L 134N while HD 157841 is in the vicinity of the MBM 151. Toward HD 141569, we detect two components in our UHRF spectra : a weak, broad b = 4.5 km s -1 component at -15 km s -1 , seen only in Ca ii K absorption and another component at 0 km s -1 , seen in Na i D 1 , Na i D 2 , Ca ii K, K i and CH absorption. The cloud represented by the -15 km s -1 component, is warm and may be located in a region close to the star. The cloud represented by the 0 km s -1 component has a
The temperature of the diffuse H I in the Milky Way - I. High resolution H I-21 cm absorption studies
Monthly Notices of the Royal Astronomical Society, 2013
We have carried out deep, high velocity resolution, interferometric Galactic H i-21 cm absorption spectroscopy towards 32 compact extra-galactic radio sources with the Giant Metrewave Radio Telescope (GMRT) and the Westerbork Synthesis Radio Telescope (WSRT). The optical depth spectra for most sources have root mean square noise values 10 −3 per 1 km s −1 velocity channel and are thus sufficiently sensitive to detect absorption by warm neutral hydrogen with Hi column densities N HI 10 20 cm −2 , spin temperatures T s 5000 K, and line widths equal to the thermal width (20 km s −1 ). Hi 21cm absorption was detected against all background sources but one, B0438−436. The spectra of sources observed separately with GMRT and WSRT show excellent agreement, indicating that spectral baseline problems and contamination from Hi 21cm emission are negligible. This paper presents the absorption spectra, the emission spectra along neighbouring sightlines from the Leiden-Argentine-Bonn survey, and the derived spin temperature spectra. On every sightline, the maximum spin temperature detected (at 3σ significance) even at a velocity resolution of 1 km s −1 is 1000 K, indicating that we are detecting the warm neutral medium along most sightlines. This is by far the largest sample of Galactic Hi 21cm absorption spectra of this quality, providing a sensitive probe of physical conditions in the neutral atomic ISM.
Probing galaxy evolution through HI 21-cm emission and absorption: current status and prospects with the Square Kilometre Array
arXiv (Cornell University), 2022
One of the major science goals of the Square Kilometre Array (SKA) is to understand the role played by atomic hydrogen (H i) gas in the evolution of galaxies throughout cosmic time. The hyperfine transition line of the hydrogen atom at 21-cm is one of the best tools to detect and study the properties of H i gas associated with galaxies. In this article, we review our current understanding of H i gas and its relationship with galaxies through observations of the 21-cm line both in emission and absorption. In addition, we provide an overview of the H i science that will be possible with SKA and its precursors and pathfinders, i.e. H i 21-cm emission and absorption studies of galaxies from nearby to high redshifts that will trace various processes governing galaxy evolution.
Compact H I Clouds from the GALFA-H I Survey
Astrophysical Journal, 2010
The Galactic Arecibo L-band Feed Array HI (GALFA-HI) survey is mapping the entire Arecibo sky at 21-cm, over a velocity range of -700 to +700 km/s (LSR), at a velocity resolution of 0.18 km/s and a spatial resolution of 3.5 arcmin. The unprecedented resolution and sensitivity of the GALFA-HI survey have resulted in the detection of numerous isolated, very compact HI clouds at low Galactic velocities, which are distinctly separated from the HI disk emission. In the limited area of ~4600 deg$^2$ surveyed so far, we have detected 96 of such compact clouds. The detected clouds are cold with a median T$_{k,max}$ (the kinetic temperature in the case in which there is no non-thermal broadening) of 300 K. Moreover, these clouds are quite compact and faint, with median values of 5 arcmin in angular size, 0.75 K in peak brightness temperature, and $5 \times 10^{18}$ cm$^{-2}$ in HI column density. Most of the clouds deviate from Galactic rotation at the 20-30 km/s level, and a significant fraction show evidence for a multiphase medium and velocity gradients. No counterparts for these clouds were found in other wavebands. From the modeling of spatial and velocity distributions of the whole compact cloud population, we find that the bulk of the compact clouds are related to the Galactic disk, and their distances are likely to be in the range of 0.1 to a few kpc. We discuss various possible scenarios for the formation and maintenance of this cloud population and its significance for Galactic ISM studies.
Probing galaxy evolution through Hi 21-cm emission and absorption: current status and prospects with square kilometre array
Journal of Astrophysics and Astronomy
One of the major science goals of the Square Kilometre Array (SKA) is to understand the role played by atomic hydrogen (H i) gas in the evolution of galaxies throughout cosmic time. The hyperfine transition line of the hydrogen atom at 21-cm is one of the best tools to detect and study the properties of H i gas associated with galaxies. In this article, we review our current understanding of H i gas and its relationship with galaxies through observations of the 21-cm line both in emission and absorption. In addition, we provide an overview of the H i science that will be possible with SKA and its precursors and pathfinders, i.e. H i 21-cm emission and absorption studies of galaxies from nearby to high redshifts that will trace various processes governing galaxy evolution.
The temperature of the diffuse H I in the Milky Way - II. Gaussian decomposition of the H I-21 cm absorption spectra
Monthly Notices of the Royal Astronomical Society, 2013
We discuss physical conditions in Galactic neutral hydrogen based on deep, high velocity resolution interferometric H i 21cm absorption spectroscopy towards 33 compact extra-galactic radio sources. The H i 21cm optical depth spectra have root-mean-square noise values 10 −3 per 1 km s −1 velocity channel, i.e., sufficiently sensitive to detect H i 21cm absorption by the warm neutral medium (WNM). Comparing these spectra with Hi 21cm emission spectra from the Leiden-Argentine-Bonn (LAB) survey, we show that some of the absorption detected on most sightlines must arise in gas with temperatures higher than that in the stable cold neutral medium (CNM). A multi-Gaussian decomposition of 30 of the H i 21cm absorption spectra yielded very few components with line widths in the temperature range of stable WNM, with no such WNM components detected for sixteen of the thirty sightlines. We find that some of the detected H i 21cm absorption along thirteen of these sightlines must arise in gas with spin temperatures larger than the CNM range. For these sightlines, we use very conservative estimates of the CNM spin temperature and the non-thermal broadening to derive strict upper limits to the gas column densities in the CNM and WNM phases. Comparing these upper limits to the total H i column density, we find that typically at least 28% of the gas must have temperatures in the thermally unstable range (200 − 5000 K). Our observations hence robustly indicate that a significant fraction of the gas in the Galactic interstellar medium has temperatures outside the ranges expected for thermally stable gas in two-phase models.
Radio H I and optical absorption-line spectra of an intermediate-velocity cloud in the general direction of the M15 globular cluster
Monthly Notices of The Royal Astronomical Society, 1998
Using H I spectra obtained with the Lovell telescope (FWHM ϳ 12 arcmin) we present maps showing the H I distribution and velocity structure of an intermediate-velocity cloud (IVC; v LSR ϳ 70 km s ¹1 ) which is observed in the general direction of the globular cluster M15. The gas is shown to be clumpy in nature and we examine its position and velocity structure.
GASS: THE PARKES GALACTIC ALL-SKY SURVEY. I. SURVEY DESCRIPTION, GOALS, AND INITIAL DATA RELEASE
The Astrophysical Journal Supplement Series, 2009
Loading Preview
Sorry, preview is currently unavailable. You can download the paper by clicking the button above.