HI 21-cm Emission Research Articles

An Image-based Tapered Gridded Estimator (ITGE) for the angular power spectrum

We present the Image-based Tapered Gridded Estimator (ITGE) to measure the angular power spectrum $(C_{\ell})$ of the sky signal directly from the visibilities measured in radio-interferometric observations. The ITGE allows us to modulate the sky response through a window function which is implemented in the image plane, and it is possible to choose a wide variety of window functions. In the context of the cosmological HI 21-cm signal, this is useful for masking out the sky signal from specific directions which have strong residual foregrounds. In the context of the ISM in external galaxies, this is useful to separately estimate the $C_{\ell}$ of different parts of the galaxy. The ITGE deals with gridded data, hence it is computationally efficient. It also calculates the noise bias internally and exactly subtracts this out to give an unbiased estimate of $C_{\ell}$. We validate the ITGE using realistic VLA simulations at $1.4 {\rm GHz}$. We have applied the ITGE to estimate the $C_{\ell}$ of HI 21-cm emission from different regions of the galaxy NGC 628. We find that the slope of the measured $C_{\ell}$ in the outer region is significantly different as compared with the inner region. This indicates that the statistical properties of ISM turbulence possibly differ in different regions of the galaxy.

Revealing H i gas in emission and absorption on pc to kpc scales in a galaxy at z ∼ 0.017

We present a detailed study of the quasar-galaxy pair: J1243+4043 - UGC07904. The sight line of the background quasar ( $z_q$= 1.5266) passes through a region of the galaxy ($z_g$=0.0169) at an impact parameter of 6.9 kpc with high metallicity (0.5 Z$_\odot$) and negligible dust extinction. We detect HI 21-cm absorption from the foreground galaxy at arcsecond and milliarcsecond scales. For typical cold neutral medium (CNM) temperatures in the Milky Way, this 21-cm absorber can be classified as a damped Ly$\alpha$ absorber (DLA). We infer the harmonic mean spin temperature of the gas to be $\sim$400 K and for a simple two-phase medium we estimate the CNM-fraction to be $f_{\rm CNM}$ = 0.27. This is remarkably consistent with the CNM fraction observed in the Galaxy and less than that of high-redshift DLAs. The quasar exhibits a core-jet morphology on milliarcsecond scales, corresponding to an overall extent of $\sim$9 pc at $z_g$. We show that the size of CNM absorbing clouds associated with the foreground galaxy is $>$5 pc and they may be part of cold gas structures that extend beyond $\sim$35 pc. Interestingly, the rotation measure of quasar J1243+4043 is higher than any other source in samples of quasars with high-$z$ DLAs. However, we do not find any detectable differences in RMs and polarization fraction of sight lines with or without high-$z$ ($z\ge2$) DLAs or low-$z$ ($z\le0.3$) 21-cm absorbers. Finally, the foreground galaxy UGC07904 is also part of a galaxy group. We serendipitously detect HI 21-cm emission from four members of the group, and a $\sim$80 kpc long HI bridge connecting two of the other members. The latter, together with the properties of the group members, suggests that the group is a highly interactive environment.

The HI Distribution Observed toward a Halo Region of the Milky Way

We use observations of the neutral atomic hydrogen (HI) 21-cm emission line to study the spatial distribution of the HI gas in a 80° × 90° region of the Galaxy halo. The HI column densities in the range of 3–11 × 10 20 cm − 2 have been estimated for some of the studied regions. In our map—obtained with a spectral sensitivity of ∼2 K—we do not detect any HI 21-cm emission line above 2 σ at Galactic latitudes higher than ∼46°. This report summarizes our contribution presented at the conference on the origin and evolution of barionic Galaxy halos.

Simulated predictions for H i at z = 3.35 with the Ooty Wide Field Array – I. Instrument and the foregrounds

Foreground removal is the most important step in detecting the large-scale redshifted HI 21-cm signal. Modelling foreground spectra is challenging and is further complicated by the chromatic response of the telescope. We present a multi-frequency angular power spectrum (MAPS) estimator for use in a survey for redshifted HI 21-cm emission from z~3.35, and demonstrate its ability to accurately characterize the foregrounds. This survey will be carried out with the two wide-field interferometer modes of the upgraded Ooty Radio Telescope, called the Ooty Wide Field Array (OWFA), at 326.5 MHz. We have tailored the two-visibility correlation for OWFA to estimate the MAPS and test it with simulated foregrounds. In the process, we describe a software model that encodes the geometry and the details of the telescope, and simulates a realistic model for the bright radio sky. This article presents simulations which include the full chromatic response of the telescope, in addition to the frequency dependence intrinsic to the foregrounds. We find that the visibility correlation MAPS estimator recovers the input angular power spectrum accurately, and that the instrument response to the foregrounds dominates the systematic errors in the recovered foreground power spectra.

Spatially resolved star formation relation in two HI-rich galaxies with central post-starburst signature

E+A galaxies are post-starburst systems that are identified from their optical spectra. These galaxies contain a substantial young A-type stellar component, but have only little ongoing star formation (SF). HI 21-cm line emission is found in approximately half of the nearby E+A galaxies, indicating that they contain a reservoir of gas that could fuel active SF. Here, we study two HI-rich galaxies, which show a typical E+A spectrum at the centre and SF at larger radii. We present new high spatial resolution radio interferometric observations of the HI 21-cm emission line using the VLA and of the CO(1-0) emission line using ALMA. We combine these data sets to predict the SFR and show that it does not correlate well with the SFR derived from H alpha on sub-kpc scales. We apply a recently developed statistical model for the small scale behaviour of the SF relation to predict and interpret the observed scatter. We find smoothly distributed, regularly rotating HI gas. The CO(1-0) emission line is not detected for both galaxies. The derived upper limit on the CO mass implies a molecular gas depletion time shorter than 20 Myr. However, due to the low metallicity, the CO-to-H2 conversion factor is highly uncertain. In the relations between the H alpha-based SFR and the HI mass, we observe a substantial scatter we demonstrate results from small-number statistics of independent star-forming regions on sub-kpc scales. This finding adds to the existing literature reporting a scale dependence of the molecular SF relation, showing that the atomic and molecular phases are both susceptible to the evolutionary 'cycling' of individual regions. This suggests that the atomic gas reservoirs host substantial substructure, which should be observable with future high-resolution observations. (abridged)

Giant Metrewave Radio Telescope observations of neutral atomic hydrogen gas in the COSMOS field atz∼ 0.37

We present the results of HI spectral stacking analysis of Giant Metrewave Radio Telescope (GMRT) observations targeting the COSMOS field. The GMRT data cube contains 474 field galaxies with redshifts known from the zCOSMOS-bright 10k catalogue. Spectra for the galaxies are co-added and the stacked spectrum allows us to make a $\sim 3\sigma$ measurement of the average HI mass. Using this average HI mass along with the integral optical $B$-band luminosity of the galaxies and the luminosity density of the COSMOS field, a volume normalisation is applied to obtain the cosmic HI mass density ($\Omega_{\rm HI}$). We find a cosmic HI mass density of $\Omega_{\rm HI}$ = (0.42 $\pm$ 0.16) $\times$ 10$^{-3}$ at $z \sim 0.37$, which is the highest-redshift measurement of $\Omega_{\rm HI}$ ever made using HI spectral stacking. The value we obtained for $\Omega_{\rm HI}$ at $z \sim 0.37$ is consistent with that measured from large blind 21-cm surveys at $z = 0$ as well as measurements from other HI stacking experiments at lower redshifts. Our measurement in conjunction with earlier measurements indicates that there has been no significant evolution of HI gas abundance over the last 4 Gyr. A weighted mean of $\Omega_{\rm HI}$ from all 21-cm measurements at redshifts $z \lesssim 0.4$ gives $\Omega_{\rm HI}$ = (0.35 $\pm$ 0.01) $\times$ 10$^{-3}$. The $\Omega_{\rm HI}$ measured (from HI 21-cm emission measurements) at $z \lesssim 0.4$ is however approximately half that measured from Damped Lyman-$\alpha$ Absorption (DLA) systems at $z \gtrsim 2$. Deeper surveys with existing and upcoming instruments will be critical to understand the evolution of $\Omega_{\rm HI}$ in the redshift range intermediate between $z \sim 0.4$ and the range probed by DLA observations.

Mapping kiloparsec-scale structures in the extended H i disc of the galaxy UGC 000439 by H i 21-cm absorption

We study the properties of HI gas in the outer regions (~2r_25) of a spiral galaxy, UGC 00439 (z = 0.01769), using HI 21-cm absorption towards different components of an extended background radio source, J0041$-$0043 (z = 1.679). The radio source exhibits a compact core coincident with the optical quasar and two lobes separated by ~7 kpc, all at an impact parameter ~25 kpc. The HI 21-cm absorption detected towards the southern lobe is found to extend over ~2 kpc^2. The absorbing gas shows sub-kpc-scale structures with the line-of-sight velocities dominated by turbulent motions. Much larger optical depth variations over 4-7 kpc-scale are revealed by the non-detection of HI 21-cm absorption towards the radio core and the northern lobe, and the detection of NaI and CaII absorption towards the quasar. This could reflect a patchy distribution of cold gas in the extended HI disc. We also detect HI 21-cm emission from UGC 00439 and two other galaxies within ~150 kpc to it, that probably form an interacting group. However, no HI 21-cm emission from the absorbing gas is detected. Assuming a linear extent of ~4 kpc, as required to cover both the core and the southern lobe, we constrain the spin temperature <~300 K for the absorbing gas. The kinematics of the gas and the lack of signatures of any ongoing in situ star formation are consistent with the absorbing gas being at the kinematical minor axis and corotating with the galaxy. Deeper HI 21-cm observations would help to map in greater detail both the large- and small-scale structures in the HI gas associated with UGC 00439.

Integral field spectroscopy of two H i-rich E+A galaxies

Approximately half of the nearby E+A galaxies followed up with 21-cm observations have detectable HI emission. The optical spectra of these galaxies show strong post-starburst stellar populations but no optical emission lines implying star-formation is not ongoing despite the presence of significant gas reservoirs. We have obtained integral field spectroscopic follow up observations of the two brightest, and nearest, of the six E+A galaxies with HI 21-cm emission in the recent sample of Zwaan et al. (2013). In the central regions of both galaxies the observations are consistent with a post-starburst population with little emission. However, outside the central regions both galaxies have strong optical emission lines, with a clumpy or knot-like distribution, indicating ongoing star-formation. We conclude that in these two cases the presence of optical spectra lacking evidence for star-formation while a large gas mass is present can be explained by an aperture effect in selecting the nearby E+A galaxies using single-fibre spectroscopy that probes only the galaxy core.

The cold gas content of post-starburst galaxies

Post-starburst galaxies, or E+A galaxies, are characterized by optical spectra showing strong Balmer absorption lines, indicating a young stellar population, and little or no emission lines, implying no active star formation. These galaxies are interpreted as a transitional population between star-forming, disk-dominated galaxies and spheroidal quiescent, non-star forming galaxies. Here, we present single dish HI 21-cm emission line measurements of a sample of eleven of these galaxies at redshifts z<0.05. We detect H I emission in six of the E+A galaxies. In combination with earlier studies, the total number of E+A galaxies with measured cold gas components is now eleven. Roughly half of the E+As studied so far have detectable HI. The gas fractions of these galaxies, measured with respect to their stellar mass, are between 1 and 10 percent and are at the high end of the gas fractions measured in gas-bearing early type galaxies and typically lower than seen in late-type galaxies with comparable stellar masses. This finding is consistent with the idea that E+As are currently evolving from the blue cloud to the red sequence. However, the question of why the star formation has ceased in these galaxies while a significant gas reservoir is still present can only be answered by higher spatial resolution observations of the cold gas.

An absorption-selected survey of neutral gas in the Milky Way halo

We aim at analysing systematically the distribution and physical properties of neutral and mildly ionised gas in the Milky Way halo, based on a large absorption-selected data set. Multi-wavelength studies were performed combining optical absorption line data of CaII and NaI with follow-up HI 21-cm emission line observations along 408 sight lines towards low- and high-redshift QSOs. We made use of archival optical spectra obtained with UVES/VLT. HI data were extracted from the Effelsberg-Bonn HI survey and the Galactic All-Sky survey. For selected sight lines we obtained deeper follow-up observations using the Effelsberg 100-m telescope. CaII (NaI) halo absorbers at intermediate and high radial velocities are present in 40-55% (20-35%) of the sightlines, depending on the column density threshold chosen. Many halo absorbers show multi-component absorption lines, indicating the presence of sub-structure. In 65% of the cases, absorption is associated with HI 21-cm emission. The CaII (NaI) column density distribution function follows a power-law with a slope of -2.2 (-1.4). Our absorption-selected survey confirms our previous results that the Milky Way halo is filled with a large number of neutral gas structures whose high column density tail represents the population of common HI high- and intermediate-velocity clouds seen in 21-cm observations. We find that CaII/NaI column density ratios in the halo absorbers are typically smaller than those in the Milky Way disc, in the gas in the Magellanic Clouds, and in damped Lyman-alpha systems. The small ratios (prominent in particular in high-velocity components) indicate a lower level of Ca depletion onto dust grains in Milky Way halo absorbers compared to gas in discs and inner regions of galaxies.

ORIGIN(S) OF THE HIGHLY IONIZED HIGH-VELOCITY CLOUDS BASED ON THEIR DISTANCES

Previous HST and FUSE observations have revealed highly ionized high-velocity clouds (HVCs) or more generally low HI column HVCs along extragalactic sightlines over 70-90% of the sky. The distances of these HVCs have remained largely unknown hampering to distinguish a "Galactic" origin (e.g., outflow, inflow) from a "Local Group" origin (e.g., warm-hot intergalactic medium). We present the first detection of highly ionized HVCs in the Cosmic Origins Spectrograph (COS) spectrum of the early-type star HS1914+7134 (l = 103, b=+24) located in the outer region of the Galaxy at 14.9 kpc. Two HVCs are detected in absorption at v_LSR = -118 and -180 km/s in several species, including CIV, SiIV, SiIII, CII, SiII, OI, but HI 21-cm emission is only seen at -118 \km. Within 17 degrees of HS1914+7134, we found HVC absorption of low and high ions at similar velocities toward 5 extragalactic sight lines, suggesting that these HVCs are related. The component at -118 km/s is likely associated with the Outer Arm of the Milky Way. The highly ionized HVC at -180 km/s is an HVC plunging at high speed onto the thick disk of the Milky Way. This is the second detection of highly ionized HVCs toward Galactic stars, supporting a "Galactic" origin for at least some of these clouds.

The H i gas content of galaxies around Abell 370, a galaxy cluster atz= 0.37

We used observations from the Giant Metrewave Radio Telescope to measure the atomic hydrogen gas content of 324 galaxies around the galaxy cluster Abell 370 at a redshift of z = 0.37 (a look-back time of ~4 billion years). The HI 21-cm emission from these galaxies was measured by coadding their signals using precise optical redshifts obtained with the Anglo-Australian Telescope. The average HI mass measured for all 324 galaxies is (6.6 +- 3.5)x10^9 solar masses, while the average HI mass measured for the 105 optically blue galaxies is (19.0 +- 6.5)x10^9 solar masses. The significant quantities of gas found around Abell 370, suggest that there has been substantial evolution in the gas content of galaxy clusters since redshift z = 0.37. The total amount of HI gas found around Abell 370 is up to ~8 times more than that seen around the Coma cluster, a nearby galaxy cluster of similar size. Despite this higher gas content, Abell 370 shows the same trend as nearby clusters, that galaxies close to the cluster core have lower HI gas content than galaxies further away. The Abell 370 galaxies have HI mass to optical light ratios similar to local galaxy samples and have the same correlation between their star formation rate and HI mass as found in nearby galaxies. The average star formation rate derived from [OII] emission and from de-redshifted 1.4 GHz radio continuum for the Abell 370 galaxies also follows the correlation found in the local universe. The large amounts of HI gas found around the cluster can easily be consumed by the observed star formation rate in the galaxies over the ~4 billion years (from z = 0.37) to the present day.

Observational Constraints on Cosmic Reionization

Observations have set the first constraints on the epoch of reionization (EoR), corresponding to the formation epoch of the first luminous objects. Studies of Gunn-Peterson (GP) absorption indicate a rapid increase in the neutral fraction of the intergalactic medium (IGM) from xHI&lt; 10−4at z ≤ 5.5, to xHI&gt; 10−3, perhaps up to 0.1, at z∼6, while the large scale polarization of the cosmic microwave background (CMB) implies a significant ionization fraction extending to higher redshifts, z∼11 ± 3. These results, as well as observations of galaxy populations, suggest that reionization is a process that begins as early as z∼14, and ends with the “percolation” phase at z∼6 to 8. Low luminosity star-forming galaxies are likely the dominant sources of reionizing photons. Future low-frequency radio telescopes will make direct measurements of HI 21-cm emission from the neutral IGM during the EoR, and measurements of secondary CMB temperature anisotropy will provide details of the dynamics of the reionized IGM.

ChandraDetection of Local Warm‐Hot Gas toward Markarian 279

We report the Chandra detection of OVII K\alpha absorption at z=0 in the direction of the z=0.03 Seyfert 1 galaxy Mkn 279. The high velocity cloud Complex C lies along this line of sight, with HI 21-cm emission and OVI 1032\AA absorption both observed at velocities of ~ -150 km/s relative to the local standard of rest. We present an improved method for placing limits on the Doppler parameter and column density of a medium when only one unresolved line can be measured; this method is applied to the OVII absorption seen here, indicating that the OVII Doppler parameter is inconsistent with that of any low-velocity (Galactic thick disk) or high-velocity OVI (OVI_HV) component. Direct association of the OVII with the OVI_HV is further ruled out by the high temperatures required to produce the observed OVI_HV/OVII ratio and the significant velocity difference between the OVII and OVI_HV lines. If the OVII absorption is associated with a very broad, undetected OVI component, then the absorption must be broadened by primarily nonthermal processes. The large velocity dispersion and possible slight redshift of the OVII absorption (as well as limits on the absorber's temperature and density) may be indicative of a local intergalactic medium origin, though absorption from a hot, low-density Galactic corona cannot be ruled out.

The HIPASS catalogue - II. Completeness, reliability and parameter accuracy

The HI Parkes All Sky Survey (HIPASS) is a blind extragalactic HI 21-cm emission line survey covering the whole southern sky from declination -90 to +25. The HIPASS catalogue (HICAT), containing 4315 HI-selected galaxies from the region south of declination +2, is presented in Meyer et al. (2004a, Paper I). This paper describes in detail the completeness and reliability of HICAT, which are calculated from the recovery rate of synthetic sources and follow-up observations, respectively. HICAT is found to be 99 per cent complete at a peak flux of 84 mJy and an integrated flux of 9.4 Jy km/s. The overall reliability is 95 per cent, but rises to 99 per cent for sources with peak fluxes >58 mJy or integrated flux > 8.2 Jy km/s. Expressions are derived for the uncertainties on the most important HICAT parameters: peak flux, integrated flux, velocity width, and recessional velocity. The errors on HICAT parameters are dominated by the noise in the HIPASS data, rather than by the parametrization procedure.

A Modern Look at ‘Interstellar Clouds’

We compare past and present modes of investigation of the structure of the interstellar gas. Many aspects of the interstellar cloud model are invalid.Interstellar optical absorption lines and Hi21-cm emission lines show a number of very large aggregates with properties similar to those of ‘cloud complexes’. At nonzero velocities especially for b&lt;0°, exist optical lines which have no Hicounterparts. These are almost certainly produced in low-density gas clouds; perhaps the intercloud medium is itself cloudy.Maps of Hicolumn density taken over large velocity ranges do not reveal much small-scale structure. This fact cannot easily be reconciled with the statistical analyses of interstellar reddening. The maps do reveal large, coherent gas structures which are often filamentary in shape and at least sometimes aligned parallel to the interstellar magnetic field.Maps of Hicolumn density over small velocity ranges show much small-scale structure, often filamentary in shape. The filaments are almost universally oriented parallel to the interstellar magnetic field and have Doppler velocity gradients along their lengths. In one area the geometry of the field and gas almost exclusively suggests Alfvén-type motions.