scholarly journals H i deficiencies and asymmetries in HIPASS galaxies

2020 ◽  
Vol 499 (3) ◽  
pp. 3233-3242
Author(s):  
T N Reynolds ◽  
T Westmeier ◽  
L Staveley-Smith
Keyword(s):  
Neutral Hydrogen ◽  
Significant Trend ◽  
Small Influence ◽  
Sky Survey ◽  
Spectral Asymmetry ◽  
The Mean

ABSTRACT We present an analysis of the sky distribution of neutral hydrogen (H i) deficiency and spectral asymmetry for galaxies detected by the H i Parkes All-Sky Survey (HIPASS) as a function of projected environment density. Previous studies of galaxy H i deficiency using HIPASS were sensitive to galaxies that are extremely H i rich or poor. We use an updated binning statistic for measuring the global sky distribution of H i deficiency that is sensitive to the average deficiencies. Our analysis confirms the result from previous studies that galaxies residing in denser environments, such as Virgo, are on average more H i deficient than galaxies at lower densities. However, many other individual groups and clusters are not found to be on average significantly H i poor, in contradiction to previous work. In terms of H i spectral asymmetries, we do not recover any significant trend of increasing asymmetry with environment density as found for other galaxy samples. We also investigate the correlation between H i asymmetry and deficiency, but find no variation in the mean asymmetry of galaxies that are H i rich, normal, or poor. This indicates that there is either no dependence of asymmetry on H i deficiency, or a galaxy’s H i deficiency only has a small influence on the measured H i asymmetry that we are unable to observe using only integrated spectra.

Southern Hemisphere Seti Survey: Five Years of Project Meta II

1997 ◽  
Vol 161 ◽  
pp. 611-621
Author(s):  
Guillermo A. Lemarchand ◽  
Fernando R. Colomb ◽  
E. Eduardo Hurrell ◽  
Juan Carlos Olalde
Keyword(s):  
Southern Hemisphere ◽  
Scattering Theory ◽  
Galactic Plane ◽  
Narrow Band ◽  
Neutral Hydrogen ◽  
Sky Survey ◽  
Inertial Frames ◽  
Interstellar Scattering ◽  
Extraterrestrial Origin ◽  
Spectral Channels

AbstractProject META II, a full sky survey for artificial narrow-band signals, has been conducted from one of the two 30-m radiotelescopes of the Instituto Argentino de Radioastronomía (IAR). The search was performed near the 1420 Mhz line of neutral hydrogen, using a 8.4 million channels Fourier spectrometer of 0.05 Hz resolution and 400 kHz instantaneous bandwidth. The observing frequency was corrected both for motions with respect to three astronomical inertial frames, and for the effect of Earths rotation, which provides a characteristic changing signature for narrow-band signals of extraterrestrial origin. Among the 2 × 1013spectral channels analyzed, 29 extra-statistical narrow-band events were found, exceeding the average threshold of 1.7 × 10−23Wm−2. The strongest signals that survive culling for terrestrial interference lie in or near the galactic plane. A description of the project META II observing scheme and results is made as well as the possible interpretation of the results using the Cordes-Lazio-Sagan model based in interstellar scattering theory.

scholarly journals Demonstrating the Tapered Gridded Estimator (TGE) for the Cosmological HI 21-cm Power Spectrum using 150 MHz GMRT observations

2020 ◽  
Author(s):  
Srijita Pal ◽  
Somnath Bharadwaj ◽  
Abhik Ghosh ◽  
Samir Choudhuri
Keyword(s):  
Power Spectrum ◽  
Unbiased Estimate ◽  
Power Spectra ◽  
Neutral Hydrogen ◽  
Statistical Error ◽  
Temperature Fluctuations ◽  
Radio Frequency Interference ◽  
Rectangular Region ◽  
Angular Power Spectrum ◽  
The Mean

Abstract We apply the Tapered Gridded Estimator (TGE) for estimating the cosmological 21-cm power spectrum from 150 MHz GMRT observations which corresponds to the neutral hydrogen (HI) at redshift z = 8.28. Here TGE is used to measure the Multi-frequency Angular Power Spectrum (MAPS) Cℓ(Δν) first, from which we estimate the 21-cm power spectrum P(k⊥, k∥). The data here are much too small for a detection, and the aim is to demonstrate the capabilities of the estimator. We find that the estimated power spectrum is consistent with the expected foreground and noise behaviour. This demonstrates that this estimator correctly estimates the noise bias and subtracts this out to yield an unbiased estimate of the power spectrum. More than $47\%$ of the frequency channels had to be discarded from the data owing to radio-frequency interference, however the estimated power spectrum does not show any artifacts due to missing channels. Finally, we show that it is possible to suppress the foreground contribution by tapering the sky response at large angular separations from the phase center. We combine the k modes within a rectangular region in the ‘EoR window’ to obtain the spherically binned averaged dimensionless power spectra Δ2(k) along with the statistical error σ associated with the measured Δ2(k). The lowest k-bin yields Δ2(k) = (61.47)2 K2 at k = 1.59 Mpc−1, with σ = (27.40)2 K2. We obtain a 2 σ upper limit of (72.66)2 K2 on the mean squared HI 21-cm brightness temperature fluctuations at k = 1.59 Mpc−1.

scholarly journals Non-Local Origin of a Substantial Portion of the Soft X-Ray Background

1984 ◽  
Vol 81 ◽  
pp. 204-210
Author(s):  
George W. Clark
Keyword(s):  
Column Density ◽  
Angular Resolution ◽  
General Trend ◽  
Neutral Hydrogen ◽  
Substantial Portion ◽  
X Ray ◽  
Local Origin ◽  
Sky Survey ◽  
Non Local ◽  
X Ray Background

Comparison of the SAS-3 soft X-ray sky survey (F. Marshall and G. Clark 1984) with the 21-cm neutral hydrogen survey of Stark et al. (1984) confirms the well-known anticorrelation between the counting rates in the C-band (0.10-0.28 keV) and the column density of neutral hydrogen, and demonstrates that this anticorrelation is significant on all angular scales ranging from that of the general trend from the galactic equator to the poles down to the angular resolution of the detector (2°.7 FWHM). Included in this general anticorrelation are numerous instances of what appear to be soft X-ray “shadows” of nearby (100-300 pc) 21-cm features, and several bright X-ray regions coincident with “holes” in the ISM.

scholarly journals X-ray Constraints on AGN Clustering

1989 ◽  
Vol 134 ◽  
pp. 492-493
Author(s):  
G. De Zotti ◽  
M. Persic ◽  
A. Franceschini ◽  
L. Danese ◽  
G.G.C. Palumbo ◽  
...  
Keyword(s):  
Space Distribution ◽  
Background Intensity ◽  
Additional Contribution ◽  
List Type ◽  
X Ray ◽  
Upper Limit ◽  
Angular Scale ◽  
Sky Survey ◽  
The Mean ◽  
X Ray Background

Studies of the HEAO–1 A2 all–sky survey data have established that the level of anisotropy of the extragalactic X–ray background (XRB) is relatively low: –The cell–to–cell XRB intensity variations can be entirely accounted for by Poisson fluctuations in the space distribution of known classes of sources; the 90% confidence upper limit to any additional contribution on a scale of 26 square degrees is 2.3% (Shafer and Fabian 1983).–No significant correlations of XRB intensity fluctuations appear to be present; the formal 90% confidence upper limit on the amplitude of autocorrelations, relative to the mean background intensity, for an angular scale of 3° is Γ(3°) ≤ 1.9 × 10−2 (Persic et al. 1988).

scholarly journals Population Synthesis of Old Neutron Stars in the Galaxy

2000 ◽  
Vol 195 ◽  
pp. 181-188
Author(s):  
S. B. Popov ◽  
M. Colpi ◽  
A. Treves ◽  
R. Turolla ◽  
V. M. Lipunov ◽  
...  
Keyword(s):  
Neutron Stars ◽  
Ambient Medium ◽  
Dynamical Evolution ◽  
Low Velocity ◽  
Mean Velocity ◽  
Secular Evolution ◽  
Sky Survey ◽  
The Galaxy ◽  
The Mean ◽  
Statistical Sample

The paucity of old, isolated accreting neutron stars in ROSAT observations is used to derive a lower limit on the mean velocity of neutron stars at birth. The secular evolution of the population is simulated following the paths of a statistical sample of stars for different values of the initial kick velocity, drawn from an isotropic, Gaussian distribution with mean velocity 0 ≤ 〈V〉 ≤ 550 km s−1. The spin-down, induced by dipole losses and the interaction with the ambient medium, is tracked together with the dynamical evolution in the Galactic potential, allowing for the determination of the fraction of stars which are, at present, in each of the four possible stages: Ejector, Propeller, Accretor, and Georotator. Taking from the ROSAT All-Sky Survey an upper limit of ~ 10 accreting neutron stars within ~ 140 pc from the Sun, we infer a lower bound for the mean kick velocity, 〈V〉 ≳ 200–300 km s−1. The same conclusion is reached for both a constant (B ~ 1012 G) and an exponentially decaying magnetic field with a timescale ~ 109 yr. Present results, moreover, constrain the fraction of low-velocity stars which could have escaped pulsar statistics to ≲ 1%.

scholarly journals The Low Surface Brightness Galaxy HIPASS1126-72

1999 ◽  
Vol 171 ◽  
pp. 204-206
Author(s):  
Virginia Kilborn ◽  
Erwin de Blok ◽  
Lister Staveley-Smith ◽  
Rachel Webster
Keyword(s):  
Surface Brightness ◽  
Neutral Hydrogen ◽  
Sky Survey ◽  
Low Surface Brightness ◽  
The Galaxy

AbstractThe low surface brightness galaxy HIPASS1126-72 was detected in the HI Parkes All Sky Survey (HIPASS). The galaxy was previously listed in the Southern Galaxy Catalogue under the name SGC1124.87221. This galaxy represents a class of galaxies that we will readily detect in the HIPASS survey, which have low surface brightness in the optical, but are easily detectable in neutral hydrogen.

scholarly journals High spin temperatures at large impact parameters: Ionisation in the outskirts of galaxies

2020 ◽  
Vol 635 ◽  
pp. A166
Author(s):  
S. J. Curran
Keyword(s):  
Temperature Distribution ◽  
High Resolution ◽  
Impact Parameter ◽  
Column Density ◽  
Milky Way ◽  
Spiral Galaxies ◽  
Neutral Hydrogen ◽  
Spin Temperature ◽  
The Mean ◽  
Impact Parameters

By including the most recent observations of H I 21-cm absorption through nearby galactic discs, we confirm our previous assertion that there is an anti-correlation between the abundance of cool neutral atomic gas and impact parameter. In comparing the measured neutral hydrogen column densities of the sample with the absorption strength, we find a peak in the mean spin temperature of ⟨Tspin/f ⟩ ≈ 2310 K at an impact parameter of ρ ≈ 14 kpc, with ⟨Tspin/f ⟩≳1000 K in the remainder of the disc. This is significantly different to the spin temperature distribution in the Milky Way, which exhibits a constant ≈250 − 400 K over ρ = 8 − 25 kpc. The measured column densities may, however, suffer from beam dilution, which we show appears to be the case for the observations of H I 21-cm emission in which the beam subtends radii of ≳10 kpc. We therefore applied the column density profile of the Milky Way, in addition to the mean of the sample, observed at sufficiently high resolution, and the mean profile for the nearby ∼1012 M⊙ galaxies in the IllustrisTNG simulations. All of the models yield a peak in the mean spin temperature at similar impact parameters (r ≈ 10 − 15 kpc) as the measured column densities. These radii are similar to those of the spiral arms where H II regions are often concentrated. We therefore suggest that the elevated spin temperatures trace the H II regions observed in the outer disc of many spiral galaxies.

scholarly journals Emulating the global 21-cm signal from Cosmic Dawn and Reionization

2020 ◽  
Vol 495 (4) ◽  
pp. 4845-4859 ◽  
Author(s):  
Aviad Cohen ◽  
Anastasia Fialkov ◽  
Rennan Barkana ◽  
Raul A Monsalve
Keyword(s):  
Neutral Hydrogen ◽  
Mean Free Path ◽  
Microwave Background ◽  
Star Forming ◽  
Global Signal ◽  
Wide Range ◽  
The Mean ◽  
Rapid Generation ◽  
Minimal Mass ◽  
Formation Efficiency

ABSTRACT The 21-cm signal of neutral hydrogen is a sensitive probe of the Epoch of Reionization (EoR), Cosmic Dawn, and the Dark Ages. Currently, operating radio telescopes have ushered in a data-driven era of 21-cm cosmology, providing the first constraints on the astrophysical properties of sources that drive this signal. However, extracting astrophysical information from the data is highly non-trivial and requires the rapid generation of theoretical templates over a wide range of astrophysical parameters. To this end emulators are often employed, with previous efforts focused on predicting the power spectrum. In this work, we introduce 21cmgem– the first emulator of the global 21-cm signal from Cosmic Dawn and the EoR. The smoothness of the output signal is guaranteed by design. We train neural networks to predict the cosmological signal using a database of ∼30 000 simulated signals which were created by varying seven astrophysical parameters: the star formation efficiency and the minimal mass of star-forming haloes; the efficiency of the first X-ray sources and their spectrum parametrized by spectral index and the low-energy cut-off; the mean-free path of ionizing photons, and the cosmic microwave background optical depth. We test the performance with a set of ∼2000 simulated signals, showing that the relative error in the prediction has an rms of 0.0159. The algorithm is efficient, with a running time per parameter set of 0.16 s. Finally, we use the database of models to check the robustness of relations between the features of the global signal and the astrophysical parameters that we previously reported.

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