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  • 1
    Electronic Resource
    Electronic Resource
    Discovery of radio emission from the brown dwarf LP944-20 (2001)
    [s.l.] : Macmillian Magazines Ltd.
    Nature 410 (2001), S. 338-340 
    Details
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] Brown dwarfs are not massive enough to sustain thermonuclear fusion of hydrogen at their centres, but are distinguished from gas-giant planets by their ability to burn deuterium. Brown dwarfs older than ∼10 Myr are expected to possess short-lived magnetic fields and to emit radio and ...
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1038/35066514
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    Paper (German National Licenses)
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  • 2
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    H I 21cm emission from the subdamped Lyman-{alpha} absorber at z = 0.0063 towards PG 1216+069 (2015)
    Oxford University Press
    Details
    Publication Date: 2015-09-05
    Description: We present H i  21 cm emission observations of the z  ~ 0.006 32 subdamped Lyman-α absorber (sub-DLA) towards PG 1216+069 made using the Arecibo Telescope and the Very Large Array (VLA). The Arecibo H i  21cm spectrum corresponds to an H i mass of ~3.2  x  10 7 M , two orders of magnitude smaller than that of a typical spiral galaxy. This is surprising since in the local Universe the cross-section for absorption at high H i column densities is expected to be dominated by spirals. The H i  21cm emission detected in the VLA spectral cube has a low signal-to-noise ratio, and represents only half the total flux seen at Arecibo. Emission from three other sources is detected in the VLA observations, with only one of these sources having an optical counterpart. This group of H i sources appears to be part of complex ‘W’, believed to lie in the background of the Virgo cluster. While several H i cloud complexes have been found in and around the Virgo cluster, it is unclear whether the ram pressure and galaxy harassment processes that are believed to be responsible for the creation of such clouds in a cluster environment are relevant at the location of this cloud complex. The extremely low metallicity of the gas, ~1/40 solar, also makes it unlikely that the sub-DLA consists of material that has been stripped from a galaxy. Thus, while our results have significantly improved our understanding of the host of this sub-DLA, the origin of the gas cloud remains a mystery.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 3
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    Circumnuclear and infalling H I gas in a merging galaxy pair at z = 0.123 (2015)
    Oxford University Press
    Details
    Publication Date: 2015-06-12
    Description: Using long-slit optical spectra obtained with the 2-m telescope at IUCAA Girawali Observatory, we show that the radio source J094221.98+062335.2 ( z  = 0.123) is associated with a galaxy pair undergoing a major merger. Its companion is a normal star-forming galaxy infalling with a velocity of 185 km s –1 at a projected separation of 4.8 kpc. Using the Westerbork Synthesis Radio Telescope (WSRT) and Giant Metrewave Radio Telescope (GMRT), we detect a strong H i 21-cm absorption at the systemic redshift of the radio galaxy with N (H i )~9 10 21  cm –2 for an assumed spin temperature of 100 K. Such a strong H i 21-cm absorption is rare and has been seen only in a few compact radio sources associated with similar merging galaxy pairs. Milliarcsecond resolution Very Long Baseline Array (VLBA) observations resolve the radio source into a compact symmetric object with the hotspot separation of 89 pc. The 21-cm absorption is detected in the VLBA spectra towards both the radio lobes albeit with a strong optical depth gradient. We show that the strong 21-cm absorption is consistent with it being arising from a clumpy circumnuclear disc/torus. We also detect two weaker absorption lines redshifted with respect to the radio source in the WSRT/GMRT spectrum. They probably represent cold (i.e. T  ≤ 10 4  K) H i gas falling into the radio source. The presence of high concentration of H i gas in the circumnuclear regions and signature of infalling cold gas allows us to conjecture that the young radio source may have been triggered by the gas infall due to the ongoing merger.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 4
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    The spin temperature of high-redshift damped Lyman {alpha} systems (2014)
    Oxford University Press
    Details
    Publication Date: 2014-02-15
    Description: We report results from a programme aimed at investigating the temperature of neutral gas in high-redshift damped Lyman α absorbers (DLAs). This involved (1) H i 21 cm absorption studies of a large sample of DLAs towards radio-loud quasars, (2) very long baseline interferometric studies to measure the low-frequency quasar core fractions, and (3) optical/ultraviolet spectroscopy to determine DLA metallicities and the velocity widths of low-ionization metal lines. Including literature data, our sample consists of 37 DLAs with estimates of the harmonic mean spin temperature T s . We find a statistically significant (4) difference between the T s distributions in the high- z ( z  〉 2.4) and low- z ( z  〈 2.4) DLA samples. The high- z sample contains more systems with high spin temperature, T s 1000 K. The T s distributions in DLAs and the Galaxy are also significantly (6) different, with more high- T s sightlines in DLAs than in the Milky Way. The high T s values in the high- z DLAs of our sample arise due to low fractions of the cold neutral medium (CNM). Only 2 of 23 DLAs at z  〉 1.7 have T s values indicating CNM fractions 〉20 per cent, comparable to the median value (27 per cent) in the Galaxy. We tested whether the H i column density measured towards the optical quasar might be systematically different from that towards the radio core by comparing the H i column densities inferred from H i 21 cm emission studies at different spatial resolutions (15 pc-1 kpc) in the Large Magellanic Cloud. The high-resolution N H i values are, on average, larger than the smoothed ones for N H i 〉 10 21 cm –2 , but lower than the smoothed N H i estimates for N H i 〈 10 21 cm –2 . Since there are far more DLAs with low N H i values than high ones, the use of the optical N H i value for the radio sightline results in a statistical tendency to underestimate DLA spin temperatures. For 29 DLAs with metallicity estimates, we confirm the presence of an anticorrelation between T s and metallicity [ Z /H], at 3.5 significance via a non-parametric Kendall-tau test. This result was obtained with the assumption that the DLA covering factor is equal to the core fraction. However, Monte Carlo simulations show that the significance of the result is only marginally decreased if the covering factor and the core fraction are uncorrelated, or if there is a random error in the inferred covering factor. We also find statistically significant evidence for redshift evolution in DLA spin temperatures even for the DLA sub-sample at z  〉 1. Since all DLAs at z  〉 1 have angular diameter distances comparable to or larger than those of their background quasars, they have similar efficiency in covering the quasars. We conclude that low covering factors in high- z DLAs cannot account for the observed redshift evolution in spin temperatures.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 5
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    J1649+2635: a grand-design spiral with a large double-lobed radio source (2014)
    Oxford University Press
    Details
    Publication Date: 2014-12-13
    Description: We report the discovery of a grand-design spiral galaxy associated with a double-lobed radio source. J1649+2635 ( z  = 0.0545) is a red spiral galaxy with a prominent bulge that it is associated with a L 1.4 GHz  ~ 10 24  W Hz –1 double-lobed radio source that spans almost 100 kpc. J1649+2635 has a black hole mass of M BH  ~ 3–7 10 8  M and SFR ~ 0.26–2.6 M  yr –1 . The galaxy hosts an ~96 kpc diffuse optical halo, which is unprecedented for spiral galaxies. We find that J1649+2635 resides in an overdense environment with a mass of $M_{\rm dyn} = 7.7^{+7.9}_{-4.3} \times 10^{13}$  M , likely a galaxy group below the detection threshold of the ROSAT All-Sky Survey. We suggest one possible scenario for the association of double-lobed radio emission from J1649+2635 is that the source may be similar to a Seyfert galaxy, located in a denser-than-normal environment. The study of spiral galaxies that host large-scale radio emission is important because although rare in the local Universe, these sources may be more common at high redshifts.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 6
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    Cold parsec-scale gas in a zabs ~ 0.1 sub-damped Lyman {alpha} with disparate H2 and 21-cm absorption (2015)
    Oxford University Press
    Details
    Publication Date: 2015-03-16
    Description: We present a detailed analysis of a H 2 -bearing metal-rich sub-damped Lyman α system at z abs  = 0.10115 towards the radio-loud quasar J0441–4313, at a projected separation of ~7.6 kpc from a star-forming galaxy. The H 2 , ${\rm C\,{\small \rm I}}$ and ${\rm Na\,{\small \rm I}}$ absorption are much stronger in the redder of the two components seen in the Hubble Space Telescope /Cosmic Origins Spectrograph spectrum. The best single-component fit to the strong H 2 component gives log  N (H 2 ) = 16.61 ± 0.05. However, possible hidden saturation in the medium-resolution spectrum can allow for log  N (H 2 ) to be as high as 18.9. The rotational excitation temperature of H 2 in this component is 133 $^{+33}_{-22}$  K. Photoionization models suggest 30–80 per cent of the total N (H i ) is associated with the strong H 2 component that has a density ≤100 cm –3 and is subject to a radiation field that is ≤0.5 times the Galactic mean field. The Very Long Baseline Array 1.4 GHz continuum image of the radio source contains only 27 per cent of the arcsecond scale emission. Using a previously published spectrum, no 21-cm absorption is found to be associated with the strong H 2 component. This suggests that either the N (H i )) associated with this component is ≤50 per cent of the total N (H i )) or the gas covering factor is ≤0.27. This is consistent with the results of the photoionization model that uses ultraviolet radiation due to stars in the associated galaxy. The 21-cm absorption previously reported from the weaker H 2 component suggests a spin temperature of ≤90 K, at odds with the weakness of H 2 , ${\rm C\,{\small \rm I}}$ and ${\rm Na\,{\small \rm I}}$ absorption in this component. From the inferred physical and chemical conditions, we suggest that the gas may be tracing a recent metal-rich outflow from the host galaxy.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 7
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    Galaxy pairs in the Sloan Digital Sky Survey - X. Does gas content alter star formation rate enhancement in galaxy interactions? (2015)
    Oxford University Press
    Details
    Publication Date: 2015-04-12
    Description: New spectral line observations, obtained with the Jansky Very Large Array (VLA), of a sample of 34 galaxies in 17 close pairs are presented in this paper. The sample of galaxy pairs is selected to contain galaxies in close, major interactions (i.e. projected separations 〈30 $h_{70}^{-1}$ kpc, and mass ratios less extreme than 4:1), while still having a sufficiently large angular separation that the VLA can spatially resolve both galaxies in the pair. Of the 34 galaxies, 17 are detected at 〉3. We compare the H i gas fraction of the galaxies with the triggered star formation present in that galaxy. When compared to the star formation rates (SFRs) of non-pair galaxies matched in mass, redshift, and local environment, we find that the star formation enhancement is weakly positively correlated (~2.5) with H i gas fraction. In order to help understand the physical mechanisms driving this weak correlation, we also present results from a small suite of binary galaxy merger simulations with varying gas fractions. The simulated galaxies indicate that larger initial gas fractions are associated with lower levels of interaction-triggered star formation (relative to an identical galaxy in isolation), but also show that high gas fraction galaxies have higher absolute SFRs prior to an interaction. We show that when interaction-driven SFR enhancements are calculated relative to a galaxy with an average gas fraction for its stellar mass, the relationship between SFR and initial gas fraction dominates over the SFR enhancements driven by the interaction. Simulated galaxy interactions that are matched in stellar mass but not in gas fraction, like our VLA sample, yield the same general positive correlation between SFR enhancement and gas fraction that we observe.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 8
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    Circumnuclear and infalling H I gas in a merging galaxy pair at z = 0.123 (2015)
    Oxford University Press
    Details
    Publication Date: 2015-06-03
    Description: Using long-slit optical spectra obtained with the 2-m telescope at IUCAA Girawali Observatory, we show that the radio source J094221.98+062335.2 ( z  = 0.123) is associated with a galaxy pair undergoing a major merger. Its companion is a normal star-forming galaxy infalling with a velocity of 185 km s –1 at a projected separation of 4.8 kpc. Using the Westerbork Synthesis Radio Telescope (WSRT) and Giant Metrewave Radio Telescope (GMRT), we detect a strong H i 21-cm absorption at the systemic redshift of the radio galaxy with N (H i )~9 10 21  cm –2 for an assumed spin temperature of 100 K. Such a strong H i 21-cm absorption is rare and has been seen only in a few compact radio sources associated with similar merging galaxy pairs. Milliarcsecond resolution Very Long Baseline Array (VLBA) observations resolve the radio source into a compact symmetric object with the hotspot separation of 89 pc. The 21-cm absorption is detected in the VLBA spectra towards both the radio lobes albeit with a strong optical depth gradient. We show that the strong 21-cm absorption is consistent with it being arising from a clumpy circumnuclear disc/torus. We also detect two weaker absorption lines redshifted with respect to the radio source in the WSRT/GMRT spectrum. They probably represent cold (i.e. T  ≤ 10 4  K) H i gas falling into the radio source. The presence of high concentration of H i gas in the circumnuclear regions and signature of infalling cold gas allows us to conjecture that the young radio source may have been triggered by the gas infall due to the ongoing merger.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 9
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    The Arecibo Galaxy Environment Survey IX: the isolated galaxy sample (2015)
    Oxford University Press
    Details
    Publication Date: 2015-12-02
    Description: We have used the Arecibo L -band Feed Array (ALFA) to map three regions, each of 5 deg 2 , around the isolated galaxies NGC 1156, UGC 2082, and NGC 5523. In the vicinity of these galaxies we have detected two dwarf companions: one near UGC 2082, previously discovered by the Arecibo Legacy Fast ALFA (ALFALFA) survey, and one near NGC 1156, discovered by this project and reported in an earlier paper. This is significantly fewer than the $15.4^{+1.7}_{-1.5}$ that would be expected from the field H i mass function from ALFALFA or the 8.9 ± 1.2 expected if the H i mass function from the Local Group applied in these regions. The number of dwarf companions detected is, however, consistent with a flat or declining H i mass function as seen by a previous, shallower, H i search for companions to isolated galaxies. We attribute this difference in H i mass functions to the different environments in which they are measured. This agrees with the general observation that lower ratios of dwarf to giant galaxies are found in lower density environments.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 10
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    H I 21-cm absorption survey of quasar-galaxy pairs: distribution of cold gas around z 〈 0.4 galaxies (2016)
    Oxford University Press
    Details
    Publication Date: 2016-12-01
    Description: We present the results from our survey of H  i 21-cm absorption, using Giant Metrewave Radio Telescope, Very Large Array and Westerbork Radio Synthesis Telescope, in a sample of 55 z  〈 0.4 galaxies towards radio sources with impact parameters ( b ) in the range ~0–35 kpc. In our primary sample (defined for statistical analyses) of 40 quasar-galaxy pairs, probed by 45 sightlines, we have found seven H  i 21-cm absorption detections, two of which are reported here for the first time. Combining our primary sample with measurements having similar optical depth sensitivity (d v ≤ 0.3 km s –1 ) from the literature, we find a weak anti-correlation (rank correlation coefficient = –0.20 at 2.42 level) between d v and b , consistent with previous literature results. The covering factor of H  i 21-cm absorbers ( C 21 ) is estimated to be 0.24 $^{+0.12}_{-0.08}$ at b ≤ 15 kpc and 0.06 $^{+0.09}_{-0.04}$ at b  = 15–35 kpc. d v and C 21 show similar declining trend with radial distance along the galaxy's major axis and distances scaled with the effective H  i radius. There is also tentative indication that most of the H  i 21-cm absorbers could be co-planar with the extended H  i discs. No significant dependence of d v and C 21 on galaxy luminosity, stellar mass, colour and star formation rate is found, though the H  i 21-cm absorbing gas cross-section may be larger for the luminous galaxies. The higher detection rate (by a factor of ~4) of H  i 21-cm absorption in z  〈 1 damped Lyman-α systems compared to the quasar-galaxy pairs indicates towards small covering factor and patchy distribution of cold gas clouds around low- z galaxies.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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