The Geometry Effects of an Expanding Universe on the Detection of Cool Neutral Gas at High Redshift

Monthly Notices of the Royal Astronomical Society

2017

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We present a comprehensive analysis of the spin temperature/covering factor degeneracy, T spin /f , in damped Lyman-α absorption systems. By normalising the upper limits and including these via a survival analysis, there is, as previously claimed, an apparent increase in T spin /f with redshift at z abs > ∼ 1. However, when we account for the geometry effects of an expanding Universe, neglected by the previous studies, this increase in T spin at z abs > ∼ 1 is preceded by a decrease at z abs < ∼ 1. Using high resolution radio images of the background continuum sources, we can transform the T spin /f degeneracy to T spin /d 2 abs , where d abs is the projected linear size of the absorber. Again, there is no overall increase with redshift, although a dip at z abs ≈ 2 persists. Furthermore, we find d 2 abs /T spin to follow a similar variation with redshift as the star formation rate, ψ * . This suggests that, although the total hydrogen column density, N HI , shows little relation to ψ * , the fraction of the cold neutral medium, τ obs dv/N HI , may. Therefore, further efforts to link the neutral gas with the star formation history should also consider the cool component of the gas.

Section: Absorber-qso Alignmentmentioning

confidence: 98%

Section: Normalisation and Inclusion Of The Limitsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

A possible connection between the spin temperature of damped Lyman α absorption systems and star formation history

Monthly Notices of the Royal Astronomical Society

2017

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“…The feature was apparent in the averaged visibilities, 6 In order to normalise the limits, each has been re-sampled to the same spectral resolution (20 km s −1 , as in Fig. 3), which is used as the FWHM to obtain the integrated optical depth limit, thus giving the N H I f /T spin limit per channel (see Curran 2012). Allison et al (2012Allison et al ( , 2015; Yan et al (2012Yan et al ( , 2016; Geréb et al (2015); Srianand et al (2015); Aditya et al (2016Aditya et al ( , 2017.…”

Section: Emission Feature In 0114+074mentioning

confidence: 99%

“…(iv) The relative paucity of detections of H I 21-cm absorption in high redshift DLAs appears to be dominated by extrinsic line-ofsight effects, rather than by any intrinsic evolution (Curran 2012;Curran et al 2016b). …”

Section: Introductionmentioning

confidence: 97%

Atomic and molecular absorption in redshifted radio sources

Monthly Notices of the Royal Astronomical Society

Whiting

Allison

et al. 2017

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We report on a survey for associated H I 21-cm and OH 18-cm absorption with the Giant Metrewave Radio Telescope at redshifts z ≈ 0.2 − 0.4. Although the low redshift selection ensures that our targets are below the critical ultra-violet luminosity (L UV ∼ 10 23 W Hz −1 ), which is hypothesised to ionise all of the neutral gas in the host galaxy, we do not obtain any detections in the six sources searched. Analysing these in context of the previous surveys, in addition to the anti-correlation with the ultra-violet luminosity (ionising photon rate), we find a correlation between the strength of the absorption and the blue -near-infrared colour, as well as the radio-band turnover frequency. We believe that these are due to the photo-ionisation of the neutral gas, an obscured sight-line being more conducive to the presence of cold gas and the compact radio emission being better intercepted by the absorbing gas, maximising the flux coverage, respectively. Regarding the photo-ionisation, the compilation of the previous surveys increases the significance of the critical ionising photon rate, above which all of the gas in the host galaxy is hypothesised to be ionised (Q H I ≈ 3 × 10 56 sec −1 ), to > 5σ. This reaffirms that this is an ubiquitous effect, which has profound implications for the detection of neutral gas in these objects with the Square Kilometre Array.

Evolution of the cold gas fraction and the star formation history: Prospects with current and future radio facilities

Publ. Astron. Soc. Aust.

2018

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It has recently been shown that the abundance of cold neutral gas may follow a similar evolution as the star formation history. This is physically motivated, since stars form out of this component of the neutral gas and if the case, would resolve the longstanding issue that there is a clear disparity between the total abundance of neutral gas and star forming activity over the history of the Universe. Radio-band 21-cm absorption traces the cold gas and comparison with the Lyman-α absorption, which traces all of the gas, provides a measure of the cold gas fraction, or the spin temperature, Tspin. The recent study has shown that the spin temperature (degenerate with the ratio of the absorber/emitter extent) appears to be anti-correlated with the star formation density, ψ * , with 1/Tspin undergoing a similar steep evolution as ψ * over redshifts of 0 < ∼ z < ∼ 3, whereas the total neutral hydrogen exhibits little evolution. Above z ∼ 3, where ψ * shows a steep decline with redshift, there is insufficient 21-cm data to determine whether 1/Tspin continues to follow ψ * . Knowing this is paramount in ascertaining whether the cold neutral gas does trace the star formation over the Universe's history. We explore the feasibility of resolving this with 21-cm observations of the largest contemporary sample of reliable damped Lyman-α absorption systems and conclude that, while today's largest radio interferometers can reach the required sensitivity at z < ∼ 3.5, the Square Kilometre Array is required to probe to higher redshifts.