Illuminating the past 8 billion years of cold gas towards two gravitationally lensed quasars

Abstract

Using the Boolardy Engineering Test Array of the Australian Square Kilometre Array Pathfinder (ASKAP BETA), we have carried out the first z = 0 − 1 survey for H I and OH absorption towards the gravitationally lensed quasars PKS B1830−211 and MG J0414+0534. Although we detected all previously reported intervening systems towards PKS B1830−211, in the case of MG J0414+0534 three systems were not found, indicating that the original identifications may have been confused with radio frequency interference. Given the sensitivity of our data, we find that our detection yield is consistent with the expected frequency of intervening H I systems estimated from previous surveys for 21-cm emission in nearby galaxies and z ∼ 3 damped Lyman α absorbers. We find spectral variability in the z = 0.886 face-on spiral galaxy towards PKS B1830−211, from observations undertaken with the Westerbork Synthesis Radio Telescope in 1997/1998 and ASKAP BETA in 2014/2015. The H I equivalent width varies by a few per cent over approximately yearly time-scales. This long-term spectral variability is correlated between the north-east and south-west images of the core, and with the total flux density of the source, implying that it is observationally coupled to intrinsic changes in the quasar. The absence of any detectable variability in the ratio of H I associated with the two core images is in stark contrast to the behaviour previously seen in the molecular lines. We therefore infer that coherent opaque H I structures in this galaxy are larger than the parsec-scale molecular clouds found at mm-wavelengths.

FLASH early science – discovery of an intervening H i 21-cm absorber from an ASKAP survey of the GAMA 23 field

“…A dedicated ASKAPSOFT data processing pipeline is currently being developed for the full FLASH survey, adapting the procedure discussed by Kleiner et al (2019). However, to facilitate processing of the early science data presented here we have followed the procedure described by Allison et al (2015Allison et al ( , 2017Allison et al ( , 2019 and refer the interested reader to that work for further details. The ingested data from the correlator were written to measurement set format and then flagged for digital glitches and autocorrelations, averaged, and split, using the CASA 3 package (Mc-Mullin et al 2007).…”

“…Several recent results from observations of radio galaxies and quasars during ASKAP commissioning have demonstrated feasibility (e.g. Allison et al 2015Allison et al , 2016aMoss et al 2017;Allison et al 2017;Glowacki et al 2019;Allison et al 2019). Here, we have used observations of the 23 hr field of the Galaxy And Mass Assembly survey (GAMA; Liske et al 2015) 1 , a 50 deg 2 area of the southern sky that contains spectroscopic information for galaxies to an i-band magnitude limit of i < 19.2 mag.…”

PKS B1740$\mathbf {-}$517: An ALMA view of the cold gas feeding a distant interacting young radio galaxy

“…Allison et al (2015). et al 1995) as described by A15, but using a higher 1 MHz resolution to form the coarse-channel data (see Allison et al 2017).…”

“…The combination of a wide instantaneous bandwidth (300 MHz), and a band that at frequencies below 1 GHz is typically clear of radio frequency interference (see e.g. Allison et al 2017), enables 21-cm line surveys to be carried out without the need for existing spectroscopicallydetermined redshifts. Future wide-field 21-cm line surveys with telescopes such as ASKAP will search for the cold gas in thousands of radio galaxies (Maccagni et al 2017;.…”

Connecting X-ray absorption and 21 cm neutral hydrogen absorption in obscured radio AGN

“…We model the absorption line as a superposition of Gaussian profiles and then determine the best fitting number of components and marginal distributions for the line parameters using a Bayesian approach as described by Allison et al (2015Allison et al ( , 2017 and references therein. We identify a single component in the absorption-line profile of PKS 1657−298, at a redshift of z = 0.420164 ± 0.000006.…”