An ASKAP survey for H I absorption towards dust-obscured quasars
Monthly Notices of the Royal Astronomical Society Oxford University Press 489:4 (2019) 4926-4943
Abstract:
Obscuration of quasars by accreted gas and dust, or dusty intervening galaxies, can cause active galactic nuclei (AGN) to be missed in optically selected surveys. Radio observations can overcome this dust bias. In particular, radio surveys searching for H I absorption inform us on how the AGN can impact on the cold neutral gas medium within the host galaxy, or the population of intervening galaxies through the observed line of sight gas kinematics. We present the results of an H I absorption line survey at 0.4 < z < 1 towards 34 obscured quasars with the Australian SKA Pathfinder (ASKAP) commissioning array. We detect three H I absorption lines, with one of these systems previously unknown. Through optical follow-up for two sources, we find that in all detections the H I gas is associated with the AGN, and hence that these AGN are obscured by material within their host galaxies. Most of our sample are compact, and in addition, are either gigahertz peaked spectrum (GPS), or steep spectrum (CSS) sources, both thought to represent young or recently re-triggered radio AGN. The radio spectral energy distribution classifications for our sample agree with galaxy evolution models in which the obscured AGN has only recently become active. Our associated H I detection rate for GPS and compact SS sources matches those of other surveys towards such sources. We also find shallow and asymmetric H I absorption features, which agrees with previous findings that the cold neutral medium in compact radio galaxies is typically kinematically disturbed by the AGN.Measuring the H I mass function below the detection threshold
Monthly Notices of the Royal Astronomical Society Oxford University Press 491:1 (2019) 1227-1242
Abstract:
We present a Bayesian stacking technique to directly measure the H i mass function (HIMF) and its evolution with redshift using galaxies formally below the nominal detection threshold. We generate galaxy samples over several sky areas given an assumed HIMF described by a Schechter function and simulate the H i emission lines with different levels of background noise to test the technique. We use Multinest to constrain the parameters of the HIMF in a broad redshift bin, demonstrating that the HIMF can be accurately reconstructed, using the simulated spectral cube far below the H i mass limit determined by the 5σ flux-density limit, i.e. down to MHI = 107.5 M⊙ over the redshift range 0 < z < 0.55 for this particular simulation, with a noise level similar to that expected for the MIGHTEE survey. We also find that the constraints on the parameters of the Schechter function, φ⋆, M⋆ and α can be reliably fit, becoming tighter as the background noise decreases as expected, although the constraints on the redshift evolution are not significantly affected. All the parameters become better constrained as the survey area increases. In summary, we provide an optimal method for estimating the H i mass at cosmological distances that allows us to constrain the H i mass function below the detection threshold in forthcoming H i surveys. This study is a first step towards the measurement of the HIMF at high (z > 0.1) redshifts.ALMACAL – VI. Molecular gas mass density across cosmic time via a blind search for intervening molecular absorbers
Monthly Notices of the Royal Astronomical Society Oxford University Press 490:1 (2019) 1220-1230
Abstract:
We are just starting to understand the physical processes driving the dramatic change in cosmic star formation rate between z ∼ 2 and the present day. A quantity directly linked to star formation is the molecular gas density, which should be measured through independent methods to explore variations due to cosmic variance and systematic uncertainties. We use intervening CO absorption lines in the spectra of mm-bright background sources to provide a census of the molecular gas mass density of the Universe. The data used in this work are taken from ALMACAL, a wide and deep survey utilizing the ALMA calibrator archive. While we report multiple Galactic absorption lines and one intrinsic absorber, no extragalactic intervening molecular absorbers are detected. However, due to the large redshift path surveyed (z = 182), we provide constraints on the molecular column density distribution function beyond z ∼ 0. In addition, we probe column densities of N(H2) > 1016 atoms cm−2, 5 orders of magnitude lower than in previous studies. We use the cosmological hydrodynamical simulation IllustrisTNG to show that our upper limits of ρ(H2) 108.3 M Mpc−3 at 0 < z ≤ 1.7 already provide new constraints on current theoretical predictions of the cold molecular phase of the gas. These results are in agreement with recent CO emission-line surveys and are complementary to those studies. The combined constraints indicate that the present decrease of the cosmic star formation rate history is consistent with an increasing depletion of molecular gas in galaxies compared to z ∼ 2.WALLABY early science - III. An HI study of the spiral galaxy NGC 1566
Monthly Notices of the Royal Astronomical Society Oxford University Press 487:2 (2019) 2797-2817
Abstract:
This paper reports on the atomic hydrogen gas (H I) observations of the spiral galaxy NGC 1566 using the newly commissioned Australian Square Kilometre Array Pathfinder radio telescope. We measure an integrated H I flux density of 180.2 Jy km s−1 emanating from this galaxy, which translates to an H I mass of 1.94×1010M⊙ at an assumed distance of 21.3 Mpc. Our observations show that NGC 1566 has an asymmetric and mildly warped H I disc. The H I-to-stellar mass fraction (MHI/M∗) of NGC 1566 is 0.29, which is high in comparison with galaxies that have the same stellar mass (1010.8 M⊙). We also derive the rotation curve of this galaxy to a radius of 50 kpc and fit different mass models to it. The NFW, Burkert, and pseudo-isothermal dark matter halo profiles fit the observed rotation curve reasonably well and recover dark matter fractions of 0.62, 0.58, and 0.66, respectively. Down to the column density sensitivity of our observations (NHI=3.7×1019 cm−2), we detect no H I clouds connected to, or in the nearby vicinity of, the H I disc of NGC 1566 nor nearby interacting systems. We conclude that, based on a simple analytic model, ram pressure interactions with the IGM can affect the H I disc of NGC 1566 and is possibly the reason for the asymmetries seen in the H I morphology of NGC 1566.ASKAP commissioning observations of the GAMA 23 field
Publications of the Astronomical Society of Australia Cambridge University Press 36 (2019) e024