[Paper Review] The MeerKAT Absorption Line Survey (MALS)

N. Gupta, R. Srianand+42 more|arXiv (Cornell University)|Aug 24, 2017

Astrophysics and Star Formation StudiesPhysics and Astronomy19 citations

TL;DR

The MeerKAT Absorption Line Survey (MALS) proposes a deep, dust-unbiased survey using MeerKAT’s L- and UHF-band receivers to detect H i 21-cm and OH 18-cm absorption lines in intervening and associated gas at 0 < z < 2. With high sensitivity (N(H i) > 10¹⁹ cm⁻²), it aims to measure the evolution of cold gas cross-sections, constrain star formation rate density drivers, and enable precise tests of fundamental constants and magnetic field evolution.

ABSTRACT

Deep galaxy surveys have revealed that the global star formation rate (SFR) density in the Universe peaks at 1 &lt; z &lt; 2 and sharply declines towards z = 0. But a clear picture of the underlying processes, in particular the evolution of cold atomic (~100 K) and molecular gas phases, that drive such a strong evolution is yet to emerge. MALS is designed to use MeerKAT's L- and UHF-band receivers to carry out the most sensitive (N(HI)&gt;10$^{19}$ cm$^{-2}$) dust-unbiased search of intervening HI 21-cm and OH 18-cm absorption lines at 0 &lt; z &lt; 2. This will provide reliable measurements of the evolution of cold atomic and molecular gas cross-sections of galaxies, and unravel the processes driving the steep evolution in the SFR density. The large sample of HI and OH absorbers obtained from the survey will (i) lead to tightest constraints on the fundamental constants of physics, and (ii) be ideally suited to probe the evolution of magnetic fields in disks of galaxies via Zeeman Splitting or Rotation Measure synthesis. The survey will also provide an unbiased census of HI and OH absorbers, i.e. cold gas associated with powerful AGNs (&gt;10$^{24}$ W Hz$^{-1}$) at 0 &lt; z &lt; 2, and will simultaneously deliver a blind HI and OH emission line survey, and radio continuum survey. Here, we describe the MALS survey design, observing plan and the science issues to be addressed under various science themes.

Motivation & Objective

• To measure the evolution of cold atomic and molecular gas cross-sections in galaxies across 0 < z < 2, addressing the unexplained decline in star formation rate density.
• To conduct the most sensitive, dust-unbiased survey of intervening H i 21-cm and OH 18-cm absorption lines, overcoming limitations of prior surveys.
• To provide a robust census of H i and OH absorbers associated with powerful AGNs (>10²⁴ W Hz⁻¹) to probe ISM physics and magnetic fields.
• To deliver a blind H i and OH emission line survey and a radio continuum survey as a byproduct, enhancing multi-wavelength synergy.
• To enable tight constraints on the variation of fundamental constants and cosmic acceleration through precise line frequency measurements.

Proposed method

• Utilize MeerKAT’s L- and UHF-band receivers to observe 300 pointings centered on radio-loud FSRQs with spectroscopically confirmed redshifts in the southern sky (−40° < δ < +30°).
• Conduct two-epoch observations per pointing, separated by a few months, to exploit Earth’s heliocentric motion and distinguish true absorption from RFI via a 20–30 km s⁻¹ Doppler shift.
• Apply wideband spectro-polarimetry to detect Zeeman splitting and Rotation Measure effects, enabling magnetic field probing in galactic disks.
• Use WISE infrared colors to pre-select high-redshift (z > 1.4) candidate quasars with ~75% efficiency, followed by optical spectroscopy with SALT and NOT to confirm redshifts.
• Implement a data pipeline (ARTIP) for calibration, flagging, and line detection, with regular public data releases of calibrated visibilities and images.
• Combine absorption line data with VLBI images of central sources to assess the impact of radio structure on absorption detectability.

Experimental results

Research questions

• RQ1 How do the cross-sections of cold atomic and molecular gas in galaxies evolve from z ≈ 0 to z ≈ 2, and what drives the steep decline in star formation rate density?
• RQ2 What is the true evolution of the H i 21-cm absorption path length function (n₂₁(z)) and OH 18-cm absorption path length function (n_OH(z)) across 0 < z < 2?
• RQ3 To what extent do intervening H i and OH absorbers trace the cold gas reservoirs in galactic disks, halos, and intra-group media?
• RQ4 Can the observed frequencies of H i and OH lines provide the tightest constraints on the variation of fundamental constants and cosmic acceleration?
• RQ5 How do magnetic fields in galactic disks evolve with redshift, as measured via Zeeman splitting and Rotation Measure synthesis in absorption lines?

Key findings

• The survey is expected to detect approximately 200 intervening H i 21-cm absorbers and ~500 associated absorbers with N(H i) > 10¹⁹ cm⁻², enabling robust path length function measurements.
• The first-year observations will focus on 70% of pointings in the declination range −40° < δ < +30°, centered on FSRQs with confirmed redshifts, to establish a reliable baseline for n₂₁(z) and n_OH(z).
• Through WISE pre-selection and optical follow-up, the team has already identified ~100 new high-redshift (z > 1.4) FSRQs over the past two years, significantly expanding the target pool.
• The survey will deliver a blind H i and OH emission line survey and a radio continuum survey as a byproduct, with data released regularly to the community.
• The use of two-epoch observations with a Doppler shift of 20–30 km s⁻¹ will allow unambiguous distinction between true absorption and RFI, ensuring catalog reliability.
• VLBI imaging of central sources will allow quantification of how radio structure affects absorption detectability and redshift evolution.