Radio Cosmology and Continuum Observations in the SKA Era: A Synergic View

I will review few basic questions which can be addressed by future and present intensity mapping experiments in the post-reionization era. Most importantly: dark energy, neutrino masses and galaxy formation aspects.

Radio continuum emission is a highly promising tool for tracing star formation activity across cosmic time and environments, by virtue of being unbiased by dust and reaching high angular resolution in interferometric imaging. I will discuss recent results, but also some of the challenges associated with using GHz continuum surveys to characterize the evolution of the star-forming galaxy population from the final Gyr of the EoR to the present day.

Galaxy clusters and superclusters are among the youngest structures forming in the Universe. The intra-cluster medium (ICM) and inter-galactic medium (IGM) are both unique plasmas that host magnetic fields and cosmic rays. Radio band observations provide the probes to these, so-called non thermal components, via the detectable synchrotron radiation. The particle acceleration mechanisms involve the role of shocks and turbulence in the magnetized plasma that can be probed with radio band continuum and polarization observations. I will present our work on studying turbulence and shocks in the high redshift, massive galaxy cluster El Gordo and the Saraswati supercluster with the precursors and pathfinders of the SKA. I will discuss our findings on the evolution of radio galaxies in superclusters and the properties of intra-cluster turbulence using radio and X-ray observations and provide a perspective on the exciting possibilities with the SKA.

With the increased sensitivity and field of view of SKA pathfinders, dynamic radio imaging (imaging the time axis) is becoming a burgeoning field, yielding rich new discoveries of transients and variable sources. MeerKAT is capable of reaching sub-150 uJy image rms in an 8s integration, which opens up studies of variability on much shorter timescales than was possible with previous radio interferometers. This also has important implications for interferometric SETI, since any potential technosignatures would be a subset of such transient events.

At the same time, imaging at such short timescales introduces its own substantial challenges. Instrumental effects that tend to average out in a traditional long synthesis observation can become limiting for dynamic imaging if not addressed correctly. I will discuss these challenges and present MeerKAT dynamic imaging of Jupiter’s radiation belts, which have led to the serendipitous discovery of a pulsar-class object named the PARROT (pulsar with abnormal refraction recurring on odd timescales).

This work has led to the development of (and given the name to) a more general dynamic imaging pipeline, developed in collaboration with the Breakthrough Listen initiative. The PARROT pipeline is capable of detecting short-duration transients in imaging data, and yielding light curves and dynamic spectra for thousands of field sources en masse. We are already starting to use it to “mine” existing archival MeerKAT data, yielding a couple of new discoveries. The longer-term plan is to develop the PARROT pipeline to a state where it can be run in real-time, commensally with any MeerKAT imaging observation. This would open the door to transient event triggers -- something that has never been done with a radio interferometer before.    

Dark matter plays a major role in the equilibrium of galaxy disks, especially the outer disks of galaxies where the stellar disk surface density declines. The hydrostatic equilibrium of the HI disk in these regions can be used to probe the dark matter distribution within the disk. In this presentation we show that the disk dark matter in nearly face-on galaxies can be probed with sensitive HI velocity dispersion observations. Alternatively, the HI disk thickness in edge-on galaxies can be used to estimate the disk dark matter. We show that both methods have assumptions and advantages/disadvantages. Assuming a form of the halo potential, we show that the disk dark matter can be used to probe the halo shape. We apply our methods to nearby galaxies and show that smaller dwarf galaxies appear to be more dark matter dominated and their halo shapes appear to be oblate, whereas massive galaxies have more spherical halos. We also discuss how HI velocity dispersion can explore the dark matter distribution of very close, low luminosity satellites such as Leo-T. The halo shape can affect galaxy disk properties, such as the stellar velocity dispersion and disk surface density, which in turn affects bar formation, gas kinematics and star formation in galaxy disks, all of which are important for galaxy evolution. Our study thus demonstrates the importance of SKA HI surveys to probe the dark matter distribution in galaxies. We discuss how the SKA sensitivity will allow us to probe HI and dark matter at much larger distances compared to present studies.

Radio galaxies associated with active galactic nuclei (AGN) have traditionally been classified into two types: Fanaroff-Riley I (FR I) and Fanaroff-Riley II (FR II). The classification is based on radio morphology but correlates strongly with their luminosities, which are separated by a sharp dividing line. However, recent observations with the most sensitive radio telescopes like LOFAR, MeerKAT, uGMRT, and JVLA are challenging this distinction. High-resolution and high-sensitivity studies have revealed that the sharp line may actully be broader band, with the luminosities of some FR I and FR II on ”wrong” side. Adding to this complexity is the discovery of Hybrid Morphology Radio Sources (HyMoRS) that show FR I morphology on one side of the active nucleus and FR II morphology on the other.
We present results from a study which used GMRT to image a sample of HyMoRS identified from the MeerKAT Galaxy Cluster Legacy Survey (MGCLS). These sources were selected based on the following criteria:
(i) difference between AGN and host galaxy cluster spectroscopic redshifts < 1000 km/s (ii) angular size > 4 arcmin, and (iii) declination 0-50 degree. The selected sources are situated at varying distances from the cluster center. Initial findings suggest a link between the host cluster environment and the occurrence of HyMoRS morphology.

Understanding the evolution of active galactic nuclei (AGN) and host galaxies across cosmic epochs is one of the key science drivers of extragalactic surveys. Although, obscuration poses a challenge to detect the complete population of AGN across cosmic epochs. The merger induced dust-obscured galaxies (DOGs) are supposedly potential hosts of AGN. However, detection of AGN in DOGs is challenging due to high absorption of optical, UV and X-ray emission arising from the AGN. Radio emission is insensitive to dust obscuration, and hence, radio continuum surveys are efficient means to detect radio AGN hosted in DOGs. I shall discuss the role of multi-frequency radio continuum surveys in uncovering the AGN population in obscured environments up to large redshifts, and therefore, shedding new insights to the cosmic evolution of AGN.

The radiation emitted by the first galaxies in our Universe ionised the hydrogen in the intergalactic medium (IGM) during the first billion years, ushering in the Epoch of Reionisation. How did this last major phase transition that governed the evolution of the galaxies we see today happen? Was it driven by the few bright or numerous faint galaxies?
Current and upcoming optical, near-infrared and radio surveys, with e.g. the Roman Space Telescope and the Square Kilometre Array, will tackle these questions: 21cm emission maps will trace the evolving distribution of ionised regions, while galaxy surveys will sketch the ionising sources and their distribution. Most importantly, combining these maps of the ionising sources and the ionisation topology opens up the possibility of constraining the ionising properties of the galaxies that are too faint to be observed. Various works have explored the benefits of synergising surveys of the 21cm signal and emission line galaxies (e.g. Lyman-alpha emitters), finding that the corresponding cross-correlation functions and power spectra trace the overall ionisation state of the IGM.
I will discuss the characteristic signatures of 21cm-galaxy cross-correlations, explaining how they trace the ionisation history and morphology and which type of 21cm and galaxy surveys can constrain these reionisation scenario characteristics.

I will talk about some recent results on the nature of radio emission in low luminosity or radio-quiet AGN. I will discuss the magnetic field structures as well as episodic AGN activity seen in the outflows of these AGN and how they impact their surroundings. SKA, with its much improved sensitivity, is expected to revolutionise our understanding of these low luminosity AGN.

Morphological summary statistics, such as Minkowski functionals, Betti numbers and Minkowski tensors, provide a route to carry out cosmological data analysis which is complementary to traditional mode decomposition based statistics such as the power spectrum. We summarize the properties of these statistics and demonstrate their information content by using the gravitational evolution of matter and 21-cm brightness temperature as physical examples.