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Register a new userStar formation across cosmic time with radio surveys. The promise of the SKA
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Added by
Gianfranco De Zotti
Publication date
2018
fields
Physics
and research's language is
English
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This lecture briefly reviews the major recent advances in radio astronomy made possible by ultra-deep surveys, reaching microJansky flux density levels. A giant step forward in many fields, including the study of the evolution of the cosmic star formation history is expected with the advent of the Square Kilometer Array (SKA).
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Radio wavelengths offer the unique possibility of tracing the total star-formation rate in galaxies, both obscured and unobscured. As such, they may provide the most robust measurement of the star-formation history of the Universe. In this chapter we highlight the constraints that the SKA can place on the evolution of the star-formation history of the Universe, the survey area required to overcome sample variance, the spatial resolution requirements, along with the multi-wavelength ancillary data that will play a major role in maximising the scientific promise of the SKA. The required combination of depth and resolution means that a survey to trace the star formation in the Universe should be carried out with a facility that has a resolution of at least ~0.5arcsec, with high sensitivity at < 1 GHz. We also suggest a strategy that will enable new parameter space to be explored as the SKA expands over the coming decade.
We investigate the balance of power between stars and AGN across cosmic history, based on the comparison between the infrared (IR) galaxy luminosity function (LF) and the IR AGN LF. The former corresponds to emission from dust heated by stars and AGN, whereas the latter includes emission from AGN-heated dust only. We find that at all redshifts (at least up to z~2.5), the high luminosity tails of the two LFs converge, indicating that the most infrared-luminous galaxies are AGN-powered. Our results shed light to the decades-old conundrum regarding the flatter high-luminosity slope seen in the IR galaxy LF compared to that in the UV and optical. We attribute this difference to the increasing fraction of AGN-dominated galaxies with increasing total infrared luminosity (L_IR). We partition the L_IR-z parameter space into a star-formation and an AGN-dominated region, finding that the most luminous galaxies at all epochs lie in the AGN-dominated region. This sets a potential `limit to attainable star formation rates, casting doubt on the abundance of `extreme starbursts: if AGN did not exist, L_IR>10^13 Lsun galaxies would be significantly rarer than they currently are in our observable Universe. We also find that AGN affect the average dust temperatures (T_dust) of galaxies and hence the shape of the well-known L_IR-T_dust relation. We propose that the reason why local ULIRGs are hotter than their high redshift counterparts is because of a higher fraction of AGN-dominated galaxies amongst the former group.
We present predictions for the evolution of radio emission from Active Galactic Nuclei (AGNs). We use a model that follows the evolution of Supermassive Black Hole (SMBH) masses and spins, within the latest version of the GALFORM semi-analytic model of galaxy formation. We use a Blandford-Znajek type model to calculate the power of the relativistic jets produced by black hole accretion discs, and a scaling model to calculate radio luminosities. First, we present the predicted evolution of the jet power distribution, finding that this is dominated by objects fuelled by hot halo accretion and an ADAF accretion state for jet powers above $10^{32}mathrm{W}$ at $z=0$, with the contribution from objects fuelled by starbursts and in a thin disc accretion state being more important for lower jet powers at $z=0$ and at all jet powers at high redshifts ($zgeq3$). We then present the evolution of the jet power density from the model. The model is consistent with current observational estimates of jet powers from radio luminosities, once we allow for the significant uncertainties in these observational estimates. Next, we calibrate the model for radio emission to a range of observational estimates of the $z=0$ radio luminosity function. We compare the evolution of the model radio luminosity function to observational estimates for $0<z<6$, finding that the predicted evolution is similar to that observed. Finally, we explore recalibrating the model to reproduce luminosity functions of core radio emission, finding that the model is in approximate agreement with the observations.
The SKA will be a transformational instrument in the study of our local Universe. In particular, by virtue of its high sensitivity (both to point sources and diffuse low surface brightness emission), angular resolution and the frequency ranges covered, the SKA will undertake a very wide range of astrophysical research in the field of nearby galaxies. By surveying vast numbers of nearby galaxies of all types with $mu$Jy sensitivity and sub-arcsecond angular resolutions at radio wavelengths, the SKA will provide the cornerstone of our understanding of star-formation and accretion activity in the local Universe. In this chapter we outline the key continuum and molecular line science areas where the SKA, both during phase-1 and when it becomes the full SKA, will have a significant scientific impact.
Building galaxy merger trees from a state-of-the-art cosmological hydrodynamics simulation, Horizon-AGN, we perform a statistical study of how mergers and smooth accretion drive galaxy morphologic properties above $z > 1$. More specifically, we investigate how stellar densities, effective radii and shape parameters derived from the inertia tensor depend on mergers of different mass ratios. We find strong evidence that smooth accretion tends to flatten small galaxies over cosmic time, leading to the formation of disks. On the other hand, mergers, and not only the major ones, exhibit a propensity to puff up and destroy stellar disks, confirming the origin of elliptical galaxies. We also find that elliptical galaxies are more susceptible to grow in size through mergers than disc galaxies with a size-mass evolution $r prop M^{1.2}$ instead of $r prop M^{-0.5} - M^{0.5}$ depending on the merger mass ratio. The gas content drive the size-mass evolution due to merger with a faster size growth for gas-poor galaxies $r prop M^2$ than for gas-rich galaxies $r prop M$.
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