S. Pinjarkar, M. Hardcastle, D. V. Lal, D. Smith, J. Afonso, D. D. Barbosa, C. L. Hale, M. J. Jarvis, S. Kolwa, E. J. Murphy, M. Vaccari, I. H. Whittam
Abstract
It has been known for many years that there is an apparent trend for the spectral index (alpha) of radio sources to steepen with redshift z, which has led to attempts to select high-redshift objects by searching for radio sources with steep spectra. In this study, we use data from the MeerKAT, Low Frequency Array survey, Giant Metre-wave Radio Telescope survey (GMRT), and uGMRT telescopes, particularly using the MeerKAT International GHz Tiered Extragalactic Exploration (MIGHTEE) and superMIGHTEE surveys, to select compact sources over a wide range of redshifts and luminosities. We investigate the relationship between spectral index, luminosity and redshift and compare our results to those of previous studies. Although there is a correlation between alpha and z in our sample for some combinations of frequency where good data are available, there is a clear offset between the alpha-z relations in our sample and those derived previously from samples of more luminous objects; in other words, the alpha-z relation is different for low and high-luminosity sources. The relationships between alpha and luminosity are also weak in our sample but in general the most luminous sources are steeper-spectrum and this trend is extended by samples from previous studies. In detail, we argue that both a alpha-luminosity relation and an alpha-z relation can be found in the data, but it is the former that drives the apparent alpha-z relation observed in earlier work, which only appears because of the strong redshift-luminosity relation in bright, flux density-limited samples. Steep-spectrum selection should be applied with caution in searching for high-z sources in future deep surveys.
Keywords
galaxies: active / galaxies: evolution / galaxies: nuclei
Monthly Notices of the Royal Astronomical Society
Volume 537, Issue 4, Page 3481
2025 March
