Search and analysis of giant radio galaxies with associated nuclei (SAGAN)

Abstract

Radio galaxies with jets of relativistic particles are usually hosted by massive elliptical galaxies with active nuclei powered by accretion of interstellar matter onto a supermassive black hole. In some rare cases (< 5%), their jets drive the overall structure to sizes larger than 700 kpc, and they are called giant radio galaxies (GRGs). A very small fraction of the population of such radio galaxies contains molecular and atomic gas in the form of rings or discs that can fuel star formation. The origin of this gas is not well known; it has sometimes been associated with a minor merger with a gas-rich disc galaxy (e.g. Centaurus A) or cooling of material from a hot X-ray atmosphere (e.g. cooling flows). The giant radio jets might be the extreme evolution of these objects, and they can teach us about the radio galaxy evolution. We selected 12 targets from a catalogue of 820 GRGs that are likely to be in a gas-accretion and star formation phase. The targets were selected from the mid-infrared to contain heated dust. We report here the results of IRAM-30m observations, the molecular gas content, and the star formation efficiency, and we discuss the origin of the gas and disc morphology. Three out of our 12 targets are detected, and for the others, we report significant upper limits. We combine our three detections and upper limits with four additional detected GRGs from the literature to discuss the results. Most of the GRG targets belong to the main sequence, and a large fraction are in the passive domain. Their star formation efficiency is comparable to normal galaxies, except for two galaxies that are deficient in molecular gas with a short (∼200 Myr) depletion time, and a quiescent gas-rich giant spiral galaxy. In general, the depletion time is much longer than the lifetime of the giant radio jet.

Jet-driven AGN feedback on molecular gas and low star-formation efficiency in a massive local spiral galaxy with a bright X-ray halo

“…Figure 1 shows that the line is well detected at the expected frequency, with a clear, although asymmetric, double-horn profile of FWHM = 684 ± 18 km s −1 and an integrated line flux of 12.6 ± 0.8 Jy km s −1 . A similar spectrum has recently been obtained by Dabhade et al (2020) who measured a somewhat greater integrated line flux of 14.0 Jy km s −1 . The difference might be due to calibration uncertainties.…”

“…The uncertainty includes only the error of the measurement, not the systematic uncertainties. Dabhade et al (2020) estimated SFR = 2.95 M yr −1 from the 22 µm flux measured with WISE, whereas Walker et al (2015) found SFR = 1.6 M yr −1 from the same GALEX UV continuum that we also used. Both adopted a Salpeter initial mass function.…”

“…They approximate the star-formation rate by setting SFR = 8.1 × 10 −2 I FUV +3.2 +1.2 −0.7 ×10 −3 I 24 . We use the FUV intensity measured with GALEX, I FUV , and the 22 µm intensity corresponding to the magnitude given by Dabhade et al (2020), m 22 µm = 7.135 mag. This gives a very similar value to our above estimate using GALEX alone, SFR = 0.9−1.4 M yr −1 , with a best estimate of SFR = 1.1 M yr −1 .…”

“…This is unusual compared to other spiral galaxies with stellar masses of few 10 11 M , which are rather poor in hot circumgalactic gas (Li et al 2018), but rich in colder baryons: Posti et al (2019) find that the most massive galaxies in the SPARCS survey (Lelli et al 2015) may have stellar-to-darkmatter mass ratios that are near the cosmic average, placing them amongst the sites of most efficient star formation known. Dabhade et al (2020) recently observed J2345−0449 as part of a sample of galaxies with giant radio jets with the IRAM 30-m telescope, and found that J2345−0449 hosts about 2 × 10 10 M of cold molecular gas, which is highly unusual for giant radio galaxies, which are generally gas-poor. This result was obtained independently from us and published well after our own IRAM single-dish, and ALMA interferometric datasets had already been taken.…”

MIGHTEE: are giant radio galaxies more common than we thought?

“…Best & Heckman 2012;Hardcastle et al 2013). Little information currently exists about the SFRs of GRGs specifically, however Clarke et al (2017) and Dabhade et al (2020c) have found evidence for moderate SFRs in several individual GRG hosts.…”