Massive galaxies on the road to quenching: ALMA observations of powerful high redshift radio galaxies

Abstract

We present 0 . 3 (band 6) and 1 . 5 (band 3) ALMA observations of the (sub)millimeter dust continuum emission for 25 radio galaxies at 1 < z < 5.2. Our survey reaches a rms flux density of ∼50 µJy in band 6 (200-250 GHz) and ∼20 µJy in band 3 (100-130 GHz). This is an order of magnitude deeper than single-dish 850 µm observations, and reaches fluxes where synchrotron and thermal dust emission are expected to be of the same order of magnitude. Combining our sensitive ALMA observations with low-resolution radio data from ATCA, higher resolution VLA data, and infrared photometry from Herschel and Spitzer, we have disentangled the synchrotron and thermal dust emission. We determine the star-formation rates and AGN infrared luminosities using our newly developed Multiresolution and multi-object/origin spectral energy distribution fitting code (Mr-Moose). We find that synchrotron emission contributes substantially at λ ∼1 mm. Through our sensitive flux limits and accounting for a contribution from synchrotron emission in the mm, we revise downward the median star-formation rate by a factor of seven compared to previous estimates based solely on Herschel and Spitzer data. The hosts of these radio-loud AGN appear predominantly below the main sequence of star-forming galaxies, indicating that the star formation in many of the host galaxies has been quenched. Future growth of the host galaxies without substantial black hole mass growth will be needed to bring these objects on the local relation between the supermassive black holes and their host galaxies. Given the mismatch in the timescales of any star formation that took place in the host galaxies and lifetime of the AGN, we hypothesize that a key role is played by star formation in depleting the gas before the action of the powerful radio jets quickly drives out the remaining gas. This positive feedback loop of efficient star formation rapidly consuming the gas coupled to the action of the radio jets in removing the residual gas is how massive galaxies are rapidly quenched.

MUSE unravels the ionisation and origin of metal-enriched absorbers in the gas halo of a z = 2.92 radio galaxy

“…This observation implies that dust has a negligible contribution to the SED at the host galaxy. Moreover, Falkendal et al (2019) have shown that the star-formation rate (SFR) in Yggdrasil, is low in comparison to that which is detected in the companion sources, collectively.…”

“…Stellar photoionisation is a possibility as well. The starformation rate (SFR) in Yggdrasil, however, is SFR=41 M yr −1 (Falkendal et al 2019) and may be insufficient in producing the star-burst activity that would ionise the absorbers. Another pos-Article number, page 14 of 21 S. Kolwa et al: MUSE unravels the ionisation and origin of metal enriched absorbers in the gas halo of a z = 2.92 radio galaxy Fig 14(d) which is a spectrum extracted from an aperture centred at the HSBR over a radius of R = 0.6 ).…”

Quenching by gas compression and consumption

“…The UV and optical continuum slopes are measured from the observed spectrum. The quoted L IR refers to that attributed to star formation only, excluding any AGN contribution (Falkendal et al 2019).…”

“…2, corresponding to moderate spectral resolutions of λ/∆λ ∼ 3200, 5000, and 4300, which were sampled at 8.9, 10.3, and 22.6 pixels per FWHM, respectively. The slit was broad enough to cover both the host galaxy and the radio lobe of PKS 0529-549 (corresponding to the W and E components as presented in Broderick et al 2007;Falkendal et al 2019). The integration times per exposure in the UVB, VIS, and NIR arms were 1800, 900, 600 s, respectively.…”

“…Here we present the most significant detection of multiple rest-frame UV photospheric absorption lines in PKS 0529-549, a radio galaxy at z = 2.57. PKS 0529-549 is a massive galaxy with a stellar mass M = (3 ± 2) × 10 11 M , (De Breuck et al 2010) and a star-formation rate of SFR = 1020 +190 −170 M yr −1 (Falkendal et al 2019). Its Lyα emission extent (∼40 kpc; Roettgering et al 1997) is well beyond its asymmetric double knot radio structure (two knots separated by ∼10 kpc; Broderick et al 2007).…”