The VLT-MUSE and ALMA view of the MACS 1931.8-2635 brightest cluster galaxy

Abstract

We reveal the importance of ongoing in situ star formation in the brightest cluster galaxy (BCG) in the massive cool-core CLASH cluster MACS 1931.8-2635 at a redshift of z = 0.35 by analysing archival VLT-MUSE optical integral field spectroscopy. Using a multi-wavelength approach, we assessed the stellar and warm ionised medium components, which were spatially resolved by the VLT-MUSE spectroscopy, and linked them to the molecular gas by incorporating sub-mm ALMA observations. We measured the fluxes of strong emission lines such as: [O II] λ3727, Hβ, [O III] λ5007, Hα, [N II] λ 6584, and [S II] λ6718, 6732, which allowed us to determine the physical conditions of the warm ionised gas, such as electron temperature, electron density, extinction, ionisation parameter, (O/H) gas metallicities, star formation rates, and gas kinematics, as well as the star formation history of the system. Our analysis reveals the ionising sources in different regions of the galaxy. The ionised gas flux brightness peak corresponds to the location of the supermassive black hole in the BCG and the system shows a diffuse warm ionised gas tail extending 30 kpc in the north-east direction. The ionised and molecular gas are co-spatial and co-moving, with the gaseous component in the tail likely falling inward, providing fuel for star formation and accretion-powered nuclear activity. The gas is ionised by a mix of star formation and other energetic processes which give rise to LINER-like emission, with active galactic nuclei emission dominant only in the BCG core. We measured a star formation rate of ∼97 M⊙ yr−1, with its peak at the BCG core. However, star formation accounts for only 50–60% of the energetics needed to ionise the warm gas. The stellar mass growth of the BCG at z < 0.5 is dominated either by in situ star formation generated by thermally unstable intracluster medium cooling or by dry mergers, with these mechanisms accounting for the build-up of 20% of the stellar mass of the system. Our measurements reveal that the most central regions of the BCG contain the lowest gas-phase oxygen abundance, whereas the Hα arm exhibits slightly more elevated values, suggesting the transport of gas out to large distances from the centre as a result of active galactic nuclei outbursts. The galaxy is a dispersion-dominated system that is typical for massive, elliptical galaxies. The gas and stellar kinematics are decoupled, with the gaseous velocity fields being more closely related to the bulk motions of the intracluster medium.

Spatially resolved properties of early-type group-dominant galaxies with MUSE: gas content, ionization mechanisms, and metallicity gradients

“…The models were created using the CLOUDY code version 17.0 (Ferland 2017) for metallicities Z/Z ⊙ = 0.2, 0.5, 0.75, 1.0, 2.0 and 3.0 and ionisation parameters U 2 in the range of -4.0 ≤ log(U) ≤ -0.5 in steps of 0.5 dex. Their assumed density of n e 500 cm −3 is typical for BCGs (e.g., Ciocan et al 2021) and it is compatible with our values from Section 3.4. In both cases, most galaxies in our sample lie in the region between Z/Z ⊙ = 0.75 (12 + log(O/H)= 8.56) and Z/Z ⊙ = 1 (12 + log(O/H) = 8.69).…”

Testing the Limits of AGN Feedback and the Onset of Thermal Instability in the Most Rapidly Star-forming Brightest Cluster Galaxies

“…In the "stimulated feedback" model (e.g., McNamara et al 2016), thermally unstable cooling happens in situ when warm (∼1 keV) X-ray gas is uplifted in the wake of AGNinflated radio bubbles as they rise buoyantly out of the central cluster potential, which increases the infall time and promotes the condensation of cold gas (see also Pope et al 2010). Evidence for this phenomenon has been provided in a number of systems where large reservoirs (∼10 10 M e ) of cold (10-100 K) molecular gas, observed with the Atacama Large Millimeter/submillimeter Array (ALMA), are projected behind or draped around the location of X-ray cavities as seen by Chandra (e.g., A1835: McNamara et al 2014Phoenix: Russell et al 2017a;MACS 1931.8-2634: Ciocan et al 2021A1664: Calzadilla et al 2019PKS 0745-191: Russell et al 2016;A2597: Tremblay et al 2018A1795: Russell et al 2017b2A 0335+096: Vantyghem et al 2016). On the other hand, several systems show cold gas in hot halos, even without a direct correlation with bubbles (e.g., Temi et al 2018;Olivares et al 2019;Rose et al 2019;Maccagni et al 2021;North et al 2021;McKinley et al 2022).…”

Gas condensation in Brightest Group Galaxies unveiled with MUSE

“…Most of the studies related to the origin of the cold gas have focused principally on the most massive and brightest elliptical galaxies located at the core of galaxy clusters, so-called brightest cluster galaxies. Observations of the ionized gas in galaxy clusters, traced with the Hα emission line, show spectacular filamentary structures extending up to ∼70 kpc from the core of the central galaxy (e.g., Heckman et al 1989;Hatch et al 2007;Mc-Donald et al 2010, 2011bHamer et al 2016;Hamer et al 2018;Tremblay et al 2018;Olivares et al 2019;Ciocan et al 2021). Systems with nebular emission, cold molecular gas, and ongoing star formation are preferentially found in galaxy clusters with short central cooling times, 1 Gyr and low central entropy values, 30 keV cm 2 (e.g., Cavagnolo et al 2008;Bildfell et al 2008;Pipino et al 2009;Rafferty et al 2008a;Pulido et al 2018;Loubser et al 2016).…”