Far-ultraviolet morphology of star-forming filaments in cool core brightest cluster galaxies

Abstract

We present a multiwavelength morphological analysis of star forming clouds and filaments in the central (< ∼ 50 kpc) regions of 16 low redshift (z < 0.3) cool core brightest cluster galaxies (BCGs). The sample spans decades-wide ranges of X-ray mass deposition and star formation rates as well as active galactic nucleus (AGN) mechanical power, encompassing both high and low extremes of the supposed intracluster medium (ICM) cooling and AGN heating feedback cycle. New Hubble Space Telescope (HST) imaging of far ultraviolet continuum emission from young (< ∼ 10 Myr), massive (> ∼ 5 M ) stars reveals filamentary and clumpy morphologies, which we quantify by means of structural indices. The FUV data are compared with X-ray, Lyα, narrowband Hα, broadband optical/IR, and radio maps, providing a high spatial resolution atlas of star formation locales relative to the ambient hot (∼ 10 7−8 K) and warm ionised (∼ 10 4 K) gas phases, as well as the old stellar population and radio-bright AGN outflows. Nearly half of the sample possesses kpc-scale filaments that, in projection, extend toward and around radio lobes and/or X-ray cavities. These filaments may have been uplifted by the propagating jet or buoyant X-ray bubble, or may have formed in situ by cloud collapse at the interface of a radio lobe or rapid cooling in a cavity's compressed shell. Many other extended filaments, however, show no such spatial correlation, and the dominant driver of their morphology remains unclear. We nevertheless show that the morphological diversity of nearly the entire FUV sample is reproduced by recent hydrodynamical simulations in which the AGN powers a self-regulating rain of thermally unstable star forming clouds that precipitate from the hot atmosphere. In this model, precipitation triggers where the cooling-to-freefall time ratio is t cool /t ff ∼ 10. This condition is roughly met at the maxmial projected FUV radius for more than half of our sample, and clustering about this ratio is stronger for sources with higher star formation rates.

Interplay Among Cooling, Agn Feedback, and Anisotropic Conduction in the Cool Cores of Galaxy Clusters

“…The snapshots are chosen to represent the diverse morphology of the cold gas, including compact, filamentary, linear, and dispersed distributions (from left to right columns, respectively). Such a wide range of structures are also found in observations of cold gas (e.g., Donahue et al 2000;McDonald et al 2011;Werner et al 2014) and stellar populations (e.g., Donahue et al 2015;Tremblay et al 2015) in CC clusters. In particular, the long, nearly isotropic filaments extending tens of kiloparsecs in the Perseus cluster (Conselice et al 2001) are reproduced (second panel in the bottom row).…”

“…On the other hand, the cold gas is condensed out only within the inner 20-30 kpc, consistent with the radius within which t c / t ff  10 (ºr TI ). Therefore, while local TI could possibly explain filaments within r TI (McDonald et al 2011), for filaments that extend beyond r TI Tremblay et al 2015), AGN uplifting could be essential.…”

ALMA observations of cold molecular gas filaments trailing rising radio bubbles in PKS 0745−191

“…Single dish observations of PKS 0745-191 detected the BCG at CO(1-0) and CO(2-1) and found a total molecular gas mass of 3.8 ± 0.9 × 10 9 M (Salomé & Combes 2003). Optical, UV and IR observations show significant star formation at a rate of ∼ 20 M yr −1 and a luminous emission line nebula with clumpy filaments extending > 10 kpc in the BCG (Fabian et al 1985;Johnstone, Fabian & Nulsen 1987;Donahue et al 2000;Hicks & Mushotzky 2005;Tremblay et al 2015). The surrounding rich cluster has a short central cooling time < 5 × 10 8 yr and X-ray spectra are consistent with several hundred solar masses per year cooling down below X-ray temperatures.…”

“…If the dusty molecular gas clouds obscured more than ∼ 50% of the galaxy light then blueshifted and redshifted gas components could be cleanly interpreted as outflow and inflow, respectively. Using the F555W HST image, we subtracted the average Tremblay et al 2015). A smooth model has been subtracted to show the dust lanes extending to the N and E of the nucleus.…”

“…Centre: Narrow band image covering the Hα+[NII] emission (Donahue et al 2000) with the 14.9 GHz VLA B array contours overlaid (Baum & O'Dea 1991). Right: F140LP SBC image of FUV continuum emission from massive, young stars (Mittal, Whelan & Combes 2015;Tremblay et al 2015) with ALMA CO(3-2) integrated intensity contours overlaid. All images are centred on the nuclear continuum point source emission (section 2).…”