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Register a new userFar Ultraviolet Morphology of Star Forming Filaments in Cool Core Brightest Cluster Galaxies
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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). New Hubble Space Telescope (HST) imaging of far ultraviolet continuum emission from young ($sim 10$ Myr), massive ($> 5$ Msol) stars reveals filamentary and clumpy morphologies, which we quantify by means of structural indices. The FUV data are compared with X-ray, Ly$alpha$, narrowband H$alpha$, broadband optical/IR, and radio maps, providing a high spatial resolution atlas of star formation locales relative to the ambient hot ($sim10^{7-8}$ K) and warm ionised ($sim 10^4$ 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 {it in situ} by cloud collapse at the interface of a radio lobe or rapid cooling in a cavitys compressed shell. 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 $t_{mathrm{cool}}/t_{mathrm{ff}}sim 10$. 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.
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We used broad-band imaging data for 10 cool-core brightest cluster galaxies (BCGs) and conducted a Bayesian analysis using stellar population synthesis to determine the likely properties of the constituent stellar populations. Determination of ongoing star formation rates (SFRs), in particular, has a direct impact on our understanding of the cooling of the intracluster medium (ICM), star formation and AGN-regulated feedback. Our model consists of an old stellar population and a series of young stellar components. We calculated marginalized posterior probability distributions for various model parameters and obtained 68% plausible intervals from them. The 68% plausible interval on the SFRs is broad, owing to a wide range of models that are capable of fitting the data, which also explains the wide dispersion in the star formation rates available in the literature. The ranges of possible SFRs are robust and highlight the strength in such a Bayesian analysis. The SFRs are correlated with the X-ray mass deposition rates (the former are factors of 4 to 50 lower than the latter), implying a picture where the cooling of the ICM is a contributing factor to star formation in cool-core BCGs. We find that 9 out of 10 BCGs have been experiencing starbursts since 6 Gyr ago. While four out of 9 BCGs seem to require continuous SFRs, 5 out of 9 seem to require periodic star formation on intervals ranging from 20 Myr to 200 Myr. This time scale is similar to the cooling-time of the ICM in the central (< 5 kpc) regions.
A fraction of brightest cluster galaxies (BCGs) shows bright emission in the UV and the blue part of the optical spectrum, which has been interpreted as evidence of recent star formation. Most of these results are based on the analysis of broadband photometric data. Here, we study the optical spectra of a sample of 19 BCGs hosted by X-ray luminous galaxy clusters at 0.15 < z < 0.3, a subset from the Canadian Cluster Comparison Project (CCCP) sample. We identify plausible star formation histories of the galaxies by fitting Simple Stellar Populations (SSPs) as well as composite populations, consisting of a young stellar component superimposed on an intermediate/old stellar component, to accurately constrain their star formation histories. We detect prominent young (~200 Myr) stellar populations in 4 of the 19 galaxies. Of the four, the BCG in Abell 1835 shows remarkable A-type stellar features indicating a relatively large population of young stars, which is extremely unusual even amongst star forming BCGs. We constrain the mass contribution of these young components to the total stellar mass to be typically between 1% to 3%, but rising to 7% in Abell 1835. We find that the four of the BCGs with strong evidence for recent star formation (and only these four galaxies) are found within a projected distance of 5 kpc of their host clusters X-ray peak, and the diffuse, X-ray gas surrounding the BCGs exhibit a ratio of the radiative cooling-to-free-fall time ($t_{c}/t_{ff}$) of < 10. These are also some of the clusters with the lowest central entropy. Our results are consistent with the predictions of the precipitation-driven star formation and AGN feedback model, in which the radiatively cooling diffuse gas is subject to local thermal instabilities once the instability parameter $t_{c}/t_{ff}$ falls below ~10, leading to the condensation and precipitation of cold gas. [Abridged]
The excitation of the filamentary gas structures surrounding giant elliptical galaxies at the center of cool-core clusters, a.k.a BCGs (brightest cluster galaxies), is key to our understanding of active galactic nucleus feedback, and of the impact of environmental and local effects on star formation. We investigate the contribution of the thermal radiation from the cooling flow surrounding BCGs to the excitation of the filaments. We explore the effects of small levels of extra-heating (turbulence), and of metallicity, on the optical and infrared lines. Using the Cloudy code, we model the photoionization and photodissociation of a slab of gas of optical depth AV{leq}30mag at constant pressure, in order to calculate self-consistently all of the gas phases, from ionized gas to molecular gas. The ionizing source is the EUV and soft X-ray radiation emitted by the cooling gas. We test these models comparing their predictions to the rich multi-wavelength observations, from optical to submillimeter. These models reproduce most of the multi-wavelength spectra observed in the nebulae surrounding the BCGs, not only the LINER-like optical diagnostics: [O iii]{lambda} 5007 {AA}/Hb{eta}, [N ii]{lambda} 6583 {AA}/H{alpha} and ([S ii]{lambda} 6716 {AA}+[S ii]{lambda} 6731 {AA})/H{alpha} but also the infrared emission lines from the atomic gas. The modeled ro-vib H2 lines also match observations, which indicates that near and mid-IR H2 lines are mostly excited by collisions between H2 molecules and secondary electrons produced naturally inside the cloud by the interaction between the X-rays and the cold gas in the filament. However, there is still some tension between ionized and molecular line tracers (i.e. CO), which requires to optimize the cloud structure and the density of the molecular zone.
We present measurements of 5-25 {mu}m emission features of brightest cluster galaxies (BCGs) with strong optical emission lines in a sample of 9 cool-core clusters of galaxies observed with the Infrared Spectrograph on board the Spitzer Space Telescope. These systems provide a view of dusty molecular gas and star formation, surrounded by dense, X-ray emitting intracluster gas. Past work has shown that BCGs in cool-core clusters may host powerful radio sources, luminous optical emission line systems, and excess UV, while BCGs in other clusters never show this activity. In this sample, we detect polycyclic aromatic hydrocarbons (PAHs), extremely luminous, rotationally-excited molecular hydrogen line emission, forbidden line emission from ionized gas ([Ne II] and [Ne III]), and infrared continuum emission from warm dust and cool stars. We show here that these BCGs exhibit more luminous forbidden neon and H2 rotational line emission than star-forming galaxies with similar total infrared luminosities, as well as somewhat higher ratios of 70 {mu}m / 24 {mu}m luminosities. Our analysis suggests that while star formation processes dominate the heating of the dust and PAHs, a heating process consistent with suprathermal electron heating from the hot gas, distinct from star formation, is heating the molecular gas and contributing to the heating of the ionized gas in the galaxies. The survival of PAHs and dust suggests that dusty gas is somehow shielded from significant interaction with the X-ray gas.
We present the results of a Spitzer/Herschel infrared photometric analysis of the largest (716) and highest-redshift (z=1.8) sample of Brightest Cluster Galaxies (BCGs), those from the Spitzer Adaptation of the Red-Sequence Cluster Survey (SpARCS). Given the tension that exists between model predictions and recent observations of BCGs at z<2, we aim to uncover the dominant physical mechanism(s) guiding the stellar-mass buildup of this special class of galaxies, the most massive in the Universe uniquely residing at the centres of galaxy clusters. Through a comparison of their stacked, broadband, infrared spectral energy distributions (SEDs) to a variety of SED model templates in the literature, we identify the major sources of their infrared energy output, in multiple redshift bins between 0 < z < 1.8. We derive estimates of various BCG physical parameters from the stacked { u}L{ u} SEDs, from which we infer a star-forming, as opposed to a red and dead population of galaxies, producing tens to hundreds of solar masses per year down to z=0.5. This discovery challenges the accepted belief that BCGs should only passively evolve through a series of gas-poor, minor mergers since z~4 (De Lucia & Blaizot 2007), but agrees with the improved semi-analytic model of hierarchical structure formation of Tonini et al. (2012), which predicts star-forming BCGs throughout the epoch considered. We attribute the star formation inferred from the stacked infrared SEDs to both major and minor wet (gas-rich) mergers, based on a lack of key signatures (to date) of the cluster cooling flows to which BCG star formation is typically attributed, as well as a number of observational and simulation-based studies that support this scenario.
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