A VLBI investigation of the discrepant image flux-density ratio in the gravitational lens JVAS B0218+357

B0218+357 is a doubly-imaged gravitational lens with the smallest known image angular separation (334 mas) amongst the galactic-scale lens systems. Besides the two compact images with flat-spectra, A and B, it has a steep-spectrum Einstein ring of a similar diameter. This work focuses on the observed systematic decline in the image flux ratio (A/B) with decreasing frequency (from ~ 4 at 15 GHz to ~ 2 at 1.7 GHz), which is not in compliance with the achromatic behaviour of gravitational lensing. In my work, I present various scenarios that can lead the observed image flux-densities to differ from those predicted by the lens model. These scenarios were tested based on multi-frequency and phase-reference VLBI observations of B0218+357. One of the possible explanations is a frequency-dependent source structure, combined with the relative image-magnification which changes significantly over the extent of the structure. I have shown that the centroids of the image brightness distributions do not reveal sufficiently large frequency-dependent shifts to account for the changing image flux ratios. A detailed examination of the interaction of the image flux density distributions with the magnification from lens models can also not explain the effect. A frequency-dependent source size could in principle interact with mass substructure in the lens but a simple model of this cannot reproduce the observed frequency trend of the image flux ratio. Refractive scattering by a screen only partially covering image A might reproduce the observed anomalous frequency-dependent flux density ratio, but there is little evidence that image A suffers strong scattering compared with image B. However, free-free absorption in a dense molecular cloud along the path of image A, known to be present from radio spectroscopic observations, can indeed reproduce the difference between the radio spectra of images A and B. I have shown that assuming an HII region present in front of image A, there is an excellent agreement between the predicted free-free absorption curve and the observed spectrum of image A, thus providing a plausible explanation for the image flux ratio anomaly seen in B0218+357

Constraining star formation rates in cool-core brightest cluster galaxies

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.Comment: 33 pages, 14 Figures, 14 Tables. Accepted for publication in MNRA

The challenging task of determining star formation rates: the case of a massive stellar burst in the brightest cluster galaxy of Phoenix galaxy cluster

Star formation in galaxies at the center of cooling-flow galaxy clusters is an important phenomenon in the context of formation and evolution of massive galaxies in the Universe. Yet, star formation rates (SFRs) in such systems continue to be elusive. We use our Bayesian-motivated spectral energy distribution (SED)-fitting code, BAYESCOOL, to estimate the plausible SFR values in the brightest cluster galaxy of a massive, X-ray luminous galaxy cluster, Phoenix. Previous studies of Phoenix have resulted in the highest measurement of SFR for any galaxy, with the estimates reaching up to 1000 solar masses/yr. However, a very small number of models have been considered in those studies. BAYESCOOL allows us to probe a large parameter space. We consider two models for star formation history, instantaneous bursts and continuous star formation, a wide range of ages for the old and the young stellar population, along with other discrete parameters, such as the initial mass function, metallicities, internal extinction and extinction law. We find that in the absence of any prior except that the maximum cooling rate < 3000 solar masses/yr, the SFR lies in the range (2230-2890) solar masses/yr. If we impose an observational prior on the internal extinction, E(B-V) < 0.6, the best-fit SFR lies in (454-494) solar masses/yr, and we consider this as the most probable range of SFR values for Phoenix. The SFR dependence on the extinction is a reflection of the standard age-extinction degeneracy, which can be overcome by using a prior on one of the two quantities in question.Comment: 12 pages, 4 figures, 1 Table, accepted for publication in MNRA

Far Ultraviolet Morphology of Star Forming Filaments in Cool Core Brightest Cluster Galaxies

We present a multiwavelength morphological analysis of star forming clouds and filaments in the central (<50 kpc) regions of 16 low redshift ($z 5$ \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 cavity's 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

Free-free absorption in the gravitational lens JVAS B0218+357

will be inserted by hand later

The challenging task of determining star formation rates: the case of a massive stellar burst in the brightest cluster galaxy of Phoenix galaxy cluster

© 2016 The Authors. Star formation in galaxies at the centre of cooling-flow galaxy clusters is an important phenomenon in the context of formation and evolution of massive galaxies in the Universe. Yet, star formation rates (SFRs) in such systems continue to be elusive. We use our Bayesianmotivated spectral energy distribution (SED)-fitting code, BAYESCOOL, to estimate the plausible SFR values in the brightest cluster galaxy of amassive,X-ray luminous galaxy cluster, Phoenix. Previous studies of Phoenix have resulted in the highest measurement of SFR for any galaxy, with the estimates reaching up to 1000M⊙ yr-1. However, a very small number of models have been considered in those studies. BAYESCOOL allows us to probe a large parameter space. We consider two models for star formation history, instantaneous bursts and continuous star formation, a wide range of ages for the old and the young stellar population, along with other discrete parameters, such as the initial mass function, metallicities, internal extinction and extinction law. We find that in the absence of any prior except that the maximum cooling rate < 3000M⊙ yr-1, the SFR lies in the range (2230 - 2890)M⊙ yr-1. If we impose an observational prior on the internal extinction, E(B-V) < 0.6, the best-fitting SFR lies in (454 - 494) M⊙ yr-1, and we consider this as the most probable range of SFR values for Phoenix. The SFR dependence on the extinction is a reflection of the standard age-extinction degeneracy, which can be overcome by using a prior on one of the two quantities in question