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Register a new userThe spatial distribution of galaxies of different spectral types in the massive intermediate-redshift cluster MACSJ0717.5+3745
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We present the results of a wide-field spectroscopic analysis of the galaxy population of the massive cluster MACSJ0717.5+3745 and the surrounding filamentary structure (z=0.55), as part of our systematic study of the 12 most distant clusters in the MACS sample. Of 1368 galaxies spectroscopically observed in this field, 563 are identified as cluster members; of those, 203 are classified as emission-line galaxies, 260 as absorption-line galaxies, and 17 as E+A galaxies (defined by $frac{H_{delta}+H_{gamma}}{2}>6$AA and no detection of [OII] and $H_{beta}$ in emission). The variation of the fraction of emission- and absorption-line galaxies as a function of local projected galaxy density confirms the well-known morphology-density relation, and becomes flat at projected galaxy densities less than $sim 20Mpc^{-2}. Interestingly, 16 out of 17 E+A galaxies lie (in projection) within the ram-pressure stripping radius around the cluster core, which we take to be direct evidence of ram-pressure stripping being the primary mechanism that terminates star-formation in the E+A population of galaxy clusters. This conclusion is supported by the rarity of E+A galaxies in the filament which rules out galaxy mergers as the dominant driver of evolution for E+A galaxies in clusters. In addition, we find the 42 e(a) and 27 e(b) member galaxies, i.e., the dusty-starburst and starburst galaxies respectively, to be spread out across almost the entire study area. Their spatial distribution, which shows a strong preference for the filament region, suggests that starbursts are triggered in relatively low-density environments as galaxies are accreted from the field population.
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Using CHANDRA, we investigate the spatial temperature distribution of the intracluster medium (ICM) within 700 kpc of the center of the massive merging cluster MACSJ0717.5+3745 at z=0.55. Combining the X-ray evidence with information about the distribution and velocities of the cluster galaxies near the core provides us with a snapshot of the three-dimensional geometry and dynamics of one of the most complex cluster studied to date. We find MACSJ0717.5+3745 to be an active triple merger with ICM temperatures exceeding 20 keV. Although radial velocity information and X-ray/optical offsets indicate that all three mergers proceed along distinctly different directions, the partial alignment of the merger axes points to a common origin in the large-scale filament south-east of the cluster core. Clear decrements in the ICM temperature observed near two of these subclusters identify the respective X-ray surface brightness peaks as remnants of cool cores; the compactness and low temperature of 5.7 keV of one of these features suggest that the respective merger, a high-velocity collision at 3,000 km/s, is still in its very early stages. Looking beyond the triple merger, we find the large-scale filament to not only provide a spatial as well as temporal arrow for the interpretation of the dynamics of the merger events near the cluster core; we also find tantalizing, if circumstantial, evidence for direct, large-scale heating of the ICM by contiguous infall of low-density gas from the filament.
We examine the latest data on the cluster MACSJ0717.5+3745 from the Hubble Frontier Fields campaign. The critically lensed area is the largest known of any lens and very irregular making it a challenge for parametric modelling. Using our Free-Form method we obtain an accurate solution, identify here many new sets of multiple images, doubling the number of constraints and improving the reconstruction of the dark matter distribution. Our reconstructed mass map shows several distinct central substructures with shallow density profiles, clarifying earlier work and defining well the relation between the dark matter distribution and the luminous and X-ray peaks within the critically lensed region. Using our free-form method, we are able to meaningfully subtract the mass contribution from cluster members to the deflection field to trace the smoothly distributed cluster dark matter distribution. We find 4 distinct concentrations, 3 of which are coincident with the luminous matter. The fourth peak has a significant offset from both the closest luminous and X-ray peaks. These findings, together with dynamical data from the motions of galaxies and gas will be important for uncovering the potentially important implications of this extremely massive and intriguing system.
We report the first weak-lensing detection of a large-scale filament funneling matter onto the core of the massive galaxy cluster MACSJ0717.5+3745. Our analysis is based on a mosaic of 18 multi-passband images obtained with ACS aboard the HST, covering an area of sim 10x20 arcmin^2. We use a weak-lensing pipeline developed for the COSMOS survey, modified for the analysis of galaxy clusters, to produce a weak-lensing catalogue. A mass map is then computed by applying a weak-gravitational-lensing multi-scale reconstruction technique designed to describe irregular mass distributions such as the one investigated here. We test the resulting mass map by comparing the mass distribution inferred for the cluster core with the one derived from strong-lensing constraints and find excellent agreement. The filament is detected within the 3 sigma detection contour of the lensing mass reconstruction, and underlines the importance of filaments for theoretical and numerical models of the mass distribution in the Cosmic Web. We measure the filaments projected length as sim 4.5 h_{74}^{-1} Mpc, and its mean density as (2.92 pm 0.66)10^8 h_{74} M_{odot} kpc^{-2}. Combined with the redshift distribution of galaxies obtained after an extensive spectroscopic follow-up in the area, we can rule out any projection effect resulting from the chance alignment on the sky of unrelated galaxy group-scale structures. Assuming plausible constraints concerning the structures geometry based on its galaxy velocity field, we construct a 3D model of the large-scale filament. Within this framework, we derive the three-dimensional length of the filament to be 18 h_{74}^{-1} Mpc, and a deprojected density in terms of the critical density of the Universe of (206 pm 46) rho_{crit}, a value that lies at the very high end of the range predicted by numerical simulations.
MACS J0717 is the most massive and extended of the Hubble Frontier Field clusters. It is one of the more difficult clusters to model, and we argue that this is in part due to the line of sight structure (LoS) at redshifts beyond 2. We show that the Grale mass reconstruction based on sources at 3<z_s<4.1 has at least 10^{13}M_sun more mass than that based on nearby sources, z_s<2.6, and attribute the excess mass to a putative LoS, which is at least 75 from the cluster center. Furthermore, the lens-model fitted z_ss of the recent Kawamata et al. reconstruction are biased systematically low compared to photometric z_ss, and the bias is a function of images distance from the cluster center. We argue that these mimic the effect of LoS. We conclude that even in the presence of 100-200 images, lens-model adjusted source redshifts can conceal the presence of LoS, demonstrating the existence of degeneracies between z_s and (sub)structure. Also, a very good fit to image positions is not a sufficient condition for having a high fidelity mass map: Kawamata et al. obtain an rms of 0.52 for 173 images of 60 sources; our Grale reconstruction of the exact same data yields a somewhat different map, but similarly low rms, 0.62. In contrast, a Grale model that uses reasonable, but fixed z_s gives a worse rms of 1.28 for 44 sources with 126 images. Unaccounted for LoS can bias the mass map, affecting the magnification and luminosity function estimates of high redshift sources.
In this paper we present the results of our search for and study of $z gtrsim 6$ galaxy candidates behind the third Frontier Fields (FF) cluster, MACSJ0717.5+3745, and its parallel field, combining data from Hubble and Spitzer. We select 39 candidates using the Lyman Break technique, for which the clear non-detection in optical make the extreme mid-$z$ interlopers hypothesis unlikely. We also take benefit from $z gtrsim 6$ samples selected using previous Frontier Fields datasets of Abell 2744 and MACS0416 to improve the constraints on the properties of very high-redshift objects. We compute the redshift and the physical properties, such emission lines properties, star formation rate, reddening, and stellar mass for all Frontier Fields objects from their spectral energy distribution using templates including nebular emission lines. We study the relationship between several physical properties and confirm the trend already observed in previous surveys for evolution of star formation rate with galaxy mass, and between the size and the UV luminosity of our candidates. The analysis of the evolution of the UV Luminosity Function with redshift seems more compatible with an evolution of density. Moreover, no robust $zge$8.5 object is selected behind the cluster field, and few $z$$sim$9 candidates have been selected in the two previous datasets from this legacy survey, suggesting a strong evolution in the number density of galaxies between $z$$sim$8 and 9. Thanks to the use of the lensing cluster, we study the evolution of the star formation rate density produced by galaxies with L$>$0.03L$^{star}$, and confirm the strong decrease observed between $z$$sim$8 and 9.
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