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تسجيل مستخدم جديدA census of the stellar content in the protocluster core SPT2349$-$56 at $z,{=},4.3$
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The protocluster core SPT2349$-$56 at $z,{=},4.3$ is one of the most actively star-forming regions known, yet constraints on the total stellar mass of this system are highly uncertain. We have therefore carried out deep optical and infrared observations of this system, probing rest-frame ultraviolet to infrared wavelengths. Using the positions of the spectroscopically-confirmed protocluster members, we identify counterparts and perform detailed source deblending, allowing us to fit spectral energy distributions in order to estimate stellar masses. We show that the galaxies in SPT2349$-$56 have stellar masses proportional to their high star-formation rates, consistent with other protocluster galaxies and field submillimetre galaxies (SMGs) around redshift 4. The galaxies in SPT2349$-$56 have on average lower molecular gas-to-stellar mass fractions and depletion timescales than field SMGs, although with considerable scatter. We construct the stellar-mass function for SPT2349$-$56 and compare it to the stellar-mass function of $z,{=},1$ galaxy clusters, finding both to be best described by a Schechter function. We measure rest-frame ultraviolet half-light radii from our {it HST/}-F160W imaging, finding that on average the galaxies in our sample are similar in size to typical star-forming galaxies around the same redshift. However, the brightest {it HST/}-detected galaxy in our sample, found near the luminosity-weighted centre of the protocluster core, remains unresolved at this wavelength. Hydrodynamical simulations predict that the core galaxies will quickly merge into a brightest cluster galaxy, thus our observations provide a direct view of the early formation mechanisms of this class of object.
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We present an extensive ALMA spectroscopic follow-up programme of the $z,{=},4.3$ structure SPT2349$-$56, one of the most actively star-forming proto-cluster cores known, to identify additional members using their [C{sc ii}] 158,$mu$m and mbox{CO(4--
3)} lines. In addition to robustly detecting the 14 previously published galaxies in this structure, we identify a further 15 associated galaxies at $z,{=},4.3$, resolving 55$,{pm},$5,per cent of the 870-$mu$m flux density at 0.5,arcsec resolution compared to 21,arcsec single-dish data. These galaxies are distributed into a central core containing 23 galaxies extending out to 300,kpc in diameter, and a northern extension, offset from the core by 400,kpc, containing three galaxies. We discovered three additional galaxies in a red {it Herschel/}-SPIRE source 1.5,Mpc from the main structure, suggesting the existence of many other sources at the same redshift as SPT2349$-$56 that are not yet detected in the limited coverage of our data. An analysis of the velocity distribution of the central galaxies indicates that this region may be virialized with a mass of (9$pm$5)$,{times},$10$^{12}$,M$_{odot}$, while the two offset galaxy groups are about 30 and 60,per cent less massive and show significant velocity offsets from the central group. We calculate the [C{sc ii}] and far-infrared number counts, and find evidence for a break in the [C{sc ii}] luminosity function. We estimate the average SFR density within the region of SPT2349$-$56 containing single-dish emission (a proper diametre of 720,kpc), assuming spherical symmetry, to be roughly 4$,{times},10^4$,M$_{odot}$,yr$^{-1}$,Mpc$^{-3}$; this may be an order of magnitude greater than the most extreme examples seen in simulations.
We present Gemini-S and {it Spitzer}-IRAC optical-through-near-IR observations in the field of the SPT2349-56 proto-cluster at $z=4.3$. We detect optical/IR counterparts for only nine of the 14 submillimetre galaxies (SMGs) previously identified by A
LMA in the core of SPT2349-56. In addition, we detect four $zsim4$ Lyman-break galaxies (LBGs) in the 30 arcsec diameter region surrounding this proto-cluster core. Three of the four LBGs are new systems, while one appears to be a counterpart of one of the nine observed SMGs. We identify a candidate brightest cluster galaxy (BCG) with a stellar mass of $(3.2^{+2.5}_{-1.4})times10^{11},{rm M}_{odot}$. The stellar masses of the eight other SMGs place them on, above, and below the main sequence of star formation at $zapprox4.5$. The cumulative stellar mass for the SPT2349-56 core is at least $(11.5pm2.9)times10^{11},{rm M}_{odot}$, a sizeable fraction of the stellar mass in local BCGs, and close to the universal baryon fraction (0.16) relative to the virial mass of the core ($10^{13},{rm M}_{odot}$). As all 14 of these SMGs are destined to quickly merge, we conclude that the proto-cluster core has already developed a significant stellar mass at this early stage, comparable to $z=1$ BCGs. Importantly, we also find that the SPT2349-56 core structure would be difficult to uncover in optical surveys, with none of the ALMA sources being easily identifiable or constrained through $g,r,$ and $i$ colour-selection in deep optical surveys and only a modest overdensity of LBGs over the extended core structure. SPT2349-56 therefore represents a truly dust-obscured phase of a massive cluster core under formation.
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Massive galaxy clusters are now found as early as 3 billion years after the Big Bang, containing stars that formed at even earlier epochs. The high-redshift progenitors of these galaxy clusters, termed protoclusters, are identified in cosmological si
mulations with the highest dark matter overdensities. While their observational signatures are less well defined compared to virialized clusters with a substantial hot intra-cluster medium (ICM), protoclusters are expected to contain extremely massive galaxies that can be observed as luminous starbursts. Recent claimed detections of protoclusters hosting such starbursts do not support the kind of rapid cluster core formation expected in simulations because these structures contain only a handful of starbursting galaxies spread throughout a broad structure, with poor evidence for eventual collapse into a protocluster. Here we report that the source SPT2349-56 consists of at least 14 gas-rich galaxies all lying at z = 4.31 based on sensitive observations of carbon monoxide and ionized carbon. We demonstrate that each of these galaxies is forming stars between 50 and 1000 times faster than our own Milky Way, and all are located within a projected region only $sim$ 130 kiloparsecs in diameter. This galaxy surface density is more than 10 times the average blank field value (integrated over all redshifts) and $>$1000 times the average field volume density. The velocity dispersion ($sim$ 410 km s$^{-1}$) of these galaxies and enormous gas and star formation densities suggest that this system represents a galaxy cluster core at an advanced stage of formation when the Universe was only 1.4 billion years old. A comparison with other known protoclusters at high redshifts shows that SPT2349-56 is a uniquely massive and dense system that could be building one of the most massive structures in the Universe today.
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