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Register a new userThe emergence of passive galaxies in the early Universe
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The emergence of passive galaxies in the early Universe results from the interplay among the processes responsible for their rapid assembly and for the abrupt shut-down of their SF. Investigating the individual properties and demographics of early passive galaxies will improve our understanding of these mechanisms. In this work we present a follow-up analysis of the z>3 passive galaxy candidates selected by Merlin et al. (2019) in the CANDELS fields. We begin by first confirming the accuracy of their passive classification by exploiting their sub-mm emission to demonstrate the lack of ongoing SF. Using archival ALMA observations we are able to confirm at least 61% of the observed candidates as passive. While the remainder lack sufficiently deep data for confirmation, we are able to validate the entire sample in a statistical sense. We then estimate the Stellar Mass Function (SMF) of all 101 passive candidates in three redshift bins from z=5 to z=3. We adopt a stepwise approach that has the advantage of taking into account photometric errors, observational incompleteness, and the Eddington bias without any a-posteriori correction. We observe a pronounced evolution in the SMF around z~4, indicating that we are witnessing the emergence of the passive population at this epoch. Massive (M>10^11Msun) passive galaxies, only accounting for a small (<10%) fraction of galaxies at z>4, become dominant at later epochs. Thanks to a combination of photometric quality, sample selection and methodology, we overall find a higher density of passive galaxies than previous works. The comparison with theoretical predictions, despite a qualitative agreement, denotes a still incomplete understanding of the physical processes responsible for the formation of these galaxies. Finally, we extrapolate our results to predict the number of early passive galaxies expected in surveys carried out with future facilities.
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The selection of red, passive galaxies in the early Universe is very challenging, especially beyond z~3, and it is crucial to constrain theoretical modelling of the processes responsible for their rapid assembly and abrupt shut-down of the star formation. We present here the analysis of ALMA archival observations of 26 out of the 30 galaxies in the deep CANDELS GOODS-South field that we identified as passive at z~3-5 by means of a careful and conservative SED fitting analysis. ALMA data are used to verify the potential contamination from red, dusty but star--forming sources that could enter the sample due to similar optical--nearIR colours. With the exception of a few marginal detections at <3sigma, we could only infer upper limits, both on individual sources and on the stacks. We translated the ALMA continuum measurements into corresponding SFRs, using a variety of far-IR models. These SFRs are compared with those predicted by secondary star-forming solutions of the optical fits and with the expected position of the star formation Main Sequence. This analysis confirms the passive nature of 9 candidates with high confidence and suggests that the classification is correct for at least half of the sample in a statistical sense. For the remaining sources the analysis remain inconclusive because available ALMA data is not deep enough, although the stacking results corroborate their passive nature. Despite the uncertainties, this work provides decisive support to the existence of passive galaxies beyond z~3.
Volonteri et al. (2011) found that the number of radio-loud quasars above redshift 4 calculated from the luminosity function (based upon Swift/BAT observations) is much smaller than the number estimated from the known high-redshift beamed sources, blazars, assuming that for every beamed source with a Lorentz factor of $Gamma$, statistically $2 Gamma^2$ non-beamed sources should exist. To explain the missing misaligned (non-beamed) population of high-redshift sources, they proposed various explanations, involving heavy optical obscuration and significantly different Lorentz factors at early cosmological epochs. Our EVN observations targeting high-redshift ($z>4$) blazar candidates revealed 3 sources not showing relativistic beaming, but rather kpc-scale double structures. These three sources have significant radio emission resolved out with the EVN, while they are compact on $sim 5-10$ arcsec scale. Our dual-frequency ($1.5$ and $5$ GHz) e-MERLIN observations of these three sources revealed a rich morphology, bending jets, and hot spots with possible sites of interaction between the jets and the surrounding medium at intermediate scales.
We search the five CANDELS fields (COSMOS, EGS, GOODS-North, GOODS-South and UDS) for passively evolving a.k.a. red and dead massive galaxies in the first 2 Gyr after the Big Bang, integrating and updating the work on GOODS-South presented in our previous paper. We perform SED-fitting on photometric data, with top-hat star-formation histories to model an early and abrupt quenching, and using a probabilistic approach to select only robust candidates. Using libraries without (with) spectral lines emission, starting from a total of more than 20,000 $z>3$ sources we end up with 102 (40) candidates, including one at $z=6.7$. This implies a minimal number density of $1.73 pm 0.17 times 10^{-5}$ ($6.69 pm 1.08 times 10^{-6}$) Mpc$^{-3}$ for $3<z<5$; applying a correction factor to account for incompleteness yields $2.30 pm 0.20 times 10^{-5}$. We compare these values with those from five recent hydrodynamical cosmological simulations, finding a reasonable agreement at $z<4$; tensions arise at earlier epochs. Finally, we use the star-formation histories from the best-fit models to estimate the contribution of the high-redshift passive galaxies to the global Star Formation Rate Density during their phase of activity, finding that they account for $sim5-10%$ of the total star formation at $3<z<8$, despite being only $sim0.5%$ of the total in number. The resulting picture is that early and strong star formation activity, building massive galaxies on short timescales and followed by a quick and abrupt quenching, is a rare but crucial phenomenon in the early Universe: the evolution of the cosmos must be heavily influenced by the short but powerful activity of these pristine monsters.
Using the Renaissance suite of simulations we examine the emergence of pristine atomic cooling haloes that are both metal-free and star-free in the early Universe. The absence of metals prevents catastrophic cooling, suppresses fragmentation, and may allow for the formation of massive black hole seeds. Here we report on the abundance of pristine atomic cooling haloes found and on the specific physical conditions that allow for the formation of these direct-collapse-black-hole (DCBH) haloes. In total in our simulations we find that 79 DCBH haloes form before a redshift of 11.6. We find that the formation of pristine atomic haloes is driven by the rapid assembly of the atomic cooling haloes with mergers, both minor and/or major, prior to reaching the atomic cooling limit a requirement. However, the ability of assembling haloes to remain free of (external) metal enrichment is equally important and underlines the necessity of following the transport of metals in such simulations. The candidate DCBH hosting haloes we find, have been exposed to mean Lyman-Werner radiation fields of J$_{LW}$ $sim$ 1 J$_{21}$ and typically lie at least 10 kpc (physical) from the nearest massive galaxy. Growth rates of the haloes reach values of greater than 10$^7$ M$_{odot}$ per unit redshift, leading to significant dynamical heating and the suppression of efficient cooling until the halo crosses the atomic cooling threshold. Finally, we also find five synchronised halo candidates where pairs of pristine atomic cooling haloes emerge that are both spatially and temporally synchronised.
We compare the sizes and luminosities of faint $z=6$-8 galaxies magnified by the Hubble Frontier Fields (HFF) clusters with star-forming regions, as well as more evolved objects, in the nearby universe. Our high-redshift comparison sample includes 333 z=6-8 galaxies, for which size measurements were made as part of a companion study where lensing magnifications were estimated from various public models. Accurate size measurements for these sources are complicated by the lens model uncertainties, but other results and arguments suggest that faint galaxies are small, as discussed in a companion study. The measured sizes for sources in our comparison sample range from <50 pc to ~500 pc. For many of the lowest luminosity sources, extremely small sizes are inferred, reaching individual sizes as small as 10-30 pc, with several sources in the 10-15 pc range with our conservative magnification limits. The sizes and luminosities are similar to those of single star cluster complexes like 30 Doradus in the lower-redshift universe and -- in a few cases -- super star clusters. The identification of these compact, faint star-forming sources in the z~6-8 universe also allows us to set upper limits on the proto-globular cluster LF at z~6. By comparisons of the counts and sizes with recent models, we rule out (with some caveats) proto-globular cluster formation scenarios favoring substantial (xi=10) post-formation mass loss and set useful upper limits on others. Our size results suggest we may be very close to discovering a bona-fide population of forming globular clusters at high redshift.
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