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Register a new userLARgE Survey -- II. The Dark Matter Halos and the Progenitors and Descendants of Ultra-Massive Passive Galaxies at Cosmic Noon
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We use a 27.6 deg$^2$ survey to measure the clustering of $gzK_s$-selected quiescent galaxies at $zsim1.6$, focusing on ultra-massive quiescent galaxies. We find that $zsim1.6$ Ultra-Massive Passively Evolving Galaxies (UMPEGs), which have $K_s(AB)<19.75$ (stellar masses of $M_{stars}$ $>sim 10^{11.4}M_{odot}$ and mean $<$$M_{stars}$$>$ = $10^{11.5}M_{odot}$), cluster more strongly than any other known galaxy population at high redshift. Comparing their correlation length, $r_0 = 29.77 pm 2.75$ $ h^{-1}$Mpc, with the clustering of dark matter halos in the Millennium XXL N-body simulation suggests that these $zsim1.6$ UMPEGs reside in dark matter halos of mass $M_{h}sim10^{14.1}h^{-1}M_{odot}$. Such very massive $zsim1.6$ halos are associated with the ancestors of $zsim0$ massive galaxy clusters such as the Virgo and Coma clusters. Given their extreme stellar masses and lack of companions with comparable mass, we surmise that these UMPEGs could be the already-quenched central massive galaxies of their (proto)clusters. We conclude that with only a modest amount of further growth in their stellar mass, $zsim1.6$ UMPEGs could be the progenitors of some of the massive central galaxies of present-day massive galaxy clusters observed to be already very massive and quiescent near the peak epoch of the cosmic star formation.
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We study the environments of a sample of 61 extremely rare z~1.6 Ultra-Massive Passively Evolving Galaxies (UMPEGs: stellar masses M_stars >10^11.5 M_sun) which -- based on clustering analysis presented in Cheema et al. (2020) -- appear to be associated with very massive (M_halo ~ 10^14.1 h^-1 M_sun) dark matter halos that are likely to be the progenitors of z~0 massive (Coma- and Virgo-like) galaxy clusters. We find that UMPEGs on average have fewer than one satellite galaxy with mass ratio M_sat : M_UMPEG >~ 1:5 (i.e., M_sat >~ 10^10.8 M_sun) within 0.5 Mpc; the large mass gap that we observe between the typical UMPEG and its most massive satellite implies that the z~1.6 UMPEGs assembled through major mergers. Using observed satellite counts with merger timescales from the literature, we estimate the growth rate due to mergers with mass ratio of >~ 1:4 to be ~13% Gyr^-1 (with a ~2x systematic uncertainty). This relatively low growth rate is unlikely to significantly affect the shape of the massive end of the stellar mass function, whose evolution must instead be driven by the quenching of new cohorts of ultra-massive star-forming galaxies. However, this growth rate is high enough that, if sustained to z~0, the typical z~1.6 M_UMPEG=10^11.6 M_sun UMPEG can grow into a M_stars~10^12 M_sun brightest cluster galaxy (BCG) of a present-day massive galaxy cluster. Our observations favour a scenario in which our UMPEGs are main-branch progenitors of some of the present-day BCGs that have first assembled through major mergers at high redshifts and grown further through (likely minor) merging at later times.
In our modern understanding of galaxy formation, every galaxy forms within a dark matter halo. The formation and growth of galaxies over time is connected to the growth of the halos in which they form. The advent of large galaxy surveys as well as high-resolution cosmological simulations has provided a new window into the statistical relationship between galaxies and halos and its evolution. Here we define this galaxy-halo connection as the multi-variate distribution of galaxy and halo properties that can be derived from observations and simulations. This connection provides a key test of physical galaxy formation models; it also plays an essential role in constraints of cosmological models using galaxy surveys and in elucidating the properties of dark matter using galaxies. We review techniques for inferring the galaxy-halo connection and the insights that have arisen from these approaches. Some things we have learned are that galaxy formation efficiency is a strong function of halo mass; at its peak in halos around a pivot halo mass of 10^12 Msun, less than 20% of the available baryons have turned into stars by the present day; the intrinsic scatter in galaxy stellar mass is small, less than 0.2 dex at a given halo mass above this pivot mass; below this pivot mass galaxy stellar mass is a strong function of halo mass; the majority of stars over cosmic time were formed in a narrow region around this pivot mass. We also highlight key open questions about how galaxies and halos are connected, including understanding the correlations with secondary properties and the connection of these properties to galaxy clustering.
Clusters, filaments, sheets and voids are the building blocks of the cosmic web. In this study, we present and compare two distinct algorithms for finding cosmic filaments and sheets, a task which is far less well established than the identification of dark matter halos or voids. One method is based on the smoothed dark matter density field, the other uses the halo distributions directly. We apply both techniques to one high resolution N-body simulation and reconstruct the filamentary/sheet like network of the dark matter density field. We focus on investigating the properties of the dark matter halos inside these structures, in particular on the directions of their spins and the orientation of their shapes with respect to the directions of the filaments and sheets. We find that both the spin and the major axes of filament-halos with masses <= 10^{13} M_sun/h are preferentially aligned with the direction of the filaments. The spins and major axes of halos in sheets tend to lie parallel to the sheets. There is an opposite mass dependence of the alignment strengths for the spin (negative) and major (positive) axes, i.e. with increasing halo mass the major axis tends to be more strongly aligned with the direction of the filament whereas the alignment between halo spin and filament becomes weaker with increasing halo mass. The alignment strengths as a function of distance to the most massive node halo indicate that there is a transit large scale environment impact: from the 2-D collapse phase of the filament to the 3-D collapse phase of the cluster/node halo at small separation. Overall, the two algorithms for filament/sheet identification investigated here agree well with each other. The method based on halos alone can be easily adapted for use with observational data sets.
We explore the buildup of quiescent galaxies using a sample of 28,469 massive ($M_star ge 10^{11}$M$_odot$) galaxies at redshifts $1.5<z<3.0$, drawn from a 17.5 deg$^2$ area (0.33 Gpc$^3$ comoving volume at these redshifts). This allows for a robust study of the quiescent fraction as a function of mass at $1.5<z<3.0$ with a sample $sim$40 times larger at log($M_{star}$/$rm M_{odot}$)$ge11.5$ than previous studies. We derive the quiescent fraction using three methods: specific star-formation rate, distance from the main sequence, and UVJ color-color selection. All three methods give similar values at $1.5<z<2.0$, however the results differ by up to a factor of two at $2.0<z<3.0$. At redshifts $1.5 < z < 3.0$ the quiescent fraction increases as a function of stellar mass. By $z=2$, only 3.3 Gyr after the Big Bang, the universe has quenched $sim$25% of $M_star = 10^{11}$M$_odot$ galaxies and $sim$45% of $M_star = 10^{12}$M$_odot$ galaxies. We discuss physical mechanisms across a range of epochs and environments that could explain our results. We compare our results with predictions from hydrodynamical simulations SIMBA and IllustrisTNG and semi-analytic models (SAMs) SAG, SAGE, and Galacticus. The quiescent fraction from IllustrisTNG is higher than our empirical result by a factor of $2-5$, while those from SIMBA and the three SAMs are lower by a factor of $1.5-10$ at $1.5<z<3.0$.
Dark matter as a Bose-Einstein condensate, such as the axionic scalar field particles of String Theory, can explain the coldness of dark matter on large scales. Pioneering simulations in this context predict a rich wave-like structure, with a ground state soliton core in every galaxy surrounded by a halo of excited states that interfere on the de Broglie scale. This de Broglie scale is largest for low mass galaxies as momentum is lower, providing a simple explanation for the wide cores of dwarf spheroidal galaxies. Here we extend these wave dark matter ($psi$DM) predictions to the newly discovered class of Ultra Diffuse Galaxies (UDG) that resemble dwarf spheroidal galaxies but with more extended stellar profiles. Currently the best studied example, DF44, has a uniform velocity dispersion of $simeq 33$km/s, extending to at least 3 kpc, that we show is reproduced by our $psi$DM simulations with a soliton radius of $simeq 0.5$ kpc. In the $psi$DM context, we show the relatively flat dispersion profile of DF44 lies between massive galaxies with compact dense solitons, as may be present in the Milky Way on a scale of 100pc and lower mass galaxies where the velocity dispersion declines centrally within a wide, low density soliton, like Antlia II, of radius 3 kpc.
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