Stress Testing Λ\LambdaCDM with High-redshift Galaxy Candidates

Early data from JWST have revealed a bevy of high-redshift galaxy candidates with unexpectedly high stellar masses. I examine these candidates in the context of the most massive galaxies expected in $\Lambda$CDM-like models, wherein the stellar mass of a galaxy is limited by the available baryonic reservoir of its host dark matter halo. For a given cosmology, the abundance of dark matter halos as function of mass and redshift sets an absolute upper limit on the number density $n(>M_{\star},z)$ and stellar mass density $\rho_{\star}(>M_{\star},z)$ of galaxies above a stellar mass limit of $M_{\star}$ at any epoch $z$. The reported masses of the most massive galaxy candidates at $z \sim 10$ in JWST observations are in tension with these limits, indicating an issue with well-developed techniques for photometric selection of galaxies, galaxy stellar mass or effective survey volume estimates, or the $\Lambda$CDM model. That the strongest tension appears at $z \sim 10$, and not (yet?) at the highest redshifts probed by JWST galaxy candidates ($z \sim 16-20$), is promising for tests of the $\Lambda$CDM model using forthcoming wider-area JWST surveys.Comment: 4 pages, 2 figures; submitted to MNRAS Letter

Mapping extragalactic dark matter structures through gamma-rays

If dark matter is composed of neutralinos, the gamma-ray radiation produced in their annihilation offers an attractive possibility for dark matter detection. This process may contribute significantly to the extragalactic gamma-ray background (EGB) radiation, which is being measured by the FERMI satellite with unprecedented sensitivity. Using the high-resolution Millennium-II simulation of cosmic structure formation we have produced the first full-sky maps of the expected contribution of dark matter annihilation to the EGB radiation. Our maps include a proper normalization of the signal according to a specific supersymmetric model based on minimal supergravity. The new simulated maps allow a study of the angular power spectrum of the gamma-ray background from dark matter annihilation, which has distinctive features associated with the nature of the annihilation process. Our results are in broad agreement with analytic models for the gamma-ray background, but they also include higher-order correlations not readily accessible in analytic calculations and, in addition, provide detailed spectral information for each pixel. In particular, we find that color maps combining different energies can reveal the cosmic large-scale structure at low and intermediate redshifts.Comment: 7 pages, 5 figures, 2009 Fermi Symposium, eConf Proceedings C09112

Dynamics of the Magellanic Clouds in a LCDM Universe

We examine Milky Way-Magellanic Cloud systems selected from the Millennium-II Simulation in order to place the orbits of the Magellanic Clouds in a cosmological context. Our analysis shows that satellites massive enough to be LMC analogs are typically accreted at late times. Moreover, those that are accreted at early times and survive to the present have orbital properties that are discrepant with those observed for the LMC. The high velocity of the LMC, coupled with the dearth of unbound orbits seen in the simulation, argues that the mass of the MW's halo is unlikely to be less than 2 x 10^12 Msun. This conclusion is further supported by statistics of halos hosting satellites with masses, velocities, and separations comparable to those of the LMC. We further show that: (1) LMC and SMC-mass objects are not particularly uncommon in MW-mass halos; (2) the apparently high angular momentum of the LMC is not cosmologically unusual; and (3) it is rare for a MW halo to host a LMC-SMC binary system at z=0, but high speed binary pairs accreted at late times are possible. Based on these results, we conclude that the LMC was accreted within the past four Gyr and is currently making its first pericentric passage about the MW.Comment: 14 pages, 13 figures; MNRAS, in press. Minor revisions, conclusions unchange

Dynamical Friction and Galaxy Merging Timescales

The timescale for galaxies within merging dark matter halos to merge with each other is an important ingredient in galaxy formation models. Accurate estimates of merging timescales are required for predictions of astrophysical quantities such as black hole binary merger rates, the build-up of stellar mass in central galaxies, and the statistical properties of satellite galaxies within dark matter halos. In this paper, we study the merging timescales of extended dark matter halos using N-body simulations. We compare these results to standard estimates based on the Chandrasekhar theory of dynamical friction. We find that these standard predictions for merging timescales, which are often used in semi-analytic galaxy formation models, are systematically shorter than those found in simulations. The discrepancy is approximately a factor of 1.7 for $M_sat/M_host \approx 0.1$ and becomes larger for more disparate satellite-to-host mass ratios, reaching a factor of $\sim 3.3$ for $M_sat/M_host\approx 0.01$. Based on our simulations, we propose a new, easily implementable fitting formula that accurately predicts the timescale for an extended satellite to sink from the virial radius of a host halo down to the halo's center for a wide range of $M_sat/M_host$ and orbits. Including a central bulge in each galaxy changes the merging timescale by \la 10%. To highlight one concrete application of our results, we show that merging timescales often used in the literature overestimate the growth of stellar mass by satellite accretion by $\approx 40$, with the extra mass gained in low mass ratio mergers.Comment: 10 pages, 7 figures; MNRAS, in press. Minor revisions, including results from additional simulations with baryonic components; conclusions unchange

Red Mergers and the Assembly of Massive Elliptical Galaxies: the Fundamental Plane and its Projections

Several recent observations suggest that gas-poor (dissipationless) mergers of elliptical galaxies contribute significantly to the build-up of the massive end of the red sequence. We perform a series of major merger simulations to investigate the spatial and velocity structure of the remnants of such mergers. Regardless of orbital energy or angular momentum, we find that the stellar remnants lie on the fundamental plane defined by their progenitors, a result of virial equilibrium with a small tilt due to an increasing central dark matter fraction. However, the locations of merger remnants in the projections of the fundamental plane -- the Faber-Jackson and R_e-M_* relations -- depend strongly on the merger orbit, and the relations steepen significantly from the canonical scalings (L sigma^4 and R_e M_*^0.6) for mergers on radial orbits. Our results imply that the projections of the fundamental plane -- but not necessarily the plane itself -- provide a powerful way of investigating the assembly history of massive elliptical galaxies, including the brightest cluster galaxies at or near the centers of galaxy clusters. We argue that most massive ellipticals are formed by anisotropic merging and that their fundamental plane projections should thus differ noticeably from those of lower mass ellipticals even though they should lie on the same fundamental plane. Current observations are consistent with this conclusion. The steepening in the L-sigma relation for luminous ellipticals may also be reflected in a corresponding steepening in the M_BH-sigma relation for massive black holes.Comment: 10 pages, 4 figures. Accepted for publication in MNRA