424 research outputs found
The bias of the submillimetre galaxy population: SMGs are poor tracers of the most massive structures in the z ~ 2 Universe
It is often claimed that overdensities of (or even individual bright)
submillimetre-selected galaxies (SMGs) trace the assembly of the most-massive
dark matter structures in the Universe. We test this claim by performing a
counts-in-cells analysis of mock SMG catalogues derived from the Bolshoi
cosmological simulation to investigate how well SMG associations trace the
underlying dark matter structure. We find that SMGs exhibit a relatively
complex bias: some regions of high SMG overdensity are underdense in terms of
dark matter mass, and some regions of high dark matter overdensity contain no
SMGs. Because of their rarity, Poisson noise causes scatter in the SMG
overdensity at fixed dark matter overdensity. Consequently, rich associations
of less-luminous, more-abundant galaxies (i.e. Lyman-break galaxy analogues)
trace the highest dark matter overdensities much better than SMGs. Even on
average, SMG associations are relatively poor tracers of the most significant
dark matter overdensities because of 'downsizing': at z < ~2.5, the
most-massive galaxies that reside in the highest dark matter overdensities have
already had their star formation quenched and are thus no longer SMGs. At a
given redshift, of the 10 per cent most-massive overdensities, only ~25 per
cent contain at least one SMG, and less than a few per cent contain more than
one SMG.Comment: 6 pages, 3 figures, 1 table; accepted for publication in MNRAS; minor
revisions from previous version, conclusions unchange
The prefrontal cortex of the bottlenose dolphin (Tursiops truncatus Montagu, 1821): a tractography study and comparison with the human
Cetaceans are well known for their remarkable cognitive abilities including self-recognition, sound imitation and decision making. In other mammals, the prefrontal cortex (PFC) takes a key role in such cognitive feats. In cetaceans, however, a PFC could up to now not be discerned based on its usual topography. Classical in vivo methods like tract tracing are legally not possible to perform in Cetacea, leaving diffusion-weighted imaging (DWI) as the most viable alternative. This is the first investigation focussed on the identification of the cetacean PFC homologue. In our study, we applied the constrained spherical deconvolution (CSD) algorithm on 3Â T DWI scans of three formalin-fixed brains of bottlenose dolphins (Tursiops truncatus) and compared the obtained results to human brains, using the same methodology. We first identified fibres related to the medio-dorsal thalamic nuclei (MD) and then seeded the obtained putative PFC in the dolphin as well as the known PFC in humans. Our results outlined the dolphin PFC in areas not previously studied, in the cranio-lateral, ectolateral and opercular gyri, and furthermore demonstrated a similar connectivity pattern between the human and dolphin PFC. The antero-lateral rotation of the PFC, like in other areas, might be the result of the telescoping process which occurred in these animals during evolution
Investigating Overdensities around z > 6 Galaxies through ALMA Observations of [C II]
We present a search for companion [C II] emitters to known luminous sources at 6 < z < 6.5 in deep, archival ALMA observations. The observations are deep enough to detect sources with L_([CII])âŒ10âž at z âŒ6. We identify three new robust line detections from a blind search of five deep fields centered on ultraluminous infrared galaxies and QSOs. We calculate the volume density of companions and find a relative overdensity of 6âșâŽââ and 86âșâ¶â°âââ when comparing to current observational constraints and theoretical predictions, respectively. These results suggest that the central sources may be highly biased tracers of mass in the early universe. We find these companion lines to have comparable properties to other known galaxies at the same epoch. All companions lie less than 650 km sâ»Âč and between 25 and 60 kpc (projected) from their central source. To place these discoveries in context, we employ a mock galaxy catalog to estimate the luminosity function for [C II] during reionization and compare to our observations. The simulations support this result by showing a similar level of elevated counts found around such luminous [C II] sources
Reproducing the Stellar Mass/Halo Mass Relation in Simulated LCDM Galaxies: Theory vs Observational Estimates
We examine the present-day total stellar-to-halo mass (SHM) ratio as a
function of halo mass for a new sample of simulated field galaxies using fully
cosmological, LCDM, high resolution SPH + N-Body simulations.These simulations
include an explicit treatment of metal line cooling, dust and self-shielding,
H2 based star formation and supernova driven gas outflows. The 18 simulated
halos have masses ranging from a few times 10^8 to nearly 10^12 solar masses.
At z=0 our simulated galaxies have a baryon content and morphology typical of
field galaxies. Over a stellar mass range of 2.2 x 10^3 to 4.5 x 10^10 solar
masses, we find extremely good agreement between the SHM ratio in simulations
and the present-day predictions from the statistical Abundance Matching
Technique presented in Moster et al. (2012). This improvement over past
simulations is due to a number systematic factors, each decreasing the SHM
ratios: 1) gas outflows that reduce the overall SF efficiency but allow for the
formation of a cold gas component 2) estimating the stellar masses of simulated
galaxies using artificial observations and photometric techniques similar to
those used in observations and 3) accounting for a systematic, up to 30 percent
overestimate in total halo masses in DM-only simulations, due to the neglect of
baryon loss over cosmic times. Our analysis suggests that stellar mass
estimates based on photometric magnitudes can underestimate the contribution of
old stellar populations to the total stellar mass, leading to stellar mass
errors of up to 50 percent for individual galaxies. These results highlight the
importance of using proper techniques to compare simulations with observations
and reduce the perceived tension between the star formation efficiency in
galaxy formation models and in real galaxies.Comment: Submitted to ApJ 9 pages, 5 figure
An Empirical Mass Function Distribution
The halo mass function, encoding the comoving number density of dark matter halos of a given mass, plays a key role in understanding the formation and evolution of galaxies. As such, it is a key goal of current and future deep optical surveys to constrain the mass function down to mass scales that typically host galaxies. Motivated by the proven accuracy of PressâSchechter-type mass functions, we introduce a related but purely empirical form consistent with standard formulae to better than 4% in the medium-mass regime, {10}^{10}\mbox{--}{10}^{13}\,{h}^{-1}Mâ. In particular, our form consists of four parameters, each of which has a simple interpretation, and can be directly related to parameters of the galaxy distribution, such as {L}_{\star }$. Using this form within a hierarchical Bayesian likelihood model, we show how individual mass-measurement errors can be successfully included in a typical analysis, while accounting for Eddington bias. We apply our form to a question of survey design in the context of a semi-realistic data model, illustrating how it can be used to obtain optimal balance between survey depth and angular coverage for constraints on mass function parameters. Open-source Python and R codes to apply our new form are provided at http://mrpy.readthedocs.org and https://cran.r-project.org/web/packages/tggd/index.html respectively
Faint dwarfs as a test of DM models: WDM versus CDM
We use high-resolution Hydro+N-Body cosmological simulations to compare the assembly and evolution of a small field dwarf (stellar mass âŒ106â7Mâ, total mass 1010Mâ) in Î-dominated cold dark matter (CDM) and 2keV warm dark matter (WDM) cosmologies. We find that star formation (SF) in the WDM model is reduced and delayed by 1-2Gyr relative to the CDM model, independently of the details of SF and feedback. Independent of the dark matter (DM) model, but proportionally to the SF efficiency, gas outflows lower the central mass density through âdynamical heating', such that all realizations have circular velocities <20âkmâsâ1 at 500pc, in agreement with local kinematic constraints. As a result of dynamical heating, older stars are less centrally concentrated than younger stars, similar to stellar population gradients observed in nearby dwarf galaxies. Introducing an important diagnostic of SF and feedback models, we translate our simulations into artificial colour-magnitude diagrams and star formation histories (SFHs) in order to directly compare to available observations. The simulated galaxies formed most of their stars in many âŒ10Myr long bursts. The CDM galaxy has a global SFH, Hâi abundance and Fe/H and alpha-elements distribution well matched to current observations of dwarf galaxies. These results highlight the importance of directly including âbaryon physics' in simulations when (1) comparing predictions of galaxy formation models with the kinematics and number density of local dwarf galaxies and (2) differentiating between CDM and non-standard models with different DM or power spectr
Bailing Out the Milky Way: Variation in the Properties of Massive Dwarfs Among Galaxy-Sized Systems
Recent kinematical constraints on the internal densities of the Milky Way's
dwarf satellites have revealed a discrepancy with the subhalo populations of
simulated Galaxy-scale halos in the standard CDM model of hierarchical
structure formation. This has been dubbed the "too big to fail" problem, with
reference to the improbability of large and invisible companions existing in
the Galactic environment. In this paper, we argue that both the Milky Way
observations and simulated subhalos are consistent with the predictions of the
standard model for structure formation. Specifically, we show that there is
significant variation in the properties of subhalos among distinct host halos
of fixed mass and suggest that this can reasonably account for the deficit of
dense satellites in the Milky Way. We exploit well-tested analytic techniques
to predict the properties in a large sample of distinct host halos with a
variety of masses spanning the range expected of the Galactic halo. The
analytic model produces subhalo populations consistent with both Via Lactea II
and Aquarius, and our results suggest that natural variation in subhalo
properties suffices to explain the discrepancy between Milky Way satellite
kinematics and these numerical simulations. At least ~10% of Milky Way-sized
halos host subhalo populations for which there is no "too big to fail" problem,
even when the host halo mass is as large as M_host = 10^12.2 h^-1 M_sun.
Follow-up studies consisting of high-resolution simulations of a large number
of Milky Way-sized hosts are necessary to confirm our predictions. In the
absence of such efforts, the "too big to fail" problem does not appear to be a
significant challenge to the standard model of hierarchical formation.
[abridged]Comment: 12 pages, 3 figures; accepted by JCAP. Replaced with published
versio
Gas Accretion and Galactic Chemical Evolution: Theory and Observations
This chapter reviews how galactic inflows influence galaxy metallicity. The
goal is to discuss predictions from theoretical models, but particular emphasis
is placed on the insights that result from using models to interpret
observations. Even as the classical G-dwarf problem endures in the latest round
of observational confirmation, a rich and tantalizing new phenomenology of
relationships between , , SFR, and gas fraction is emerging both in
observations and in theoretical models. A consensus interpretation is emerging
in which star-forming galaxies do most of their growing in a quiescent way that
balances gas inflows and gas processing, and metal dilution with enrichment.
Models that explicitly invoke this idea via equilibrium conditions can be used
to infer inflow rates from observations, while models that do not assume
equilibrium growth tend to recover it self-consistently. Mergers are an overall
subdominant mechanism for delivering fresh gas to galaxies, but they trigger
radial flows of previously-accreted gas that flatten radial gas-phase
metallicity gradients and temporarily suppress central metallicities. Radial
gradients are generically expected to be steep at early times and then
flattened by mergers and enriched inflows of recycled gas at late times.
However, further theoretical work is required in order to understand how to
interpret observations. Likewise, more observational work is needed in order to
understand how metallicity gradients evolve to high redshifts.Comment: Invited review to appear in Gas Accretion onto Galaxies, Astrophysics
and Space Science Library, eds. A. J. Fox & R. Dav\'e, to be published by
Springer. 29 pages, 2 figure
Conformal linear gravity in de Sitter space II
From the group theoretical point of view, it is proved that the theory of
linear conformal gravity should be written in terms of a tensor field of rank-3
and mixed symmetry [Binegar, et al, Phys. Rev. D 27, (1983) 2249]. We obtained
such a field equation in de Sitter space [Takook, et al, J. Math. Phys. 51,
(2010) 032503]. In this paper, a proper solution to this equation is obtained
as a product of a generalized polarization tensor and a massless scalar field
and then the conformally invariant two-point function is calculated. This
two-point function is de Sitter invariant and free of any pathological
large-distance behavior.Comment: 16 pages, no figure, published versio
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