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Register a new userThe Skeleton: Connecting Large Scale Structures to Galaxy Formation
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We report on two quantitative, morphological estimators of the filamentary structure of the Cosmic Web, the so-called global and local skeletons. The first, based on a global study of the matter density gradient flow, allows us to study the connectivity between a density peak and its surroundings, with direct relevance to the anisotropic accretion via cold flows on galactic halos. From the second, based on a local constraint equation involving the derivatives of the field, we can derive predictions for powerful statistics, such as the differential length and the relative saddle to extrema counts of the Cosmic web as a function of density threshold (with application to percolation of structures and connectivity), as well as a theoretical framework to study their cosmic evolution through the onset of gravity-induced non-linearities.
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We used the Nanc{c}ay Radio Telescope (NRT) to measure the 21 cm line emission of near-infrared bright galaxies in the northern Zone of Avoidance (ZoA) without previous redshift determinations. We selected galaxies with extinction-corrected magnitudes $K_s^o leq 11hbox{$.!!^{rm m}$}25$ from the 2MASS Extended Source Catalog. These data will complement the existing 2MASS Redshift Survey (2MRS; first data release) as well as the ongoing 2MASS Tully-Fisher survey, both of which exclude the inner ZoA ($|b|< 5^{circ}$), where the identification of galaxy candidates is the hardest. Of the $sim$1000 identified 2MASX galaxy candidates we have so far detected 252 to our 3.0 mJy rms sensitivity limit and the velocity limit of 10500 km/s. The resulting redshift distribution reveals various new structures that were hitherto uncharted. They seem to form part of the larger Perseus-Pisces Supercluster (PPS). The most conspicuous is a ridge at about $ellapprox 160^{circ}$,$v approx 6500$ km/s. Within this wall-like structure, two strong radio galaxies (3C 129 and 3C 129.1) are embedded which lie at the same distance as the ridge. They seem to form part of an X-ray cluster. Another prominent filament has been identified crossing the ZoA at $ell approx 90^circ$, hence suggesting the second Perseus-Pisces arm is more extended than previously thought.
Intensity mapping of the neutral hydrogen (HI) is a new observational tool that can be used to efficiently map the large-scale structure of the Universe over wide redshift ranges. The power spectrum of the intensity maps contains cosmological information on the matter distribution and probes galaxy evolution by tracing the HI content of galaxies at different redshifts and the scale-dependence of HI clustering. The cross-correlation of intensity maps with galaxy surveys is a robust measure of the power spectrum which diminishes systematics caused by instrumental effects and foreground removal. We examine the cross-correlation signature at redshift z=0.9 using a variant of the semi-analytical galaxy formation model SAGE (Croton et al. 2016) applied to the Millennium simulation in order to model the HI gas of galaxies as well as their optical magnitudes based on their star-formation history. We determine the clustering of the cross-correlation power for different types of galaxies determined by their colours, acting as a proxy for their star-formation activity. We find that the cross-correlation coefficient for red quiescent galaxies falls off more quickly on smaller scales k>0.2h/Mpc than for blue star-forming galaxies. Additionally, we create a mock catalogue of highly star-forming galaxies using a selection function to mimic the WiggleZ survey, and use this to predict existing and future cross-correlation measurements of the GBT and Parkes telescope. We find that the cross-power of highly star-forming galaxies shows a higher clustering on small scales than any other galaxy type and that this significantly alters the power spectrum shape on scales k>0.2h/Mpc. We show that the cross-correlation coefficient is not negligible when interpreting the cosmological cross-power spectrum. On the other hand, it contains information about the HI content of the optically selected galaxies.
[Abridged] We present the first results of hydrodynamical simulations that follow the formation of galaxies to z=0 in spherical regions of radius ~20 Mpc/h drawn from the Millennium Simulation. The regions have overdensities that deviate by (-2, -1, 0, +1, +2)sigma from the cosmic mean, where sigma is the rms mass fluctuation on a scale of ~20Mpc/h at z=1.5. The simulations have mass resolution of up to 10^6 Msun/h, cover the entire range of large-scale environments and allow extrapolation of statistics to the entire 500 (Mpc/h)^3 Millennium volume. They include gas cooling, photoheating from an ionising background, SNe feedback and winds, but no AGN. We find that the specific SFR density at z <~ 10 varies systematically from region to region by up to an order of magnitude, but the global value, averaged over all volumes, reproduces observational data. Massive, compact galaxies, similar to those observed in the GOODS fields, form in the overdense regions as early as z=6, but do not appear in the underdense regions until z~3. These environmental variations are not caused by a dependence of the star formation properties on environment, but rather by a strong variation of the halo mass function from one environment to another, with more massive haloes forming preferentially in the denser regions. At all epochs, stars form most efficiently in haloes of circular velocity ~ 250 km/s. However, the star formation history exhibits a form of downsizing (even in the absence of AGN): the stars comprising massive galaxies at z=0 have mostly formed by z=1-2, whilst those comprising smaller galaxies typically form at later times. However, additional feedback is required to limit star formation in massive galaxies at late times.
The large-scale structure of the Universe formed from initially small perturbations in the cosmic density field, leading to galaxy clusters with up to 10^15 Msun at the present day. Here, we review the formation of structures in the Universe, considering the first primordial galaxies and the most massive galaxy clusters as extreme cases of structure formation where fundamental processes such as gravity, turbulence, cooling and feedback are particularly relevant. The first non-linear objects in the Universe formed in dark matter halos with 10^5-10^8 Msun at redshifts 10-30, leading to the first stars and massive black holes. At later stages, larger scales became non-linear, leading to the formation of galaxy clusters, the most massive objects in the Universe. We describe here their formation via gravitational processes, including the self-similar scaling relations, as well as the observed deviations from such self-similarity and the related non-gravitational physics (cooling, stellar feedback, AGN). While on intermediate cluster scales the self-similar model is in good agreement with the observations, deviations from such self-similarity are apparent in the core regions, where numerical simulations do not reproduce the current observational results. The latter indicates that the interaction of different feedback processes may not be correctly accounted for in current simulations. Both in the most massive clusters of galaxies as well as during the formation of the first objects in the Universe, turbulent structures and shock waves appear to be common, suggesting them to be ubiquitous in the non-linear regime.
We have resimulated the six galaxy-sized haloes of the Aquarius Project including metal-dependent cooling, star formation and supernova feedback. This allows us to study not only how dark matter haloes respond to galaxy formation, but also how this response is affected by details of halo assembly history. In agreement with previous work, we find baryon condensation to lead to increased dark matter concentration. Dark matter density profiles differ substantially in shape from halo to halo when baryons are included, but in all cases the velocity dispersion decreases monotonically with radius. Some haloes show an approximately constant dark matter velocity anisotropy with $ beta approx 0.1-02$, while others retain the anisotropy structure of their baryon-fre
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