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Register a new userNeutral hydrogen surveys for high redshift galaxy clusters and proto-clusters
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We discuss the possibility of performing blind surveys to detect large-scale features of the universe using 21cm emission. Using instruments with approx. 5-10 resolution currently in the planning stage, it should be possible to detect virialized galaxy clusters at intermediate redshifts using the combined emission from their constituent galaxies, as well as less overdense structures, such as proto-clusters and the `cosmic web, at higher redshifts. Using semi-analytic methods we compute the number of virialized objects and those at turnaround which might be detected by such surveys. We find a surprisingly large number of objects might be detected even using small (approx. 5%) bandwidths and elaborate on some issues pertinent to optimising the design of the instrument and the survey strategy. The main uncertainty is the fraction of neutral gas relative to the total dark matter within the object. We discuss this issue in the context of the observations which are currently available.
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We discuss the possibility of performing a substantial spectroscopic galaxy redshift survey selected via the 21cm emission from neutral hydrogen using the Five-hundred metre Aperture Spherical Telescope (FAST) to be built in China. We consider issues related to the estimation of the source counts and optimizations of the survey, and discuss the constraints on cosmological models that such a survey could provide. We find that a survey taking around two years could detect ~10^7 galaxies with an average redshift of ~0.15 making the survey complementary to those already carried out at optical wavelengths. These conservative estimates have used the z=0 HI mass function and have ignored the possibility of evolution. The results could be used to constrain Gamma = (Omega_m h) to 5 per cent and the spectral index, n_s, to 7 per cent independent of cosmic microwave background data. If we also use simulated power spectra from the Planck satellite, we can constrain w to be within 5 per cent of -1.
We present images obtained with LABOCA on the APEX telescope of a sample of 22 galaxies selected via their red Herschel SPIRE 250-, 350- and $500textrm{-}mutextrm{m}$ colors. We aim to see if these luminous, rare and distant galaxies are signposting dense regions in the early Universe. Our $870textrm{-}mutextrm{m}$ survey covers an area of $approx0.8,textrm{deg}^2$ down to an average r.m.s. of $3.9,textrm{mJy beam}^{-1}$, with our five deepest maps going $approx2times$ deeper still. We catalog 86 DSFGs around our signposts, detected above a significance of $3.5sigma$. This implies a $100pm30%$ over-density of $S_{870}>8.5,textrm{mJy}$ DSFGs, excluding our signposts, when comparing our number counts to those in blank fields. Thus, we are $99.93%$ confident that our signposts are pinpointing over-dense regions in the Universe, and $approx95%$ confident that these regions are over-dense by a factor of at least $ge1.5times$. Using template SEDs and SPIRE/LABOCA photometry we derive a median photometric redshift of $z=3.2pm0.2$ for our signposts, with an interquartile range of $z=2.8textrm{-}3.6$. We constrain the DSFGs likely responsible for this over-density to within $|Delta z|le0.65$ of their respective signposts. These associated DSFGs are radially distributed within $1.6pm0.5,textrm{Mpc}$ of their signposts, have median SFRs of $approx(1.0pm0.2)times10^3,M_{odot},textrm{yr}^{-1}$ (for a Salpeter stellar IMF) and median gas reservoirs of $sim1.7times10^{11},M_{odot}$. These candidate proto-clusters have average total SFRs of at least $approx (2.3pm0.5)times10^3,M_{odot},textrm{yr}^{-1}$ and space densities of $sim9times10^{-7},textrm{Mpc}^{-3}$, consistent with the idea that their constituents may evolve to become massive ETGs in the centers of the rich galaxy clusters we see today.
By means of zoom-in hydrodynamic simulations we quantify the amount of neutral hydrogen (HI) hosted by groups and clusters of galaxies. Our simulations, which are based on an improved formulation of smoothed particle hydrodynamics (SPH), include radiative cooling, star formation, metal enrichment and supernova feedback, and can be split in two different groups, depending on whether feedback from active galactic nuclei (AGN) is turned on or off. Simulations are analyzed to account for HI self-shielding and the presence of molecular hydrogen. We find that the mass in neutral hydrogen of dark matter halos monotonically increases with the halo mass and can be well described by a power-law of the form $M_{rm HI}(M,z)propto M^{3/4}$. Our results point out that AGN feedback reduces both the total halo mass and its HI mass, although it is more efficient in removing HI. We conclude that AGN feedback reduces the neutral hydrogen mass of a given halo by $sim50%$, with a weak dependence on halo mass and redshift. The spatial distribution of neutral hydrogen within halos is also affected by AGN feedback, whose effect is to decrease the fraction of HI that resides in the halo inner regions. By extrapolating our results to halos not resolved in our simulations we derive astrophysical implications from the measurements of $Omega_{rm HI}(z)$: halos with circular velocities larger than $sim25~{rm km/s}$ are needed to host HI in order to reproduce observations. We find that only the model with AGN feedback is capable of reproducing the value of $Omega_{rm HI}b_{rm HI}$ derived from available 21cm intensity mapping observations.
In the local Universe, globular clusters (GCs) with metallicities $[{rm Fe}/{rm H}]<-2.5$ are extremely rare. In this Letter, the close connection between GC formation and galaxy evolution is used to show that this GC metallicity `floor results from the galaxy mass-metallicity relation of ultra low-luminosity galaxies (ULLGs) at high redshift, where the most metal-poor GCs must have formed. Galaxies with metallicities $[{rm Fe}/{rm H}]lesssim-2.5$ have too low masses to form GCs with initial masses $M_{rm i}gtrsim10^5~{rm M}_odot$, needed to survive for a Hubble time. This translates the galaxy mass-metallicity relation into a maximum initial cluster mass-metallicity relation for $[{rm Fe}/{rm H}]lesssim-1.8$, which naturally leads to the observed colour-magnitude relation of metal-poor GCs at $z=0$ (the `blue tilt). Its strength traces the slope of the gas phase mass-metallicity relation of ULLGs. Based on the observed blue tilt of GCs in the Virgo and Fornax Clusters, the galaxy mass-metallicity relation is predicted to have a slope of $alpha=0.4pm0.1$ for $10^5lesssim M_star/{rm M}_odotlesssim10^7$ at $zgtrsim2$. The GC metallicity floor implies a minimum host galaxy mass and a maximum redshift for GC formation. Any proto-GCs that may be detected at $z>9$ are most likely to end up in galaxies presently more massive than the Milky Way, whereas GCs in low-mass galaxies such as the Fornax dSph ($M_starapprox4times10^7~{rm M}_odot$) formed at $zlesssim3$.
Francis & Hewett (1993) identified two 10-Mpc scale regions of the high redshift universe that were seemingly very overdense in neutral hydrogen. Subsequent observations showed that at least one of these gas-rich regions enveloped a cluster of galaxies at redshift 2.38. We present improved observations of the three background QSOs with sightlines passing within a few Mpc of this cluster of galaxies. All three QSOs show strong neutral hydrogen absorption at the cluster redshift, suggesting that this cluster (and perhaps all high redshift clusters) may be surrounded by a ~5 Mpc scale region containing ~ 10^12 solar masses of neutral gas. If most high redshift clusters are surrounded by such regions, we show that the gas must be in the form of many small (< 1 kpc), dense (> 0.03 cm^-3) clouds, each of mass < 10^6 solar masses. These clouds are themselves probably gathered into > 20 kpc sized clumps, which may be galaxy halos or protogalaxies. If this gas exists, it will be partially photoionised by the UV background. We predict the diffuse Ly-alpha flux from this photoionisation, and place observational limits on its intensity.
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