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Register a new userThe History of Galaxies and Galaxy Number Counts
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(Abridged) A simple quantitative model is presented for the history of galaxies to explain galaxy number counts, redshift distributions and some other related observations. We first infer that irregular galaxies and the disks of spiral galaxies are young, probably formed at $zapprox 0.5-2$ from a simultaneous consideration of colours and gas content under a moderate assumption on the star formation history. Assuming that elliptical galaxies and bulges of spiral galaxies, both called spheroids in the discussion, had formed early in the universe, the resulting scenario is that spiral galaxies formed as intergalactic gas accreting onto pre-existing bulges mostly at $zapprox 1-2$; irregular galaxies as seen today formed by aggregation of clouds at $zapprox 0.5-1.5$. Taking the formation epochs thus estimated into account, we construct a model for the history of galaxies employing a stellar population synthesis model. We assume that the number of galaxies does not change except that some of them (irregulars) were newly born, and use a morphology-dependent local luminosity function to constrain the number of galaxies. The predictions of the model are compared with the observation of galaxy number counts and redshift distributions for the $B$, $I$ and $K$ colour bands. It is shown that young irregular galaxies cause the steep slope of the $B$-band counts. The fraction of irregular galaxies increases with decreasing brightness: at $B=24$ mag, they contribute as much as spiral galaxies. Thus, ``the faint blue galaxy problem is solved by invoking young galaxies. This interpretation is corroborated by a comparison of our prediction with the morphologically-classified galaxy counts in the $I$ band.
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The far-ultraviolet (FUV) number counts of galaxies constrain the evolution of the star-formation rate density of the universe. We report the FUV number counts computed from FUV imaging of several fields including the Hubble Ultra Deep Field, the Hubble Deep Field North, and small areas within the GOODS-North and -South fields. These data were obtained with the Hubble Space Telescope Solar Blind Channel of the Advance Camera for Surveys. The number counts sample a FUV AB magnitude range from 21-29 and cover a total area of 15.9 arcmin^2, ~4 times larger than the most recent HST FUV study. Our FUV counts intersect bright FUV GALEX counts at 22.5 mag and they show good agreement with recent semi-analytic models based on dark matter merger trees by Somerville et al. (2011). We show that the number counts are ~35% lower than in previous HST studies that use smaller areas. The differences between these studies are likely the result of cosmic variance; our new data cover more lines of sight and more area than previous HST FUV studies. The integrated light from field galaxies is found to contribute between 65.9 +/-8 - 82.6 +/-12 photons/s/cm^2/sr/angstrom to the FUV extragalactic background. These measurements set a lower limit for the total FUV background light.
We use the number counts of X-ray selected normal galaxies to explore their evolution by combining the most recent wide-angle shallow and pencil-beam deep samples available. The differential X-ray number counts, dN/dS, for early and late-type normal galaxies are constructed separately and then compared with the predictions of the local X-ray luminosity function under different evolution scenarios. The dN/dS of early type galaxies is consistent with no evolution out to z~0.5. For late-type galaxies our analysis suggests that it is the sources with X-ray--to--optical flux ratio logfx/fopt>-2 that are evolving the fastest. Including these systems in the late-type galaxy sample yields evolution of the form ~(1+z)^{2.7} out to z~0.4. On the contrary late-type sources with logfx/fopt<-2 are consistent with no evolution. This suggests that the logfx/fopt>-2 population comprises the most powerful and fast evolving starbursts at moderate and high-z. We argue that although residual low-luminosity AGN contamination may bias our results toward stronger evolution, this is unlikely to modify our main conclusions.
We derive and test an approximation for the angular power spectrum of galaxy number counts in the flat sky limit. The standard density and redshift space distortion (RSD) terms in the resulting approximation are distinct to the Limber approximation, providing an accurate result for multipoles as low as $ellsimeq10$, where the corresponding Limber approximation is completely inaccurate. At equal redshift the accuracy of the density and RSD (standard) terms is around 0.2% for $z<3$ and 0.5% at $z=5$, even to $ell<50$. At unequal redshifts, if we consider the total power spectrum, the precision is better than 5% only for very small redshift differences, $delta <delta_0 (simeq 3.6times10^{-4}(1+z)^{2.14})$ where the standard terms are well-approximated, or for large enough redshift differences $delta >delta_1 (simeq 0.33(r(z)H(z))/(z+1))$ where the lensing terms dominate. The flat sky expressions for the pure lensing and the lensing-density cross-correlation terms are equivalent to the Limber approximation. For arbitrary redshift differences, the Limber approximation achieves an accuracy of 0.5% (above $ellsimeq 40$ for pure lensing and $ellsimeq 80$ for density-lensing). Besides being very accurate, the flat sky approximation is computationally much simpler and can therefore be very useful for data analysis and forecasts with MCMC methods. This will be particularly crucial for upcoming galaxy surveys that will measure the power spectrum of galaxy number counts.
We present a fully nonlinear and relativistically covariant expression for the observed galaxy density contrast. Building on a null tetrad tailored to the cosmological observers past light cone, we find a decomposition of the nonlinear galaxy over-density into manifestly gauge-invariant quantities, each of which has a clear physical interpretation as a cosmological observable. This ensures that the monopole of the galaxy over-density field is properly accounted for. We anticipate that this decomposition will be useful for future work on nonlinearities in galaxy number counts, for example, deriving the relativistic expression for the galaxy bispectrum. We then specialise our results to conformal Newtonian gauge, with a Hubble parameter either defined globally or measured locally, illustrating the significance of the different contributions to the observed monopole of the galaxy density.
(abridged) A detailed comparison is performed of the LFs compiled at infrared, radio and optical wavelengths and converted into XLFs using available relations with the XLF directly estimated in the 0.5--2 keV energy band from X-ray surveys (Norman et al). We find that the XLF from the local sample of IRAS galaxies (Takeuchi et al) provides a good representation of all available data samples; pure luminosity evolution of the form (1+z)^eta, with eta< ~3, is favoured over pure density. The local X-ray luminosity density is also well defined. We discuss different estimates of the galaxies LogN-LogS, selected from the Chandra Deep Fields with different selection criteria: these have similar slopes, but normalisations scattered within a factor ~2, of the same order of the Poissonian error on the counts. We compare the observed LogN-LogS with the counts predicted by integrating our reference z=0 XLF. By using number counts alone, it is not possible to discriminate between density and luminosity evolution; however, the evolution of galaxies must be stopped in both cases at z~1-2. The contribution of galaxies to the X-ray background is found to be in the range 6%--12%. Making use of cosmic star formation models, we find that the X-ray LogN-LogS might be not compatible with very large star formation rates at z ~ 3 as suggested by sub-mm observations in Blain et al. 1999. As to the content of current and, possibly, future X-ray surveys, we determine the fraction of galaxies around the current flux limit: (30+-12 %). At fainter fluxes the fraction of galaxies will probably rise, and overcome the counts from AGN at fluxes < ~10^{-17} erg/s/cm^2.
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