Galaxy Number Counts Research Articles

Testing general relativity with cosmological large scale structure

In this paper I investigate the possibility to test Einstein’s equations with observations of cosmological large scale structure. I first show that we have not tested the equations in observations concerning only the homogeneous and isotropic Universe. I then show with several examples how we can do better when considering the fluctuations of both, the energy momentum tensor and the metric. This is illustrated with galaxy number counts, intensity mapping and cosmic shear, three examples that are by no means exhaustive.

Questioning Planck-selected star-forming high-redshift galaxy protoclusters and their fate

About 2100 star-forming galaxy protocluster candidates at z ∼ 1 − 4 were identified at sub-millimetre wavelengths in the Planck all-sky survey. Follow-up spectroscopic observations of a few candidates have confirmed the presence of actual galaxy overdensities with large star formation rates (SFRs). In this work, we use state-of-the-art hydrodynamical simulations to investigate whether the Planck high-z sub-millimetre sources (PHz) are progenitors of massive clusters at z = 0. To match the PHz sources with simulated halos, we select the most star-forming (SF) halos in 19 redshift bins from z = 3 to z = 1.3 in the TNG300 simulation of the IllustrisTNG project. At each redshift, the total SFR of the simulated protocluster candidates is computed from the SFR of all the galaxies within an aperture corresponding to the Planck beam size, including those along the line of sight (LOS). The simulations reproduce the Planck-derived SFRs as the sum of both the SFR of at least one of the most SF high-z halos and the average contribution from SF sources along the LOS. Focusing on the spectroscopically confirmed z ∼ 2 PHz protoclusters, we compare the observed properties of their galaxy members with those in the most SF simulated halos. We find a good agreement in the stellar mass and SFR distributions, and in the galaxy number counts, but the SFR-stellar mass relation of the simulated galaxies tends to be shifted to lower SFRs with respect to the observed galaxies. Based on the estimated final masses of the simulated halos, we infer that between 63% and 72% of the Planck-selected protoclusters will evolve into massive galaxy clusters by z = 0. Despite contamination from star-forming galaxies along the LOS, we thus confirm the efficiency of Planck in selecting star-forming protoclusters at cosmic noon with the simulations, and provide a new criterion for selecting the most massive cluster progenitors at high-z, using observables such as the number of galaxy members and their SFR distribution.

Improvement of cosmological constraints with the cross-correlation between line-of-sight optical galaxy and FRB dispersion measures

Fast Radio Bursts (hereafter FRBs) can be used in cosmology by studying the Dispersion Measure (hereafter DM) as a function of redshift. The large scale structure of matter distribution is regarded as a major error budget for such application. Using optical galaxy and dispersion measure mocks built from N-body simulations, we have shown that the galaxy number density can be used as a tracer for large scale electron density and help improve the measurement of DM as a function of redshift. We have shown that, using the line-of-sight galaxy number counts within 1' around the given localized FRB source can help improve the cosmological parameter constraints by more than 20%.

An estimator for the lensing potential from galaxy number counts

We derive an estimator for the lensing potential from galaxy number counts which contains a linear and a quadratic term. We show that this estimator has a much larger signal-to-noise ratio than the corresponding estimator from intensity mapping. We show that this is due to the additional lensing term in the number count angular power spectrum which is present already at linear order. We estimate the signal-to-noise ratio for future photometric surveys. We find that particularly at high redshifts, z ≳ 1.5, the signal to noise ratio can become of order 30. We therefore claim that number counts in photometric surveys are an excellent means to measure tomographic lensing spectra.

Galaxy and Mass Assembly (GAMA): The Weak Environmental Dependence of Quasar Activity at 0.1 < z < 0.35

Abstract Understanding the connection between nuclear activity and galaxy environment remains critical in constraining models of galaxy evolution. By exploiting the extensive cataloged data from the Galaxy and Mass Assembly survey, we identify a representative sample of 205 quasars at 0.1 < z < 0.35 and establish a comparison sample of galaxies, closely matched to the quasar sample in terms of both stellar mass and redshift. On scales <1 Mpc, the galaxy number counts and group membership of quasars appear entirely consistent with those of the matched galaxy sample. Despite this, we find that quasars are ∼1.5 times more likely to be classified as the group center, indicating a potential link between quasar activity and cold gas flows or galaxy interactions associated with rich group environments. On scales of ∼a few Mpc, the clustering strengths of both samples are statistically consistent, and beyond 10 Mpc, we find no evidence that quasars trace large-scale structures any more than the galaxy control sample. Both populations are found to prefer intermediate-density sheets and filaments to either very high-density environments or very low-density environments. This weak dependence of quasar activity on galaxy environment supports a paradigm in which quasars represent a phase in the lifetime of all massive galaxies and in which secular processes and a group-centric location are the dominant triggers of quasars at low redshift.

The impact of galaxy selection on the splashback boundaries of galaxy clusters

ABSTRACTWe explore how the splashback radius (Rsp) of galaxy clusters, measured using the number density of the subhalo population, changes based on various selection criteria using the IllustrisTNG cosmological galaxy formation simulation. We identify Rsp by extracting the steepest radial gradient in a stacked set of clusters in 0.5 dex wide mass bins, with our clusters having halo masses 1013 ≤ M200,mean/M⊙ ≤ 1015. We apply cuts in subhalo mass, galaxy stellar mass, i-band absolute magnitude, and specific star formation rate. We find that, generally, galaxies of increasing mass and luminosity trace smaller measured splashback radii relative to the intrinsic dark matter radius. We also show that quenched galaxies may be used to reliably reconstruct the dark matter splashback radius. This trend is likely due to changes in the galaxy population. Additionally, we are able to reconcile different observational predictions that Rsp based upon galaxy number counts and dark matter may either align or show significant offset (e.g. those using optically or SZ-selected clusters) through the selection functions that these studies employ. Finally, we demonstrate that changes in Rsp measured through number counts are not due to a simple change in galaxy abundance inside and outside of the cluster.

Forecasts for broad-band intensity mapping of the ultraviolet-optical background with CASTOR and SPHEREx

ABSTRACT Broad-band tomography statistically extracts the redshift distribution of frequency dependent emission from the cross-correlation of intensity maps with a reference catalog of galaxy tracers. We make forecasts for the performance of future all-sky UV experiments doing broad-band tomography. We consider the Cosmological Advanced Survey Telescope for Optical-UV Research (castor) and the Spectro-Photometer for the History of the Universe, Epoch of Reionization, and Ices Explorer (SPHEREx). The dominant uncertainty is from variability in the photometric zero-point, which scales with limiting magnitude and mirror size. With this scaling and assuming a galaxy number density characteristic of future spectroscopic data sets, we find that castor measures the UV background SED 2–10 times better than existing data. The applicable redshift range will expand from the current z < 1 to z ≈ 0–3 with castor and z = 5–9 with SPHEREx. We show that castor can provide competitive constraints on the EBL monopole to those available from galaxy number counts and direct measurement techniques. At high redshift especially, these results will help understand galaxy formation and reionization. Our modelling code and chains are publicly available.

High-redshift predictions from IllustrisTNG – III. Infrared luminosity functions, obscured star formation, and dust temperature of high-redshift galaxies

ABSTRACT We post-process galaxies in the IllustrisTNG simulations with skirt radiative transfer calculations to make predictions for the rest-frame near-infrared (NIR) and far-infrared (FIR) properties of galaxies at z ≥ 4. The rest-frame K- and z-band galaxy luminosity functions from TNG are overall consistent with observations, despite ${\sim}0.5\, \mathrm{dex}$ underprediction at z = 4 for MK ≲ −25 and Mz ≲ −24. Predictions for the JWST MIRI observed galaxy luminosity functions and number counts are given. Based on theoretical estimations, we show that the next-generation survey conducted by JWST can detect 500 (30) galaxies in F1000W in a survey area of $500\, {\rm arcmin}^{2}$ at z = 6 (z = 8). As opposed to the consistency in the UV, optical, and NIR, we find that TNG, combined with our dust modelling choices, significantly underpredicts the abundance of most dust-obscured and thus most luminous FIR galaxies. As a result, the obscured cosmic star formation rate density (SFRD) and the SFRD contributed by optical/NIR dark objects are underpredicted. The discrepancies discovered here could provide new constraints on the sub-grid feedback models, or the dust contents, of simulations. Meanwhile, although the TNG predicted dust temperature and its relations with IR luminosity and redshift are qualitatively consistent with observations, the peak dust temperature of z ≥ 6 galaxies are overestimated by about $20\, {\rm K}$. This could be related to the limited mass resolution of our simulations to fully resolve the porosity of the interstellar medium (or specifically its dust content) at these redshifts.

Covariant decomposition of the non-linear galaxy number counts and their monopole

We present a fully non-linear and relativistically covariant expression for the observed galaxy density contrast. Building on a null tetrad tailored to the cosmological observer's past light cone, we find a decomposition of the non-linear 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 (the mean galaxy density as a function of observed redshift) is properly accounted for. We anticipate that this decomposition will be useful for future work on non-linearities 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.

Virial Halo Mass Function in the Planck Cosmology

Abstract We study halo mass functions with high-resolution N-body simulations under a ΛCDM cosmology. Our simulations adopt the cosmological model that is consistent with recent measurements of the cosmic microwave backgrounds with the Planck satellite. We calibrate the halo mass functions for 108.5 ≲ M vir/(h −1 M ⊙) ≲ 1015.0–0.45 z , where M vir is the virial spherical-overdensity mass and redshift z ranges from 0 to 7. The halo mass function in our simulations can be fitted by a four-parameter model over a wide range of halo masses and redshifts, while we require some redshift evolution of the fitting parameters. Our new fitting formula of the mass function has a 5%-level precision, except for the highest masses at z ≤ 7. Our model predicts that the analytic prediction in Sheth & Tormen would overestimate the halo abundance at z = 6 with M vir = 108.5–10 h −1 M ⊙ by 20%–30%. Our calibrated halo mass function provides a baseline model to constrain warm dark matter (WDM) by high-z galaxy number counts. We compare a cumulative luminosity function of galaxies at z = 6 with the total halo abundance based on our model and a recently proposed WDM correction. We find that WDM with its mass lighter than 2.71 keV is incompatible with the observed galaxy number density at a 2σ confidence level.

Galaxy number counts at second order in perturbation theory: a leading-order term comparison

The Galaxy number density is a key quantity to compare theoretical predictions to the observational data from current and future large scale structure surveys. The precision demanded by these stage IV surveys requires the use of second order cosmological perturbation theory. Based on the independent calculation published previously, we present the result of the comparison with the results of three other groups at leading order. Overall we find that the differences between the different approaches lie mostly on the definition of certain quantities, where the ambiguity of signs results in the addition of extra terms at second order in perturbation theory.

Measuring anisotropic stress with relativistic effects

One of the main goal of large-scale structure surveys is to test the consistency of General Relativity at cosmological scales. In the $\Lambda$CDM model of cosmology, the relations between the fields describing the geometry and the content of our Universe are uniquely determined. In particular, the two gravitational potentials -- that describe the spatial and temporal fluctuations in the geometry -- are equal. Whereas large classes of dark energy models preserve this equality, theories of modified gravity generally create a difference between the potentials, known as anisotropic stress. Even though measuring this anisotropic stress is one of the key goals of large-scale structure surveys, there are currently no methods able to measure it directly. Current methods all rely on measurements of galaxy peculiar velocities (through redshift-space distortions), from which the time component of the metric is inferred, assuming that dark matter follows geodesics. If this is not the case, all the proposed tests fail to measure the anisotropic stress. In this letter, we propose a novel test which directly measures anisotropic stress, without relying on any assumption about the unknown dark matter. Our method uses relativistic effects in the galaxy number counts to provide a direct measurement of the time component of the metric. By comparing this with lensing observations our test provides a direct measurement of the anisotropic stress.

The flat-sky approximation to galaxy number counts — redshift space correlation function

We study the flat-sky approximation for galaxy number counts including relativistic effects, and numerically assess its performance and accuracy with respect to the full-sky result. We find an agreement of up to 5% for the local and lensing contributions to the 2-point correlation function and its multipoles at z > 0.5, and up to 1% for the multipoles alone at z > 1 and separations ≲ 250 Mpc/h, with a speed-up of over a factor of 1000. Using a semi-analytic method, which has been implemented in a new version of the code COFFE,[Available at https://github.com/JCGoran/coffe.] along with the Limber approximation for the integrated contributions, we further increase the performance, allowing the computation of the flat-sky multipoles to be done over 10000 times faster than in the full-sky calculation, which could be used to greatly speed-up Markov chain Monte Carlo sampling for cosmological parameter estimation.

Cosmological parameter forecasts by a joint 2D tomographic approach to CMB and galaxy clustering

The cross-correlation between the cosmic microwave background (CMB) fields and matter tracers carries important cosmological information. In this paper, we forecast by a signal-to-noise ratio analysis the information contained in the cross-correlation of the CMB anisotropy fields with source counts for future cosmological observations and its impact on cosmological parameters uncertainties, using a joint tomographic analysis. We include temperature, polarization and lensing for the CMB fields and galaxy number counts for the matter tracers. By restricting ourselves to quasi-linear scales, we forecast by a Fisher matrix formalism the relative importance of the cross-correlation of source counts with the CMB in the constraints on the parameters for several cosmological models. We obtain that the CMB-number counts cross-correlation can improve the dark energy Figure of Merit (FoM) at most up to a factor $\sim 2$ for LiteBIRD+CMB-S4 $\times$ SKA1 compared to the uncorrelated combination of both probes and will enable the Euclid-like photometric survey to reach the highest FoM among those considered here. We also forecast how CMB-galaxy clustering cross-correlation could increase the FoM of the neutrino sector, also enabling a statistically significant ($\gtrsim$ 3$\sigma$ for LiteBIRD+CMB-S4 $\times$ SPHEREx) detection of the minimal neutrino mass allowed in a normal hierarchy by using quasi-linear scales only. Analogously, we find that the uncertainty in the local primordial non-Gaussianity could be as low as $\sigma (f_{\rm NL}) \sim 1.5-2$ by using two-point statistics only with the combination of CMB and radio surveys such as EMU and SKA1. Our results highlight the additional constraining power of the cross-correlation between CMB and galaxy clustering from future surveys which is mainly based on quasi-linear scales and therefore sufficiently robust to non-linear effects.

Galaxy shape statistics in the effective field theory

Intrinsic galaxy alignments yield an important contribution to the observed statistics of galaxy shapes. The general bias expansion for galaxy sizes and shapes in three dimensions has been recently described by Vlah, Chisari & Schmidt using the general perturbative effective field theory (EFT) framework, in analogy to the clustering of galaxies. In this work, we present a formalism that uses the properties of spherical tensors to project galaxy shapes onto the observed sky in the flat-sky approximation, and compute the two-point functions at next-to-leading order as well as the leading-order three-point functions of galaxy shapes and number counts. The resulting expressions are given in forms that are convenient for efficient numerical implementation. For a source redshift distribution typical of Stage IV surveys, we find that nonlinear intrinsic alignment contributions to galaxy shape correlations become relevant at angular wavenumbers l ≳ 100.

On the importance of lensing for galaxy clustering in photometric and spectroscopic surveys

We study the importance of gravitational lensing in modelling the number counts of galaxies for the first time in spectroscopic surveys. We confirm previous results for photometric surveys, showing that lensing cannot be neglected in a survey like LSST since it would infer a significant shift of standard cosmological parameters. For a spectroscopic survey like SKA2, we find that neglecting lensing in the monopole, quadrupole and hexadecapole of the correlation function can also induce an important shift of cosmological parameters. For ΛCDM parameters, the shift is moderate, of the order of 0.6σ or less. However, for a model-independent analysis, that measures the growth rate of structure in each redshift bins, neglecting lensing introduces a shift of up to 2.3σ at high redshift. Since the growth rate is directly used to test the theory of gravity, such a strong shift would wrongly be interpreted as the breakdown of General Relativity. This shows the importance of including lensing in the analysis of future surveys. For a survey like DESI, we find on the other hand that lensing is not important, mainly due to the value of the magnification bias parameter of DESI, s(z), which strongly reduces the lensing contribution at high redshift. This result relies on our theoretical modelling of s(z) in DESI and should therefore be confirmed with measurements of s(z) in simulations.We also propose a way of improving the analysis of spectroscopic surveys, by including the cross-correlations between different redshift bins (which is neglected in spectroscopic surveys) from the spectroscopic survey or from a different photometric sample. We show that including the cross-correlations in the SKA2 analysis does not improve the constraints. On the other hand replacing the cross-correlations from SKA2 by cross-correlations measured with LSST improves the constraints by 10% to 20%.The marginal improvement is mainly due to the density correlations between nearby bins and, therefore, does notstrongly depend on our knowledge of the magnification bias. Interestingly, for standard cosmological parameter estimation, the photometric survey LSST in its 12 redshift bin configurationand the spectroscopic SKA2 survey are highly complementary, since they are affected by different degeneracies between parameters:LSST yields the tightest constraints on Ωcdm, h and n s, while SKA2 better constrains Ωbaryon, A s and the bias.

The Evolution of the IR Luminosity Function and Dust-obscured Star Formation over the Past 13 Billion Years

Abstract We present the first results from the Mapping Obscuration to Reionization with ALMA (MORA) survey, the largest Atacama Large Millimeter/submillimeter Array (ALMA) blank-field contiguous survey to date (184 arcmin2) and the only at 2 mm to search for dusty star-forming galaxies (DSFGs). We use the 13 sources detected above 5σ to estimate the first ALMA galaxy number counts at this wavelength. These number counts are then combined with the state-of-the-art galaxy number counts at 1.2 and 3 mm and with a backward evolution model to place constraints on the evolution of the IR luminosity function and dust-obscured star formation in the past 13 billion years. Our results suggest a steep redshift evolution on the space density of DSFGs and confirm the flattening of the IR luminosity function at faint luminosities, with a slope of . We conclude that the dust-obscured component, which peaks at z ≈ 2–2.5, has dominated the cosmic history of star formation for the past ∼12 billion years, back to z ∼ 4. At z = 5, the dust-obscured star formation is estimated to be ∼35% of the total star formation rate density and decreases to 25%–20% at z = 6–7, implying a minor contribution of dust-enshrouded star formation in the first billion years of the universe. With the dust-obscured star formation history constrained up to the end of the epoch of reionization, our results provide a benchmark to test galaxy formation models, to study the galaxy mass assembly history, and to understand the dust and metal enrichment of the universe at early times.

GAMA/DEVILS: constraining the cosmic star formation history from improved measurements of the 0.3–2.2 μm extragalactic background light

ABSTRACT We present a revised measurement of the optical extragalactic background light (EBL), based on the contribution of resolved galaxies to the integrated galaxy light (IGL). The cosmic optical background radiation (COB), encodes the light generated by star formation, and provides a wealth of information about the cosmic star formation history (CSFH). We combine wide and deep galaxy number counts from the Galaxy And Mass Assembly survey (GAMA) and Deep Extragalactic VIsible Legacy Survey (DEVILS), along with the Hubble Space Telescope (HST) archive and other deep survey data sets, in nine multiwavelength filters to measure the COB in the range from 0.35 μm to 2.2 μm. We derive the luminosity density in each band independently and show good agreement with recent and complementary estimates of the optical-EBL from very high-energy (VHE) experiments. Our error analysis suggests that the IGL and γ-ray measurements are now fully consistent to within $\sim 10{{\ \rm per\ cent}}$, suggesting little need for any additional source of diffuse light beyond the known galaxy population. We use our revised IGL measurements to constrain the CSFH, and place amplitude constraints on a number of recent estimates. As a consistency check, we can now demonstrate convincingly, that the CSFH, stellar mass growth, and the optical-EBL provide a fully consistent picture of galaxy evolution. We conclude that the peak of star formation rate lies in the range 0.066–0.076 M⊙ yr−1 Mpc−3 at a lookback time of 9.1 to 10.9 Gyr.

Galaxy number counts at second order: an independent approach

Next generation surveys will be capable of determining cosmological parameters beyond percent level. To match this precision, theoretical descriptions should look beyond the linear perturbations to approximate the observables in large scale structure. A quantity of interest is the Number density of galaxies detected by our instruments. This has been focus of interest recently, and several efforts have been made to explain relativistic effects theoretically, thereby testing the full theory. However, the results at nonlinear level from previous works are in disagreement. We present a new and independent approach to computing the relativistic galaxy number counts to second order in cosmological perturbation theory. We derive analytical expressions for the full second order relativistic observed redshift, for the angular diameter distance and for the volume spanned by a survey. Finally, we compare our results with previous works which compute the general distance-redshift relation, finding that our result is in agreement at linear order.

The flat sky approximation to galaxy number counts

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 ℓ ≃ 10, 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 ℓ < 50. At unequal redshifts, if we consider the total power spectrum, the precision is better than 5% only for very small redshift differences, δ <δ0 (≃ 3.6×10−4(1+z)2.14) where the standard terms are well-approximated, or for large enough redshift differences δ >δ1 (≃ 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 ℓ ≃ 40 for pure lensing and ℓ ≃ 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.