Nature of Dark Matter


Our third key goal is to understand the nature (mass and interactions) of the elusive Dark Matter that makes up 80% of the matter content of the Universe. Given that the earliest galaxies are the key building blocks of all structure and the key sources of reionization, changing the nature of the Dark Matter that they form in should have an effect on both structure formation and the reionization history of the Universe. We have developed a framework for  high-redshift (z>5) galaxy formation that traces their dark matter (DM) and baryonic assembly in four cosmologies: cold dark matter (CDM) and warm dark matter (WDM) with particle masses of mx = 1.5, 3, and 5 keV.


Using the same astrophysical parameters regulating star formation and feedback, chosen to match current observations of the evolving ultraviolet luminosity function (UV LF), we find that the assembly of observable (with current and upcoming instruments) galaxies in CDM and mx>3 keV WDM results in similar halo mass-to-light ratios (M/L), stellar mass densities (SMDs), and UV LFs. However, the suppression of small-scale structure leads to a notably delayed and subsequently more rapid stellar assembly in the 1.5 keV WDM model. Thus, galaxy assembly in mx<=2 keV WDM cosmologies is characterized by a dearth of small-mass halos hosting faint galaxies and a younger, more UV-bright stellar population, for a given stellar mass. Using this model, we have shown that the redshift evolution of the SMD is a powerful probe of the nature of DM (Early galaxy formation in warm dark matter cosmologies; Dayal, Mesinger & Pacucci, 2015, ApJ, 806, 67).


Redshift evolution of the stellar mass density (SMD) in CDM and WDM models. In each panel we show results using the fiducial 1.5 keV and CDM models (solid purple and black lines, respectively) and a model with no SN feedback in the 1.5 keV WDM scenario (grey line). The red and blue dashed lines show 1.5 keV results obtained by varying the two free parameters by 50% as compared to the fiducial model. The left and right panels show the SMD from galaxies that have already been detected and galaxies that are expected to be detected by the JWST, respectively. As seen, varying the free parameter values only affects the normalization of the SMD with the slope remaining unchanged. The slope of the SMD is independent of the astrophysics implemented, presenting a robust observable to distinguish between DM models.

We have also modeled reionization in different cosmologies (Reionization and galaxy formation in Warm Dark Matter cosmologiesDayal, Choudhury, Bromm & Pacucci, 2017, ApJ, 836, 16). In this work, we showed that the delay in the start of reionization in light (1.5 keV) WDM models can be compensated by a steeper redshift evolution of the ionizing photon escape fraction and a faster mass assembly, resulting in reionization ending at comparable redshifts (z ~ 5.5) in all the dark matter models considered. We find that the bulk of the reionization photons come from galaxies with a (log) halo mass less than 9 solar masses and a UV magnitude of between -15 and -10 in CDM. The progressive suppression of low-mass halos with decreasing mx leads to a shift in the “reionization” population to larger halo masses (log halo masses larger than 9 solar masses) and a UV magnitude between -17 and -13 for 1.5 keV WDM. Interestingly, current observations of the CMB (cosmic microwave background) electron scattering optical depth are equally compatible with all the (cold and warm) dark matter models considered in this work. We therefore propose that the JWST can be used as a DM-machine to shed light on the Dark Matter particle mass by pinning down the evolution of the stellar mass density and halo-mass-stellar mass ratios. 

The CMB electron scattering optical depth as a function of redshift for the 4 DM models considered in this paper, as marked. In each panel, lines show results using four different values of the escape fraction of ionizing photons: 1.0 (dot-dashed green line), 0.5 (dashed red line), 0.2 (dotted blue line) and the fiducial z-dependent value marked (solid black line). The horizontal dashed line shows the central value for the optical depth inferred by Planck combining polarisation, temperature and lensing data (Planck 2015) with the gray shaded region showing the 1-sigma errors. The horizontal dotted and dot-dashed orange lines show the central values from Planck (2014) and Planck (2016), respectively; the inclined and vertically shaded (blue) regions show the associated 1-sigma error bars. As seen, while the Planck-2014 results ruled out WDM as light as 1.5 keV, although requiring a very steep z-evolution of the escape fraction, this WDM model is allowed by the latest Planck (2015, 2016) results.  

We have now extended these works to study the metal enrichment of the intergalactic medium and the formation of direct collapse black holes (DCBHs) in both CDM and WDM cosmologies as part of the DELPHI project.  

1. We have published a paper on "Probing the nature of dark matter through the metal enrichment of the intergalactic medium" (Bremer, Dayal & Ryan-Weber, 2018, MNRAS, 477, 2154). Incorporating two different metallicity prescriptions, we We find that while galaxies brighter than a UV magnitude of -15 contribute half of all IGM metals in the CDM model by z~4.5, given the suppression of low-mass haloes, larger haloesprovide about 80 per cent of the IGM metal budget in 1.5 keV WDM. Our results also show that the only models compatible with two different high-redshift data sets, provided by the evolving ultraviolet luminosity function (UV LF) at z~6-10 and the IGM metal density, are standard CDM and 3 keV WDM that do not include any reionization feedback. Tightening the error bars on the IGM metal enrichment, future observations, at z>5.5, could therefore represent an alternative way of shedding light on the nature of DM.

2. In "Warm dark matter constraints from high-z direct collapse black holes using the JWST" (Dayal, Choudhury, Pacucci &Bromm, 2017, MNRAS, 472, 4414), we again used the Delphi model to identify pristine halos irradiated by a sufficient Lyman-Werner (LW) background as direct collapse black hole (DCBH) hosts. DCBHs are of crucial importance in that they could be the massive seeds that could explain the super-massive black holes seen in the first billion years. We find that a combination of delayed structure formation and an accelerated assembly of galaxies results in a later metal-enrichment and an accelerated build-up of the LW background in the 1.5 keV WDM model, resulting in DCBH hosts persisting down to much lower redshifts (z~5) as compared to CDM where DCBH hosts only exist down to z~8. We show how the expected colours in three different bands of the Near Infrared Camera (NIRCam) onboard the forthcomingJWST can be used to hunt for potential z~5-9 DCBHs, allowing hints on the WDM particle mass.  












The number density of DCBH hosts as a function of z, for CDM (shaded region), 3.5 keV (dashed lines) and 1.5 keV WDM (solid lines) for three values of the critical LW background corresponding to 30 (blue lines), 100 (red lines) and and 300 J21 (yellow lines), respectively. Potential DCBH candidates can be identified by their JWST colours shown on the right axes: here, symbols show the AB magnitude range expected for a DCBH mass between 10^5-6 solar masses in the F070 band (black), F090 band (brown) and F115 band (purple) using the sensitivity threshold for a 10 ks observation. DCBH hosts are undetectable in the F070W filter at z>7, i.e. before the end of the reionization, independent of their mass. We therefore predict that while DCBHs could be detectable in all three JWST bands considered for z~5-9 for 1.5 keV WDM, DCBHs would not be detectable in the F070W band at z>7 in CDM and 3.5 keV WDM. Such colors therefore offer a testable means of hunting for DCBH hosts with further spectroscopy, for example with the JWST, required to unambiguously pin down their true nature.