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Register a new userThe imprint of dark subhaloes on the circumgalactic medium
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The standard model of cosmology, the LCDM model, robustly predicts the existence of a multitude of dark matter subhaloes around galaxies like the Milky Way. A wide variety of observations have been proposed to look for the gravitational effects such subhaloes would induce in observable matter. Most of these approaches pertain to the stellar or cool gaseous phases of matter. Here we propose a new approach, which is to search for the perturbations that such dark subhaloes would source in the warm/hot circumgalactic medium (CGM) around normal galaxies. With a combination of analytic theory, carefully-controlled high-resolution idealised simulations, and full cosmological hydrodynamical simulations (the ARTEMIS simulations), we calculate the expected signal and how it depends on important physical parameters (subhalo mass, CGM temperature, and relative velocity). We find that dark subhaloes enhance both the local CGM temperature and density and, therefore, also the pressure. For the pressure and density, the fluctuations can vary in magnitude from tens of percent (for subhaloes with M_sub=10^10 Msun) to a few percent (for subhaloes with M_sub=10^8 Msun), although this depends strongly on the CGM temperature. The subhaloes also induce fluctuations in the velocity field ranging in magnitude from a few km/s up to 25 km/s. We propose that X-ray, Sunyaev-Zeldovich effect, radio dispersion measure, and quasar absorption line observations can be used to measure these fluctuations and place constraints on the abundance and distribution of dark subhaloes, thereby placing constraints on the nature of dark matter.
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The majority of baryons reside beyond the optical extent of a galaxy in the circumgalactic and intergalactic media (CGM/IGM). Gaseous halos are inextricably linked to the appearance of their host galaxies through a complex story of accretion, feedback, and continual recycling. The energetic processes, which define the state of gas in the CGM, are the same ones that 1) regulate stellar growth so that it is not over-efficient, and 2) create the diversity of todays galaxy colors, SFRs, and morphologies spanning Hubbles Tuning Fork Diagram. They work in concert to set the speed of growth on the star-forming Main Sequence, transform a galaxy across the Green Valley, and maintain a galaxys quenched appearance on the Red Sequence. Most baryons in halos more massive than 10^12 Msolar along with their high-energy physics and dynamics remain invisible because that gas is heated above the UV ionization states. We argue that information on many of the essential drivers of galaxy evolution is primarily contained in this missing hot gas phase. Completing the picture of galaxy formation requires uncovering the physical mechanisms behind stellar and SMBH feedback driving mass, metals, and energy into the CGM. By opening galactic hot halos to new wavebands, we not only obtain fossil imprints of >13 Gyrs of evolution, but observe on-going hot-mode accretion, the deposition of superwind outflows into the CGM, and the re-arrangement of baryons by SMBH feedback. A description of the flows of mass, metals, and energy will only be complete by observing the thermodynamic states, chemical compositions, structure, and dynamics of T>=10^6 K halos. These measurements are uniquely possible with a next-generation X-ray observatory if it provides the sensitivity to detect faint CGM emission, spectroscopic power to measure absorption lines and gas motions, and high spatial resolution to resolve structures.
Upcoming measurements of the highly redshifted 21cm line with next-generation radio telescopes such as HERA and SKA will provide the intriguing opportunity to probe dark matter (DM) physics during the Epoch of Reionization (EoR), Cosmic Dawn, and the Dark Ages. With HERA already under construction, there is a pressing need to thoroughly understand the impact of DM physics on the intergalactic medium (IGM) during these epochs. We present first results of a hydrodynamic simulation suite with $2 times 512^3$ particles in a $(100 h^{-1} text{Mpc})^3$ box with DM annihilation and baryonic cooling physics. We focus on redshift $z sim 11$, just before reionization starts in our simulations, and discuss the imprint of DM annihilation on the IGM and on structure formation. We find that whereas structure formation is not affected by thermal WIMPs heavier than $m_chi gtrsim 100 text{MeV}$, heating from $mathcal{O}$(GeV) DM particles may leave a significant imprint on the IGM that alters the 21cm signal. Cold gas in low density regions is particularly susceptible to the effects of DM heating. We note, however, that delayed energy deposition is not currently accounted for in our simulations.
We propose the use of the kinetic Sunyaev-Zeldovich (kSZ) effect to probe the circumgalactic medium (CGM), with the aid of a spectroscopic survey covering the same area of a SZ survey. One can design an optimal estimator of the kSZ effect of the CGM with a matched filter, and construct the cross correlation between the estimator and the peculiar velocity recovered from the galaxy survey, which can be measured by stacking a number of galaxies. We investigate two compelling profiles for the CGM, the MB profile (Maller & Bullock 2004) and the $beta$ profile, and estimate the detectability against the synergy of a fiducial galaxy survey with number density $10^{-3}h^3,$ Mpc$^{-3}$ and an ACT-like SZ survey. We show that the shape of the filter does not change much with redshift for the $beta$ profile, while there are significant side lobes at $z<0.1$ for the MB profile. By stacking $sim 10^4$ Milky Way-size halos around z $sim 0.5$, one can get $gtrsim$ 1 $sigma$ signal to noise (S/N) for the both profiles. The S/N increases with decreasing redshift before it reaches a maximum ($sim$ 7.5 at z $simeq$ 0.15 for the MB profile, $sim 19$ at $zsimeq 0.03$ for the $beta$ profile). Due to the large beam size, a Planck-like CMB survey can marginally detect the kSZ signal by stacking the same number of galaxies at $z<0.1$. The search for the CGM in realistic surveys will involve dividing the galaxies into subsamples with similar redshift and mass of host halos, and scaling the results presented here to obtain the S/N.
We assess the metal content of the cool (10^4 K) circumgalactic medium (CGM) about galaxies at z<1 using an H I-selected sample of 28 Lyman limit systems (LLS, defined here as absorbers with 16.2<log N(H I)<18.5) observed in absorption against background QSOs by the Cosmic Origins Spectrograph on-board the Hubble Space Telescope. The N(H I) selection avoids metallicity biases inherent in many previous studies of the low-redshift CGM. We compare the column densities of weakly ionized metal species (e.g., O II, Si II, Mg II) to N(H I) in the strongest H I component of each absorber. We find that the metallicity distribution of the LLS (and hence the cool CGM) is bimodal with metal-poor and metal-rich branches peaking at [X/H]=-1.6 and -0.3 (or about 2.5% and 50% solar metallicities). The cool CGM probed by these LLS is predominantly ionized. The metal-rich branch of the population likely traces winds, recycled outflows, and tidally stripped gas; the metal-poor branch has properties consistent with cold accretion streams thought to be a major source of fresh gas for star forming galaxies. Both branches have a nearly equal number of absorbers. Our results thus demonstrate there is a significant mass of previously-undiscovered cold metal-poor gas and confirm the presence of metal enriched gas in the CGM of z<1 galaxies.
Early-type galaxies (ETGs) are observed to be more compact, on average, at $z gtrsim 2$ than at $zsimeq 0$, at fixed stellar mass. Recent observational works suggest that such size evolution could reflect the similar evolution of the host dark matter halo density as a function of the time of galaxy quenching. We explore this hypothesis by studying the distribution of halo central velocity dispersion ($sigma_0$) and half-mass radius ($r_{rm h}$) as functions of halo mass $M$ and redshift $z$, in a cosmological $Lambda$-CDM $N$-body simulation. In the range $0lesssim zlesssim 2.5$, we find $sigma_0propto M^{0.31-0.37}$ and $r_{rm h}propto M^{0.28-0.32}$, close to the values expected for homologous virialized systems. At fixed $M$ in the range $10^{11} M_odot lesssim Mlesssim 5.5 times 10^{14} M_odot$ we find $sigma_0propto(1+z)^{0.35}$ and $r_{rm h}propto(1+z)^{-0.7}$. We show that such evolution of the halo scaling laws is driven by individual haloes growing in mass following the evolutionary tracks $sigma_0propto M^{0.2}$ and $r_{rm h}propto M^{0.6}$, consistent with simple dissipationless merging models in which the encounter orbital energy is accounted for. We compare the $N$-body data with ETGs observed at $0lesssim zlesssim3$ by populating the haloes with a stellar component under simple but justified assumptions: the resulting galaxies evolve consistently with the observed ETGs up to $z simeq 2$, but the model has difficulty reproducing the fast evolution observed at $zgtrsim 2$. We conclude that a substantial fraction of the size evolution of ETGs can be ascribed to a systematic dependence on redshift of the dark matter haloes structural properties.
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