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Where the cosmic baryons lie in and around galactic dark matter haloes is only weakly constrained. We develop a method to quickly paint on models for their distribution. Our approach uses the statistical advantages of N-body simulations, while painting on the profile of gas around individual haloes in ways that can be motivated by semi-analytic models or zoom-in hydrodynamic simulations of galaxies. Possible applications of the algorithm include extragalactic dispersion measures to fast radio bursts (FRBs), the Sunyaev–Zeldovich effect, baryonic effects on weak lensing, and cosmic metal enrichment. As an initial application, we use this tool to investigate how the baryonic profile of foreground galactic-mass haloes affects the statistics of the dispersion measure (DM) towards cosmological FRBs. We show that the distribution of DM is sensitive to the distribution of baryons in galactic haloes, with viable gas profile models having significantly different probability distributions for DM to a given redshift. We also investigate the requirements to statistically measure the circumgalactic electron profile for FRB analyses that stack DM with impact parameter to foreground galaxies, quantifying the size of the contaminating ‘two-halo’ term from correlated systems and the number of FRBs for a high significance detection. Publicly available python modules implement our CGMBrush algorithm.

 

The distribution of gas in the circumgalactic medium (CGM) of galaxies of all types is poorly constrained. Foreground CGMs contribute an extra amount to the dispersion measure (DM) of fast radio bursts (FRBs). We measure this DM excess for the CGMs of 10¹¹–10¹³M_⊙halos using the CHIME/FRB first data release, a halo mass range that is challenging to probe in any other way. Because of the uncertainty in the FRBs’ angular coordinates, only for nearby galaxies is the localization sufficient to confidently associate them with intersecting any foreground halo. Thus we stack on galaxies within 80 Mpc, optimizing the stacking scheme to approximately minimize the stack’s variance and marginalize over uncertainties in FRB locations. The sample has 20–30 FRBs intersecting halos with masses of 10¹¹–10¹²M_⊙and also of 10¹²–10¹³M_⊙, and these intersections allow a marginal 1σ–2σdetection of the DM excess in both mass bins. The bin of 10¹¹–10¹²M_⊙halos also shows a DM excess at 1–2 virial radii. By comparing data with different models for the CGM gas profile, we find that all models are favored by the data up to 2σlevel compared to the null hypothesis of no DM excess. With 3000 more bursts from a future CHIME data release, we project a 4σdetection of the CGM. Distinguishing between viable CGM models by stacking FRBs with CHIME-like localization would require tens of thousands of bursts.

 

Motivated by integral field units (IFUs) on large ground telescopes and proposals for ultraviolet-sensitive space telescopes to probe circumgalactic medium (CGM) emission, we survey the most promising emission lines and how such observations can inform our understanding of the CGM and its relation to galaxy formation. We tie our emission estimates to both HST/COS absorption measurements of ions around z ≈ 0.2 Milky Way mass haloes and models for the density and temperature of gas. We also provide formulas that simplify extending our estimates to other samples and physical scenarios. We find that O iii 5007 Å and N ii 6583 Å, which at fixed ionic column density are primarily sensitive to the thermal pressure of the gas they inhabit, may be detectable with KCWI and especially IFUs on 30 m telescopes out to half a virial radius. O v 630 Å and O vi 1032,1038 Å are perhaps the most promising ultraviolet lines, with models predicting intensities >100 γ cm−2 s−1 sr−1 in the inner 100 kpc of Milky Way-like systems. A detection of O vi would confirm the collisionally ionized picture and constrain the density profile of the CGM. Other ultraviolet metal lines constrain the amount of gas that is actively cooling and mixing. We find that C iii 978 Å and C iv 1548 Å may be detectable if an appreciable fraction of the observed O vi column is associated with mixing or cooling gas. H α emission within $100\,$ kpc of Milky Way-like galaxies is within reach of current IFUs even for the minimum signal from ionizing background fluorescence, while hydrogen n > 2 Ly-series lines are too weak to be detectable.

 

Abstract The Ly-α forest is the large-scale structure probe for which we appear to have modeling control to the highest wavenumbers. This makes the Ly-α forest of great interest for constraining the warmness/fuzziness of dark matter and the timing of reionization processes. However, the standard statistic, the Ly-α forest power spectrum, is unable to strongly constrain the IGM temperature-density relation, and this inability further limits how well other high wavenumber-sensitive parameters can be constrained. With the aim of breaking these degeneracies, we measure the power spectrum of the Ly-β forest and its cross correlation with the coeval Ly-α forest using the one hundred spectra of z = 3.5 − 4.5 quasars in the VLT/X-Shooter XQ-100 Legacy Survey, motivated by the Ly-β transition’s smaller absorption cross section that makes it sensitive to somewhat higher densities relative to the Ly-α transition. Our inferences from this measurement for the IGM temperature-density relation appear to latch consistently onto the recent tight lower-redshift Ly-α forest constraints. The z = 3.4 − 4.7 trends we find using the Ly-α–Ly-β cross correlation show a flattening of the slope of the temperature-density relation with decreasing redshift. This is the trend anticipated from ongoing He ii reionization and there being sufficient time to reach the asymptotic temperature-density slope after hydrogen reionization completes. Furthermore, our measurements provide a consistency check on IGM models that explain the Ly-α forest, with the cross correlation being immune to systematics that are uncorrelated between the two forests, such as metal line contamination.