Exploring gas thermodynamics around galaxies from the Sunyaev-Zel'dovich effects: impact of galaxy-halo connection, 2D projection and velocity field

Abstract

A complete picture of the gas thermodynamics around galaxies is imprinted on the cosmic microwave background (CMB). Indeed, the thermal, kinematic, and relativistic Sunyaev-Zel'dovich effects (tSZ, kSZ, rSZ) measure the gas density, temperature, pressure of baryonic feedback and bulk velocity around galaxies, along with the gravitational potential it sits in. This full thermodynamic picture promises to constrain galaxy formation models and gas related uncertainties in the impact on galaxy lensing. Recent kSZ measurements around galaxies suggest that the gas may be more extended than anticipated, pointing to powerful feedback processes and large baryonic corrections to lensing. How robust are these conclusions about the galaxy-halo connection, including satellite fraction and high-mass outliers, or to 2D projection effects and large-scale velocity modes? In this paper, we give estimates for these effects using a simulated sample of DESI-like luminous red galaxies within the IllustrisTNG hydrodynamical simulation and the Abacus N-body simulation. We show that analyzing projected 2D profiles can lead to biases when computing quantities like the gas fraction. We also find that in the absence of spatial filtering, the 2-halo term is non-negligible for kSZ even at the smaller radii where the 1-halo term dominates. We show that a 1% uncertainty in the satellite fraction of galaxies can propagate into uncertainties of $\pm$1%, $\pm$3% and $\pm$5% in the 1-halo terms of the kSZ, tSZ, and rSZ signals, respectively. We show that masking the 2% most massive objects in the sample reduces the profile amplitudes by up to 10%, 40%, and 75% for the kSZ, tSZ, and rSZ signals, respectively. Finally, we show that naive simulations of the kSZ effect can be biased by an artificial Doppler term, which is automatically removed when high-pass or compensated aperture filtering is applied.

Figures (10)

• Figure 1: From top to bottom, the panels illustrate tests for the convergence scales of the 2-halo term contributions to the 3D and 2D matter distributions, as well as to velocities. The curves are plotted such that the integral corresponds to the area under the curve, accounting for the logarithmic scaling of the radius. Top panel: When calculated using 3D spherical shells, the mass contribution from the two-halo term, computed analytically using both the linear (continuous) and nonlinear (dashed) correlation functions, exhibits poor convergence, with significant contributions extending beyond approximately $\sim 5$Gpc/$h$. Middle panel: In contrast, the 2D version of this mass term, calculated using cylindrical shells, converges at much smaller scales, around 150 cMpc/$h$ (cMpc stands for comoving Mpc). This indicates that the box size of the IllustrisTNG-300 simulation is sufficient for studies requiring convergence of projected mass profiles. Lower panel: For analyses involving the kSZ effect, the velocity correlation function, shown for both linear and non-linear theory (black), does not fully converge within the volume of IllustrisTNG-300 but does so in larger simulations such as AbacusSummit. This convergence is critical for constraining the contribution of the Doppler term to the kSZ signal, as $\xi_{vv}$ serves as the integrand for the Doppler component (see Eq. \ref{['eq:kSZ_doppler_other']} and note the constant mean number density).
• Figure 2: From left to right, profiles of projected gas density, projected pressure, and projected relativistic corrections to the pressure as a function of radius, as defined in Eqs. \ref{['eq:kSZ']}, \ref{['eq:compton_tSZ']} and \ref{['eq:compton_rSZ']}, respectively. These profiles are separated into the 1-halo term (solid line), which dominates at smaller scales ($\lesssim$ 1 cMpc$/h$), and the projected 2-halo term (dashed line), which dominates at larger scales. From left to right, as the mass dependence increases ($M$, $M^{5/3}$, and $M^{7/3}$, respectively), we observe a decreasing contribution from the 2-halo term, as the projected profile becomes increasingly dominated by massive halos from neighboring clusters. This trend is accompanied by increasing noise, which is further exaggerated by the logarithmic scale of the plots. In the specific case of the projected relativistic SZ, the contribution of the 2-halo term is on the order of a few percent.
• Figure 3: The first two panels show the 2D projected and 3D density profiles: baryons are plotted with dash-dotted lines, and total matter (baryons + dark matter) with dashed lines. The upper panel presents the 2D profiles, and the middle panel the 3D ones, with the corresponding 1-halo and 2-halo terms shown faintly. The lower panel displays the ratio of gas to total matter density for both 2D (black) and 3D (blue) profiles. While the 3D ratio converges to the expected value of $\Omega_{\rm b}/\Omega_{\rm m}$ (indicated by the dotted black line), the 2D ratio does not and instead overestimates it, illustrating that gas fractions may not be trivially inferred from projected profiles.
• Figure 4: Similar to Fig. \ref{['fig:2d_summary']}, except this figure splits the LRG sample into central (black) and satellite (grey) galaxies. Satellites tend to reside in larger halos, which have higher mass and are surrounded by more massive neighbors. Their 1-halo and 2-halo terms are thus larger than for centrals. This effect is especially pronounced in pressure and projected relativistic SZ, as these quantities have an even steeper mass dependence. However, given the total number of satellites is lower than the centrals, their contribution to the average profiles for centrals plus satellites is reduced by their fraction compared to the curves shown.
• Figure 5: The top panels show the projected 2D profiles of gas density, pressure, and relativistic SZ for varying satellite fractions, and their fractional errors within a $\pm1\%$ (white), $\pm5\%$ (light gray) and $\pm10\%$ (gray) bands in the lower panels. Each profile represents a linear combination of the central and satellite profiles from Fig. \ref{['fig:sat_vs_cent_summary']}, with a satellite fraction of 20$\% \pm 1\%$, $20\pm 3\%$ and $20\pm 7\%$. The fiducial satellite fraction, denoted as $f_{\rm sat}$, corresponds to 20$\%$. From left to right, a $\pm$1$\%$ change in the fiducial satellite fraction results in an increasing fractional error, reaching up to 2$\%$ for the projected gas density, $\sim 4 \%$ for the projected pressure, and 5$\%$ for the relativistic SZ profile. This allows us to quantify the maximum allowable uncertainty in $f_{\rm sat}$ to ensure that its propagated error to the SZ profiles (shape and amplitude) remains constrained by certain percentage.