A measurement of gas rotation in galaxy groups via the kinetic Sunyaev-Zeldovich effect

Abstract

We utilise the kinetic Sunyaev-Zeldovich effect (kSZ) to measure the rotation of ionised gas within galaxy groups defined in the SDSS-DR7 galaxy sample, via their dipolar imprint on the cosmic microwave background (CMB). We estimate the direction of the projected angular momentum for each group by measuring the redshift dipole of satellite galaxies around their group centre. We find a clear redshift dipole in the stacked data for the SDSS groups. We then perform oriented stacking of the Planck CMB temperature map using the group centres and directions of angular momenta. We report a $2.3σ$ measurement of the coherent rotational kSZ effect (rkSZ) within the virial radii of SDSS groups with an average mass of $10^{14}h^{-1} \rm M_{\odot}$. We estimate the averaged rotational velocity of the sample to be $\sim 100-200 ~\rm km ~s^{-1}$, peaking at approximately half the virial radius. Our results are consistent within the errors with predictions based on the ELUCID constrained realisation simulation, with the predicted amplitude of the rkSZ signal being slightly lower near the centre. We also identify a systematic bias when estimating rotational velocities using the observed redshifts of galaxies, but find it to be subdominant for our analysis.

Figures (5)

• Figure 1: Mollweide projection of the sky positions of the selected 134 groups from the SDSS-DR7 parent sample overplotted on the Planck CMB temperature map. Each white dot represents the group centre, and the associated red circle represents a $2\times r_{\rm vir}$ region around it. These are chosen to be fast-rotators with their rotation axes being approximately in the plane of the sky. They are individually matched to the haloes from the ELUCID constrained realisation simulation. The centres of the matched ELUCID haloes are indistinguishable from the white dots. The details of the algorithm for the selection of the groups, and their matching with the simulation are presented in Section \ref{['sec: selection']}. The CMB temperature map with a mask applied is shown with the resolution parameter nside$~=2048$.
• Figure 2: Histograms of the virial masses, radii and redshifts of the 134 selected SDSS-DR7 groups from the observation (blue) and their individually matched haloes from the ELUCID simulation (red). The mean redshift of the observed (simulated) sample is $\langle z \rangle \approx$ 0.066 (0.066), the mean virial radius $\langle r_{\rm vir} \rangle \approx 1.04~(0.98)$$\textrm{Mpc}/h$, and the mean mass is $\langle \log_{10}{M_{\rm vir}~(h^{-1}{\rm M}_{\odot}) \rangle} \approx 14.19~(14.10)$. See Section \ref{['sec: selection']} for the details.
• Figure 3: Top-left: The oriented stacked redshift map of simulated sub-haloes relative to the redshift of the centres of the 134 selected main haloes. Top-right: the oriented stacked redshift map of SDSS satellite galaxies relative to the redshift of the 134 selected SDSS group centres. The virial masses and positions of group centres from the data are individually matched to the halo centres from the ELUCID simulation. All maps are viewed edge-on with the projected rotation axes aligned with the $y$-axis, pointing downwards in the direction of $-\vec{y}$. Bottom: the oriented stacked simulated and observed rkSZ temperature map of the 134 haloes (left) and groups (right). The rkSZ temperature fluctuation around each simulated halo is computed with Equation \ref{['eqn::rksz_eq_in_simulation']}. The signs of the temperature fluctuations at the bottom panels -- negative on the left and positive on the right -- are consistent with the signs of the redshift fluctuations -- redshift on the left and blueshift on the right at the top panels, if the redshift dipoles are responsible for the temperature dipoles via the rkSZ effect.
• Figure 4: Left: The horizontal redshift profiles extracted from the top panels of Fig. \ref{['zredtot_sim_delta_T_four_panels']}. For each radial bin, we draw an annulus from the centre of the stack and take the difference between the average redshift within the semi-annulus on the left versus the one on the right. Orange -- redshifts traced by subhaloes in the ELUCID simulation; Red -- redshifts traced by the dark matter field in the simulation; Blue -- the observed redshifts of SDSS satellite galaxies. The shaded regions represent the error on the mean of the 134 groups. Right: The horizontal temperature profiles extracted from the stacked simulated (red-solid line) and observed (blue) $\Delta \rm T$ map from the bottom panel of Fig. \ref{['zredtot_sim_delta_T_four_panels']}, respectively. Shaded regions represent the 1-$\sigma$ errors estimated with 10 000 Gaussian realisations of the CMB temperature map. The red-dashed line is the prediction using the simulated dark matter field, but assuming that their rotation follows the rotation curve measured from the observed redshift dipole of the galaxy field -- the blue curve on the left-hand panel.
• Figure 5: PDFs of the temperature difference $\Delta T_{\rm CMB}$ between the left and the right semi-circular regions of the stacked CMB noise maps. Blue is the result obtained from 10 000 simulated CMB maps, including instrumental noise. Red is the result from the observed Plancksmica-noSZ CMB temperature map with randomly offset group centres such that there is no rkSZ signal. The vertical dashed line indicates the same measurement for the observed signal from Planck, at $\Delta T=25.5\,\mu$K. The standard deviation is $10.7\,\mu$K with the noise estimated from observations (or $9.3\,\mu$K with the noise estimated from simulations). The corresponding probability of this signal arising due to statistical fluctuations is 0.96% (or 0.35% from simulated noise maps), at the $\sim$$2.3\sigma$ (or $\sim$$2.7\sigma$) level.