Measuring the growth rate of structure with cosmic voids

Transcription

1 Measuring the growth rate of structure with cosmic voids Adam James Hawken Centre de physique des particules de Marseille 53rd rencontres de Moriond La Thuile

2 Outline Cosmic voids and RSD A linear model for void RSD Voids in GAMA Testing on mocks Application to data

3 What are cosmic voids?

4 Redshift space distortions Galaxies have peculiar velocities due to gravitational interactions. These add an additional component to their apparent recessional velocity and thus their redshift. These redshift space distortions lead to anisotropies in the clustering pattern of galaxies. Kaiser 1987, Hamilton 1992 etc

5 Redshift space distortions The beta parameter relates the velocity field to the integrated matter density field The ratio of the growth of structure to galaxy bias In GR γ = 0.55

6 How are voids affected by RSD? Affects the hight of the peak Solid lines show line of sight component. Affects void shape Affects depth Dotted lines show tangential component. Undistorted profile is a stretched exponential toy model.

7 Why measure the growth rate around voids? Plenty of evidence for RSD around voids. Largest component by volume of the cosmic web. Micheletti, Iovino, AJH, Granett et al '14 Dark Energy lives in voids. Some modified gravity theories, like f(r), predict that the growth rate of structure should deviate from GR in low density environments. Less affected by non-linearities. A complimentary probe of the growth of structure. Hamaus et al '16 Paz et al '13

8 Multipoles of the void-galaxy crosscorrelation The void-galaxy cross correlation can be decomposed into multipoles. Multipoles higher than the monopole are a result of RSD. See Nadathur and Percival 2017, Hamaus et al 2017, Cai et al 2016

9 A linear model for void RSD Mohammad et al 2016 apply this to group-galaxy cross correlations

10 A linear model for void RSD In this model the higher multipoles are a function of the monopole. The two beta parameters are degenerate. We need prior knowledge of one in order to gain knowledge of the other. Other people have looked into void beta before (see e.g. Chuang et al 2017).

11 Galaxy And Mass Assembly G09 We used the public DR2 galaxy catalogue, studying the three equatorial fields. G12 G15 Baldry et al 2018 Lisk et al 2015 Carried out using the AAOmega multiobject spectrograph on the AngloAustralian Telescope (AAT). Observations are now completed.

12 Searching for voids in GAMA First we define a volume limited catalogue. This increases the contrast between high and low density regions. We then look for empty nonoverlapping spheres in this volume limited catalogue. The algorithm is described in Micheletti et al '15 and Hawken et al '17

13 G09 G12 G15

14 Abundance of GAMA voids Voids are small compared to other void catalogues. This is because of the sizes of the fields and also the void finder. Fields are in good agreement with each other.

15 Void galaxy cross-correlation in GAMA We select voids with r > 12 Mpc/h and measure the cross correlation with ALL galaxies

16 Mock galaxy catalogues We used 26 realisations of the 3 equatorial GAMA fields. Galaxy formation model of Gonzalez-Perez Lightcone construction described in Merson et al Applied the same magnitude cut. Ran the void finder. Measured the void-galaxy cross correlation and the multipoles. Constructed covariance matrices (these are noisy so we applied tapering).

17 Result from mocks We know the cosmology in the mocks (Planck 2015) and the galaxy bias. It appears that in the mocks β_v is non-zero and negative.

18 Results from GAMA data The coefficient β_v also appears to be non zero and negative in the data. This implies that out void sample has a strong negative bias. There is some slight disagreement with the expected value from the mocks.

19 Other measurements of the growth rate Hawken et al (2017) VIPERS is the highest redshift measurement of the growth rate from voids. Low compared to Planck prediction could this be because of void bias? However, not significantly in tension with other VIPERS measurements. 19

20 Summary We have measured the multipoles of the void galaxy cross correlation function in mock galaxy catalogues and in GAMA. It appears that a linear model for the RSD with stationary void centres does not fully describe the multipoles. There is evidence that there is a growth rate and a bias associated with the void centres. This conclusion applies to spherical voids in small volume surveys. Void bias must be understood in order to get accurate measurements of the growth rate from voids.