Khaled Said Soliman

Do we really live in a giant void?

The main goal of this project is to confirm/Rule a recently proposed idea that we live in a giant void. Wong et al. (2021) proposed the idea of a Local Hole that covers 90% of the whole sky and goes out to 0.075, showing 20-21% under-density. This idea is backed by Whitbourn & Shanks (2016); Shanks et al. (2019) who detected an under-density of 15% in number-magnitude counts, redshift distributions, and Bulk flow motions. Another evidence comes from B ̈ohringer et al. (2020) who used the density distribution of X-ray clusters for 34% of the entire sky and found evidence for an underdensity of 30-40%, out to z ~ 0.04. Confirming this under-density means local measurements of the Hubble Constant have been over-estimated by 3%. That will solve the current Hubble tension. However, such under-density is in tension with a global ΛCDM cosmology. Confirming such a huge void will influence the course of billions of dollars worth of major scientific missions around the world. The best way to confirm/rule out this proposal is through a peculiar velocity survey. However, the main limi- tation of the current surveys analysed to date is that they do not reach far enough in redshift to map the volume containing the most massive known over-densities in the local universe, such as the Shapley, Vela superclusters, and Sloan Great Wall. DESI peculiar velocity survey will use Tully-Fisher (Spirals), Fundamental Plane (Ellipticals), and Metric Plane (BCGs) to provide a peculair velocity survey that reachs four times as far as the proposed void.

Reconstruction of the whole Universe with DESI and Wallaby redshift surveys

The peculiar velocity of a galaxy can be measured two main ways: (1)~`Directly', via so-called distance indicators, a difficult and expensive method requiring many observations of the same object at different wavelengths. Consequently the largest homogeneous sample of directly-measured peculiar velocities to date is the 6-degree Field Galaxy Survey of $\sim$9000 galaxies (6dFGSv), covering only the southern hemisphere. (2)~`Indirectly', by measuring the density of the nearby universe and using that to predict the peculiar velocities. This relies on the fact that high-density regions will continue to accrete more matter while low-density regions will lose more matter. This method is known as {\em velocity field reconstruction} and is observationally much cheaper and has higher fidelity than `direct' measurements, and so is our focus here.

The main objective of this project is to use the positions of all galaxies that will be available from the new galaxy surveys to produce a density map that covers the full sky and as deep as possible. Subsequently, convert that map into a full sky velocity map that can be, globally, used as a hub to predict peculiar velocities for any cosmological analysis. This goal will be addressed through linked aims:

• How to combine redshifts from DESI (optical) with WALLABY (21 cm) to produce a nearly full sky sample that we can use for reconstruction
• Creating the first-ever all-sky maps of density and peculiar velocity for galaxies in the nearby universe using these datasets.

Coma Fundamental plane from DESI

Although peculiar velocity surveys usually use either Tully-Fisher for spirals or Fundamental Plane for ellipticals, DESI will be able to combine these two distance indicators in an unprecedented way.

The first sample of galaxies will be a magnitude limited sample of elliptical galaxies within the footprint of DESI survey. For this sample we will measure the traditionally measured central velocity dispersion but with some repeat observations to build SNR as well as control systematic. The DESI Legacy Imaging Surveys will provide a photometric catalog of three optical bands ($g$, $r$, and $z$) and four far-IR $WISE$ channels. The optimal filter for our purpose is the $r-$band but we will also need $g-$band for the sample selection and color separation. This photometric catalog already include the surface brightness and the angular radius. With all data in hand (velocity dispersion, surface brightness, and effective angular radius) one can establish the FP and then measure distances to elliptical galaxies in the sample.

The second sample will include spiral galaxies in the DESI footprint which will be observed with several fibers per pass in both dark and bright time. The obvious way to space the fibers will be along the major axis as much as possible. For spirals, that will allow us to measure the rotation velocity. Again, the DESI legacy Imaging will provide the apparent magnitude for all galaxies in our sample as well as inclinations. With rotation velocities and magnitudes in hand one can measure distances to these galaxies using the TF relation.

In this project, the candidate will identify all elliptical galaxies in the Coma cluster, measure the velocity dispersion using DESI spectra, investigate the best magnitude to use as a probe for the total luminosity, correct all observational effects, establish the FP relation, measure distances to all galaxies in the Coma cluster, and compare the measured distances to Coma using FP with the one from TF sample and with the literature.