[Paper Review] Schwarzschild Lecture 2014: HectoMAPping The Universe

Margaret J. Geller, Ho Seong Hwang|arXiv (Cornell University)|Jul 22, 2015

Galaxies: Formation, Evolution, Phenomena51 references3 citations

TL;DR

This paper presents HectoMAP, a large-scale cosmological survey combining a dense redshift survey from the Hectospec instrument on the MMT with weak gravitational lensing maps from Subaru's HyperSuprime-Cam. By merging spectroscopic galaxy distributions with dark matter maps, HectoMAP enables robust probing of how galaxies trace dark matter across cosmic time, particularly in voids and large-scale structures, with a target redshift range of z ≈ 0.55 and comoving scales up to 1.3 Gpc.

ABSTRACT

During the last three decades progress in mapping the universe from an age of 400,000 years to the present has been stunning. Instrument/telescope combinations have naturally determined the sampling of various redshift ranges. Here we outline the impact of the Hectospec on the MMT on exploration of the universe in the redshift range 0.2 < z < 0.8. We focus on dense redshift surveys, SHELS and HectoMAP. SHELS is a complete magnitude limited survey covering 8 square degrees. The HectoMAP survey combines a red-selected dense redshift survey and a weak lensing map covering 50 square degrees. Combining the dense redshift survey with a Subaru HyperSuprimeCam (HSC) weak lensing map will provide a powerful probe of the way galaxies trace the distribution of dark matter on a wide range of physical scales.

Motivation & Objective

• To map the large-scale distribution of galaxies and dark matter across cosmic time using a combination of spectroscopic redshifts and weak lensing.
• To investigate how galaxies trace the underlying dark matter distribution, especially in underdense regions like voids.
• To extend previous pilot surveys by covering a significantly larger volume (up to 1.3 Gpc × 55 Mpc) and higher redshift (z ≈ 0.7–0.8).
• To provide a robust dataset for testing models of structure formation and galaxy evolution using both spectroscopic and photometric probes.
• To address unresolved uncertainties in photometric redshifts by using a dense spectroscopic survey as a calibration benchmark.

Proposed method

• Utilizes the Hectospec multi-object spectrograph on the MMT to obtain redshifts for ~2000 galaxies per night at median redshift z ≈ 0.3.
• Combines a redshift survey (HectoMAP) with weak lensing mass maps from Subaru HyperSuprime-Cam (HSC) imaging over 50 square degrees.
• Employs a red-selected sample to target galaxies in the redshift range 0.2 ≲ z ≲ 0.8, focusing on dense, contiguous regions of the sky.
• Applies statistical techniques to compare galaxy density fields from spectroscopy with projected dark matter maps from weak lensing.
• Uses the SHELS survey as a foundation, which is a complete magnitude-limited survey covering 8 square degrees with spectroscopic redshifts.
• Leverages the high dynamic range of the Hectospec (300 fibers, 1° field of view) to efficiently survey rich clusters and large-scale structures.

Experimental results

Research questions

• RQ1 How do galaxies trace the distribution of dark matter across a wide range of physical scales, from clusters to voids?
• RQ2 To what extent do voids in the galaxy distribution align with underdense regions in the dark matter distribution as revealed by weak lensing?
• RQ3 Can a dense spectroscopic redshift survey improve the calibration and reliability of photometric redshifts used in weak lensing studies?
• RQ4 What is the evolution of the void size distribution function from z ≈ 0.55 to higher redshifts (z ≈ 0.7–0.8)?
• RQ5 How do the clustering properties of galaxies relate to the underlying matter power spectrum in large-scale structures?

Key findings

• The HectoMAP survey covers a comoving transverse dimension of approximately 1.3 Gpc × 55 Mpc at z ≈ 0.55, significantly larger than previous surveys like VIPERS.
• The combination of a dense redshift survey with weak lensing maps enables robust, direct comparison of galaxy and dark matter distributions on large scales.
• The survey is expected to provide reliable estimates of the void size distribution function and its evolution up to z ≈ 0.7–0.8.
• The foreground spectroscopic survey will allow for observational testing of the alignment between galaxy troughs and mass troughs in weak lensing maps.
• The HectoMAP project builds on prior pilot surveys (e.g., Geller et al. 2005, 2014a; Hwang et al. 2014) and extends their scope in area and depth.
• The survey is designed to test the hypothesis that voids in the galaxy distribution correspond to underdense regions in the dark matter field, as suggested by van Waerbeke et al. (2013) using photometric redshifts.