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# Low Surface Brightness Galaxies in Nearby Perseus Cluster

Abstract

We propose a systematic study of Ultra Diffuse Galaxies (UDGs) in very nearby massive clusters, compiling different mass and dynamical states, using deep HSC images in g and i bands. The unique population of UDGs has emerged into a serious problem that confronts the standard picture of structural formation and galaxy evolution in dense environment. To understand UDGs, we aim to: (1) accurately model their photometric structure, and extract color information; (2) compare their number density, spatial distribution, and key properties among different clusters to shed light on their relation to the evolution of their host clusters, especially on how can they survive this harsh environment; (3) compare their structures with other populations of dwarf galaxies in the cluster, and other low surface brightness galaxies in the field, to explore possible evolutionary connection. We advocate that observation like this can help HSC make a mark in this emerging topic.

## Introduction

Despite their potential large number density and importance in verifying galaxy formation theory under $$\Lambda$$-cold dark matter ($$\Lambda$$-CDM) cosmology, the Low Surface Brightness (LSB) galaxies are still largely unexplored even in the low redshift universe. Early this year, the Dragonfly camera array uncovered a population of large and diffuse dwarf galaxies (referred as Ultra Diffuse Galaxies, or UDGs) within the virial radius of the Coma cluster at $$z=0.023$$ (Dokkum 2015). These UDGs have the typical luminosity of a dwarf elliptical galaxy, yet their effective radius can be as large as our Milky Way. These are very surprising discoveries not only because they populate a parameter space that was occupied by very few galaxies (see Fig.1); More importantly, it was considered that galaxies like them are impossible to exist in massive clusters. Due to their diffuse nature, such vulnerable systems should not be able to survive the harsh environment. However, after the initial discoveries, (Koda 2015) spotted many more UDGs in the Coma cluster using images from Suprime-Cam on Subaru telescope, some of which even locate closely to the line-of-sight center of the cluster; Later, (Mihos 2015) found three UDGs with more extreme properties in the inner region of the much closer and less massive Virgo cluster; More recently, one of the largest UDG in Coma was confirmed to be a real cluster member van Dokkum (2015b). So far, they are found in clusters with different total mass, and occupy a large region on the luminosity-size plane (Fig.1) ; Except for one case in Virgo, they rarely show any sign of tidal disruption; Based on their optical color, they are most likely quiescent systems, and locate close to the red-sequence of the cluster.

In less than year, this unique population of galaxy has emerged into a serious problem that may even confront the standard picture of structural formation and galaxy evolution in dense environment, assuming that most UDGs identified so far are indeed living inside the cluster. Until now, we still know very little about their properties and their cosmological nature, and only two nearby clusters have been searched. \ulAre they really the “failed $$L^{\ast}$$ galaxies” that dwell inside a large dark matter halo? Or maybe they have lived most their life in isolation, and have only been accreted into cluster very recently? How many more of them that have already been disrupted during the past 10 Gyr? Are their number density, spatial distribution, or other characters contain interesting information regarding the evolution of their host clusters? And, finally, can we successfully explain their formation and properties using the established $$\Lambda$$-CDM cosmology? All these imminent questions shall be answered by new observations. First and foremost, we need to carefully search for UDGs in more nearby clusters with different total mass and dynamical states. These halo properties are essential for us to understand the formation and survival of these UDGs. The halo mass defines the tidal force exerted on galaxies, while the dynamical state can affect the infall of the UDGs. Hereby, we propose that we can already achieve the above goal efficiently through carefully designed deep, multi-band imaging survey using the Hyper-Surpime Camera (HSC) on the Subaru telescope.

## Data Reduction

Basic HSC pipeline reduction.

Summary of the SExtractor run.