DRAFT VERSION MARCH 30, 2018 Typeset using LATEX twocolumn style in AASTeX61 THE NEXT GENERATION FORNAX SURVEY (NGFS): III. REVEALING THE SPATIAL SUBSTRUCTURE OF THE DWARF GALAXY POPULATION INSIDE HALF OF FORNAX’S VIRIAL RADIUS. 1, 2 , ∗ 1, 3 ,† 4 ,‡ 1 YASNA ORDENES-BRICENO˜ , PAUL EIGENTHALER, MATTHEW A. TAYLOR, THOMAS H. PUZIA, 1 ,§ 1 1 1, 3 ,§ 2 KARLA ALAMO-MART´INEZ, KAREN X. RIBBECK, ROBERTO P. MUNOZ˜ , HONGXIN ZHANG, EVA K. GREBEL, 1 5 5 6 7 8 SIMON´ A´ NGEL, PATRICK COTˆ E´, LAURA FERRARESE, MICHAEL HILKER, ARIANE LANC¸ ON, STEFFEN MIESKE, 9 1, 3 ,† 10 BRYAN W. MILLER, YU RONG, AND RUBEN SANCHEZ´ -JANSSEN 1Institute of Astrophysics, Pontificia Universidad Catolica´ de Chile, Av. Vicuna˜ Mackenna 4860, 7820436 Macul, Santiago, Chile 2Astronomisches Rechen-Institut, Zentrum fur¨ Astronomie der Universitat¨ Heidelberg, Monchhofstraße¨ 12-14, D-69120 Heidelberg, Germany 3Chinese Academy of Sciences South America Center for Astronomy and China-Chile Joint Center for Astronomy, Camino El Observatorio 1515, Las Condes, Santiago, Chile 4Gemini Observatory, Northern Operations Center, 670 North A’ohoku Place, Hilo, HI 96720, USA 5NRC Herzberg Astronomy and Astrophysics, 5071 West Saanich Road, Victoria, BC V9E 2E7, Canada 6European Southern Observatory, Karl-Schwarzchild-Str. 2, D-85748 Garching, Germany 7Observatoire astronomique de Strasbourg, Universite´ de Strasbourg, CNRS, UMR 7550, 11 rue de l’Universite, F-67000 Strasbourg, France 8European Southern Observatory, 3107 Alonso de Cordova,´ Vitacura, Santiago 9Gemini Observatory, South Operations Center, Casilla 603, La Serena, Chile 10STFC UK Astronomy Technology Centre, Royal Observatory, Blackford Hill, Edinburgh, EH9 3HJ, UK (Accepted by The Astrophysical Journal on 26th March, 2018) ABSTRACT We report the discovery of 271 previously undetected dwarf galaxies in the outer Fornax cluster regions at radii rvir 4 ′ ′ ′ r rvir 2 using data from the Next Generation Fornax Survey (NGFS) with deep coadded u , g and i images obtained with Blanco/DECam at Cerro Tololo Interamerican Observatory. From the 271 dwarf candidates we find 39 to be nucleated. Together~ < with< our~ previous study of the central Fornax region, the new dwarfs detected with NGFS data are 392, of which 56 are nucle- ated. The total Fornax dwarf galaxy population from NGFS and other catalogs rises, therefore, to a total of 643 with 181 being nucleated, yielding an overall nucleation fraction of 28%. The absolute i′-band magnitudes for the outer NGFS dwarfs are in the range 18:80 Mi′ 8:78 with effective radii reff;i′ 0:18 2:22 kpc and an average Sersic index n i′ 0:81. Non- nucleated dwarfs are found to be fainter and smaller by ∆ Mi′ 2:25 mag and ∆ reff;i′ 0:4 kpc than the nucleated dwarfs. We demonstrate− a significant≤ ≤ clustering− of dwarf galaxies on scales= 100− kpc, and projected surface number density⟨ ⟩ = profile esti- mates, ΣN r , show a concentration of dwarfs in the Fornax⟨ core⟩= region within r⟨.350⟩kpc.= ΣN r has a flat distribution up to 350 kpc, beyond which it declines for the non-nucleated dwarfs. The≲ nucleated dwarfs have a steeper ΣN r distribution, being more concentrated( ) towards NGC 1399 and decreasing rapidly outwards. This is the first time the( transition) from cluster to field environment∼ has been established for the very faint dwarf galaxy population with robust sample statistics. ( ) Keywords: galaxies: clusters: individual (Fornax) — galaxies: dwarf — galaxies: elliptical and lenticular, cD arXiv:1803.10784v1 [astro-ph.GA] 28 Mar 2018 Corresponding author: Yasna Ordenes-Briceno˜ [email protected] ∗ PUC-HD Graduate Student Exchange Fellow † CASSACA Postdoctoral Fellow ‡ Gemini Science Fellow § FONDECYT Postdoctoral Fellow 2 Y. ORDENES-BRICENO˜ ET AL. 1. INTRODUCTION In the present work we attempt to lay the foundation for ad- Large numbers of faint, low surface brightness dwarf dressing these issues by investigating the faint dwarf galaxy galaxies are rapidly being discovered in different environ- population in the Fornax galaxy cluster out to half of its ments throughout the local Universe (e.g., van Dokkum et virial radius (rvir), using data obtained as part of the Next al. 2015; Munoz˜ et al. 2015;M uller¨ et al. 2017; Wittmann Generation Fornax Survey (NGFS). The most prominent sur- et al. 2017). The rise of large detector arrays in present-day vey covering the galaxy cluster outskirts (Ferguson 1989), observatories such as the Dark Energy Camera (DECam; while seminal, is showing signs of age and suffers from shal- low detection limits (mB 20 mag for point-sources and Flaugher et al. 2015), enables us to survey large areas of −2 the sky down to ultra low surface brightness levels, pro- µB 24 mag arcsec in surface-brightness sensitivity) com- viding the exciting opportunity to search for undiscovered pared to the potential of modern≲ instrumentation. Given the faint dwarf galaxies in massive clusters, galaxy groups, and faint≲ surface brightness of dwarf galaxies recently detected in the field (e.g., Munoz˜ et al. 2015; Ferrarese et al. 2016; in various galaxy aggregates, the goal of this paper is to up- Muller¨ et al. 2017; Venhola et al. 2017). Dwarf galaxies are date the known population of Fornax dwarf galaxies out to . found in galaxy group and cluster environments, which are rvir 2, where one might expect to witness the transition from numerically dominated by early-type dwarf galaxies with the central galaxy population to those residing in the clus- characteristically smooth morphologies, exponential surface ter outskirts.~ The Fornax cluster is the nearest high-density brightness profiles, and stellar populations consistent with region in the Southern hemisphere (m M 31:51 mag or red-sequence galaxies (Sandage & Binggeli 1984; Binggeli DL 20:0 Mpc, Blakeslee et al. 2009), and given its proxim- et al. 1985; Conselice et al. 2003; Misgeld et al. 2008; den ity, twice the central galaxy density, and− a larger= early-type Brok et al. 2011; Ordenes-Briceno˜ et al. 2016; Roediger et galaxy= (ETG) fraction than its Northern hemisphere counter- al. 2017; Eigenthaler et al. 2018). Early-type dwarf galax- part the Virgo Cluster, Fornax is an important nearby labora- ies have typically been classified as dwarf ellipticals (dE), tory to investigate the dependence of galaxy evolution on the but are also known as dwarf spheroidals (dSph) at fainter dynamical state of the environment. magnitudes (Grebel et al. 2003). They exhibit absolute B- 2. OBSERVATIONS AND IMAGE PROCESSING band magnitudes fainter than MB 16, corresponding to The data presented in this paper is part of the observed log ⋆ M⊙ . 9, and effective radii smaller than 1 kpc (cf. Fig. 8 in Eigenthaler et al. 2018≃ −). A further morpho- Next Generation Fornax Survey (NGFS; Munoz˜ et al. 2015), 2 logical(M distinction~ ) among dwarf galaxies is whether∼ or not an ongoing, panchromatic 30 deg survey of the Fornax they host a central nuclear star cluster. Recent findings show galaxy cluster using the Dark Energy Camera (DECam; that dwarf nucleation probability is strongly dependent on its Flaugher et al. 2015) mounted∼ on the 4m Blanco telescope at spheroid luminosity (Munoz˜ et al. 2015; Ordenes-Briceno˜ et Cerro Tololo Interamerican Observatory (CTIO). Figure1 il- al. 2018), with the fraction of nucleated dwarfs systemati- lustrates the Fornax cluster region that is covered by our inner cally increasing toward brighter magnitudes. NGFS footprint, which consists of seven tiles centered on the Large populations of low-mass dwarf galaxies are ideal for dominant NGC 1399 galaxy, and homogeneously mapping studying the dependence of galaxy formation and evolution the cluster out to 50% of its virial radius (rvir 1:4 Mpc; processes in the transition zones between field and cluster Drinkwater et al. 2001). The dwarf galaxy population in the environments, especially in rich galaxy clusters. Statistically central tile corresponding∼ to r rvir 4 was already≃ studied significant samples allow us to study their clustering prop- in Munoz˜ et al.(2015) and Eigenthaler et al.(2018). All tiles erties on large scales (Munoz˜ et al. 2015) and potentially are observed in three optical bands≤ reaching~ point-source de- probe the dark matter (DM) fine-structure within the cluster tections with S N & 5 at 26.5, 26.1, and 25.3 AB mag in the ′ ′ ′ halo. This distribution in return serves as an ideal laboratory u -, g -, and i -band, respectively. for comparisons with predictions from structure formation Initial image~ processing is carried out by the CTIO Com- models (e.g. Bovill et al. 2016). munity Pipeline (CP; Valdes et al. 2014), focusing mainly Recently, observations have revealed that low-mass dwarf on instrumental signature removal (e.g., bias subtraction, galaxies appear to form surprisingly thin planes in the local flat-fielding, cross-talk correction). After the CP-processing, 1 Universe (Pawlowski et al. 2012; Ibata et al. 2013; Tully et we apply further processing using the ASTROMATIC soft- al. 2015). The observations of such planes challenge current ware suite to astrometrically calibrate and stack the individ- ΛCDM models of hierarchical structure formation (Kroupa ual frames, and conduct basic photometry (SCAMP, SWARP, et al. 2005; Pawlowski et al. 2014). The frequency of the oc- SOURCE EXTRACTOR, hereafter SE; Bertin & Arnouts 1996; currence of such anisotropic distributions in dense environ- Bertin et al. 2002; Bertin 2006). Astrometric and photomet- ments, will inform an updated view of structure formation ric calibration has been performed using the 2MASS astro- in modern galaxy formation models. These findings show metric Point Source Catalogue (Skrutskie et al. 2006) and ′ ′ ′ that deep homogeneous surveys are necessary to highlight SDSS u g i stripe 82 standard stars, respectively. To ensure the spatial distribution of the faint dwarf galaxies in nearby accurate photometric calibration, we cross-verified it with the groups and clusters of galaxies without completeness. 1 www.astromatic.net/software NGFS DWARF GALAXIES INSIDE HALF OF FORNAX’S VIRIAL RADIUS.