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1. INTRODUCTION Dwarf Galaxies Are the Most Common Type of Galaxies in Ferguson 1989; Bothun, Impey, & Malin 1991; Groups: the Local Universe

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1. INTRODUCTION Dwarf Galaxies Are the Most Common Type of Galaxies in Ferguson 1989; Bothun, Impey, & Malin 1991; Groups: the Local Universe THE ASTRONOMICAL JOURNAL, 121:148È168, 2001 January ( 2001. The American Astronomical Society. All rights reserved. Printed in U.S.A. [ 1 THE DWARF GALAXY POPULATION OF THE DORADO GROUP DOWN TO MV B 11 ELEAZAR R. CARRASCO AND CLA UDIA MENDES DE OLIVEIRA InstitutoAstronoü mico e Geof• sico, Universidade de Sa8 oSa Paulo C.P. 3386, 01060-970,8 o Paulo, Brazil; rcarrasc=pushkin.iagusp.usp.br, oliveira=iagusp.usp.br LEOPOLDO INFANTE2,3 Departamento deAstronom• a y Astrof• sica, Facultad de F• sica, PontiÐcia Universidad Cato lica de Chile, Casilla 306, Santiago 22, Chile; linfante=astro.puc.cl AND MICHAEL BOLTE UCO/Lick Observatory, University of California at Santa Cruz, Santa Cruz, CA 95064; bolte=ucolick.org Received 2000 August 15; accepted 2000 October 3 ABSTRACT We present V and I CCD photometry of suspected low surface brightness dwarf galaxies detected in a survey covering D2.4 deg2 around the central region of the Dorado group of galaxies. The low surface brightness galaxies were chosen based on their sizes and magnitudes at the limiting isophote of 26 V k. The selected galaxies have magnitudes brighter than V B 20(M B [11 for an assumed distance to the V ~2 group of 17.2 Mpc), with central surface brightnesses k0 [ 22.5 V mag arcsec , scale lengths h [ 2A, and diameters º14A at the limiting isophote. Using these criteria, we identiÐed 69 dwarf galaxy candi- dates. Four of them are large very low surface brightness galaxies that were detected on a smoothed image, after masking high surface brightness objects. Monte Carlo simulations performed to estimate completeness, photometric uncertainties and to evaluate our ability to detect extended low surface brightness galaxies show that the completeness fraction is, on average, [80% for dwarf galaxies with [ [ ~2 17 \ MV \ 10.5and 22.5 \k0 \ 25.5 V mag arcsec , for the range of sizes considered by us (D º 14A). The V [I colors of the dwarf candidates vary from [0.3 to 2.3 with a peak on V [I \ 0.98, suggesting a range of di†erent stellar populations in these galaxies. The projected surface density of the dwarf galaxies shows a concentration toward the group center similar in extent to that found around Ðve X-ray groups and the elliptical galaxy NGC 1132 studied by Mulchaey & Zabludo†, suggesting that the dwarf galaxies in Dorado are probably physically associated with the overall potential well of the group. Key words: galaxies: clusters: general È galaxies: clusters: individual (Dorado Group) È galaxies: dwarf È galaxies: fundamental parameters È galaxies: luminosity function 1. INTRODUCTION Dwarf galaxies are the most common type of galaxies in Ferguson 1989; Bothun, Impey, & Malin 1991; groups: the local universe. In addition, they are thought to be the Ferguson & Sandage 1991). From these works two general single systems with the largest dark-matter contents, with trends were uncovered: (1) clusters and rich groups usually M/L ratios as high as that of groups and poor clusters (e.g., have a luminosity function with a faint end that is steeper, Carignan & Freeman 1988), and hence their spatial dis- i.e., has more low-luminosity galaxies, than that for poorer tribution and mass spectrum may give us important groups, and (2) dwarf elliptical and dwarf spheroidal gal- insights into the spatial scales over which mass is distrib- axies (dE/dS0) seem to dominate the faint end of the uted. However, they are also the hardest galaxies to number counts in clusters, while the dwarf-irregular popu- observe, because of their low surface brightnesses and low lation (dIrr) are more representative in poor groups and in luminosities. Only recently has it become possible to obtain the Ðeld. Point 1 was well illustrated by Ferguson & deep wide-Ðeld CCD photometry of signiÐcant numbers of Sandage (1991) who found a faint-end slope (Ðt to a Schec- these galaxies, mostly in clusters. Spectroscopy of such low ter function) of a \[1.3 in the composite luminosity func- luminosity systems is still a challenge even with large-class tion of seven nearby groups (the sample included the telescopes, unless emission lines are present. Dorado group), with a tendency for the richer groups to Dwarf galaxies in nearby clusters and groups have been present a larger ratio of dwarf to giant galaxies than the studied in detail with photographic material (e.g., Coma: poorer groups. Point 2 has been challenged by observations Thompson & Gregory 1993; Virgo: Sandage, Binggeli, & of the most well-known group of all, our own Local Group. Tammann 1985, Impey, Bothun, & Malin 1988; Fornax: Despite being a poor group with just over 30 members, the Local Group has a larger number of dE/dS0s than of dIrrs ÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈ (Pritchet & van den Bergh 1999). The majority of the dwarf 1 Based on the data collected at the Cerro Tololo Inter-American spheroidal galaxies are concentrated around M31 and the Observatory (CTIO) and Las Campanas Observatory, Chile. Milky Way, with a slope of the faint-end of the luminosity 2 Visiting astronomer CTIO. CTIO is operated by Association of Uni- function (Ðt to a Schechter function) of approximately [1.3, versities for Research in Astronomy (AURA), Inc., under cooperative agreement with the National Science Foundation (NSF). while the dwarf irregulars are less concentrated with a Ñat 3 Visiting astronomer, Las Campanas Observatory. Las Campanas is luminosity distribution (van den Bergh 1999; Grebel 2000). operated by the Carnegie Institution of Washington. This seems to contradict the results obtained for the CfA 148 THE DORADO GROUP 149 groups and Ðeld survey where the largest contribution to observed on November 1È2, with the 0.9 m Curtis/Schmidt the steep faint end was from the irregular galaxies (Marzke telescope at the Cerro Tololo Inter-American Observatory et al. 1998). Recent results coming from CCD studies in (CTIO). The images were obtained using a standard STIS nearby groups up to D \ 10 Mpc show an increasing 2048 ] 2048 pixel CCD with 21 km pixels corresponding to number of new dwarf members (e.g., Jerjen, Binggeli, & 2A.028pixel~1 on the sky. The total Ðeld of view was 1¡.15 Freeman 2000) with a spatial distribution similar to that ] 1¡.15. The second set of observations was performed with found in the Local Group (i.e., the dwarf spheroidals con- the 1.0 m Swope telescope at the Las Campanas Observa- centrated around the brightest galaxies, while the irregulars tory (LCO40), Chile, in the nights of 1996 December 2È7. In distributed throughout the group). this case the Tektronix 5 CCD, with 2048 ] 2048 21 km This is the Ðrst paper of a series dedicated to survey pixels (0A.696 on the sky) was used. The total Ðeld of view nearby groups with the aim of identifying low surface per image was 24@ ] 24@. brightness dwarf galaxies (hereafter LSBDs) down to MV D The Ðelds were imaged through standard Johnson V and [11. Our primary goal is to study the physical properties Cousins I Ðlters. Typical total exposure times were between and luminosity distributions of the LSBDs and ultimately 30 and 60 minutes per Ðeld in both Ðlters. All data were determine the luminosity function of the groups as a func- taken under photometric conditions, with a seeing ranging tion of their richness and other group parameters. Here we from1A.3 to 1A.9 in the LCO40 frames. Because of the large report the results of the CCD photometry of the dwarf pixel size of the CCD used in the observations taken at galaxy candidates of the nearby group of galaxies Dorado. CTIO(2A.028 pixel~1), the image quality in the central Ðeld A further study will analyze other nearby groups of gal- is much worse than that for the LCO40 images (D3A). Table axies. 1 shows the observation log (Ðeld identiÐcation, equatorial The Dorado group is a loose concentration of galaxies coordinates, Ðlters, observatories, exposure times, the centered at D4h15m, [55¡41@ (epoch J2000.0). It is also observed area, and the area used on the analysis after the known as Shk 18 (Shakhbazian 1957), G16 (de Vaucouleurs masked pixels were disregarded), where D01 is the central 1975) and HG3 (Huchra & Geller 1982). Dorado was Ðrst Ðeld observed in CTIO and D02 to D11 are the Ðelds identiÐed by de Vaucouleurs (1975) as a ““ large and complex observed in LCO40. The total area covered by the obser- nebulae ÏÏ in the region of Dorado. In 1982, Huchra & vations wasD2¡.6 (D1¡.2 in the central region and D1¡.4 Geller (1982) measured velocities for 18 galaxies in the spread over the outskirts of the group) in each of the Ðlters. group. Maia, da Costa, & Latham (1989) extended the After subtraction and masking the bright objects in the number of known members to 46. Finally, Ferguson & Ðelds, the total area used in the analysis wasD2¡.4. Figure 1 Sandage (1990, 1991), in a study of the galaxy properties in shows the sizes and locations of the observed CCD images. seven nearby groups, identiÐed 34 possible members of The symbols indicate the galaxies with known velocities Dorado with magnitudesBT [ 19 in photographic plates. taken from the NED. In addition, we observed two Ðelds The central part of the group is dominated by two almost 10¡ north (B3) and 15¡ south (B4) of the Fornax galaxy equally bright early-type systems: NGC 1549 (an E1 galaxy NGC 1399, in order to estimate the background contami- withV \ 1247 km s~1) and NGC 1553 (an S0 galaxy with nation of low surface brightness galaxies. These obser- \ r ~1 Vr 1280 km s ). No detailed distance determination has vations were performed in the second run (LCO40), with the been made for Dorado, and therefore we will adopt a group ~1 \ distance of 17.2 h Mpc, whereh H0/75.
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