The Astronomical Journal, 127:2522–2543, 2004 May # 2004. The American Astronomical Society. All rights reserved. Printed in U.S.A. OPTICAL IMAGING OF VERY LUMINOUS INFRARED GALAXY SYSTEMS: PHOTOMETRIC PROPERTIES AND LATE EVOLUTION Santiago Arribas,1,2 Howard Bushouse, and Ray A. Lucas Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218; [email protected], [email protected], [email protected] Luis Colina Instituto de Estructura de la Materia, CSIC, Serrano 119, E-28006 Madrid, Spain; [email protected] and Kirk D. Borne George Mason University, School of Computational Sciences; and NASA Goddard Space Flight Center, Greenbelt, MD 20771; [email protected] Received 2003 November 7; accepted 2004 February 17 ABSTRACT 11 A sample of 19 low-redshift (0:03 < z < 0:07), very luminous infrared galaxy [VLIRG: 10 L < L(8– 12 1000 m) < 10 L ] systems (30 galaxies) has been imaged in B, V,andI using ALFOSC with the Nordic Optical Telescope. These objects cover a luminosity range that is key to linking the most luminous infrared galaxies with the population of galaxies at large. As previous morphological studies have reported, most of these objects exhibit features similar to those found in ultraluminous infrared galaxies (ULIRGs), which suggests that they are also undergoing strong interactions or mergers. We have obtained photometry for all of these VLIRG systems, the individual galaxies (when detached), and their nuclei, and the relative behavior of these classes has been studied in optical color-magnitude diagrams. The observed colors and magnitudes for both the systems and the nuclei lie parallel to the reddening vector, with most of the nuclei having redder colors than the galaxy disks. Typically, the nuclei compose 10% of the total flux of the system in B and 13% in I. The photometric properties of the sample are also compared with previously studied samples of ULIRGs. The mean observed optical colors and magnitudes agree well with those of cool ULIRGs. The properties of the nuclei also agree with those of warm ULIRGs, although the latter show a much larger scatter in both luminosity and color. Therefore, the mean observed photometric properties of VLIRG and ULIRG samples, considered as a whole, are indistinguishable at optical wavelengths. This suggests that not only ULIRGs, but also the more numerous population of VLIRGs, have similar rest-frame optical photometric properties to the submillimeter galaxies (SMGs), reinforcing the connection between low-z LIRGs and high-z SMGs. When the nuclei of the young and old interacting systems (classified according to a scheme based on morphological features) are considered separately, some differences between the VLIRG and the ULIRG samples are found. In particular, although the young VLIRGs and ULIRGs seem to share similar properties, the old VLIRGs are less luminous and redder than old ULIRG systems. If confirmed with larger samples, this behavior suggests that the late-stage evolution is different for VLIRGs and ULIRGs. Specifically, as suggested from spectroscopic data, the present photometric observations support the idea that the activity during the late phases of VLIRG evolution is dominated by starbursts, while a higher proportion of ULIRGs could evolve into a QSO type of object. Key words: galaxies: evolution — galaxies: interactions — galaxies: photometry — galaxies: starburst 1. INTRODUCTION advanced merger systems. The high IR luminosities are at- Luminous infrared galaxies have been the subject of nu- tributed to dust emission, with the heating source being varying combinations of interaction-induced starbursts and active ga- merous studies over the past years (see, e.g., Sanders & lactic nuclei (AGNs). ULIRGs also appear to be forming Mirabel 1996; Veilleux, Kim, & Sanders 2002, and refer- ences therein). Many of these works, both from the ground moderately massive (L*) field elliptical galaxies (e.g., Genzel et al. 2001 and references therein). and space-based, have been focused on the most energetic At a lower energy, the very luminous infrared galaxies objects: the ultraluminous infrared galaxies [ULIRGs: L ¼ ir (VLIRGs: 1011 L < L < 1012 L ;see 2.1) have not been L(8 1000 m) > 1012 L ] (e.g., Sanders et al. 1988; Melnick ir x the subject of as much scrutiny (although see, e.g., Lawrence & Mirabel 1990; Leech et al. 1994; Murphy et al. 1996; et al. 1989; Kim et al. 1995; Wu et al. 1998a,1998b). However, Clements et al. 1996a, 1996b; Surace et al. 1998, Surace, this subclass of lower luminosity objects has become in- Sanders, & Evans 2000; Borne et al. 2000; Colina et al. 2001; creasingly interesting for several reasons. First, the above- Farrah et al. 2001; Kim, Veilleux, & Sanders 2002; Bushouse mentioned studies on ULIRGs have shown that many of the et al. 2002 and references therein). These studies have found fundamental properties of these objects, such as the frequency that the vast majority of ULIRGs are strongly interacting or of interactions, the interaction phase, and the frequency of AGNs, all appear to correlate with the IR luminosity 1 ADliated with the Space Telescope Division of the European Space Agency, ESTEC, Noordwijk, Netherlands. (Sanders & Mirabel 1996 and references therein). VLIRG 2 On leave from the Instituto de Astrofı´sica de Canarias and from the studies offer the possibility to analyze these properties over a Consejo Superior de Investigaciones Cientificas (CSIC), Spain. wider luminosity range. In addition, VLIRGs represent a much 2522 VERY LUMINOUS INFRARED GALAXY SYSTEMS 2523 larger fraction of galaxies in the local universe than ULIRGs. Murphy et al (1996, five objects in the rÀGunn), and Melnick & In particular, the local density of VLIRGs is 2ordersof Mirabel (1990; one VLIRG imaged in R). Clements et al. magnitude higher than the density of ULIRGs (see, e.g., Soifer (1996a) obtained optical imaging (R-band) of 60 objects, but, as et al. 1987; Saunders et al. 1990). Moreover, the infrared lu- claimed by these authors, the sample contains basically À1 minosities of VLIRGs are between those of the ULIRGs ULIRGs, not VLIRGs (note that they used H0 ¼ 100 km s 10 11 À1 and local field spirals, for which typically Lir ¼ 10 10 L Mpc ). In summary, most of the previous optical imaging (Rieke & Lebofsky 1986). Therefore, VLIRGs represent a key studies of luminous infrared galaxies are related to the most link between the ULIRGs and the population of galaxies at luminous objects (i.e., ULIRGs). They were focused on infer- large. VLIRGs are also believed to be low-redshift analogs of ring morphological properties (e.g., interaction, merger rates) the galaxies that give rise to the far-IR background (Hauser based on single filter imaging (generally R), and no photometric et al. 1998; e.g., high-redshift submillimeter galaxies, SMGs or or color values were reported. SCUBA sources; Smail et al. 1998). The nature of the SMGs is Here, we present multiwavelength optical imaging and pho- not yet clear. Some of them could be ULIRGs, as has recently tometry for a sample of 19 low-redshift (hzi¼0:047, 0:03 < been suggested by Frayer et al. (2004) from their near-infrared z < 0:07) VLIRG systems (30 galaxies). The objects were (rest-frame optical) magnitudes and colors. However, the ma- mainlyselectedfromthe1Jyand2Jyredshiftsamplesof jority could be related to the significantly more numerous class Straus et al. (1992) and Fisher et al. (1995), respectively, with a of VLIRGs. Therefore, well-studied local samples of VLIRGs few cases from other sources (see Table 1). From all potential are valuable when establishing the relationship with the high- candidates, those suitable for observations from La Palma redshift populations. This is especially true in light of the (latitude +28) during the summer were finally selected. The ongoing deep surveys such as, for instance, the Great Obser- current sample is not complete, but it likely covers the full vatories Origin Deep Survey (GOODS, Dickinson et al. 2003; range of properties of these objects. Eight of these objects have Giavalisco et al. 2004), and the Ultra Deep Field (Beckwith only upper limits for their f25 and/or f60 IRAS fluxes, and et al. 2003). therefore for them it was not possible to compute their inte- This paper presents recently obtained ground-based optical grated [8–1000 m] infrared luminosity following the pre- B, V,andI images for a sample of VLIRGs. In x 2, we com- scription given by Sanders & Mirabel (1996; i.e., only upper ment on previous optical imaging surveys of VLIRGs, limits). Twelve systems in the sample can be classified as cool describe the characteristics of our sample, and give details (i.e., f25=f60 < 0:2), two as warm, and five are undefined due to about the observations. Section 3 describes the data reduction the fact that only an upper limit is available for f25. Six objects process. In x 4, the photometric properties are presented and have optical spectroscopy by Veilleux et al. (1995) and/or Wu compared with previously studied samples of ULIRGs. In x 5, et al. (1998b). Further details about the objects in the sample we present our conclusions. An appendix gives additional can be found in Table 1. comments on individual objects. 2.2. Observations 2. SAMPLE AND OBSERVATIONS We have used the Nordic Optical Telescope (NOT) in 2.1. Previous Optical Imaging VLIRG Studies: combination with ALFOSC (Andalucia Faint Object Spectro- The Properties of the Sample graph and Camera) to observe our sample in imaging mode on the nights of 2002 July 8–11. ALFOSC is a 2048 Â 2048 pixel Previous optical imaging studies of luminous infrared gal- Loral/Lesser CCD with a pixel size of 15 m and a plate scale axies have included some VLIRGs. When reviewing previous of 0B188 pixelÀ1. Ten of the 19 galaxies, primarily those works, it is important to establish a common definition of exhibiting detailed, extended structure on the Digitized Sky VLIRGs.