Draft version July 9, 2008 Preprint typeset using LATEX style emulateapj v. 4/9/03 STELLAR DISKS OF COLLISIONAL RING GALAXIES I. NEW MULTIBAND IMAGES, RADIAL INTENSITY AND COLOR PROFILES, AND CONFRONTATION WITH N-BODY SIMULATIONS R. Romano1, Y.D. Mayya1 and E. I. Vorobyov2 (Accepted in Astronomical Journal; astroph/0807.1477) Draft version July 9, 2008 ABSTRACT We present new multi-band imaging data in the optical (BV RI and Hα) and near infrared bands (JHK) of 15 candidate ring galaxies from the sample of Appleton & Struck-Marcell (1987). We use these data to obtain color composite images, global magnitudes and colors of both the ring galaxy and its companion(s), and radial profiles of intensity and colors. We find that only nine of the observed galaxies have multi-band morphologies expected for the classical collisional scenario of ring formation, indicating the high degree of contamination of the ring galaxy sample by galaxies without a clear ring morphology. The radial intensity profiles, obtained by masking the off-centered nucleus, peak at the position of the ring, with the profiles in the continuum bands broader than that in the Hα line. The images as well as the radial intensity and color profiles clearly demonstrate the existence of the pre-collisional stellar disk outside the star-forming ring, which is in general bluer than the disk internal to the ring. The stellar disk seems to have retained its size, with the disk outside the ring having a shorter exponential scale length as compared to the values expected in normal spiral galaxies of comparable masses. The rings in our sample of galaxies are found to be located preferentially at around half-way through the stellar disk. The most likely reason for this preference is bias against detecting rings when they are close to the center (they would be confused with the resonant rings), and at the edge of the disk the gas surface density may be below the critical density required for star formation. Most of the observed characteristics point to relatively recent collisions (< 80 Myr ago) according to the N-body simulations of Gerber et al. (1996). Subject headings: galaxies: photometry — galaxies: interactions 1. introduction been tested observationally in the Cartwheel, the pro- Ring galaxies are a class of objects whose optical ap- totype ring galaxy. The ring in this galaxy is expand- pearance is dominated by a ring or a ring-like structure. ing (Fosbury & Hawarden 1977), and is forming massive In one of the earliest discussions of ring galaxies, Bur- stars (Higdon 1995). A radial color gradient was also bidge & Burbidge (1959) suggested that these objects noticed by Marcum et al. (1992), which was found to be may be the aftermath of a close collision between an el- in agreement with the sequential ordering of stellar ages liptical and spiral galaxy. On the other hand, Freeman & (Korchagin et al. 2001; Vorobyov & Bizyaev 2001). Ap- de Vaucouleurs (1974) suggested that the ring is proba- pleton & Marston (1997) established the color gradient bly the result of a collision between a spiral galaxy and an in some other ring galaxies. Marston & Appleton (1995) intergalactic cloud of Hi. Theys & Spiegel (1976, 1977) carried out Hα imaging observations of eight ring galax- systematically studied the basic observational properties ies and found that majority of the star-forming regions of a sample of ring galaxies and suggested that rings are are located exclusively in the ring. formed when an intruding galaxy passes nearly through In the collisional scenario proposed by Lynds & the center of a normal disk galaxy. Lynds & Toomre Toomre (1976), a stellar density wave is set off as the (1976) used numerical simulations and settled the is- collective response of the stars that were present in the sue regarding their origin. They demonstrated that ring pre-collisional disk of the target galaxy. However, we galaxies are formed as a result of an on-axis collision know very little about the underlying stellar disk of the between an intruder galaxy and a gas-rich disk-galaxy. target galaxy. It is generally believed that the observed In this scenario, the collision sets off an expanding ring rings, especially in the near infrared continuum bands, wave, which in turn triggers star formation in its wake. trace the location of the stellar density waves. The stel- Due to the expanding nature of the wave, the ring of star lar disk outside the ring should carry important informa- formation also advances successively to larger radii with tion on the nature of the pre-collisional disk. However, time. This scenario has stood the test of the multi-band such a disk has not been traced even in the Cartwheel. data those have become available since then (Appleton Appleton & Marston (1997) tried to infer an outer disk & Struck-Marcell 1996 and references therein). using radial profiles of surface brightness and color in Many of the predictions from theoretical models have four galaxies, and found such a disk in two cases (IIHz4 and VIIZw466). Their images were not deep enough to 1 Instituto Nacional de Astrof´ısica Optica y Electronica, Luis study the azimuthal structure of the outer disks. Enrique Erro No. 1, Tonantzintla, Apdo Postal 51 y 216, 72840, Korchagin et al. (2001) have developed a method to Puebla, Mexico: [email protected], [email protected] estimate the contribution of the newly formed stars in 2 The Institute for Computational Astrophysics, Saint Mary’s University, Halifax NS, B3H 3C3, Canada; and Institute of Physics, different wavelength bands as the density wave expands South Federal University, Stachki 194, Rostov-on-Don, Russia. in the target galaxy. The density wave in the underlying 2 Romano et al. stellar disk can be recovered by subtracting the contri- Table 2 contains a detailed log of the observations. bution of the newly formed stars. Availability of Hα and The observing runs for each galaxy are given in column continuum images are fundamental in achieving this ob- 2, followed by the exposure times in the broadband fil- jective. The continuum images should reach at least 2 ters. These broadband filters correspond to the standard magnitudes deeper than the brightness of the ring in or- Johnson-Cousins BV RI system. The central wavelength der to register the stellar disk on either side of the star- of the Hα filter for each galaxy is given in column 7, fol- forming ring. With this goal in mind, we carried out lowed by a column containing the exposure times in these new imaging observations of a sample of 15 candidate filters. The Hα filters were of square-shape with typically ring galaxies in the BV RIJHK broad bands and in the a width of 100A,˚ and include both the [Nii] lines flanking emission line of Hα. Some of the candidate galaxies may the Hα. For five galaxies, observations were carried out not have formed by the scenario proposed by Lynds & in emission-line free narrow bands to facilitate subtrac- Toomre (1976), and these galaxies were included in our tion of the in-band continuum from the Hα-filter images. observing list with the hope that the new observations The central wavelength of the off-band continuum filters would allow us to filter out contaminating galaxies. The used for these galaxies are given in column 9. For the data and detailed surface photometric analysis of all sam- rest of the galaxies, the R-band images were used for the ple galaxies are presented in this paper. In a forthcom- purpose of continuum subtraction. Twilight sky expo- ing paper (Paper II), we will discuss the results obtained sures were taken for flat-fielding purposes. Several bias from the extraction of the underlying stellar density wave frames were obtained at the start and end of each night. in galaxies that are most likely formed by the classical collisional scenario. 2.2. Near infrared imaging Sample and the details of our observations are dis- cussed in § 2. Morphological descriptions and one di- All near infrared observations were carried out in the mensional profiles of individual galaxies are presented in J, H and K bands with the Observatorio Astronomico § 3. In § 4, we compare the photometric properties of Nacional 2.1-m telescope at San Pedro Martir, Mexico. ring galaxies with that of normal galaxies. Results ob- The CAMILA instrument (Cruz-Gonzalez et al. 1994), tained from the new data are discussed in the context of that hosts a NICMOS 3 detector of 256×256 pixel for- N-body simulations in § 5. Conclusions from our study mat, was used in the imaging mode with the focal reducer configuration f/4.5. This results in a spatial sampling of are presented in § 6. Gray scale maps in the Hα and B- ′′ −1 ′ ′ bands and one dimensional surface brightness and color 0. 85 pixel and a total field of view of 3.6 × 3.6. Each profiles for each galaxy are presented in the Appendix. observation consisted of a sequence of object and sky ex- Throughout this paper, all distance scaling assumes a posures, with the integration time of an individual expo- value for the Hubble Constant of 75 km s−1 Mpc−1. sure limited by the sky counts, which was kept well below the non-linear regime of the detector. A typical image sequence consisted of 10 exposures, six on the object and 2. observations and reductions four on the sky. NIR observations could not be carried Ring galaxies from the sample of Appleton & Struck- out for three of the sample galaxies. Though of lesser sen- Marcell (1987) were selected for optical and NIR pho- sitivity, we used the 2MASS3 images of these three galax- tometric study.