The HST Key Project on the Extragalactic Distance Scale XIV. The Cepheids in NGC 13651 N. A. Silbermann,2 Paul Harding,3 Laura Ferrarese,4 Peter B. Stetson,5 Barry F. Madore,6 Robert C. Kennicutt, Jr.,3 Wendy L. Freedman,7 Jeremy R. Mould,8 Fabio Bresolin,3 Holland Ford,9 Brad K. Gibson,8 John A. Graham,10 Mingsheng Han,11 John G. Hoessel,11Robert J. Hill,12 John Huchra,13 Shaun M.G. Hughes,14 Garth D. Illingworth,15 Dan Kelson,15 Lucas Macri,13 Randy Phelps,7Daya Rawson,8 Shoko Sakai,2and Anne Turner3 ABSTRACT We report the detection of Cepheid variable stars in the barred spiral galaxy NGC 1365, located in the Fornax cluster, using the Hubble Space Telescope Wide Field and Planetary Camera 2. Twelve V (F555W) and four I (F814W) epochs of observation were obtained. The two photometry packages, ALLFRAME and DoPHOT, were separately used to obtain profile-fitting photometry of all the stars in the HST field. The search for Cepheid variable stars resulted in a sample of 52 variables, with periods between 14 and 60 days, in common with both datasets. ALLFRAME photometry and light curves of the Cepheids are presented. A subset of 34 Cepheids were selected on the basis of period, light curve shape, similar ALLFRAME and DoPHOT periods, color, and relative crowding, to fit the Cepheid period-luminosity relations in V and I for both ALLFRAME and DoPHOT. The measured distance modulus to NGC 1365 from the ALLFRAME photometry is 31.31 ± 0.20 (random) ± 0.18 (systematic) mag, corresponding to a distance of 18.3 ± 1.7 (random) ± 1.6 (systematic) Mpc. The reddening is measured to be E(V–I) = 0.16 ± 0.08 mag . These values are in excellent agreement with those 1Based on observations with the NASA/ESA Hubble Space Telescope obtained at the Space Telescope Science Institute, which is operated by AURA, Inc. under NASA Contract No. NAS5-26555. 2Infrared Processing and Analysis Center, Jet Propulsion Laboratory, California Institute of Technology, MS 100-22, Pasadena, CA 91125 3Steward Observatory, University of Arizona, Tucson, AZ 85721 arXiv:astro-ph/9806017v1 1 Jun 1998 4Hubble Fellow, California Institute of Technology, Pasadena, CA 91125 5Dominion Astrophysical Observatory, Victoria, British Columbia V8X 4M6 Canada 6NASA Extragalactic Database, Infrared Processing and Analysis Center, California Institute of Technology, MS 100-22, Pasadena, CA 91125 7Observatories of the Carnegie Institution of Washington, Pasadena CA 91101 8Mount Stromlo and Siding Spring Observatories, Institute of Advanced Studies, ANU, ACT 2611 Australia 9Johns Hopkins University and Space Telescope Science Institute, Baltimore, MD 21218 10Dept. of Terrestrial Magnetism, Carnegie Institution of Washington, Washington D.C. 20015 11University of Wisconsin, Madison, Wisconsin 53706 12Laboratory for Astronomy & Solar Physics, NASA Goddard Space Flight Center, Greenbelt MD 20771 13Harvard Smithsonian Center for Astrophysics, Cambridge, MA 02138 14Royal Greenwich Observatory, Cambridge CB3 0EZ England 15Lick Observatory, University of California, Santa Cruz CA 95064 –2– obtained using the DoPHOT photometry, namely a distance modulus of 31.26 ± 0.10 mag, and a reddening of 0.15 ± 0.10 mag (internal errors only). Subject headings: galaxies: individual (NGC 1365) — galaxies: distances — stars: Cepheids 1. Introduction The observations presented in this paper are part of the Hubble Space Telescope Key Project on the Extragalactic Distance Scale, a detailed description of which can be found in Kennicutt, Freedman & Mould (1995). The main goal of the Key Project is to measure the Hubble Constant, H0, to an accuracy of 10%, by using Cepheids to calibrate several secondary distance indicators such as the planetary nebula luminosity function, the Tully–Fisher relation, surface brightness fluctuations, and methods using supernovae. These methods will then be used to determine the distances to more distant galaxies whereby the global Hubble Constant can be measured. Published results from other Key Project galaxies include M81 (Freedman et al. 1994), M100 (Ferrarese et al. 1996), M101 (Kelson et al. 1996), NGC 925 (Silbermann et al. 1996), NGC 3351 (Graham et al. 1997), NGC 3621 (Rawson et al. 1997), NGC2090 (Phelps et al. 1998), NGC 4414 (Turner et al. 1998), NGC 7331 (Hughes et al. 1998), and NGC2541 (Ferrarese et al. 1998). h m ◦ ′ NGC 1365 (α1950 = 3 31 , δ1950 = −36 18 ) is a large, symmetric, barred spiral galaxy with a measured heliocentric velocity of 1652 km s−1 (Sandage & Tammann 1981) located in the Fornax cluster of galaxies. It is classified as an SBb(s)I galaxy by Sandage & Tammann and as an SBs(b) galaxy by de Vaucouleurs et al. (1991). Work by Veron et al. (1980) found that NGC 1365 contains a hidden Seyfert 1 nucleus. NGC 1365 has been extensively mapped in HI by Ondrechen & van der Hulst (1989) and more recently by J¨ors¨ater & van Moorsel (1995). In particular, J¨ors¨ater & van Moorsel found that the inner disk of NGC 1365 has a significantly different inclination angle (40◦) compared to previous work using optical isophotes (55◦) by Linblad (1978). Other inclination angles found in the literature are 56◦(Bartunov et al. 1994), 61◦(Schoniger & Sofue 1994, Aaronson et al. 1981), 44◦ ± 5◦ (Bureau, Mould, & Staveley-Smith 1996), 46◦± 8◦ (Ondrechen & van der Hulst 1989) and 63◦ (Tully 1988). This scatter is a result of the warped nature of the NGC1365 disk (J¨ors¨ater & van Moorsel 1995) combined with the various observational methods the authors used to determine the the major and minor axis diameters. The true inclination angle of NGC 1365 is not vital for our Cepheid work but is critial for photometric and HI line-width corrections for the Tully-Fisher (TF) relation. If one uses infrared (H band) absolute magnitudes the effect of using the various inclination angles between 40◦ to 63◦ is minimal as the extinction correction in the infrared is small. However, the correction to the line-width, (sin i)−1, is not small. The corrected HI line width will decrease by 40% if one uses 63◦ instead of 40◦. Detailed discussion of the TF method is beyond the scope of this paper but we caution readers to accurately determine the inclination angle of NGC 1365 before using it as a TF calibrator. Overall, Fornax is a relatively compact cluster of about 350 galaxies with a central, dense concentration of elliptical galaxies. The center of the cluster is dominated by the large E0 galaxy NGC1399, with NGC 1365 lying ∼0.5 Mpc from NGC1399 as projected on the sky. The heliocentric radial velocity of the Fornax cluster is 1450 km s−1 with a dispersion of 330 km s−1 (Held & Mould 1994). The Fornax cluster has been the target of many studies involving secondary distance indicators, as reviewed by Bureau, Mould, & Staveley-Smith (1996). A Cepheid distance to the cluster will be an important step forward in calibrating the extragalactic distance scale. –3– This is the first of two papers on the Cepheid distance to NGC 1365 and the Fornax cluster. This paper describes the HST observations of Cepheid variable stars in NGC 1365 and the distance derived to the galaxy. A companion paper by Madore et al. (1998a) discusses the implications for the distance to the Fornax cluster and the calibration of the extragalactic distance scale. The location of NGC 1365 within the Fornax cluster and the geometry of the local universe is discussed in Madore et al. (1998b). We note that the Key Project has recently observed two other galaxies within the Fornax cluster, NGC 1425 (Mould et al. 1998) and NGC1326A (Prosser et al. 1998). 2. Observations NGC 1365 was imaged using the Hubble Space Telescope’s Wide Field and Planetary Camera 2 (WFPC2). A description of WFPC2 instrument is given in the HST WFPC2 Instrument Handbook (Burrows et al. 1994). The camera consists of four 800×800 pixel CCDs. Chip 1 is the Planetary Camera with 0.046 arcsec pixels and an illuminated ∼ 33 × 31 arcsec field of view. The three other CCDs make up the Wide Field Camera (Chips 2-4), each with 0.10 arcsec pixels and a ∼ 1.25 × 1.25 arcmin illuminated field of view. Each CCD has a readout noise of about 7 e−. A gain setting of 7 e−/ADU was used for all of the NGC 1365 observations. WFPC2 imaged the eastern part of NGC1365, as seen in Figure 1, from 1995 August through September. As with all the galaxies chosen for the Key Project, the dates of observation were selected using a power-law time series to minimize period aliasing and maximize uniformity of phase coverage for the expected range of Cepheid periods from 10 to 60 days (Freedman et al. 1994). Twelve epochs in V (F555W) and four in I (F814W) were obtained. Each epoch consisted of two exposures, taken on successive orbits, with typical integration times of 2700 seconds. All observations were made with the camera at an operating temperature of −88◦ C. Table 1 lists the image identification code, Heliocentric Julian Date of observation (mid-exposure), exposure time, and filter, of each observation. On 1995 August 28 the focus of HST was changed, occurring between the 5th and 6th epoch of NGC 1365 observations. The effect of this refocus is discussed in Section 3.1. 3. Photometric Reductions All observations were preprocessed through the standard Space Telescope Science Institute (STScI) pipeline as described by Holtzman et al. (1995b). The images were calibrated with the most up-to-date version of the routine reference files provided by the Institute at the time the images were taken. Our post-pipeline processing included masking out the vignetted edges, bad columns, and bad pixels. The images were then multiplied by a pixel area map to correct for the WFPC2 geometric distortion (Hill et al.