c The HST Key Project on the Extragalactic Distance Scale XIV. The Cepheids in NGC 1365' N. A. Silbermann,2 Paul Laura Ferrare~e,~Peter B. Stetson,5Barry F. Madore,' Robert C. Kennicutt, Jr.,3 Wendy L. ,Freedman,7.Jeremy R. i\/lould,' Fabio Bre~olin,~ HollandBrad E;. Gibson,' John A. Graham,"Mingsheng Han," John G. 'Based on observations with the NASA/ESA HubbleSpace Telescope obtained at the Space Telescope Science Institute, which is operated by AURA, Inc. under NASA Contract No. NAS5-265.55. 21nfrared Processing and Analysis Center, Jet Propulsion Laboratory, California Institute of Technology, MS 100-22, Pasadena, CA 91125 3Steward Observatory, University of Arizona, Tucson, A2 85721 4Hubble Fellow, California Institute of Technology, Pasadena, CA 91125 5Dominion Astrophysical Observatory, Victoria, British Columbia V8X 4M6 Canada 'NASA ExtragalacticDatabase, Infrared Processing and A4nalysis Center, California Institute of Technology, MS 100-22, Pasadena, CA 91125 70bservatories of the Carnegie Institution of Washington, Pa,sadena CA 91101 'Mount Stromlo and Siding Spring Observatories, Institute of Advanced Studies, ANU, ACT 2611 Australia 'Johns Hopkins University and Space Telescope Science Institute, Baltimore, MD 21218 "Dept. of Terrestrial Magnetism, Carnegie Institution of \Va.shington, Washington D.C. 20015 "University of Wisconsin, Madison, Wisconsin 53706 -- .j ~ ABSTRACT We report the detection of Clepheicl 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 136.5 from the ALLFRAME photometry is 31.31 f 0.20 (random) It 0.18 (systema,tic) mag, corresponding to a distance of 18.3 f 1.7 (random) f 1.6 (systematic) Mpc. The reddening is measured to be E( V-I) = 0.16 f 0.08 mag. These values are in excellent agreement with those obtained using the DoPHOT photometry, namely a distance modulus of 31.26 f 0.10 mag, and a reddening of 0.15 f 0.10 mag (internal errors only). Subject headings: galaxies:individual (NGC 136.5) - galaxies:distances - stars:Cepheids Hoesse1,"Rohert .J. .John Hu~hra,~~Shaun M.G. Hughes,'" Garth D. Illingworth,'s Dan Iielson," Lucas Macri,'3 Randy Phelps.'Daya Rawson,' Shoko Sakai,2ancl Anne Turner3 Received ; accepted "Laboratory for Astronomy k Solar Physics, NASA Goddard Space FlightCenter, Greenbelt MD 20771 13Harvard Smithsonian Center for Astrophysics, Cambridge, MA 02138 '"Royal Greenwich Observatory, Chmbriclge CU:3 OEZ England 15Lick Observatory, University of Chlifornia, Santa. Cruz C.4 95064 1. Introduction The observations presented in this paper are part of the Huhhle Space Telescope Key Project on the Extragalactic Distance Scale, a detailed description of which can be found in Kennicutt, Freedman & Mould (199.5). The main goal of the Key Project is to measure the Hubble Constant, Ho, to an accuracy of lo%, 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), MlOO (Ferrarese et al. 1996), MlOl (Kelson et al. 1996), NGC925 (Silbermann et al. 1996), NGC33.51 (Graham et al. 1997), NGC 3621 (Rawson et al. 1997),NGC2090 (Phelps et al. 1998),NGC4414 (Turner et al. 199S), NGC 7331 (Hughes et al. 1998), and NGC 2541 (Ferrarese et al. 1998a). NGC 136.5 (cy1950 = 3h31m, 61950 = -36'18') is a large, symmetric, barred spiral galaxy with a measured heliocentric velocity of 1652 km s-l (Sandage & Tammann 1991) 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 (19S9) and more recently by Jorsater tk van Moorsel (1995). In particular, Jorsater k van Moorsel found that the inner disk of NGC 1i36.5 has a significantly different inclination angle (40") compared to previous work using optical isophotes (rj.5") by Linblad (197s). Ot,her inclination angles found in the literature are 5Go(Bartunov et al. 1994), 61"(Schoniger S: Sofue 1994, Aaronson et al. 19S1), 31" f 5" (Bureau, Mould, k Shveley-Smith 1996), 46"f 8" (Ondrechen tk van cler I-Iulst 1W)and 63" (Tully 1988). This scatter is a result or t,lw warped nature of the NGC; 1365 tlisk (.Jijrs$tcr k van Moorsel 199.5) combined with t,hc various ohscrvatioml mc?thocls the authors used to determine the the major and minor axis diameters. The'true inclination angle of NGC) 136.5 is not vital for our Cepheid work hut is critial for photometric and HI line-width corrections for the Tully-Fisher (TF) relation. If one uses infrared (H hand) 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 z)-', 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 EO galaxy NGC 1399, with NGC 1365 lying -0.5 Mpc from NGC 1399 as projected on the sky. The heliocentric radial velocity of the Fornax cluster is 1450 km s-l with a dispersion of 330 km s-l (Held & Mould 1994). The Fornm cluster has been the target of many studies involving secondary distance indicators, as reviewed by Bureau, Mould, k Staveley-Smith (1996). A Cepheid distance to the cluster will be an important step forward in calibrating the extragalactic distance scale. 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 companionpaper by kIaclore et nl. (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 t,he Fornax cluster and the geometry of the local universe is tliscussecl in Maclore €1 nl. (1998h). We note that the Iiey Project has recently observed two other galaxics within the Fornax cluster, NC:C 1125 2. Observations NGC 136.5 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 800x800 pixel CCDs.Chip 1 is the Planetary Camera with 0.046 arcsecpixels and an illuminated - 33 x 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 x 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 136.5 observations. i WFPC2 imaged the eastern part of NGC 136.5, 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 (F555kV) and four in I (F814W) were obtained. Each epoch consisted of two exposures, taken on successive orbits, with typical integmtion times of 2700 seconds. All observations were made with the cameraat 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. PhotometricReductions 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.