Royal Greenwich Observatory, Madingley Road, Cambridge CB3 0EZ, UK
Mingsheng Han, John Hoessel
University of Wisconsin, Madison, Wisconsin 53706, USA
Keywords: galaxies, cepheids, distance scale
NGC 7331 is one of 18 galaxies chosen by the HST Extragalactic Distance Scale Key Project to calibrate secondary distance estimators, using HST to obtain an accurate distance via Cepheid variables, with the ultimate goal of using these to measure H to an external accuracy of 10%. NGC 7331 is in the constellation of Pegasus, at position 22 37 05.2, +34 25 10 (J2000). It is an early type spiral classified as Sb(rs)I-II in the Revised Shapley Ames scheme by Sandage & Bedke (1985), and given a Hubble type T = 3 by de Vaucouleurs et al. (1991) with an inclination of 71 (Huchtmeier & Richter 1989), and was chosen as one of the calibrators for the luminosity-line width (aka Tully-Fisher) relation.
The WFPC2 field lies 3.5 arcmin north of the galaxy's nucleus, along the major axis. Variable stars were detected from 15 epochs of F555W cosmic-ray split exposures, and their colors were measured from 4 epochs of F814W exposures, over the interval of 1994 June 18 to 1995 August 17. All of these epochs are thus taken after the decrease in WFPC2's operating temperature (which occurred on 1994 April 23). As such, no correction for a charge transfer effect has been attempted, as it adds a maximum of 0.01 magnitudes rms uncertainty, and as most of our WFPC2 field has a high background, any charge transfer effect is likely to be even smaller than this. The photometry was derived independently by SH and MH, each using programs based on DoPHOT (Schechter, Mateo & Saha 1993, Saha et al. 1994) and DAOPHOT/ALLFRAME (Stetson 1994), respectively, in a similar way as described in detail for M100 by Hill et al. (1996). IRAF routines were used for all image arithmetic.
Similar methods as described by Ferrarese et al. (1996) (for M100) were also used to detect Cepheids in NGC 7331. Master lists of objects obtained from combined medianed images of all epochs gave a total of 6720, 7215, 8193, and 9026 objects for chips 1--4, respectively. A 1.5 sigma cut, with a minimum of 10 or more epochs was applied, resulting in 179, 313, 229 and 299 variables for chips 1--4. The lightcurves of each of these were then searched for those that are periodic with periods between 10 and 80 days, rejecting any that were obviously crowded or near a saturated star. The number of good Cepheid candidates (i.e., those with regular, Cepheid-like lightcurves) obtained for each chip is 0, 11, 3, and 5, and a sample are shown in Figure 1.
Figure: Phased lightcurves for 4 of the Cepheids in NGC 7331.
Corrections to the intensity averaged I mags were made, as described in Ferrarese et al. (1996), to account for the sparser sampling of the I data as compared to the V data.
The reasons for using two independent photometry programs to search for Cepheids is to provide a check on any systematic errors that may be present in deriving photometry from WFPC2 images. The Cepheid lists and periods were obtained independently from each photometry set and are consistent with each other. We are still in the process of defining an absolute calibration for the photometry and defer to a later publication the presentation of the distance to NGC 7331 (Hughes et al. 1996). But we can show that the Cepheids found have period and color properties that are consistent with LMC Cepheids.
The mean magnitudes from all epochs has been used to produce the deep I vs V-I color-magnitude diagram in Figure 2.
Figure: I vs V-I CMD for chips 2--4 of the WFPC2 field (no Cepheids were found on the PC chip). For clarity, only every 10th star is plotted. Open circles are the Cepheids. Those outside the instability strip are marked by open squares.
This shows that most of our Cepheids lie within the classic Cepheid instability strip. The two that lie outside the edge will not be used in deriving distance moduli, as they are likely to be either contaminated by a companion, or else have unusual amounts of reddening.
The Period-Luminosity relations in V and I are shown in Figure 3.
Figure: Period-Luminosity relations in V (top panel) and I (bottom panel). Solid circles are NGC 7331 Cepheids found with HST, open circles are LMC Cepheids from Madore (1985). The straight line is a least squares fit, with slope set independently by the LMC Cepheids only, and the offset by the NGC 7331 Cepheids. The variables outside the instability strip and excluded from the fit are indicated by `R' (numbers are the V-I colors).
Also plotted are the LMC Cepheids that have periods within the range of the NGC 7331 Cepheids, with their magnitudes shifted to minimize the least squares fit. All of the LMC Cepheids are within the above color limits. The two empirical PL relations for the LMC and NGC 7331 are very alike in both V and I, with no obvious differences in slope. Hence, we can be confident that the relative distance modulus between the LMC and NGC 7331 will be accurate, once the DoPHOT/ALLFRAME photometric zeropoint calibration has been derived.
Any project as ambitious as the Extragalactic Distance Scale requires a lot of work to produce the proposals, the development of the software, and the careful analysis and calibration of the photometry and lightcurves, and fitting procedures. Hence this work would not be possible without the efforts of the rest of the H team: Wendy Freedman, Jeremy Mould, Robert Kennicutt Jr., Sandra Faber, Laura Ferrarese, Holland Ford, John Graham, James Gunn, Paul Harding, Bob Hill, John Huchra, Garth Illingworth, Dan Kelson, Barry Madore, Randy Phelps, Abhijit Saha, Shoko Sakai, Nancy Silbermann, Peter Stetson, and Anne Turner. We are also indebted to the very able support provided by the staff at STScI, in particular Bob Williams and Doug Van Orsow. Support for this work was provided by NASA through grant number 2227-87A from STScI which is operated by AURA, Inc. under NASA Contract NAS5-26555.
de Vaucouleurs, G. et al. 1991, Third Reference Catalogue of Bright Galaxies
Ferrarese, L. et al. 1996, ApJ, submitted
Hill, R. et al. 1996, ApJ, submitted
Huchtmeier, W.K. & Richter, O.-G. 1989, A General Catalog of HI Observations of Galaxies, New York: Springer
Hughes, S.M. et al. 1996, in preparation
Madore, B.F. 1985, in IAU Coll. 82, Cepheids, Theory and Observations, ed. B.F.Madore, Cambridge: Cambridge University Press, p. 166
Sandage, A. & Bedke, J. 1985, AJ, 90, 1992
Schechter, P.L., Mateo, M., & Saha, A. 1993, PASP, 105, 1342
Stetson, P.B. 1994, PASP, 106, 250