Randy L. Phelps

The Hubble Space Telescope Key Project on the Extragalactic Distance Scale. XIII. The Metallicity Dependence of the Cepheid Distance Scale

Uncertainty in the metal-abundance dependence of the Cepheid variable period-luminosity (PL) relation remains one of the outstanding sources of systematic error in the extragalactic distance scale and in the Hubble constant. To test for such a metallicity dependence, we have used the Wide Field Planetary Camera 2 (WFPC2) on the Hubble Space Telescope (HST) to observe Cepheids that span a range in oxygen abundance of 0.7 ± 0.15 dex in two fields in the nearby spiral galaxy M101. A differential analysis of the PL relations in V and I in the two fields yields a marginally significant change in the inferred distance modulus on metal abundance, with δ(m-M)0/δ[O/H] = -0.24 ± 0.16 mag dex-1. The trend is in the theoretically predicted sense that metal-rich Cepheids appear brighter and closer than metal-poor stars. External comparisons of Cepheid distances with those derived from three other distance indicators, in particular from the tip of the red giant branch method, further constrain the magnitude of any Z-dependence of the PL relation at V and I. The overall effects of any metallicity dependence on the distance scale derived with HST will be of the order of a few percent or less for most applications, though distances to individual galaxies at the extremes of the metal abundance range may be affected at the 10% level.

The Extragalactic Distance Scale Key Project. IV. The Discovery of Cepheids and a New Distance to M100 Using the Hubble Space Telescope

This paper presents initial observations, including the discovery of 30 Cepheids in the nearby galaxy M81, made using the Wide Field Camera (WFC).

The Extragalactic Distance Scale Key Project. XVI. Cepheid Variables in an Inner Field of M101

We report on the identification of 255 candidate variable stars in a field located some 17 from the center of the late-type spiral galaxy M101 = NGC 5457, based on observations made with the Wide Field and Planetary Camera 2 on board the Hubble Space Telescope. Photometric measurements in the F555W and F814W filters—analyzed independently with the DAOPHOT/ALLFRAME and DoPHOT software suites—have been transformed to the Johnson V and Kron-Cousins I standard magnitude systems. Periods and intensity-averaged mean magnitudes for 61 carefully selected candidate Cepheid variables with periods in the range 10-48 days indicate a reddening-corrected mean distance modulus (m - M)0 = 29.05 ± 0.14 (if the true modulus of the Large Magellanic Cloud is 18.50 ± 0.10, and if there is no dependence of the period-luminosity relation on metal abundance); results consistent with this are obtained whether or not the sample is expanded to include a larger fraction of the candidates. Applying a metallicity-dependent correction of +0.16 ± 0.10 mag would increase this estimate to (m - M)0 = 29.21 ± 0.17 mag.

The Hubble Space Telescope Key Project on the Extragalactic Distance Scale. XV. A Cepheid Distance to the Fornax Cluster and Its Implications

Using the Hubble Space Telescope, 37 long-period Cepheid variables have been discovered in the Fornax Cluster spiral galaxy NGC 1365. The resulting V and I period-luminosity relations yield a true distance modulus of ?0=31.35?0.07 mag, which corresponds to a distance of 18.6?0.6 Mpc. This measurement provides several routes for estimating the Hubble constant. (1) Assuming this distance for the Fornax Cluster as a whole yields a local Hubble constant of 70?18 (random) ?7 (systematic) km s?1 Mpc?1. (2) Nine Cepheid-based distances to groups of galaxies out to and including the Fornax and Virgo Clusters yield H0=73?16 (random) ?7 (systematic) km s?1 Mpc?1. (3) Recalibrating the I-band Tully-Fisher relation using NGC 1365 and six nearby spiral galaxies, and applying it to 15 galaxy clusters out to 100 Mpc, give H0=76?3 (random) ?8 (systematic) km s?1 Mpc?1. (4) Using a broad-based set of differential cluster distance moduli ranging from Fornax to Abell 2147 gives H0=72?3 (random) ?6 (systematic) km s?1 Mpc?1. Finally, (5) assuming the NGC 1365 distance for the two additional Type Ia supernovae in Fornax and adding them to the SN Ia calibration (correcting for light-curve shape) gives H0=67?6 (random) ?7 (systematic) km s?1 Mpc?1 out to a distance in excess of 500 Mpc. All five of these H0 determinations agree to within their statistical errors. The resulting estimate of the Hubble constant, combining all of these determinations, is H0=72?5 (random) ?7 (systematic) km s?1 Mpc?1. An extensive tabulation of identified systematic and statistical errors, and their propagation, is given.

The Hubble Space Telescope Extragalactic Distance Scale Key Project. VII. The Discovery of Cepheids in the Leo I Group Galaxy NGC 3351

We report of the discovery and properties of Cepheid variable stars in the barred spiral galaxy NGC 3351 which is a member of the Leo I group of galaxies. NGC 3351 is one of 18 galaxies being observed as part of the Hubble Space Telescope (HST) Key Project on the Extragalactic Distance Scale which aims to determine the Hubble constant to 10% accuracy. Our analysis is based on observations made with the Wide Field Planetary Camera 2 during 1994 and early 1995. The Leo I group contains several bright galaxies of diverse types and is very suitable for linking together a number of secondary calibrators which can be employed at much greater distances than the Cepheid variables. We identify 49 probable Cepheids within NGC 3351 in the period range 10-43 days which have been observed at 12 epochs with the F555W filter and 4 epochs using the F814W filter. The HST F555W and F814W data have been transformed to the Johnson V and Cousins I magnitude systems, respectively. Photometry has principally been carried out using the DAOPHOT/ALLFRAME package. Reference is made to parallel measurements being made with the DoPHOT package. Apparent period-luminosity functions for V and I have been constructed assuming values of μ0 = 18.50 ± 0.10 mag and E(B - V) = 0.10 mag for the distance modulus and reddening of the Large Magellanic Cloud. A true distance modulus of 30.01 ± 0.19 mag is derived corresponding to a distance of 10.05 ± 0.88 Mpc with a reddening E(V - I) = 0.15 mag. A comparison is made with distances estimated for other galaxies in the Leo I group using various distance indicators. There is good agreement with the surface brightness fluctuation and planetary nebula luminosity function methods as calibrated by the Cepheids in M31.

The Extragalactic Distance Scale Key Project. V. Photometry of the Brightest Stars in M100 and the Calibration of WFPC2

We present multiepoch VI observations of the brightest stars in the Virgo spiral galaxy M100 obtained with the WFPC2 aboard the Hubble Space Telescope. Particular attention has been given to the WFPC2 calibration process, and a new calibration is presented here. We find that a calibration based on short exposure (t ≤ 60 sec) observations of the galactic globular clusters ω Cen, Pal 4, and NGC 2419 yields results that are in excellent agreement with the calibration of Holtzmann et al. However, a calibration based on longer exposures (t ≥ 1300 s) yields magnitudes that are systematically brighter by 0.05 mag, on average, in both V and I. We have independently employed both the DAOPHOT/ALLFRAME and DoPHOT photometry routines in reducing the M100 data and have found that the two packages yield magnitudes that agree to better than 0.05 mag except for the WFPC2 chip 3 and WFPC2 chip 4 I data. Severe crowding appears to be the problem in the case of the WFPC2 chip 3 data, but we have been unable to determine the source of the discrepancy for the WFPC2 chip 4 I data. We consider the better than 0.05 mag agreement for the other chip/filter combinations to be quite good under the circumstances prevailing in this data set: an undersampled point-spread function (PSF), faint stars, a rapidly varying background, and moderate to severe crowding. A color-magnitude diagram for the brightest stars in M100 is also presented, which shows a plume of bright blue supergiants extending to at least V ~ 23 mag, or MV ~ -8 mag.

The Extragalactic Distance Scale Key Project. XVIII. The Discovery of Cepheids and a New Distance to NGC 4535 Using the Hubble Space Telescope

We report on the discovery of Cepheids in the Virgo spiral galaxy NGC 4535, based on observations made with the Wide Field and Planetary Camera 2 on board the Hubble Space Telescope (HST). NGC 4535 is one of 18 galaxies observed as a part of The HST Key Project on the Extragalactic Distance Scale, which aims to measure the Hubble constant to 10% accuracy. NGC 4535 was observed over 13 epochs using the F555W filter, and over 9 epochs using the F814W filter. The HST F555W and F814W data were transformed to the Johnson V and Kron-Cousins I magnitude systems, respectively. Photometry was performed using two independent programs, DoPHOT and DAOPHOT II/ALLFRAME. Period-luminosity relations in the V and I bands were constructed using 39 high-quality Cepheids present in our set of 50 variable candidates. We obtain a distance modulus of 31.02 ± 0.26 mag, corresponding to a distance of 16.0 ± 1.9 Mpc. Our distance estimate is based on values of μ0 = 18.50 ± 0.10 mag and E(V-I) = 0.13 mag for the distance modulus and reddening of the LMC, respectively.

The Hubble Space Telescope Extragalactic Distance Scale Key Project. X. The Cepheid Distance to NGC 7331

The distance to NGC 7331 has been derived from Cepheid variables observed with the Hubble Space Telescope (HST) WFPC2, as part of the Extragalactic Distance Scale Key Project. Multiepoch exposures in F555W ( ~V) and F814W ( ~I), with photometry derived independently from DoPHOT and DAOPHOT/ALLFRAME programs, were used to detect a total of 13 reliable Cepheids, with periods between 11 and 42 days. The relative distance moduli between NGC 7331 and the LMC, derived from the V and I magnitudes, imply an extinction to NGC 7331 of AV = 0.47 ± 0.15 mag and an extinction-corrected distance modulus to NGC 7331 of 30.89 ± 0.14 (random), equivalent to a distance of 15.1+1.0−0.9 Mpc. There are additional systematic uncertainties in the distance modulus of ±0.12 mag resulting from the calibration of the Cepheid period-luminosity relation and a systematic offset of +0.05 ± 0.04 mag if the metallicity correction inferred from the M101 results of Kennicutt et al. are applied.

The HST Key Project on the Extragalactic Distance Scale XI. The Cepheids in NGC 4414

We report the discovery of Cepheid variable stars in the galaxy NGC 4414, as part of the Hubble Space Telescope (HST ) Key Project on the Extragalactic Distance Scale. Observations were obtained with the HST Wide-Field and Planetary Camera 2 (WFPC2) for 13 epochs at V (F555W) and 4 at I (F814W). Photometry was performed using two independent programs, DoPHOT and DAOPHOT/ ALLFRAME. We -nd 11 Cepheids with high-quality light curves and well-determined periods of 19E70 days. Nine of these Cepheids are used in -tting the period-luminosity relation. Assuming an LMC dis- tance modulus of 18.50 mag and E(B(V ) \ 0.10 mag, we derive a reddening-corrected distance modulus for NGC 4414 of 31.41 ^ 0.17 (random) ^ 0.16 (systematic) mag, corresponding to a distance of 19.1 ^ 1.5 (random) ^ 1.4 (systematic) Mpc. We derive a mean color excess for NGC 4414 of E(V (I) \ 0.02 ^ 0.05 mag. NGC 4414 is a calibrator of the Tully-Fisher and Type Ia supernova sec- ondary distance indicators, and we brieNy discuss the implications of the new distance for these methods. Subject headings: Cepheids E galaxies: distances and redshifts E galaxies: individual (NGC 4414)

The Hubble Space Telescope Key Project on the Extragalactic Distance Scale. XIV. The Cepheids in NGC 1365

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.