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Register a new userThe Cepheid Period-Luminosity relation and the maser distance to NGC 4258
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F. Caputo
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In a recent paper describing HST observations of Cepheids in the spiral galaxy NGC 4258, Newman et al. (2001) report that the revised calibrations and methods for the Key Project on the Extragalactic Distance Scale yield that the true distance modulus of this galaxy is 29.40+-0.09 mag, corresponding to a metric distance of 7.6+-0.3 Mpc. This Cepheid distance, which holds for 18.50 mag as the true distance modulus of the LMC, is not significantly larger than 7.2+-0.5 Mpc, the value determined by Herrnstein et al. (1999) from purely geometric considerations on the orbital motions of water maser sources. However, if the metallicity difference D[O/H]~0.35 between NGC 4258 and LMC is taken into account, then the Key Project methods lead to a metallicity-corrected value of 29.47+-0.09 mag, with 18.50 mag for the LMC, namely to a Cepheid distance of 7.8+-0.3 Mpc, which is 1.2 sigma from the maser determination. In this paper we show that the metallicity correction on Cepheid distance determinations, as suggested by pulsation models, might provide the natural way of reaching a close agreement between Cepheid and maser distance to NGC 4258 for a wide variety of LMC distance determinations.
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Distances measured using Cepheid variable stars have been essential for establishing the cosmological distance scale and the value of the Hubble constant. These stars have remained the primary extragalactic distance indicator since 1929 because of the small observed scatter in the relationship between their pulsation period and luminosity, their large numbers, which allow many independent measures of the distance to a galaxy, and the simplicity of the basic physics underlying their variability. Potential systematic uncertainties in the use of the LMC-calibrated Cepheid period-luminosity relation to determine distances using HST are estimated to be 8-10%. Here we describe the results of a search for Cepheids in the nearby galaxy NGC 4258, which has an independently determined geometric distance of 7.2 +/- 0.5 Mpc (Herrnstein et al. 1999). We obtain a Cepheid distance of 8.1 +/- 0.4 (excluding possible systematic errors affecting all HST Cepheid distances) Mpc; there is a 1.3 sigma difference between the two measurements. If the maser-based distance is adopted and other HST Cepheid distances are revised according to our results, the derived value of the Hubble constant would be increased by 12 +/- 9%, and the corresponding age of the Universe would decrease by the same factor.
We identify and phase a sample of 81 Cepheids in the maser-host galaxy NGC 4258 using the Large Binocular Telescope (LBT), and obtain calibrated mean magnitudes in up to 4 filters for a subset of 43 Cepheids using archival HST data. We employ 3 models to study the systematic effects of extinction, the assumed extinction law, and metallicity on the Cepheid distance to NGC 4258. We find a correction to the Cepheid colors consistent with a grayer extinction law in NGC 4258 compared to the Milky Way ($R_V =4.9$), although we believe this is indicative of other systematic effects. If we combine our Cepheid sample with previously known Cepheids, we find a significant metallicity adjustment to the distance modulus of $gamma_1 = -0.61 pm 0.21$ mag/dex, for the Zaritsky et al. (1994) metallicity scale, as well as a weak trend of Cepheid colors with metallicity. Conclusions about the absolute effect of metallicity on Cepheid mean magnitudes appear to be limited by the available data on the metallicity gradient in NGC 4258, but our Cepheid data require at least some metallicity adjustment to make the Cepheid distance consistent with independent distances to the LMC and NGC 4258. From our ensemble of models and the geometric maser distance of NGC 4258 ($mu_{N4258} = 29.40 pm 0.06$ mag), we estimate $mu_{LMC} = 18.57 pm 0.14$ mag ($51.82 pm 3.23$ kpc).
In a previous paper (Maoz et al. 1999), we reported a Hubble Space Telescope (HST) Cepheid distance to the galaxy NGC 4258 obtained using the calibrations and methods then standard for the Key Project on the Extragalactic Distance Scale. Here, we reevaluate the Cepheid distance using the revised Key Project procedures described in Freedman et al. (2001). These revisions alter the zero points and slopes of the Cepheid Period-Luminosity (P-L) relations derived at the Large Magellanic Cloud (LMC), the calibration of the HST WFPC2 camera, and the treatment of metallicity differences. We also provide herein full information on the Cepheids described in Maoz et al. 1999. Using the refined Key Project techniques and calibrations, we determine the distance modulus of NGC 4258 to be 29.47 +/- 0.09 mag (unique to this determination) +/- 0.15 mag (systematic uncertainties in Key Project distances), corresponding to a metric distance of 7.8 +/- 0.3 +/- 0.5 Mpc and 1.2 sigma from the maser distance of 7.2 +/- 0.5 Mpc. We also test the alternative Cepheid P-L relations of Feast (1999), which yield more discrepant results. Additionally, we place weak limits upon the distance to the LMC and upon the effect of metallicity in Cepheid distance determinations.
In this paper, we derive the period-luminosity (P-L) relation for Large Magellanic Cloud (LMC) Cepheids based on mid-infrared AKARI observations. AKARIs IRC sources were matched to the OGLE-III LMC Cepheid catalog. Together with the available I band light curves from the OGLE-III catalog, potential false matches were removed from the sample. This procedure excluded most of the sources in the S7 and S11 bands: hence only the P-L relation in the N3 band was derived in this paper. Random-phase corrections were included in deriving the P-L relation for the single epoch AKARI data, even though the derived P-L relation is consistent with the P-L relation without random-phase correction, though there is a sim 7 per-cent improvement in the dispersion of the P-L relation. The final adopted N3 band P-L relation is N3 = -3.246 log(P) + 15.844, with a dispersion of 0.149.
We have assessed the influence of the stellar iron content on the Cepheid Period-Luminosity (PL) relation by relating the V band residuals from the Freedman et al (2001) PL relation to [Fe/H] for 68 Galactic and Magellanic Cloud Cepheids. The iron abundances were measured from FEROS and UVES high-resolution and high signal-to-noise optical spectra. Our data indicate that the stars become fainter as metallicity increases, until a plateau or turnover point is reached at about solar metallicity. This behavior appears at odds both with the PL relation being independent from iron abundance and with Cepheids becoming monotonically brighter as metallicity increases (e.g. Kennicutt et al 1998, Sakai et al 2004).
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