The Cepheids of Centaurus A NGC 5128 and Implications for H0 - Astrophysics > Cosmology and Nongalactic AstrophysicsReportar como inadecuado

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Abstract: An analysis based on new OGLE observations reaffirms Ferrarese et al.discovery of 5 Type II Cepheids in NGC 5128. The distance to that comparativelyunreddened population is d=3.8+-0.4se Mpc. The classical Cepheids in NGC 5128are the most obscured in the extragalactic sample n=30 surveyed, whereasgroups of Cepheids tied to several SNe host galaxies feature negativereddenings. Adopting an anomalous extinction law for Cepheids in NGC 5128 owingto observations of SN 1986G Rv~2.4 is not favoured, granted SNe Ia may followsmall Rv. The distances to classical Cepheids in NGC 5128 exhibit a dependenceon colour and CCD chip, which may arise in part from photometric contamination.Applying a colour cut to mitigate contamination yields d~3.5 Mpc V-I<1.3,while the entire sample-s mean is d~3.1 Mpc. The distance was established viathe latest VI Galactic Wesenheit functions that include the 10 HST calibrators,and which imply a shorter distance scale than Sandage et al.2004 by ~>10% atP~25d. HST monitored classical Cepheids in NGC 5128, and the SNe hosts NGC 3021and NGC 1309, follow a shallower VI Wesenheit slope than ground-basedcalibrations of the Milky Way, LMC, NGC 6822, SMC, and IC 1613. The discrepancyis unrelated to metallicity since the latter group share a common slope over asizeable abundance baseline a=-3.34+-0.08,dFe-H~1. A negligible distanceoffset between OGLE Cepheids and RR Lyr variables in the LMC, SMC, and IC 1613bolsters assertions that VI-based Wesenheit functions are relativelyinsensitive to chemical abundance. In sum, a metallicity effect VI is not thechief source of uncertainty associated with the Cepheid distance to NGC 5128 orthe establishment of Hubble-s constant, but rather it may be the admittedlychallenging task of obtaining precise, commonly standardized, multiepoch,multiband, comparatively uncontaminated extragalactic Cepheid photometry.

Autor: Daniel J. Majaess


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