Neural Networks and the Classification of Active Galactic Nucleus Spectra

The use of Artificial Neural Networks (ANNs) as a classifier of digital spectra is investigated. Using both simulated and real data, it is shown that neural networks can be trained to discriminate between the spectra of different classes of active galactic nucleus (AGN) with realistic sample sizes and signal-to-noise ratios. By working in the Fourier domain, neural nets can classify objects without knowledge of their redshifts.Comment: 11 pages, LaTeX, including two postscript figures, 41 kb. Accepted for publication in Publ. AS

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 1.7 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

The HST 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. 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 1365 from the ALLFRAME photometry is 31.31 +/- 0.20 (random) +/- 0.18 (systematic) mag, corresponding to a distance of 18.3 +/- 1.7 (random) +/- 1.6 (systematic) Mpc. The reddening is measured to be E(V-I) = 0.16 +/- 0.08 mag. These values are in excellent agreement with those obtained using the DoPHOT photometry, namely a distance modulus of 31.26 +/- 0.10 mag, and a reddening of 0.15 +/- 0.10 mag (internal errors only).Comment: 48 pages, 8 tables, 8 figures, to appear in Ap

The Extragalactic Distance Scale Key Project VIII. The Discovery of Cepheids and a New Distance to NGC3621 Using the Hubble Space Telescope

We report on the discovery of Cepheids in the field spiral galaxy NGC 3621, based on observations made with the Wide Field and Planetary Camera 2 on board the Hubble Space Telescope (HST). NGC 3621 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. Sixty-nine Cepheids with periods in the range 9--60 days were observed over 12 epochs using the F555W filter, and 4 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 packages, DAOPHOT II/ALLFRAME and DoPHOT. Period-luminosity relations in the V and I bands were constructed using 36 fairly isolated Cepheids present in our set of 69 variables. Extinction-corrected distance moduli relative to the LMC of 10:63 \Sigma 0:09 mag and 10:56 \Sigma 0:10 mag were obtained using the ALLFRAME and DoPHOT dat..

The Cepheid Period-Luminosity Relation in the Large Magellanic Cloud

A homogeneous set of ground-based BVRI observations of ~600 Cepheids is presented to check the Cepheid period-luminosity zero point for the Large Magellanic Cloud (LMC). The sample of Cepheids is completely self-contained and has been reduced consistently to eliminate photometric differences caused by combining multiple sources of photometry. The Cepheid distances to nearby galaxies obtained by the Hubble Space Telescope (HST) Key Project on the Extragalactic Distance Scale were computed assuming a "standard" PL relation based on a compilation of photoelectric observations of 34 Cepheids and a standard distance modulus to the LMC of 18.50. The final Key Project results substituted a statistically stronger PL relation obtained by the OGLE collaboration. In this paper we compare our data with the OGLE PL relation; the subsample of LMC Cepheids with similar periods to those discovered in the nearby galaxies is fainter at the relevant periods by 0.04 ± 0.02 mag than the OGLE sample. Substituting in turn this PL relation for that of the OGLE collaboration would raise the Hubble constant by 2% ± 1%, a correction which is not significant. The multicolor data set will also be useful in considering the effects of reddening and chemical composition and deriving an improved Cepheid period-luminosity relation for the LMC

The Hubble Space Telescope key project on the extragalactic distance scale XXI: the Cepheid distance to NGC 1425

The distance to NGC 1425 has been derived from Cepheid variables, as part of the Hubble Space Telescope Key Project on the Extragalactic Distance Scale. Thirteen F555W (V) and eight F814W (I) epochs of cosmic-ray-split Wide Field Planetary Camera 2 observations were obtained. Twenty-nine Cepheids were discovered, with periods ranging from 16 to 63 days. Adopting a Large Magellanic Cloud distance modulus and extinction of 18.50 ± 0.10 mag and E(V-I) = 0.13 mag, respectively, a true reddening-corrected distance modulus (based on an analysis employing the ALLFRAME software package) of 31.73 ± 0.16 (random) ± 0.17 (systematic) mag was determined for NGC 1425. The corresponding distance of 22.2 ± 1.0 (random) ± 1.0 (systematic) Mpc is in satisfactory agreement with that found with an independent analysis based on the DoPHOT photometry package