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Twenty years ago, there was disagreement at a level of a factor of two as regards the value of the expansion rate of the Universe. Ten years ago, a value that was good to 10% was established using the Hubble Space Telescope (HST), completing one of the primary missions that NASA designed and built the HST to undertake. Today, after confronting most of the systematic uncertainties listed at the end of the Key Project, we are looking at a value of the Hubble constant that is plausibly known to within 3%. In the near future, an independently determined value of H0 good to 1% is desirable to constrain the extraction of other cosmological parameters from the power spectrum of the cosmic microwave background in defining a concordance model of cosmology. We review recent progress and assess the future prospects for those tighter constraints on the Hubble constant, which were unimaginable just a decade ago.
We present the deepest colour-magnitude diagram (CMD) of M32 to date, obtained from deep (F435W, F555W) photometry of HST ACS/HRC images. Due to the high resolution of our images, the small photometric errors, and the completeness level of our data we obtain the most detailed resolved photometric study of M32 to date. The CMD of M32 displays a wide colour distribution of red giant branch stars, mainly due to a metallicity spread, a strong red clump and bright asymptotic giant branch stars. The detection of a “blue plume” in M32 indicates the presence of a very young stellar population. There is not a noticeable presence of blue horizontal branch stars, suggesting that an old population with [Fe/H] < −1.5 does not significantly contribute to the light or mass of M32 in our observed fields.
We observed two fields near M32 with the ACS/HRC (Program GO-10572, PI: T. Lauer) on board the Hubble Space Telescope, located at distances of about 1.8' and 5.4' (hereafter F1 and F2, respectively) from the center of M32. To obtain a very detailed and deep color-magnitude diagram (CMD) and to look for short period variability, we obtained time-series imaging of each field in 32-orbit-long exposures using the F435W (B) and F555W (V) filters, spanning a temporal range of 2 days per filter. We focus on our detection of variability on RR Lyrae variable stars, which represents the only way to obtain information about the presence of a very old population (larger than 10 Gyr) in M32 from optical data. Here we present results obtained from the detection of 31 RR Lyrae in these fields: 17 in F1 and 14 in F2. We claim we detected 7+4−3 RR Lyrae variables belonging to M32 in F1 thus indicating the presence of a metal-poor ancient population in M32.
Cosmology is the study of the origin, evolution, composition, and structure of the Universe. As a scientific discipline cosmology began only in the twentieth century. Among the fundamental theoretical and observational developments that established the Big-Bang model were the general theory of relativity proposed by Albert Einstein in 1915, the development of the theory of an expanding relativistic cosmology by Alexander Friedmann in 1922 and Georges LemaÎtre in 1927, the observation of the expansion of the Universe by Edwin Hubble in 1929, the development of the theory of Big-Bang nucleosynthesis by Ralph Alpher, George Gamow, and Robert Herman in the early 1950s, and the discovery of the cosmic background radiation by Arno Penzias and Robert Wilson in 1964.
Traditionally, cosmology has been a data-starved science, but cosmology today is experiencing a fertile interplay between observation and theory. Precision measurements of the expansion rate of the Universe, the large-scale homogeneity and isotropy of the distribution of galaxies, the existence and high degree of isotropy of the 3-K cosmic microwave background radiation, and the abundances of the light elements support the basic picture of an expanding hot-Big-Bang Universe.
As this conference has attested to, cosmology is a rapidly maturing field, currently experiencing a very healthy and vigorous confrontation between theory and experiment. This rapid progress in many different areas of cosmology has not removed the longstanding interest in measuring many of the fundamental cosmological parameters - rather, the increasingly detailed predictions of current theory highlight the critical importance of independently, accurately measuring the cosmological parameters which define the basic model for the dynamical evolution of the Universe. I present here the final results of the Hubble Space Telescope (HST) Key Project to measure the Hubble constant, summarizing our method, the results and the uncertainties. The Key Project results are based on a Cepheid calibration of several secondary distance methods applied over the range of about 60 to 400 Mpc. Based on the Key Project Cepheid calibration and its application to five secondary methods (type Ia supernovae, the Tully-Fisher relation, surface brightness fluctuations, type II supernovae, and the fundamental plane for elliptical galaxies), a combined value of H0 = 72 ± 8 km/sec/Mpc is obtained. Comparing to current estimates of the ages of Galactic globular clusters, an age conflict is avoided for this high a value of H0 if we live in a Λ-dominated (or other form of dark energy) universe.
We have obtained HST-NICMOS observations of five of M31's most metal-rich globular clusters: G1, G170, G174, G177 & G280. For the two clusters farthest from the nucleus, G1 and G280, we statistically subtract the field population and estimate metallicities using K-(J - K) color-magnitude diagrams (CMDs). Based on the slopes of their infrared giant branches we estimate [Fe/H] = −1.22 ± 0.43 dex for G1 and −0.15 ± 0.37 dex for G280. We combine our infrared observations of G1 with two epochs of optical HST-WFPC2 F-band data and identify at least one LPV based on color and variability. The location of G1's giant branch in the K, (V - K) CMD is very similar to that of M107, indicating a somewhat higher metallicity than our purely infrared CMD; [Fe/H]= −0.9 ± 0.2 dex.
A Joint Discussion on the extragalactic distance scale and the Hubble constant took place fifteen years ago, at the 1982 XVIIIth General Assembly of the IAU, held in Patras, Greece. At that time, the newest applications of infrared photometers to Tully-Fisher measurements (Aaronson 1983) and Cepheid distances (Madore 1983) were reported. CCDs were just coming into use and had not yet been applied to extragalactic distance determinations; all of the extragalactic Cepheid distances were based on photographic Argelander (eye-estimated) photometry (Tammann and Sandage 1983 and references therein). No Cepheid distances to type Ia supernova-host galaxies were available.
The Hubble Space Telescope key project to determine the extragalactic distance scale aims to measure H0 to 10%. To achieve this goal will require Cepheid distance measurements for some 20 galaxies within a redshift of approximately 103 km/s. These galaxies in turn will calibrate five secondary distance indicators which will extend the volume over which the expansion rate has been measured to some 106 Mpc3.
NGC 6822 was the first galaxy outside the Galaxy where the Cepheid period-luminosity relation was applied as a distance indicator (Hubble 1925, ApJ, 62, 409). Later, thirteen Cepheids in this galaxy were studied by Kayser (1967, AJ, 72, 134) using BV photographic photometry. We have obtained BVRI CCD photometry of stars in two 2.2' X 3.5' fields of NGC 6822. Field 3) using Figure 1 shows an I – (V – I) diagram of ∼ 6,400 measured stars in NGC 6822 including seven known Cepheids. The reddening is estimated to E(B – V) = 0.28 ± 0.03 from the (B – V)-(V – I) diagram of the stars with V < 20 mag. The distance to NGC 6822 has been estimated using the random-phase I band photometry of seven Cepheids (V7(I=16.31), V5(I=18.97), V6(I=18.44), V3(I=17.88), V1(I=18.05), V28(I=17.61), and V2(I=17.29)) combined with their periods given by Kayser.
WLM (DD0221) is a highly resolved dwarf irregular galaxy in the Local Group. Fifteen Cepheids in this galaxy have been studied by Sandage & Carlson (1985, AJ, 90, 1464) using photographic photometry. We have obtained BVRI CCD photometry of stars in the central area (2.2' x 3.5') of WLM. Figure 1 shows an I–(V – I) diagram of ∼ 2,600 measured stars including five known Cepheids. The distance to WLM has been estimated using the random-phase I band photometr y of five Cepheids (V7(I=20.52), V29(I=20.77), V48(I=20.43), V66(I=21.19), V67(I=21.14)) combined with their periods given by Sandage & Carlson. We have obtained Δ(m - M)II(WLM – LMC) = 6.25 ± 0.11 and (m - M)0 = 24.92 ± 0.21, adopting (m - M)o(LMC) = 18.5 and AI(WLM) = 0.04.
In the course of the last decade significant advances have been made in the observations of Cepheid variables and in their successful application to the extragalactic distance scale. Much of this progress has come about as a result of new CCD and near-infrared photometry. These recent improvements are discussed, and a comparison is given of Population I Cepheids and Population II distances. The correspondence is good, with the zero points agreeing at a level of better than 15% in distance. At this same level of significance, a systematic difference between these distances scales may exist, in the sense that the RR Lyrae distances appear to be smaller than the Cepheid distances (if it is assumed, as has generally been done for extragalactic studies of RR Lyraes, that Mv(RR) = 0.77 mag, independent of [Fe/H]). However, several recently-published calibrations of Mv(RR) significantly reduce this discrepancy. Finally, new Cepheid data for the nearby galaxy M81 are presented based on recent Hubble Space Telescope observations.
UBV CCD photometry has been obtained for 14 OB associations in the Large and Small Magellanic Clouds. The data have been used to construct color-magnitude diagrams for the purpose of investigating the massive-star content of these extragalactic associations.
An active debate continues over whether elliptical galaxies are primarily old stellar systems or whether they have had major star formation events in the recent past. Not only is this question of interest with regard to understanding the stellar populations and star formation history of nearby systems, but the resolution of this issue influences the interpretation of the spectra of high-redshift galaxies and has profound consequences for our understanding of galaxy, and therefore ultimately, of cosmological evolution.
Our lack of understanding of the stellar make-up in elliptical galaxies has persisted for some time because there are no giant elliptical galaxies near enough to allow the study of their stellar populations directly. Most information on the stellar populations of elliptical galaxies rely on the interpretation of integrated light. However, direct information on the bright stellar content of low-luminosity elliptical galaxies can be obtained from a study of the Local Group dwarf ellipticals. The nearby Andromeda galaxy, M31 has four low-luminosity elliptical companions: M32, NGC 205, NGC 185 and NGC 147, the subjects of this review.
This review will begin with a broad summary of population characteristics of dwarf elliptical galaxies (dE's), it will briefly summarize what is known about the stellar populations of the four Andromeda companions, and then discuss the specific case of M32 in detail. M32, the highest surface brightness Andromeda companion, has characteristics very simliar to the giant ellipticals, and has therefore been the focus of much of the controversy surrounding the issue of the ages of elliptical galaxies. Studies of its integrated light, in combination with new studies of its brightest resolved giants, particularly in the near-infrared, may help to resolve many of the outstanding questions regarding the stellar populations in elliptical galaxies.
We are undertaking empirical tests for the effects of metallicity on the zero-point of the Period-Luminosity relations for Cepheids in M31 and in the Small Magellanic Cloud (SMC), as compared to the Large Magellanic Cloud (LMC). For M31 we have used BVRI data on comparable sets of Cepheids in three radially separated, and chemically different, fields in order to solve simultaneously for the reddenings and the true distance moduli to each of these fields. Recent theory predicts that the reddening-corrected moduli should differ by 0.8 mag given the observed metallicity range in our M31 Cepheid sample; we observe a maximum range of only 0.17 mag. For the SMC BVRIJHK data, treated in a similar fashion, the predicted theoretical effects on the short wavelength (B and V) data may be visible, but with low statistical certainty and at a reduced amplitude with respect to the predictions.
As part of a long-term program to study the distance and stellar content of M33, CCD frames of a number of fields centered on known Cepheids in this galaxy (Hubble 1926, Sandage and Carlson 1983) have been obtained, in collaboration with Madore, McAlary, and Davis. The observations were made at the prime focus of the Kitt Peak 4m telescope, and reduced using the Kitt Peak RICHFLD profile fitting programs. These data are still preliminary, but quite sufficient to illustrate the power in the application of our method.
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