An international team of astronomers using NASA's Hubble Space Telescope announced today the most accurate measurement yet of the distance of the remote galaxy M100, located in the Virgo cluster of galaxies.
This measurement will help provide a precise calculation of the expansion rate of the universe, called the Hubble Constant, which is crucial to determining the age and size of the universe.
"Although this is only the first step in a major systematic program to measure accurately the scale, size, and age of the universe," noted Dr. Wendy L. Freedman, of the Observatories of the Carnegie Institution of Washington, "a firm distance to the Virgo cluster is a critical milestone for the extragalactic distance scale, and it has major implications for the Hubble Constant."
HST's detection of Cepheid variable stars in the spiral galaxy M100, a member of the Virgo cluster, establishes the distance to the cluster as 56 million light-years (with an uncertainty of +/- 6 million light-years). M100 is now the most distant galaxy in which Cepheid variables have been measured accurately.
The precise measurement of this distance allows astronomers to calculate that the universe is expanding at the rate of 80 km/sec per megaparsec (+/- 17 km/sec). For example, a galaxy one million light-years away will appear to be moving away from us at approximately 60,000 miles per hour. If it is twice that distance, it will be seen to be moving at twice the speed, and so on. This rate of expansion is the Hubble Constant.
These results are being published in the October 27 issue of the journal Nature. The team of astronomers is jointly led by Freedman, Dr. Robert Kennicutt (Steward Observatory, University of Arizona), and Dr. Jeremy Mould (Mount Stromlo and Siding Spring Observatories, Australian National University).
Dr. Mould noted, "Those who pioneered the development of the Hubble Space Telescope in the 1960s and 1970s recognized its unique potential for finding the value of the Hubble Constant. Their foresight has been rewarded by the marvelous data that we have obtained for M100."
Using Hubble's Wide-Field and Planetary Camera (WFPC2), the team of astronomers repeatedly imaged a field where much star formation recently had taken place, and was, therefore, expected to be rich in Cepheids a class of pulsating stars used for determining distances. Twelve one-hour exposures, strategically placed in a two-month observing window, resulted in the discovery of 20 Cepheids. About 40,000 stars were measured in the search for these rare, but bright, variables. Once the periods and intrinsic brightness of these stars were established from the careful measurement of their pulsation rates, the researchers calculated a distance of 56 million light-years to the galaxy. (The team allowed for the dimming effects of distance as well as that due to dust and gas between Earth and M100.)
Many complementary projects are currently being carried out from the ground with the goal of providing values for the Hubble Constant. However, they are subject to many uncertainties which HST was designed and built to circumvent. For example, a team of astronomers using the Canada-France-Hawaii telescope at Mauna Kea recently have arrived at a distance to another galaxy in Virgo that is similar to that found for M100 using HST but their result is tentative because it is based on only three Cepheids in crowded star fields.
"Only Space Telescope can make these types of observations routinely," Freedman explained. "Typically, Cepheids are too faint and the resolution too poor, as seen from ground-based telescopes, to detect Cepheids clearly in a crowded region of a distant galaxy."
Although M100 is now the most distant galaxy in which Cepheid variables have been discovered, the Hubble team emphasized that the HST project must link into even more distant galaxies before a definitive number can be agreed on for the age and size of the universe. This is because the galaxies around the Virgo Cluster are perturbed by the large mass concentration of galaxies near the cluster. This influences their rate of expansion.
Refining the Hubble Constant
These first HST results are a critical step in converging on the true value of the Hubble Constant, first developed by the American astronomer Edwin Hubble in 1929. Hubble found that the farther a galaxy is, the faster it is receding away from us. This "uniform expansion" effect is strong evidence the universe began in an event called the "Big Bang" and that it has been expanding ever since.
To calculate accurately the Hubble Constant, astronomers must have two key numbers: the recession velocities of galaxies and their distances as estimated by one or more cosmic "mileposts," such as Cepheids. The age of the universe can be estimated from the value of the Hubble Constant, but it is only as reliable as the accuracy of the distance measurements.
The Hubble constant is only one of several key numbers needed to estimate the universe's age. For example, the age also depends on the average density of matter in the universe, though to a lesser extent.
A simple interpretation of the large value of the Hubble Constant, as calculated from HST observations, implies an age of about 12 billion years for a low-density universe, and 8 billion years for a high-density universe. However, either value highlights a long-standing dilemma. These age estimates for the universe are shorter than the estimated ages of some of the oldest stars found in the Milky Way and in globular star clusters orbiting our Milky Way. Furthermore, small age values pose problems for current theories about the formation and development of the observed large-scale structure of the universe.
Cepheid variable stars rhythmically change in brightness over intervals of days (the prototype is the fourth brightest star in the circumpolar constellation Cepheus). For more than half a century, from the early work of the renowned astronomers Edwin Hubble, Henrietta Leavitt, Allan Sandage, and Walter Baade, it has been known that there is a direct link between a Cepheid's pulsation rate and its intrinsic brightness. Once a star's true brightness is known, its distance is a relatively straightforward calculation because the apparent intensity of light drops off at a geometrically predictable rate with distance. Although Cepheids are rare, once found, they provide a very reliable "standard candle" for estimating intergalactic distances, according to astronomers.
Besides being an ideal hunting ground for the Cepheids, M100 also contains other distance indicators that can in turn be calibrated with the Cepheid result. This majestic, face-on, spiral galaxy has been host to several supernovae, which are also excellent distance indicators. Individual supernovae (called Type II, massive exploding stars) can be seen to great distances, and, so, can be used to extend the cosmic distance scale well beyond Virgo.
As a crosscheck on the HST results, the distance to M100 has been estimated using the Tully-Fisher relation (a means of estimating distances to spiral galaxies using the maximum rate of rotation to predict the intrinsic brightness) and this independent measurement also agrees with both the Cepheid and supernova "yardsticks."
HST Key Projects are scientific programs that have been widely recognized as being of the highest priority for the Hubble Space Telescope and have been designated to receive a substantial amount of observing time on the telescope. The Extragalactic Distance Scale Key Project involves discovering Cepheids in a variety of important calibrating galaxies to determine their individual distances. These distances then will be used to establish an accurate value of the Hubble Constant.
Ray Villard, STScI
Dr. Wendy L. Freedman