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.
Cosmic Mileposts
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.
BACKGROUND INFORMATION: COSMIC YARDSTICKS
Astronomers gauge the dimensions of space by using "distance indicators" – celestial objects with unique properties that allow for their distances to be deduced. Reliable distance measurements are a crucial factor in determining a precise value for the universe's expansion rate (called the Hubble Constant) which is needed to estimate the size and age of the universe. (To calculate the Hubble Constant, astronomers also need to know how fast a galaxy is moving away from us, measured by spectral redshift.)
Measuring the distance to a faraway galaxy involves a complicated set of closely-linked steps. First, distance indicators within our galaxy are used as a stepping stone to calibrate other distance indicators in nearby galaxies, which in turn creates yet another stepping stone to calibrate distances to even more faraway galaxies.
The first rung in the "distance scale ladder" can be found in our Milky Way neighborhood, in nearby open star clusters such as Hyades and the Ursa Major cluster. An open cluster is a collection of young stars with a common motion in space. Because the Hyades and the Ursa Major cluster are close to us, their distances can be derived using radial velocity (motion toward or away from us) and proper motion measurements of member stars. This allows astronomers to obtain the intrinsic brightness, or luminosity, of different types of stars in these open clusters.
Astronomers then measure the brightness of stars with similar properties in more distant open clusters. By assuming that these stars would have the same intrinsic brightness as their nearby counterparts, a distance to the remote open clusters is calculated by comparing the apparent and intrinsic brightness of their member stars.
To obtain distances to nearby galaxies, astronomers use "primary distance indicators." These are objects that can be observed within our galaxy or have characteristics that can be theoretically modeled. Examples include Cepheid variable stars, novae, supernovae, and RR Lyrae stars.
Two well-defined primary distance indicators, or "standard candles," are the Cepheids and fainter RR Lyrae stars. They have a regular variation in brightness, and the period of this pulsation is closely linked to the star's intrinsic brightness. So, if the pulsation period of a star is known, its true brightness can be deduced. The distance to the star can then be calculated by comparing its true brightness with its apparent brightness.
Cepheid variable stars are often used as distance calibrators for nearby galaxies. They are very luminous yellow giant or supergiant stars, regularly varying in brightness with periods ranging from 1 to 70 days. This type of star is in a late evolutionary stage, pulsating due to an imbalance between its inward gravitational pull and outward pressure.
Cepheids are found in remote open clusters whose distances are known from comparison with nearby open clusters. It is, therefore, possible to calibrate these Cepheids with an independently obtained ruler ot yardstick.
In the past, the best ground-based observations have detected Cepheids in nearby galaxies within 12 million light-years. However, all galaxies in this region have motions due to gravitational attraction of neighboring galaxies. In order to study the overall expansion of the universe, it is necessary to reach out to Cepheids in galaxies at least 30 million light-years away.
Until the recent Hubble Space Telescope observations of Cepheids in M100, there were no well-calibrated standard candles observable over this distance. Therefore, astronomers have been using other kinds of objects, called "secondary distance indicators," to probe even deeper into the universe.
Secondary distance indicators, such as planetary nebulae, supernovae, and the brightest stars are used in galaxies that are so remote that only prominent objects can be discerned. (These secondary indicators are calibrated in nearer galaxies, where distances are known from resident primary distance indicators, before being applied to more remote galaxies.) The galaxies themselves can also be used as secondary distance indicators. One widely-used strategy, the Tully-Fisher method, uses a correlation between the internal motions within galaxies (from radio observations of cold interstellar gas) with their luminosities. Another method, the Faber-Jackson relation, looks at the random motions of stars in a galaxy obtained from spectroscopic measurements. These relationships are based on the fact that a more massive galaxy would be more luminous, and would rotate faster than a less massive galaxy.
BACKGROUND INFORMATION: THE HUBBLE CONSTANT
The Hubble Constant (Ho) is one of the most important numbers in cosmology because it is needed to estimate the size and age of the universe. This long-sought number indicates the rate at which the universe is expanding, from the primordial "Big Bang."
The Hubble Constant can be used to determine the intrinsic brightness and masses of stars in nearby galaxies, examine those same properties in more distant galaxies and galaxy clusters, deduce the amount of dark matter present in the universe, obtain the scale size of faraway galaxy clusters, and serve as a test for theoretical cosmological models.
In 1929, American astronomer Edwin Hubble announced his discovery that galaxies, from all directions, appeared to be moving away from us. This phenomenon was observed as a displacement of known spectral lines towards the red-end of a galaxy's spectrum (when compared to the same spectral lines from a source on Earth). This redshift appeared to have a larger displacement for faint, presumably further, galaxies. Hence, the farther a galaxy, the faster it is receding from Earth.
The Hubble Constant can be stated as a simple mathematical expression, Ho = v/d, where v is the galaxy's radial outward velocity (in other words, motion along our line-of-sight), d is the galaxy's distance from earth, and Ho is the current value of the Hubble Constant.
However, obtaining a true value for Ho is very complicated. Astronomers need two measurements. First, spectroscopic observations reveal the galaxy's redshift, indicating its radial velocity. The second measurement, the most difficult value to determine, is the galaxy's precise distance from earth. Reliable "distance indicators," such as variable stars and supernovae, must be found in galaxies. The value of Ho itself must be cautiously derived from a sample of galaxies that are far enough away that motions due to local gravitational influences are negligibly small.
The units of the Hubble Constant are "kilometers per second per megaparsec." In other words, for each megaparsec of distance, the velocity of a distant object appears to increase by some value. (A megaparsec is 3.26 million light-years.) For example, if the Hubble Constant was determined to be 50 km/s/Mpc, a galaxy at 10 Mpc, would have a redshift corresponding to a radial velocity of 500 km/s.
The value of the Hubble Constant initially obtained by Edwin Hubble was around 500 km/s/Mpc, and has since been radically revised because initial assumptions about stars yielded underestimated distances.
For the past three decades, there have been two major lines of investigation into the Hubble Constant. One team, associated with Allan Sandage of the Carnegie Institutions, has derived a value for Ho around 50 km/s/Mpc. The other team, associated with Gerard DeVaucouleurs of the University of Texas, has obtained values that indicate Ho to be around 100 km/s/Mpc. A long-term, key program for HST is to refine the value of the Hubble Constant.
