Science Nugget—Imaging Stars in Nearby Galaxies with IRIS + NFIRAOS
  T. J. Davidge
  Herzberg Institute of Astrophysics/National Research Council of Canada

August 31, 1885, was a day of great significance in the field of extragalactic astronomy. On that day, Ernst Hartwig of Dorpat Observatory reported the discovery of S Andromedae. While this was only the second variable star to be discovered in the constellation of Andromeda (and over 400 are known today), it was located within the great spiral nebula in Andromeda, also known as M31.

A number of other contemporary astronomers recorded observations, and a light curve was obtained. The extragalactic nature of S And (and M31) was not appreciated for another four decades. However, it is now known that S And was a supernova, and the work done in 1885 was the first recorded detection and characterization of an individual star in a large spiral galaxy other than the Milky Way.

In the 120 years since the detection of S And, there have been a number of studies of resolved stars in M31. These studies have enabled an accurate distance to M31 to be determined and—perhaps more importantly—have revealed that the star-forming history of M31 has been very different from that of the Milky Way. The rate at which stars have formed during the past billion years is much lower in M31 than in the Milky Way. There is also a growing body of evidence that M31 has interacted significantly with its companions, and that these interactions have almost certainly shaped its evolution. For comparison, the Milky Way has had only comparatively modest interactions with its companions.

A number of basic questions come to mind when comparing the past histories of M31 and the Milky Way. Is either one of these galaxies a typical example of spiral galaxy evolution? How much of a role has environment played in their evolution? When did the evolutionary paths of the Milky Way and M31 diverge? When did these galaxies develop their present appearance?

The Infrared Imaging Spectrometer (IRIS) is a first-generation TMT instrument that will be used with the NFIRAOS adaptive optics system. IRIS has two basic observing modes: imaging and integral field spectroscopy. In this article, we focus on the imaging capability, and its potential for studying the resolved stellar contents of nearby galaxies.

When used with NFIRAOS, IRIS will record images with angular resolutions set by the diffraction limit of the TMT, and this will have a major impact on the study of stars in nearby galaxies. The glow from the night sky, coupled with thermal emission from the telescope, impedes our ability to detect and characterize faint objects in the near-infrared.

An obvious benefit of a large telescope is that the increased collecting area allows more photons to be detected. However, even greater benefits can be reaped if the observations are made at the telescope diffraction limit. In this case, a stellar image has a projected area on the sky that scales as the inverse of the primary mirror diameter—larger telescopes will thus produce more compact images, and the contrast with respect to the sky is improved.

The increase in mirror collecting area and the compact nature of diffraction-limited images together yield an increase in sensitivity that scales with the fourth power of the telescope diameter. This so-called D4 advantage is of particular importance in the study of resolved populations, as many stars that provide key age and chemical abundance information are inherently faint.

This is demonstrated in the figure to the right, where the K-band brightnesses of various diagnostic populations are shown as a function of distance modulus (a measure of distance used by astronomers, which is 5_log(r)—5, where r is the distance in parsecs). The dotted lines show the brightness of a source that would be detected with a 10:1 signal-to-noise ratio after a three hour exposure time through the K filter with 4-meter, 8-meter, and 30-meter telescopes; in each case an adaptive optics system that delivers a Strehl ratio of 0.6 is assumed. It is evident from this projection that the TMT will allow the study of stars such as those evolving on the red giant branch, which are key probes of chemical content, to be measured at distances beyond the Local Group.

Extending the survey of resolved stars beyond the Local Group is of critical importance for increasing the sample of galaxies, and determining how the evolutionary histories of the Milky Way and M31 fit within the larger context of galaxy evolution.

The Virgo cluster is a juicy target, as it is the closest large cluster of galaxies. Not only is there a large number of galaxies in this cluster, spanning a range of types, but Virgo is also similar to the large clusters seen at high redshifts; thus, it is a key to decoding and understanding galaxy evolution.

The images that will be obtained with IRIS + NFIRAOS will allow a major step beyond what can be achieved with 8-meter facilities. This is demonstrated in the figure to the right, which shows simulations of a field in a Virgo cluster elliptical galaxy as observed with an 8-meter telescope (left hand panel) and 30- meter telescope (right hand panel) with a three-hour exposure time. As above, a Strehl ratio of 0.6 is assumed.

The differences between the two images are dramatic. Many more stars are detected in the 30 meter image, and these objects are the RGB stars that will provide the crucial information that is needed to probe the chemical enrichment history of this system. In addition, bright groups of stars merge together to form what appear to be single objects in the 8 meter image, but these are resolved into there individual components when observed with a 30 meter telescope.

Not only will IRIS + NFIRAOS allow individual stars to be resolved in more distant galaxies than is currently possible, but they will also allow stars to be resolved in the crowded, central regions of nearby galaxies, where the angular resolving power of existing facilities is such that only the brightest stars can be studied. The area around S And is a case in point. This supernova was located only 15 arcsec from the nucleus of the galaxy, and only the very brightest AGB stars can be resolved in this part of the galaxy with the current generation of telescopes. Only with the state-of-the-art imaging capabilities offered by IRIS + NFIRAOS will it be possible to resolve individual RGB stars in this part of M31, and thereby probe the environment that gave birth to this object.