MassesofGalaxies

Measuring the Masses of Distant Galaxies with Extremely Large TelescopesE. J. Barton (NRC/HIA): 

Measuring the Masses of Distant Galaxies with Extremely Large Telescopes E. J. Barton (NRC/HIA) I. The rotation of spiral galaxies. Fig. 1 TOP - An image of a nearby spiral galaxy. The horizontal line shows the position of the slit (aperture) of the spectrograph for the observation. Deep images detect very distant galaxies at early stages in their evolutionary histories, the precursors to present-day galaxies. These pictures reveal the unusual shapes of the objects, but much more detailed knowledge is required to understand their nature. Studies of local galaxies show that the motions of stars and gas inside the galaxies reveal important properties of the objects. These motions are difficult to observe. Measurements of internal motions of distant galaxies require the sensitivity and image quality available from 20 to 30-meter telescopes with adaptive optics and special instrumentation. 1 III. Very distant and young objects: 'Lyman break' galaxies and beyond. II. Stellar motions inside elliptical galaxies. Fig. 1 MIDDLE - A spectrum showing the rotation of the galaxy (z=0). The spectrum measures the velocity of each piece of galaxy along the slit. The shifts in the spectral lines indicate that right side of the galaxy is moving away from us faster than the left side. The difference between the velocities of the left and right sides is the rotation speed. Fig. 1 BOTTOM TWO PANELS- Simulated spectra of the nearby galaxy at redshifts of 1.4 and 1.8 as they would look through a 20-meter telescope with a long (10-hour) exposure. The rotation speed can be measured easily from each of the spectra. Spectroscopic observations of local galaxies show that more luminous spiral (disk) galaxies spin faster than intrinsically fainter spirals. Known as the Tully-Fisher luminosity-linewidth relationship, it is thought to arise because more luminous galaxies are more massive and exert stronger forces on orbiting stars. Just as the Sun is weighed by measuring the rotation speeds of the planets, a spiral galaxy is weighed by measuring the rotation speeds of stars orbiting its center. The empirical relationship provides a basis for the direct detection of evolution in spiral galaxies. Galaxies of different ages that have the same rotation speeds can be compared directly. The differences in luminosities are a direct measure of the amount of evolution between one epoch in the Universe and another. With existing telescopes and spectrographs, we can only measure the rotation speeds of the brightest distant spiral galaxies, and only to redshifts less than about 1.2; these observations require large amounts of telescope time. 20 to 30-meter telescopes could survey a large sample of typical spiral galaxies to redshifts of almost 2 and could provide a direct measures of the luminosities of spiral galaxies as a function of time in the evolution of the Universe. Fig. 3 - An example of a 'Lyman break' galaxy in the Hubble deep field. These four lumps may be four galaxies merging together or four star-forming regions inside the same galaxy. A 20 to 30-meter telescope would enable us to distinguish between these possibilities for large samples of similar objects, using measurements of the relative velocities of these kinds of lumps. Fig. 2b - An absorption line in the spectrum an elliptical galaxy. The feature is broadened by the motions of the hundreds of millions or billions of stars it arises from, which are each Doppler shifted in wavelength by their motions inside the galaxy. The total width of the absorption feature, labeled s here, reveals the aggregate motions of the stars. Fig. 2a - An image of a nearby elliptical galaxy. Stellar motions are much less organized in these galaxies than in rotating spirals. SPECTRUM SLIT Elliptical galaxies have very different stellar contents and formation histories from spiral galaxies. Although motions inside ellipticals are much less organized than in spirals, the range of velocities of stars in an elliptical provides a measure of the true size of the galaxy that is similar to the luminosity-linewidth relation for spiral galaxies. This relation is currently used to measure the evolution of elliptical galaxies in clusters to redshifts of about 1. If high-redshift ellipticals have strong absorption lines, 20 to 30-meter telescopes would extend these measures to redshifts beyond 2 for the brightest galaxies. Recent studies of very distant objects reveal numerous galaxies at a redshift of 3. Named for the technique by which they were discovered, the 'Lyman break' galaxies are a varied population of objects. Some appear round, others are amorphous or lumpy like the object on the right. The nature of Lyman break galaxies is heavily debated. They may be intrinsically large galaxies or smaller galaxies that are very luminous due to recent star formation. In some cases, measurements of the internal motions of the galaxies could resolve this degeneracy. For most Lyman break galaxies, these measurements require both the sensitivity and the resolving power of large-aperture telescopes. Measurements of the internal motions in Lyman break galaxies are possible with a 20 or 30-meter telescope and an efficient spectrograph. If the image quality is near the diffraction limit in the near-infrared, a 20-meter telescope could measure the velocity of an unresolved lump of star formation with a very low star-formation rate in 3 hours (0.1 solar masses per year of OII flux at a redshift of about 3). A 30-meter telescope could measure the velocity of a lump with about only 40% of this flux. For lumpy objects with very high star formation rates, velocities in other emission lines could be measured at much higher redshifts. z = 0 z = 1.4 20-m telescope z = 1.8 20-m telescope