Discovering New Frontiers in Astronomy with TMT/IRMS
By Bahram Mobasher
The author would also like to acknowledge helpful discussions with Don Figer and Brian Siana.
With the commissioning of the Thirty Meter Telescope (TMT) later this decade, a new era in observational astronomy will begin.
Among the first-light instruments planned for the TMT, is the Infra-Red Multi-object Spectrograph (IRMS), which would enable detail study of the nature and properties of the faintest objects in our Universe. The IRMS will be placed behind an Adaptive Optics system on TMT and will provide much sharper images with higher sensitivity over 2 arcmin diameter field-of-view (FoV). The combination of the multiplexing capability of the IRMS, its sensitivity and TMT aperture, provides a unique opportunity to address the most fundamental questions in astronomy.
Understanding properties of massive star clusters is essential in addressing a range of questions from establishing local distance scale and studying star formation activity, to the early Universe where the first generation of stars were formed and contributed to the re-ionization process. They control the dynamical and chemical evolution of their local environment through their effect on the interstellar medium, dominate early evolution of the first galaxies and are likely progenitors of the most energetic processes in the Universe, the Gamma Ray Bursts. The most massive of these objects end their lives as supernovae, producing the heavy elements that build the elements for formation of planets and eventually, life. These star clusters are expected to be in Galactic disks, where detailed observations are difficult due to high extinction and limited spatial
and spectral resolution available (Figure 1). The combination of IRMS and TMT will be a powerful tool for discovering new massive star clusters in our Galaxy. With only a few minutes of exposure time, we can easily verify the presence of massive stars down to a mass of ~10 Msun, within 8 kpc from the center of the star clusters. This allows measurement of the stellar content, age, mass and metallicity of a currently unidentified population of massive clusters in the Galaxy.
One outstanding question in observational cosmology is the evolution of star formation, metallicity and stellar mass in galaxies with cosmic time. Star formation in galaxies builds up their stellar mass and enriches their metal content. Therefore, these parameters are inter-related. Figure 2 shows changes in metallicity with star formation for galaxies in different mass intervals, taken from the Sloan Digital Sky Survey (SDSS). A clear trend exists for nearby galaxies in that, galaxies with lower star formation rate (SFR) have higher stellar mass and higher metallicities. The trend appears to continue to z~2, although with larger scatter (Figure 2). However, there is very little overlap between the low- and high-redshift galaxies, with high-redshift objects only sampling the high SFR and high mass end of the distribution. It is not clear how the mass-metallicity relation behaves for low star forming and low mass galaxies at high redshifts (where the bulk of star formation activity is taking place). Furthermore, there are some indications for evolution in the mass-metallicity-SFR relation beyond z~3, but there are only a handful of sources with available data at that redshift. The TMT aperture, combined with the sensitivity, wavelength coverage and multiplexing capability of the IRMS allows, via measurement of the [OII], [OIII], and H-beta line fluxes, a determination of the oxygen abundances in galaxies up to z~3.8. This also enables study of the metallicity-mass-SFR relation for low star-forming/low mass galaxies at high-redshifts.
Over the last decade, a large number of discoveries were made by complementary observations between the Hubble Space Telescope (HST) and ground-based Keck telescopes. Similarly, the combined capabilities of the James Webb Space Telescope (JWST) and the TMT/IRMS could provide exceptional opportunity for discovery. Compared to the JWST, the TMT will provide ~25 times larger light collecting area. Furthermore, the relatively small field of view of the IRMS with adaptive optics, limits the contamination by sky background when long exposures are taken. This makes the TMT/IRMS an ideal instrument for high S/N ratio follow-up spectroscopy of high redshift candidates found by JWST. Many of these sources have Lyman-alpha emission lines. Figure 3 shows the simulated image of a nearby merging system, shifted to z=5 and 12, as expected to be seen at 3.8 microns by JWST. Follow-up high spatial resolution spectroscopy with IRMS of such merging systems allows study of the nature of individual components (i.e. by searching for He 1640 lines), which would grow to form larger and more massive galaxies we observe today. The IRMS will enable measurement of velocity dispersions of UV lines in both absorption and emission, probing gas infall/outflow and winds in multi-component systems at z>4. This will reveal the nature of the first generation of galaxies.
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