Draft (Jan. 18, 2010) Executive Summary




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4.6Near Field Cosmology and Stellar Astrophysics

4.6.1Resolved stellar populations and kinematics in nearby galaxies


Martin Smith (KIAA), Eric Peng (KIAA), Richard de Grijs (KIAA), Jinliang Hou (SHAO)
Key questions:


  1. Can we understand the origins of the first galaxies and study dark matter distribution on the smallest scales?

  2. How does accretion affect the evolution of galactic discs and how important are these effects in comparison to secular processes?

  3. How accurately can CDM simulations match observations, especially when they are confronted with detailed analyses of the kinematics and chemistry of stars in nearby galactic haloes?


Introduction:

Theories of galaxy evolution have converged on the hierarchical growth scenario, where structures are built up from the aggregation of smaller building blocks. Whilst this general picture has been reaching a consensus, the next crucial step it is to identify and then analyse the signatures which remain today. This process of ‘digging for fossils’ is not a trivial one, especially in distant galaxies. In recent years, the most promising avenue for finding such fossils has been in our very own Galaxy; only for our own Galaxy is it possible to study large numbers of individual stars in detail. Spectrographs on 10m-class telescopes have enabled such studies to tentatively explore M31, but further progress is hampered by the limitations of the current-generation of telescopes. TMT will revolutionise this field by allowing us to probe more distant galaxies and in much greater detail than is currently possible. Such work will allow us to push theoretical understanding to its limits, moving the field into an era where progress is driven by observational discoveries.


Main science areas:
1. Ultra-faint satellites of the Milky Way

One of the most exciting recent developments in this field has been the discovery of a new class of satellite galaxies around the Milky Way, the so-called ultra-faint (UF) dwarf galaxies [e.g. 1]. These galaxies have very low-luminosity and as a consequence are believed to be the most dark matter dominated objects in the universe, with mass-to-light ratios claimed to be as high as 1,000 [2]. This means that they are important laboratories for studying dark matter on the smallest scales. They play a crucial role in our understanding of the processes which control galaxy evolution, in particular the influence of feedback and reionization on the gas content (and hence star formation histories) of such low-mass galaxies.


However, spectroscopic analyses using the current generation of telescopes are limited to only a handful of stars in these objects. With TMT it will be possible to obtain velocities for much larger samples, allowing robust mass-to-light ratios to be determined from the analysis of velocity dispersions. For example, TMT will allow us to extend at least two magnitudes below the main-sequence for UFs within 100 kpc. Their small sizes (half-light radii of 1 to 20 arcmin) are ideal for the WFOS field-of-view. Furthermore, in the coming years photometric surveys such as Pan-Starrs will identify these UFs out to much larger distances; only telescopes such as TMT will be able to carry out spectroscopic analyses of these objects. In addition to the kinematic analyses, these spectra will be provide metallicity information, which will be vital if we are to understand the formation histories of these UFs. For example, recent studies are finding that these galaxies host some of the most metal-poor stars in the universe [3]. Large samples of medium-resolution spectra from WFOS will allow us to identify many candidate metal-poor stars which can be followed up with, for example, HROS (see Section 4.6.2).
2. Dissecting the Andromeda galaxy

Although the Milky Way is an very good target to carry out detailed studies of the stellar populations, there is one major drawback - because we are located within the galaxy is it difficult to build up a full panoramic understanding of its structure. However, this is not a problem for more distant galaxies, such as Andromeda (M31). This is an ideal target for TMT because M31 is the Milky Way’s closest neighbour and, as TMT will be the only 30m-class telescope located in the Northern Hemisphere, it will be the only one which will be able to study this important galaxy.


The current generation of large telescopes are only beginning to unveil the full complexity of M31 [4]. Some of the most important spectroscopic work in this field has been carried out using the DEIMOS instrument on Keck [e.g. 5, 6], although these studies are only able to probe the tip of the giant branch (which, at a distance of 785 kpc, lies at the limit of what is capable with a 10m telescope). Given its comparable field of view with the DEIMOS instrument, a survey with WFOS will be ideal to illuminate the processes which are governing the evolution of M31. Using TMT, a study similar to the previous DEIMOS analyses will be able to measure the velocities and [Fe/H] for much fainter depths, possibly extending to the red clump region. For the brightest stars WFOS will be able to estimate the alpha-element abundance, which can be used to further illuminate the chemical history of this galaxy (see [7] and also Section 4.6.2). Therefore TMT will be able to amass a vast and rich set of data, considerably greater than what is possible with the current generation of telescopes. With this we can make detailed studies of the structures in the halo of M31, for example deconstructing the numerous streams and investigating how the recent encounter between M33 and M31 has shaped the evolution of these two galaxies. For the first time we will be able to understand the complexities of the M31 disc, which is known to extend out to ~70 kpc [8]. It is unclear how the violent merger history of M31 has influenced the disc evolution, in particular how the disc reacts to (or is formed by) these numerous accretion events. Only through detailed kinematic and chemical studies of large samples of stars will we finally be able to decipher the disc’s history, assessing the importance of accretion versus secular processes. By constructing metallicity distribution functions for the disc at different locations we can constrain the star formation and accretion history across the disc, simultaneously investigating the significance of stellar radial migration which is currently an area of much debate.
Another important area of study for M31 will be its satellite population. Owing to the large distance to M31, it is very difficult to determine reliable mass-to-light ratios for these objects using velocity dispersions. One of the most comprehensive recent studies [9] looked at one cluster with DEIMOS on Keck, but they were only able to place a very tentative upper limit on the mass-to-light ratio due to their restricted sample size of only six stars. The satellite population of M31 is especially important because of the differences to that of the Milky Way; one notable discrepancy is the presence of recently-identified extended clusters around M31 [10]. The nature of these objects (which intriguingly do not appear to be present around the Milky Way) is unclear, in particular the question of whether they posses dark matter, i.e. whether they can be classified as dwarf galaxies or star clusters. TMT will be able to obtain samples of the order of 100 stars, from which dispersion profiles and mass models can be constructed.
Lastly, TMT will provide a major advance in the study of the M31 globular cluster system.  Globular clusters (GCs) are the oldest and most prominent stellar remains of some of the earliest star formation in galaxies. These compact, massive star clusters are generally simple stellar populations, older than 10 Gyr, and have played a crucial role in a wide range of astrophysics, including the distance scale, the age of the Universe, stellar evolution, dynamics, and as tracers of stellar halo formation. With a few exceptions, however, the ability to resolve GCs into stars and study their detailed colour-magnitude diagrams has been limited to the Milky Way and its nearby satellites.  Even with HST, colour-magnitude diagrams of GCs at the distance of M31 are plagued by crowding.  With adaptive optics, the spatial resolution and the field of view of TMT/IRIS is well-matched to GCs in M31 (comparable to ~1 arcsec seeing and a ~10 arcmin field of view at 12 kpc). TMT/IRIS will enable a deep, detailed study of halo GC population in M31, roughly comparable to pre-HST studies of Galactic GCs, and complementing studies of the M31 field stars, providing an independent measure of the star formation, merging, and chemical enrichment history of M31.
3. Our more distant neighbours

For near-field cosmology, one of the most valuable aspects of a telescope such as TMT will be its ability to study the kinematics and chemistry for resolved stellar populations in a number of galaxies beyond the Milky Way. With velocity information for the streams in these systems it will be possible to undertake modelling of a diverse array of streams in many different environments [11]. For the first time we will be able to address the statistical significance of our findings, comparing them to theoretical predictions from CDM simulations [e.g. 12, 13] across a wide variety of galaxies. It will be possible to carry out spectroscopic surveys for many galaxies within the Local Volume, for example building on the current work of the ACS Nearby Galaxy Survey Treasury [14] which has constructed uniform photometric catalogues of resolved stellar populations in galaxies out to 4 Mpc.


One such target is the M81 group which lies at 3.5 Mpc. This group is interesting because the central members (M81, M82, and NGC3077) are known to be interacting [15], and so TMT will allow us to see the effects of accretion using the kinematics of individual stars. Furthermore, at these distances the WFOS field-of-view allows us to efficiently map large areas of the halo so that we can use bright tracers to carry out detailed analyses of the mass distribution in these galaxies, applying techniques developed for the Milky Way [e.g. 16] to more distant galaxies.

Possible TMT programs:

  1. Velocity dispersion profile analysis of MW ultra-faint dwarf population, accurately determining their mass-to-light ratios and investigating the chemical properties of these extreme galaxies.

  2. Unravelling the accretion history of M31 by combining [Fe/H] and [alpha/Fe] with kinematics for photometrically selected halo substructures.

  3. Detailed dissection of M31 disc, using a number of well-chosen pointings along the major axis to probe the kinematics and chemical history of the disc.

  4. A spectroscopic survey of a number of more distant galaxies, probing the accretion history across a diverse range of galaxies and placing strong constraints on CDM simulations.


China’s strengths and weakness in this area:

The science case that is presented here is a natural extension to work that is currently being undertaken in the field of Galactic archaeology and near-field cosmology. Importantly, this is a rapidly growing field in China. The current growth is being driven by the LAMOST telescope, which will carry out a large spectroscopic survey of stars in the Milky Way and hence make a significant contribution to the field. As a consequence, over the coming years the local knowledge base will be built up so that by the time TMT reaches first-light this field will be flourishing in China. The seeds of this growth are currently being planted through the recruitment drives such as those funded by CAS, which aim to bring world-leading young scientists to China, although there is still a deficiency in the number of experts that are crucial to train the current-generation of students; it is these current students who will be the ones to lead the exploitation of TMT data and so, if they are going to be able to make an impact on the international stage, it is vital for these to receive the best possible training.

In the near-future LAMOST will carry out a survey of the M31 region. This will be particularly important as it will allow Chinese students to actively carry-out science using a world-class dataset, which is clearly the best way for them to learn how to excel in this field. Furthermore, this work will build on the collaborations which already exist between the Chinese community and international scientists who are at the forefront of M31 research, for example groups in the Herzberg Institute of Astrophysics (Victoria, Canada) and the Institute of Astronomy (Cambridge, UK).

At the present time the most active institutes are KIAA, NAOC and SHAO, all of which contain numerous people working on multi-object spectroscopy. This list is rapidly growing and by the middle of the next decade China has the potential to be a major power in the area of near-field cosmology.


References:

  1. Belokurov et al., 2007, ApJ, 654, 897

  2. Geha et al., 2009, ApJ, 692, 1464

  3. Kirby et al., 2008, ApJ, 685, L43

  4. McConnachie et al., 2009, Nature, 461, 66

  5. Chapman et al., 2008, MNRAS, 390, 1437

  6. Gilbert et al., 2009, ApJ, 705, 1275

  7. Venn et al., 2004, AJ, 128, 1177

  8. Ibata et al., 2005, ApJ, 634, 287

  9. Collins et al., 2009, MNRAS, 396, 1619

  10. Huxor et al., 2005, MNRAS, 360, 1007

  11. Martinez-Delgado et al., 2008, in ”Highlights of Spanish Astrophysics V”, Proceedings of the VIII Scientific Meeting of the Spanish Astronomical Society (SEA), Springer (arXiv:0812.3219)

  12. Bullock & Johnston, 2005, ApJ, 635, 931

  13. Cooper et al., 2009, MNRAS, submitted (arXiv:0910.3211)

  14. Dalcanton et al., 2009, ApJS, 183, 67

  15. Mouhcine & Ibata, 2009, MNRAS, 399, 737

  16. Xue et al., 2008, ApJ, 684, 1143



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