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Register a new userGalactic Stellar Populations in the Era of SDSS and Other Large Surveys
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Studies of stellar populations, understood to mean collections of stars with common spatial, kinematic, chemical, and/or age distributions, have been reinvigorated during the last decade by the advent of large-area sky surveys such as SDSS, 2MASS, RAVE, and others. We review recent analyses of these data that, together with theoretical and modeling advances, are revolutionizing our understanding of the nature of the Milky Way, and galaxy formation and evolution in general. The formation of galaxies like the Milky Way was long thought to be a steady process leading to a smooth distribution of stars. However, the abundance of substructure in the multi-dimensional space of various observables, such as position, kinematics, and metallicity, is by now proven beyond doubt, and demonstrates the importance of mergers in the growth of galaxies. Unlike smooth models that involve simple components, the new data reviewed here clearly show many irregular structures, such as the Sagittarius dwarf tidal stream and the Virgo and Pisces overdensities in the halo, and the Monoceros stream closer to the Galactic plane. These recent developments have made it clear that the Milky Way is a complex and dynamical structure, one that is still being shaped by the merging of neighboring smaller galaxies. We also briefly discuss the next generation of wide-field sky surveys, such as SkyMapper, Pan-STARRS, Gaia and LSST, which will improve measurement precision manyfold, and comprise billions of individual stars. The ultimate goal, development of a coherent and detailed story of the assembly and evolutionary history of the Milky Way and other large spirals like it, now appears well within reach.
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The study of galaxies has changed dramatically over the past few decades with the advent of large-scale astronomical surveys. These large collaborative efforts have made available high-quality imaging and spectroscopy of hundreds of thousands of systems, providing a body of observations which has significantly enhanced our understanding not only of cosmology and large-scale structure in the universe but also of the astrophysics of galaxy formation and evolution. Throughout these changes, one thing that has remained constant is the role of galaxy morphology as a clue to understanding galaxies. But obtaining morphologies for large numbers of galaxies is challenging; this topic, Morphology in the era of large surveys, was the subject of a recent discussion meeting at the Royal Astronomical Society, and this Astronomy and Geophysics article is a report on that meeting.
Recent advances from astronomical surveys have revealed spatial, chemical, and kinematical inhomogeneities in the inner region of the stellar halo of the Milky Way Galaxy. In particular, large spectroscopic surveys, combined with Gaia astrometric data, have provided powerful tools for analyzing the detailed abundances and accurate kinematics for individual stars. Despite these noteworthy efforts, however, spectroscopic samples are typically limited by the numbers of stars considered; their analysis and interpretation are also hampered by the complex selection functions that are often employed. Here we present a powerful alternative approach $-$ a synoptic view of the spatial, chemical, and kinematical distributions of stars in the Milky Way based on large photometric survey databases, enabled by a well-calibrated technique for obtaining individual stellar metal abundances from broad-band photometry. We combine metallicities with accurate proper motions from the Gaia mission along the Prime Meridian of the Galaxy, and find that various stellar components are clearly separated from each other in the metallicity versus rotation-velocity space. The observed metallicity distribution of the inner-halo stars deviates from the traditional single-peaked distribution, and exhibits complex substructures comprising varying contributions from individual stellar populations, sometimes with striking double peaks at low metallicities. The substructures revealed from our less-biased, comprehensive maps demonstrate the clear advantages of this approach, which can be built upon by future mixed-band and broad-band photometric surveys, and used as a blueprint for identifying the stars of greatest interest for upcoming spectroscopic studies.
Until recently our knowledge of the Galactic Bulge stellar populations was based on the study of a few low extinction windows. Large photometric and spectroscopic surveys are now underway to map large areas of the bulge. They probe several complex structures which are still to be fully characterized as well as their links with the inner disc, the thick disc and the inner halo. I will review our current, rapidly increasing, knowledge of the bulge stellar populations and the new insight expected towards the Gaia era to disentangle the formation history of the Galactic inner regions.
In the coming decade the Gaia satellite will precisely measure the positions and velocities of millions of stars in the Galactic halo, including stars in many tidal streams. These streams, the products of hierarchical accretion of satellite galaxies by the Milky Way (MW), can be used to infer the Galactic gravitational potential thanks to their initial compactness in phase space. Plans for observations to extend Gaias radial velocity (RV) measurements to faint stars, and to determine precise distances to RR Lyrae (RRLe) in streams, would further extend the power of Gaias kinematic catalog to characterize the MWs potential at large Galactocentric distances. In this work I explore the impact of these extra data on the ability to fit the potential using the method of action clustering, which statistically maximizes the information content (clumpiness) of the action space of tidal streams, eliminating the need to determine stream membership for individual stars. Using a mock halo in a toy spherical potential, updated post-launch error models for Gaia, and estimates for RV and distance errors for the tracers to be followed up, I show that combining either form of additional information with the Gaia catalog greatly reduces the bias in determining the scale radius and total mass of the Galaxy, compared to the use of Gaia data alone.
This is the first paper in a series devoted to review the main properties of galaxies designated S0 in the Hubble classification system. Our aim is to gather abundant and, above all, robust information on the most relevant physical parameters of this poorly-understood morphological type and their possible dependence on the environment that could later be used to assess their possible formation channel(s). The adopted approach combines the characterisation of the fundamental features of the optical spectra of $68{,}043$ S0 with heliocentric $zlesssim 0.1$ with the exploration of a comprehensive set of their global attributes. A principal component analysis is used to reduce the huge number of dimensions of the spectral data to a low-dimensional space facilitating a bias-free machine-learning-based classification of the galaxies. This procedure has revealed that objects bearing the S0 designation consist, despite their similar morphology, of two separate sub-populations with statistically inconsistent physical properties. Compared to the absorption-dominated S0, those with significant nebular emission are, on average, somewhat less massive, more luminous with less concentrated light profiles, have a younger, bluer and metal-poorer stellar component, and avoid high-galaxy-density regions. Noteworthy is the fact that the majority of members of this latter class, which accounts for at least a quarter of the local S0 population, show star formation rates and spectral characteristics entirely similar to those seen in late spirals. Our findings suggest that star-forming S0 might be less rare than hitherto believed and raise the interesting possibility of identifying them with plausible progenitors of their quiescent counterparts.
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