With GALAH, we will chemically tag stars into coeval groups, identifying individual members of star clusters which have long since dispersed. Using the stellar relics of ancient star formation and accretion events, we can reconstruct the Galactic accretion history, and dynamical and chemical evolution.
The GALAH data sets will yield a comprehensive view of the formation and evolution of the Milky Way disk and address the following basic questions:
- What were the conditions of star formation during early stages of Galaxy assembly?
- When and where were the major episodes of star formation in the disk and what drove them?
- To what extent is the Galactic disk composed of stars from merger events?
- Under what conditions and in what types of systems did accreted stars form?
- How have the stars that formed in situ in the disk evolved dynamically since their birth?
- Where are the solar siblings that formed together with our Sun?
After their birth-clusters or birth-galaxies disperse, stars may change their dynamical behavior thanks to mechanisms like heating and radial migration. However, the chemical composition of these stars, which reflect the conditions of their birth locations, remain largely unchanged. We plan to use the chemical abundance distribution of our targets to identify relic stars from old long-dispersed star formation sites, a technique known as chemical tagging. Since most stars are born in dense clusters, the formation and evolution of galaxies today must involve millions of discrete cluster events throughout their history. These clusters are known to be chemically homogeneous in the heavier elements, and the abundances vary from cluster to cluster. By chemically tagging stars by their cluster "fingerprint," we can establish the evolving mass function of star clusters, their chemical composition, formation and survival rate as a function of cosmic time. Most importantly, understanding these clusters helps us understand the overall development of the Milky Way.
Below, we describe some of the interesting science projects we are pursuing with GALAH.
Changes with Time
When fossil relic stars of dispersed star forming events are identified, we will need to estimate their ages in order to develop a picture of the assembly of the Milky Way. By working with Gaia, K2, and CoRoT, we will be able to get accurate ages for the GALAH sample. We then hope to place these reconstructed stellar clusters into an evolutionary sequence, i.e. a family tree, based on their chemical signatures. This allows us to estimate a timeline of the Galaxy's chemical and dynamical evolution, including its accretion history. Additionally, we can investigate the variation in the star formation rate and the cluster mass function with time. Previously, this was only possible for a small volume of stars in the solar neighborhood, and this was plagued by small sample size.
GALAH focuses its observations on the Milky Way disk and bulge, which contain almost all of the stellar mass of the Galaxy. These components, in addition to evolving dynamically, may also be influenced by the infall of small evolved satellite systems. For example, the Galactic disk near the Sun shows some kinematical clumping or substructure. Some of these clumps are likely the debris of old disrupted star clusters in the disk, which are now dispersed into extended regions of the Galaxy. Others may be remnants of satellites cannibalized by the Milky Way. Cold Dark Matter (CDM) simulations of galaxy formation predict a high level of merger activity, but observations of disk galaxies are unclear about the number and influence of mergers. Using chemical tagging we can detect and isolate the debris of such systems within the indigenous populations of the Milky Way, putting observational limits on the satellite accretion history of the Galaxy.
The GALAH sample will be useful for examining the dominant dynamical processes that occur during Galaxy development. For example, radial migration has been proposed to form the thick component of the disk and disrupt open clusters in the inner Galaxy. Driven by the torques of the Galactic bar and spiral arms, this mechanism can move stars radially from one near-circular orbit to another without significantly heating the disk (Sellwood & Binney 2002). Radial mixing would spread the phase-mixed azimuthally dispersed debris of individual clusters over several kpc in radius (Roskar et al. 2008). Without radial mixing, the dispersed debris from open clusters detected by GALAH would be on near-circular orbits and confined to a fairly narrow annulus around the Galaxy, reflecting their early dynamical properties. Using GALAH, we can examine the radial extent of the chemically-tagged debris of disrupted clusters; we will then have a direct measurement of how important radial mixing, and other dynamical processes, has been to Milky Way evolution.
The large, homogeneously observed and analyzed GALAH stellar sample will help to delineate the different nucleosynthetic processes that lie behind the chemical evolution of the Galaxy. By examining how the different elements relate to one another using principal component analysis (PCA), we can examine the structure of the different dominant nucleosynthetic processes. Furthermore, we can learn about the contributions of the various nucleosynthetic processes in each Galactic component, as a function of position, velocity, and orbital integrals.
After HERMES was commissioned, we began a Pilot Survey consisting of a number of small science projects. Although these projects are narrow in scope, they provide valuable tests of our data pipelines and survey design in addition to interesting science. These include things such as observing large open clusters for chemical homogeneity, kinematics, and membership, observing stars with asteroseismology information from CoRoT and Kepler, and examining the geometric structure of the Milky Way disk.
The APOGEE survey uses high-resolution, high signal-to-noise infrared spectroscopy to examine stars in the inner Galaxy. By observing some of their 100,000 red giant stars with HERMES, we will cross correlate with their abundance measurements. As APOGEE is in the H-band, it will be well-suited for highly reddened fields, providing a valuable check of both our stellar abundances and extinction determinations. By combining APOGEE and GALAH information, we can develop a detailed multiwavelength picture of regions of the Milky Way. In addition, combining information from APOGEE and GALAH can give us a more complete picture of the Milky Way structure from the center out to large Galactocentric distances.
Giant stars are bright, allowing us to examine the chemical properties of the Milky Way at large distances from the Sun. However, it is very difficult to determine the ages for these stars using isochrones. The CoRoT mission has a large sample of giants with asteroseismology data, which will allow them to determine surface gravities and estimate ages. Without abundance information, the age errors from asteroseismology are about 30%. With abundance information from HERMES, the age errors drop to better than 15%. These stars would allow us to study the age-metallicity-velocity relation for the Milky Way disk.
When fossil relic stars of dispersed star-forming events are identified, we will need to estimate their ages in order to build up a picture of how the Milky Way was assembled. Measuring accurate ages for individual stars is very difficult. Near the Sun, we can compare subgiants and stars near the main sequence turnoff with isochrones. However, these stars are too faint to study at large distances. Consequently, we know little about the age-metallicity distribution of the disk beyond the solar neighborhood.
Gaia will provide us with precision astrometry, transverse motions, and accurate stellar distances (expected 1% errors) for all of the GALAH stars. By combining this information with our spectroscopically-determined metallicities and temperatures, we can estimate the ages of our targets from color-magnitude diagrams, and determine the timeline of Galaxy development.
Gaia will also provide radial velocities for a subsample of stars. By observing these targets as part of GALAH, we can ensure that our pipeline radial velocity measurements are consistent with Gaia and accurate. Combining GALAH radial velocities with Gaia astrometry and distances, we can determine the 3D velocities for the entire survey sample. This will allow us to examine the Milky Way in chemical, geometric, and orbital space.
Gaia-ESO is primarily a lower-resolution spectroscopic survey of various Galactic fields using the Very Large Telescope and FLAMES-GIRAFFE; a subset of Gaia-ESO stars will also be observed with UVES at high resolution. This sample is complementary to GALAH, probing farther distances. Combining data from the two surveys will allow us to investigate the geometric structure and chemistry of Milky Way structures over a significant range of Galactocentric distance.
In addition, this survey will observe a number of star clusters. With overlapping stars in the field and clusters, GALAH can relate the parameter scales of the two surveys. This will provide a check on our analysis pipeline, in addition to widening the scope of scientific analysis.
The Kepler mission's new life -- K2 -- brings a whole new dimension to galactic archaeology using asteroseismic estimates of surface gravity and age of several tens of thousands of red giants. The K2 observations cover the ecliptic, hence sampling regions in the Galaxy ranging from its plane to its poles and from the galactic centre to the anticentre. The observations cover a number of open and globular clusters, which are key for calibrating seismic and spectroscopic results. Most K2 fields are observable with GALAH.
The RAVE Survey provides accurate and precise radial velocity measurements over 20,000 square degrees of the southern sky. They also determine atmospheric parameters, elemental abundances, and distance estimates for their sample, making them a valuable data set to cross correlate GALAH information with.
SkyMapper is a wide field survey telescope working on the first digital photometric survey of the entire southern sky. With six filters, Skymapper photometry will complement our 2MASS and APASS photometry. With this data set, we can determine initial estimates of the stellar parameters and extinction. In addition, SkyMapper can help us to identify particular stars of interest, such as subgiants and extremely metal-poor stars.