LSST: Mapping the Milky Way and Local Volume Structure

Individual stars in the Milky Way and the galaxies nearby can be resolved by the LSST. These stars then provide a fossil record—a Rosetta Stone—that can be decoded to determine how these galaxies were formed. LSST will revolutionize the study of this fossil record.

The region of space within a distance of about 10 Megaparsecs (Mpc), or 32 million light years, from the Milky Way is called the Local Volume (LV) because, astronomically speaking, it is so nearby. The last decade has seen a renaissance in the study of our own and other galaxies in the LV, based in large part on the multi-dimensional maps provided by the vast numbers of stars cataloged by the Two Micron All-Sky Survey (2MASS), Sloan Digital Sky Survey (SDSS), and others. This renaissance has revolutionized our view of the Milky Way by facilitating cross-sectional views of its global structure and revealing a vast menagerie of substructures, including a new population of satellite galaxies with a millionth the luminosity of our Galaxy and a halo replete with lumps and streams that betray its formation.

LSST will provide an excellent resource for mapping the structure and accretion history of the Milky Way and LV in a way that the present generation of surveys has only hinted at. LSST is anticipated to catalog over 10 billion stars, for 200 million of which we will also have measured photometric chemical compositions. (For comparison, SDSS measured about 50 million stars.)

These maps, which will include information about the spatial distribution, motions, and chemical compositions of individual stars are key to understanding what our Galaxy looks like, how far it extends, how it and other galaxies formed, and how much dark matter exists and where it is located.

Science enabled by LSST includes mapping the 3D distribution of dust throughout the Milky Way’s disk; understanding the smooth distribution of stars in the MW and other nearby galaxies; understanding large-scale chemical gradients in the MW; discovering lumps and streams in metallicity and phase-space; inferring the mass distribution in the MW; discovering ultra-faint galaxies throughout the Local Volume as overdensities of resolved stars.

A Milky Way Map

Chapter 7 says that LSST will provide a “uniform, multidimensional, star-by-star phase space map of all Milky Way components.” What this means is that LSST will measure the positions, proper motions (i.e., motions across the plane of the sky), and chemical compositions of millions of individual stars in the Milky Way. The Galaxy consists of three main parts: a spherical bulge in the central region, a disk of young and old stars, and a low density halo of stars and dark matter that extends about half way to the Andromeda galaxy. The LSST survey will provide a complete picture of the spatial, kinematic, and chemical makeup of the Galaxy and its components (halo, bulge, and disk). LSST will enable a complete characterization of these three components, which leads to a better understanding of how the Galaxy as a whole formed and evolved.

LSST will be able to find two orders of magnitude more stars than the number of stars currently catalogued from all previous sky surveys. The analysis of these stars, with LSST’s unique combination of an ultraviolet u-band filter, near-IR y band, well-sampled time domain information, which enables the study of variable stars, proper motion measurements of faint stars, and the depth and wide-area coverage make the exceptional mapping possible.

Questions that LSST data will answer include:

• How have mergers with other galaxies affected the structure of the Milky Way? Small galaxies that have been swallowed up by the Milky Way leave traces in the form of narrow streams of stars with common proper motions and similar chemical compositions.
• What is the total mass of the Galaxy? How far out does it extend? Does the dark matter form a spherical halo around the Galaxy or does it have a flattened shape?
• Are there truly any “missing” Galactic satellites?

The Milky Way and Cosmology

Proper motion measurements for millions of main sequence stars and hundreds of tracer objects in the outer halo will provide the data for major steps in understanding the Milky Way in a cosmological context.

One open issue in galaxy formation today is the seeming contradiction between the hierarchical model of galaxy formation, which says that large galaxies are built by mergers of many small galaxies, and the observations of the preponderance of Milky Way sized galaxies, which have thin, cold (in this context, cold means that the stars are moving at low velocities relative to one another) disks. Large numbers of violent mergers should stir up the orbits of stars, making the disk thicker and the stellar velocities higher than are actually observed. Detailed studies of the Milky Way can provide valuable boundary conditions for models of the formation and evolution of galaxies with disks.

Evidence from all-sky surveys and theoretical models suggest that the LV is filled with low surface brightness structures such as faint dwarf galaxies, tidal streams, and exotic objects. Resolved stars can be used to map these objects and explore outer disks, external galaxies, and even discover new galaxies and intragroup stars. The measurements of the outer halo and orbits of Milky Way satellite galaxies are critical to the modeling of the formation of dwarf galaxies.

Finally, globular clusters are found associated with all but the faintest dwarf galaxies. They can be used to probe formation epochs, assembly mechanisms, and evolution of galaxies. “LSST will give a complete photometric characterization of globular cluster systems of essentially every galaxy within ~ 30 Mpc.”

The Darkest Galaxies

The discovery of ultra-faint dwarf galaxies around the Milky Way and Andromeda galaxies has made available new paths of investigation into galaxy formation and cosmology. Some have absolute magnitudes as faint as that of a single red giant and contain so few stars one might think they’re star clusters and not galaxies at all. Yet, studies of their internal motions imply that unlike star clusters, these objects do contain dark matter. How did such small, faint galaxies form? Are they remnants of more luminous objects? Did they ever contain more stars than the number they do today? Why is there a discrepancy between the number of dark matter halos predicted to orbit around the Milky Way and the number lit up by the eleven dwarf galaxies?

Willman says, “These ultra-faint dwarfs consist of only a few thousand stars and a luminosity range below the average globular cluster. They are also the most dark matter rich and metal poor galaxies known. We want to find out why the luminosity is so low—is it nature or nurture—were they formed that way or did something happen in their evolution?”

The LSST’s deep and wide field survey will enable a complete census of the Milky Way’s satellite galaxies and reveal distant ultra-faint dwarfs. The analyses of these data will provide insights into the nature of dark matter and the limits of galaxy formation.

The challenge of the coming decade will be to move beyond the past decade’s “checking of models” and instead to use resolved stellar structure throughout the LV to untangle galaxy formation in general—to use the LV as “a laboratory for testing how stars form over a range of timescales, within a variety of masses of dark matter halos, in different environments in the early Universe, and with different interaction histories.” LSST’s unparalleled maps of the stellar distribution in the LV will provide a census of structures (galaxies, velocity streams, etc.) and show how the properties of structures (morphology, density, and extent) vary by location. With LSST’s ability to observe faint stars at large distances, scientists will be able to generalize results from high-resolution spectroscopic studies of nearby Milky Way stars to larger scales. Using various tracers within the Milky Way and LV, scientists will make new maps and address fundamental questions about formation of the structures of the Universe.

This article written by Anna H. Spitz, Beth Willman, and Sidney Wolff