Searching for Dwarf Satellites around Milky Way – Analog Galaxies with the SAGA survey
Ben Weiner, Steward Observatory
Dwarf satellites of massive galaxies are a probe of many issues in galaxy evolution and cosmology, including the nature of low-mass galaxies, star formation at early times, accretion into halos, and the abundance of low-mass dark matter halos. Much attention has been devoted to the number and nature of Milky Way and M31 dwarf satellites, especially the “missing satellites problem.” However, we know very little about dwarf satellites outside the Local Group below the mass of the LMC, and we don’t know if the MW and M31 satellite systems are typical. The SAGA (Satellites Around Galactic Analogs) survey collaboration aims to address this with both observational and theoretical studies of satellite abundances and properties around Milky Way analog central galaxies. I will present results from our MMT/Hectospec wide field spectroscopic surveys for satellites. We have surveyed the fields of several nearby galaxies that are similar to the Milky Way to detect and spectroscopically confirm dwarf satellites. We find a range of numbers of satellites, suggesting that there is a significant variance in halo histories. We also find that not all dwarf systems resemble the Milky Way and M31 systems. I will discuss these results and some of the implications on the life cycle of satellites that we can infer from satellite abundances and properties, including their images and spectra.
New Tools for Galactic Archaeology from the Milky Way
Gail Zasowski, John Hopkins University
One of the critical components for understanding galaxy evolution is understanding the Milky Way Galaxy itself — its detailed structure and chemodynamical properties, as well as fundamental stellar physics, which we can only study in great detail locally. This field is currently undergoing a dramatic expansion towards the kinds of large-scale statistical analyses long used by the extragalactic and other communities, thanks in part to an enormous influx of data from space- and ground-based surveys. I will describe the Milky Way and Local Group in the context of general galaxy evolution and highlight some recent developments in Galactic astrophysics that take advantage of these big data sets and analysis techniques. In particular, I will focus on two diverse approaches: one to characterize the distribution and dynamics of the carbon-rich, dusty diffuse ISM, and one to map the resolved bulk stellar properties of the inner disk and bulge. The rapid progress in these areas promises to continue, with the arrival of data sets from missions like SDSS, Gaia, LSST, and WFIRST.
Do star formation laws break in the center of the Galaxy?
Betsy Mills, University of Arizona
I will review our understanding of molecular gas conditions in the central 500 parsecs of the Milky Way, and summarize recent studies that find that the Galactic center deviates from universal star formation relations. It is suggested that the amount of star formation in the Galactic center is less than expected, given the quantity of dense gas in this region. However, in order to conclude that the Galactic center truly breaks these ‘laws’ of star formation, two possibilities must be ruled out: that our indicators in this region could underestimate the amount of star formation, and that prior observations could have overestimated the amount of dense gas. I will analyze new evidence for ongoing star formation in the Galactic center and present new measurements of the gas densities in the Galactic center that show it to be less dense than originally thought. However, I will ultimately argue that the average density of the gas is less relevant to explaining the dearth of star formation than the fraction of gas at each density.
Simulations of the interstellar medium at high redshift: What does [CII] trace?
Dr. Karen Olsen, Arizona State University
We are in an exciting era were simulations on large, cosmological scales meet modeling of the interstellar medium (ISM) on sub-parsec scales. This gives us a way to predict and interpret observations of the ISM, and in particular the star-forming gas, in high-redshift galaxies, useful for ongoing and future ALMA/VLA projects.
In this talk, I will walk you though the current state of simulations targeting the the fine structure line of [CII] at 158 microns, which has now been observed in several z>6 galaxies. [CII] can arise throughout the interstellar medium (ISM), but the brightness of the [CII] line depends strongly on local environment within a galaxy, meaning that the ISM phase dominating the [CII] emission can depend on galaxy type. This complicates the use of [CII] as a tracer of either SFR or ISM mass and calls for detailed modeling following the different ways in which [CII] can be excited.
I will present SÍGAME (Simulator of GAlaxy Millimeter/submillimeter emission) – a novel method for predicting the origin and strength of line emission from galaxies. Our method combines data from cosmological simulations with sub-grid physics that carefully calculates local radiation field strength, pressure, and ionizational/thermal balance. Preliminary results will be shown from recent modeling of [CII] emission from z~6 star-forming galaxies with SÍGAME. We find strong potential for using the total [CII] luminosity to derive the ISM and molecular gas mass of galaxies during the Epoch of Reionization (EoR).