Letters of Intent for 2015

LoI 2015-167
Focus Meeting: The physics of the stellar halos around galaxies

Date: 3 August 2015 to 14 August 2015
Location: Honolulu, United States
Contact: Magda Arnaboldi (marnabol@eso.org)
Coordinating division: Division J Galaxies and Cosmology
Co-Chairs of SOC: Magda Arnaboldi (ESO)
Ortwin Gerhard (MPE)
Chair of LOC: ()



1. Do all galaxies have halos?
2. What is the shape of their density distribution?
3. What is the radial gradient of their density surface brightness profile?
4. Are the stellar halos in dynamical equilibrium? Out to which radius?
5. What is the angular momentum distribution of stars in these stellar halos?
6. What is the frequency of substructures and the relative fraction in stellar mass of substructures vs. smooth population?
7. Stellar tracers of halos in nearby galaxies
8. How do stellar population age and metallicity vary as function of radius? As function of morphological types, i.e. disks vs ellipticals?
9. Do we see halos forming at high z? How do they grow their mass over time?
10. How do the halos form? What fraction forms in situ vs. accretion?



Stellar halos are ubiquitous in luminous galaxies, but because of their faint surface brightness the study of their physical properties has been difficult and confined to the Milky Way only. Since the advent of large cameras and surveys, both from ground and space, our view on stellar halos has changed. We now know that disk galaxies like our own Milky Way and the closest neighbor the Andromeda galaxy have halos that extend out to hundred kiloparsec, while only ten years ago the luminous boundary of galaxies were placed out to (of order) ten kiloparsec only. Extended surveys show that these halos have complex morphologies with a complicated web of tails and substructures. These morphologies resemble the density maps from cosmological simulations of galaxy formation in a hierarchical universe and a large effort is now spent to understand how we go from a qualitative resemblance to quantitative measurements of these substructures. Because dynamical times are of the order of Gyrs at distance of 50 kpc or larger, the stellar halos may contain the fossil records of the events that led to their formation. By combining information on the amount of substructures and the dynamical status of the stellar halos as function of distance from the bright galaxy host with the morphology of halos at different redshift, we can set constraints on the epoch of most likely assembly of these components of galaxies. It is of importance to understand how the halos set in: which part formed together with the bright regions of galaxies (the in-situ stars) and which ones are acquired later (the accreted part), and whether kinematics and stellar population parameters may be able to tell them apart. The proposed focused meeting on the physical properties of stellar halos at the XXIX IAU GA in 2015 is very well timed to bring together theorists and observation astronomers to discuss the results from the space telescopes’ (HST, Spitzer) surveys of stellar halos in disks and ellipticals (S4G*, GHOSTS**, EDGES***), the first results of the GAIA ESO survey of the MW halo, the results from the PANDAS survey of M31 and from the NGVS survey for the ellipticals in the Virgo cluster, which are well underway and whose results can be discussed in this dedicated meeting.

*S4G=> Spitzer Survey of Stellar Structure in Galaxies
**GHOST => Galaxy Halos Outer disks Substructure Thick disks Star clusters – HST survey
***EDGES=> Extended Disk Galaxies Exploration Science – Spitzer survey
****NGVS=> Next Generation Virgo Cluster Survey – CFHT survey