AST 1420 Galactic Structure and Dynamics
M51
Cen A
NGC 1300
M81
NGC 3923
Why study galaxies? • Fascinating cosmic objects! • Great application of fundamental physics: GR: galaxy formation in expanding Universe; Newtonian gravity dominating the evolution of bound galaxies; radiation, hydrodynamics, magnetic fields,… • Our own cosmic genesis: how did the Milky Way that contain our solar system form? Where did the solar system travel over the lifetime of the Sun? • Cosmic laboratories for investigating dark matter
Why study Galactic Structure and Dynamics? • Gravity is the dominant force in galaxies: most of the mass only* feels gravity (stars and dark matter) • Could just run large simulations but: • Running large, gravity-only simulations still very expensive, don’t always lead to a very good understanding of gravitational effects • Additional physics (“baryonic physics”) of star-formation, feedback from stellar winds, supernovae, active galactic nuclei very uncertain and difficult to simulate • Newtonian gravity + dark matter: simple framework to understand complex phenomenology of galaxies • Only well-understood physical systems can lead to big discoveries : e.g., dark matter, dark energy
Golden age of galactic dynamics • Gaia satellite is scanning the sky and making high-precision measurements of stellar positions over five years —> measure stellar distances, motions, and stellar properties for >1 billion stars! • First major data release in April 2018! • Will provide incredibly detailed view of all aspects of galactic dynamics: detailed kinematics in the disk, most precise measurement of structure of dark matter halo of any galaxy, internal kinematics of clusters, star- forming regions, globular clusters, orbits of all satellite galaxies, …
Objectives of this course • To know and understand the basic physical properties of galaxies: constituents of galaxies, their dynamics, and relation to each other • Up-to-date overview of types of tools available for studying galaxy formation and evolution • Hone astrophysical problem solving skills: combination of analytical thinking, numerical approaches, simulations, and data analysis
Course details • Full details on the website: https://github.com/jobovy/AST1420 • Meeting time / room: 11am-1pm Fri, AB 113 • Email: jo.bovy@utoronto.ca • Office hours: drop by / email appointment. Please do drop by if you have any question!
Lecture notes • Linked to from course webpage • New notes will be posted one week ahead of class • Some webpages have lots of content / math-to- typeset; you might want to keep these pages open in different tabs
Additional reading • Essential reference book: Binney & Tremaine, Galactic Dynamics, 2nd Edition , 2008, Princeton University Press • Goes into more detail on some topics than the notes will + advanced material • Must-have for the galactic dynamicist!
Additional reading • Binney & Merrifield, Galactic Astronomy , 1998, Princeton University Press • Will use for galaxy phenomenology and topics related to galaxy evolution / formation • Additional readings indicated on the course website
Code • Lecture notes contain code examples in Python • Assignments will require some coding as well, preferably done in Python (e.g., jupyter notebook) • Necessary environment and a small course- specific code package given on the course webpage
https://stackoverflow.blog/2017/09/06/incredible-growth-python/
Code • Require: • and the latest version of galpy • Code package includes environment.yml and requirements.txt that easily allow you to setup a conda environment for this course that contains everything you need
Marking scheme • Assignments: 3 assignments throughout the semester —-> total 30% • Presentation: Each student gives a short presentation in week 11 (Nov. 24) on topic on “Galactic Structure and Dynamics”; we’ll discuss possible topics later —-> 20% of total • Take-home final + oral —-> 30% • Participation —-> 20%
(preliminary) Schedule
What is the diameter of the Milky Way disk? A. 3 kpc B. 10 kpc C. 30 kpc D. 100 kpc
What is the diameter of the Milky Way disk? A. 3 kpc B. 10 kpc C. 30 kpc D. 100 kpc
How thick is the Milky Way disk? A. 100 pc B. 600 pc C. 2 kpc D. 20 kpc
How thick is the Milky Way disk? A. 100 pc B. 600 pc C. 2 kpc D. 20 kpc
How many stars does the Milky Way contain? A. 10 5 B. 10 7 C. 10 11 D. 10 13
How many stars does the Milky Way contain? A. 10 5 B. 10 7 C. 10 11 D. 10 13
What is the ratio of (dark matter) / (stellar matter) in total in the Milky Way? A. 0.3 B. 1 C. 3 D. 15
What is the ratio of (dark matter) / (stellar matter) in total in the Milky Way? A. 0.3 B. 1 C. 3 D. 15
What is the orbital period of the Sun around the Galactic center? A. 1 Gyr B. 100 Myr C. 50 Myr D. 250 Myr
What is the orbital period of the Sun around the Galactic center? A. 1 Gyr B. 100 Myr C. 50 Myr D. 250 Myr
Overview of a typical galaxy: the Milky Way
NGC 4565 ~ MW
Radial distribution of stars in disks: exponential light distribution Freeman (1970)
Type I: pure exponential Type II: Outer steeper profile Pohlen & Trujillo (2006)
Type II: Outer steeper profile Type III: Outer shallower profile Pohlen & Trujillo (2006)
Does an exponential light profile imply an exponential stellar mass profile?
Color-magnitude diagram of stars • From Gaia DR1 • Most of the light comes from rare, luminous stars (depends on wavelength) • Most of the mass is in abundant, dim stars Bovy (2017)
Local stellar mass distribution Initial mass function Present-day mass function Bovy (2017) Converting a light distribution to a mass distribution requires taking into relation between mass and light for stellar populations —> Mass-to-light ratio M/L
Radial distribution of stars in disks: exponential mass distribution Total mass of the disk: ~5 x 10 10 M sun Bovy & Rix (2013)
Vertical distribution of stars in disks: ~exponential • To first approximation, light drops exponentially when going away from the mid-plane • Slight turn-over at small heights —> sech^2(z) • Profile ~ independent of R —> constant thickness • Typical thickness: 300 pc (old stars)
Bovy (2017)
• Looking in more detail, vertical profile is much more complicated • Thin disk, thick disk, …, eventually halo Juric et al. • Exact structure does (2008) not matter greatly for orbits and dynamical modeling
But there is more to stars in the Milky Way than the disk! Bulge
The bulge • The bulge is the central region of a galaxy, rounder than the disk • Surface-brightness profile well represented by Sersic profile (similar to elliptical galaxies) • Between galaxies, ranges from ~spherical, “classical” bulge to flattened, “pseudo-bulge” / bar • Dominates dynamics within a few kpc from the center • Milky Way bulge (/bar): approx. 10 10 M sun
Putting disk + bulge together
Different components dominate different regions Courteau et al. (2011)
Most galaxies are surrounded by stellar halos • Very important from the point of galaxy formation • Little mass (e.g., MW ~10 9 Msun) • Tracer population where DM dominates Merritt et al. (2016); Dragonfly
Merritt et al. (2016); Dragonfly
and then there’s the dark matter halo
Via Lactea; Diemand et al. (2008)
Mass profile of the Milky Way
Mass profile of the Milky Way
Galaxies also contain gas
Gas in galaxies • Gas is found in various phases in the interstellar medium of galaxies —-> covered in ISM course • For our purposes, important: • Gas ~10% of mass in stars for galaxies like the MW, more in lower-mass galaxies • Mostly distributed in thin layer, ~100 pc thick • Because of thinness, important for local density : local density: half gas / half stars (+sprinkling of DM) • Kinematics of gas plays very important role in galactic dynamics!
(show interactive figure of GMCs)
The Milky Way only represents one type among many different types of galaxies
Next week • General theory of gravitational potentials for smooth mass distributions • Spherical potentials • Orbits in spherical potentials • Please read the notes before class
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