AST 1420 Galactic Structure and Dynamics M51 Cen A NGC 1300 M81 - - PowerPoint PPT Presentation

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AST 1420 Galactic Structure and Dynamics M51 Cen A NGC 1300 M81 - - PowerPoint PPT Presentation

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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

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AST 1420 Galactic Structure and Dynamics

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M51

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Cen A

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NGC 1300

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M81

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NGC 3923

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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
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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

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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

  • f dark matter halo of any galaxy, internal kinematics of clusters, star-

forming regions, globular clusters, orbits of all satellite galaxies, …

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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

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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!

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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

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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!

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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
  • n the course website
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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

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https://stackoverflow.blog/2017/09/06/incredible-growth-python/

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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

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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%
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(preliminary) Schedule

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What is the diameter of the Milky Way disk?

  • A. 3 kpc
  • B. 10 kpc
  • C. 30 kpc
  • D. 100 kpc
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What is the diameter of the Milky Way disk?

  • A. 3 kpc
  • B. 10 kpc
  • C. 30 kpc
  • D. 100 kpc
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How thick is the Milky Way disk?

  • A. 100 pc
  • B. 600 pc
  • C. 2 kpc
  • D. 20 kpc
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How thick is the Milky Way disk?

  • A. 100 pc
  • B. 600 pc
  • C. 2 kpc
  • D. 20 kpc
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How many stars does the Milky Way contain?

  • A. 105
  • B. 107
  • C. 1011
  • D. 1013
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How many stars does the Milky Way contain?

  • A. 105
  • B. 107
  • C. 1011
  • D. 1013
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What is the ratio of (dark matter) / (stellar matter) in total in the Milky Way?

  • A. 0.3
  • B. 1
  • C. 3
  • D. 15
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What is the ratio of (dark matter) / (stellar matter) in total in the Milky Way?

  • A. 0.3
  • B. 1
  • C. 3
  • D. 15
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What is the orbital period of the Sun around the Galactic center?

  • A. 1 Gyr
  • B. 100 Myr
  • C. 50 Myr
  • D. 250 Myr
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What is the orbital period of the Sun around the Galactic center?

  • A. 1 Gyr
  • B. 100 Myr
  • C. 50 Myr
  • D. 250 Myr
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Overview of a typical galaxy: the Milky Way

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NGC 4565 ~ MW

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Radial distribution of stars in disks: exponential light distribution

Freeman (1970)

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Pohlen & Trujillo (2006) Type I: pure exponential Type II: Outer steeper profile

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Pohlen & Trujillo (2006) Type II: Outer steeper profile Type III: Outer shallower profile

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Does an exponential light profile imply an exponential stellar mass profile?

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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)

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Local stellar mass distribution

Bovy (2017)

Initial mass function Present-day mass function 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

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Radial distribution of stars in disks: exponential mass distribution

Bovy & Rix (2013) Total mass of the disk: 
 ~5 x 1010 Msun

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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)

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Bovy (2017)

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Juric et al. (2008)

  • Looking in more detail,

vertical profile is much more complicated

  • Thin disk, thick disk,

…, eventually halo

  • Exact structure does

not matter greatly for

  • rbits and dynamical

modeling

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But there is more to stars in the Milky Way than the disk!

Bulge

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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. 1010 Msun
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Putting disk + bulge together

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Different components dominate different regions

Courteau et al. (2011)

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Most galaxies are surrounded by stellar halos

Merritt et al. (2016); Dragonfly

  • Very important from

the point of galaxy formation

  • Little mass (e.g.,

MW ~109 Msun)

  • Tracer population

where DM dominates

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Merritt et al. (2016); Dragonfly

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and then there’s the dark matter halo

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Via Lactea; Diemand et al. (2008)

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Mass profile of the Milky Way

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Mass profile of the Milky Way

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Galaxies also contain gas

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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!
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(show interactive figure of GMCs)

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The Milky Way only represents one type among many different types of galaxies

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Next week

  • General theory of gravitational potentials for smooth

mass distributions

  • Spherical potentials
  • Orbits in spherical potentials
  • Please read the notes before class