Galactic Astronomy - PKU

Galactic Astronomy

星系天⽂学

Course Information:

• Professor Eric Peng (彭逸西)

• Teaching Assistant: Mr. Kaixiang Wang 王凯翔 [email protected]

• Scheduled meeting time, Tuesdays, 08:00-11:00, SUBJECT TO CHANGE

• Course web site: http://github.com/ewpeng/PKUGalaxies20

Galactic Astronomy (星系天⽂学). E. Peng

mailto:[email protected]

http://vega.bac.pku.edu.cn/~peng/teaching/galaxies/index.html

https://github.com/ewpeng/PKUGalaxies20

Galactic Astronomy

星系天⽂学Course Information: • Professor Eric Peng (彭逸西)• Aim: To develop the knowledge and skills you need to understand and conduct current research in the field of galaxy formation and evolution.• I will teach this class in English.• Your work for this class will also be done in English.

Why English? • English is the international language of science: papers and talks are all in English.• If your English is better, then everything you do will be easier.• You will be a better scientist.• 你们的英⽂⽐我的中⽂好很多！• If you do not understand me, or if I talk too fast, please tell me!


Galactic Astronomy

星系天⽂学

Course Philosophy • Science is not about solving questions with known answers. Science is also rarely done alone.

• Classwork will consist mainly of short assignments, presentations, and projects.

• Project and presentation work will be done in groups. This will be a collaboration, and everyone is expected to contribute.


• Grading will be:★ Class Participation: 20%★ Debate: 30%★ Projects and Assignments: 30%★ Final Exam: 20%

Galactic Astronomy

星系天⽂学

• Course information (schedule, assignments, grading, etc) can be found on course web site: https://github.com/ewpeng/PKUGalaxies20

Coursework • Assignments will be homework tasks and reading.• Presentations: Pairs of groups will debate each other on current controversial topics in galactic astronomy. Classmates will judge winners.• Participation: Ask questions in class, ask me, ask your TA, ask your classmates. Use the Slack channel.



Who am I?

• Born and raised in New York City, parents from Taiwan• Princeton, Johns Hopkins, DAO (Canada), STScI• Joined the astronomy faculty at PKU in 2008 as Bairen Research Professor (百⼈计划研究员).

• Currently a visiting scholar at Harvard-Smithsonian Center for Astrophysics during COVID-19

Eric Peng 彭逸西

Research Interests•Galaxy Formation and Evolution

•Galactic Structure, Dynamics, Stellar Populations

•Observational: Hubble, CFHT, VLT, Gemini, Keck

•Email: [email protected]

•Office: KIAA, Rm 212 (normally)

mailto:[email protected]


Who are you? New Graduate Student

• This is your education. This is your profession.

• YOU DO NOT HAVE TO BE HERE, so get what you came for!

• What kinds of problems interest you? How do you like to solve them?

• Who might be a good match, as an advisor?

• Learning a lot about whatever interests me. Don’t be narrow-minded.

• Participate! Don’t be afraid.

• Work with data and get dirty. Only way to learn how to design good experiments.

• Work with good, new data: HST, large telescopes, large surveys

• Learn to use spectroscopy and large databases! Prepare for LSST, CSST, JWST, ELTs.

• Be international. Be comfortable with English. (I don’t care if you make mistakes).

What should you (the new student) be looking for?

What would I be doing if I were you?

What is a Galaxy?


• Stars• Gas• Dust• Dark Matter• Black Holes• Active Nuclei

M31

Foreground stars

Not a galaxy!Why?

What is a Galaxy?


• Stars• Gas• Dust• Dark Matter• Black Holes• Active Nuclei

M31Irwin et al 2005

Why Study Galaxies?


Initial Conditions: Density fluctuations 300,000 years after the Big BangSeen in the Cosmic Microwave Background (WMAP)

The formation and evolution of galaxies is one of the most interesting and important problems in astrophysics

Why Study Galaxies?


Initial Conditions: Density fluctuations 300,000 years after the Big BangSeen in the Cosmic Microwave Background (WMAP)


Today

?

Why Study Galaxies?


Big Bang + 300,000 years


Dark Matter

Millennium fly-through

Why Study Galaxies?


Big Bang + 300,000 years


Today

?

Dark MatterYoung (high redshift) Universe

?

?

Fundamental Questions!• What is dark matter?

• How do galaxies (and stars) first form from gravitational collapse?

• How do galaxies evolve into what we see today?

How Do We Study Galaxies?


The Milky Way (Our Galaxy)

Advantages:• The closest, most detailed look at a galaxy• Can see individual, faint stars, measure their distances, measure their 3-dimensional space motions• Our only truly 3-dimensional view of a galaxy

Disadvantages:• We are in the Galactic plane, where there is a lot of dust. Cannot see entire galaxy.• It is only one galaxy.• We are inside! How do you know what your home looks like if you cannot leave?



Nearby Galaxies

Advantages:• Larger variety of galaxy types• Can still study in detail (stars, star clusters, gas)• Can see entire galaxies from the outside

Disadvantages:• Can only resolve bright stars. Rely on integrated properties.• Cannot see galaxies in 3-dimensions (depth, velocities)• We can only study galaxies as they are today.

Spirals dwarf Irregulars

dwarf Ellipticals

Giant Ellipticals



Large Surveys of (slightly) more Distant Galaxies

Advantages:• Power of large numbers - can study galaxy properties as a function of mass, environment, star formation history

Disadvantages:• Can only measure rough integrated properties.

Sloan Digital Sky Survey (SDSS)



Very Distant Galaxies

Advantages:• At large distances (high redshift), we look back in time. Can see galaxies as they were in the distant past.• Can track the evolution of galaxy populations back to when the Universe was only a few hundred million years old.

Disadvantages:• Very faint. Need space telescopes and 8-10m ground based telescopes to study.• Can only get very basic properties of galaxies.

Hubble Deep Field (HDF)

Hubble Ultra Deep Field (UDF)



Simulations

Advantages:• We can use our knowledge of physics!• Can run “experiments” in the computer.

Disadvantages:• Maybe we don’t have all the right physics?• Do not necessarily provide unique solutions.

Merger Simulation (Summers, Mihos, Hernquist)

What Tools Do You Need?


SAO/NASA Astrophysics Data System (ADS)(http://ui.adsabs.harvard.edu/)

Tips: • Check references and citations to find related papers.• ^Wang, J = first author is Wang, J• Make yourself an account and use private libraries• We will have a library for our class

http://labs.adsabs.harvard.edu/adsabs/



arXiv.org preprint server (“astro-ph”)(https://arxiv.org/archive/astro-ph)

Papers that are not yet published, but also a free archive.

https://arxiv.org/archive/astro-ph



NASA/IPAC Extragalactic Database (http://nedwww.ipac.caltech.edu/)

Compiles published information on extragalactic objects

http://nedwww.ipac.caltech.edu



SIMBAD databaseCompiles published

information on both Galactic and extragalactic objects

The Language of Astrophysics


Like most sciences, astrophysics has many specialized concepts. These are not difficult, but you need to learn them.

Spectral Energy Distribution (SED). The energy emitted by an astrophysical object as a function of wavelength (or frequency). [erg s-1 cm-2 nm-1]

1 Angstrom = 10-10 m



Photometry: measurement of the brightnesses of objects (stars, galaxies, etc), usually over a specific wavelength range.

In optical astronomy, we use a logarithmic measure of flux, the magnitude:

• Why? The Greeks measured star brightnesses on a scale from 1 to 6, with 1 = brightest, 6 = faintest that the eye could see. • Pogson determined that this was roughly logarithmic with a factor of 100 over 5 magnitudes (constant = -2.5).• Useful for describing the large range of luminosities for astronomical objects.• The magnitude is always measured relative to a standard source flux.• This standard traditionally was the star Vega, which is defined to have magnitude = 0 in all bandpasses. Now, AB mags (Oke & Gunn) uses a flat spectrum.• e.g., V = 10 mag, means that the V magnitude of an object is 10-4 times fainter than Vega.



Photometry is often done using filters. There are many filter “systems” that are optimized for different astrophysical problems.

Johnson-Morgan-Cousins (UBVRI JHK LM)Still one of the most common systems, and the standard system for half a century. “V” is closest to visual and is a green-ish filter.

Sloan Digital Sky Survey (ugriz)Designed for full coverage of the optical spectrum, and is now a standard system due to the success of the SDSS.

Colors• What is the meaning of a color?• Astronomical colors are flux ratios across different parts of the spectrum• Ratios in flux are differences in magnitudes.

• a large color means that the SED is “red”, small means “blue”• e.g., g-z = 1.0 mag, means that object is 2.5 times brighter in z than in g.

telescope/instrument/filter response

SED



The apparent brightness of an object depends on its distance. How do we measure distance?

Trigonometric Parallax: The fundamental way to measure astronomical distances.The positions of stars shift with respect to the background as the Earth orbits the Sun

• Angles on the sky are measured in degrees, arcminutes (deg/60), and arcseconds (deg/3600). Also in radians. [Abbreviation: deg, arcmin, arcsec, rad].• A parsec (pc) is the distance at would have a parallax of 1” (one arcsec).• 1 pc = 206265 AU = 3.086 x 1018 cm• Distances between stars, sizes of star clusters ~ a few pc• Sizes of galaxies ~ 1-100 kpc• Distances between galaxies, sizes of galaxy clusters ~ Mpc

http://www.astro.umd.edu/resources/introastro/parallax.html

http://www.astro.umd.edu/resources/introastro/parallax.html



The apparent brightness of an object depends on its distance, but intrinsic brightness (luminosity) does not.

Absolute Magnitude• A measure of the intrinsic luminosity of an object.• The magnitude (M) an object would have if moved to a standard distance (D).• For an apparent flux f at distance d:

(m-M) is called the distance modulus• The absolute magnitude in a given band, for example V, is MV.• MV of the Sun is +4.83 mag.

The difference between the apparent and absolute magnitude is:

d in pc

d in MpcD=10 pc



The brightness per unit area of an extended object is the surface brightness.

Surface Brightness • To a first approximation, the surface brightness of an object is constant with distance. Why?• Surface brightness is often described using units of [mag arcsec-2].• 21 mag arcsec-2 means a 1”x1” square contains same brightness as 21 mag star.• These can be very confusing units for adding or subtracting surface brightnesses.



Spectroscopy

• Measuring the flux as a function of wavelength at higher spectral resolution than photometry.• Measured using a spectrograph.

Radial Velocity (vr) and Redshift (z)• Objects that are moving relative to us along the line of sight have the spectrum Doppler shifted to the blue (approaching) or red (receding).• For low velocities:



Spectroscopy

Spectroscopy can also give us a lot of information about the chemical composition and temperature of an object (metallicity, age).

Metallicity: describes the abundance of elements heavier than H and He. Often represented by iron (Fe).

[Fe/H] = 0.0, solar metallicity[Fe/H] = -1.0, 1/10 solar[Fe/H] = -2.3, most metal-poor globular cluster



Astrometry• Measuring the positions of stars and other astronomical objects.• Important for parallax.• Stars in the galaxy move, and we can see them move. Proper motion.

Galactic Astronomy - PKU

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