Oct 30, 2017

• More on tidal interactions • Large galaxy surveys and some surprising things they have revealed • Next: Evidence for dark matter

• HW#7 is due next Wed. Part of it relates to your final project;

the first two parts require short answers based on class notes, the textbook and PE#16 and should be easy! Final project: Finding good references can be tricky.

• Discuss with Dominik or me • Use “ADS Advanced” search not straight Google etc.

http://adsabs.harvard.edu/abstract_service.html

• Next week: 2nd 30-min test; similar to first one.

Encounter geometry

Barnes, 1988, ApJ 331, 699

• Mass ratio ~1 • Disks shown in their original position. • Each disk inclined 60o with respect to orbital plane

N-body simulations

1. Adopt a model for each galaxy as a system of N “mass particles”. - N is less than the number of stars/gas clouds in the galaxy but

still can be big (millions). - The bigger N the better the simulation

2. Adopt a model for the rotation (spin) and orientation of the disk(s). 3. Adopt a model for the orbital motion of the two galaxies. 4. Compute the gravitational force on each mass particle due to every

other particle; 5. Figure out how the particle will move in response to that force and

move the particle there. 6. Then recompute the gravitational force (4) and the new position(5) 7. Keep repeating, to track what happens!

• Orientation and direction of disk spin

• Orientation and direction of orbit

M81/M82 encounter

M81/M82 encounter

https://www.astro.umass.edu/~myun/movie.gif

How likely are encounters? • Slow encounters are unlikely in dense clusters.

• Simulated passages are unlikely to be hyperbolic.

• Tails and bridges are the least observed in dense clusters.

• Close encounters unlikely in loose groups.

• Most tidal effects must have been created by galaxies that are gravitationally bound.

• Disruption damage

• Tidal force F ∝ rp-3 where rp is the separation at perigalacticon

• Duration T ∝ rp/v where v is the relative velocity

• In the impulse approximation (I = FT) , the “damage” is ~ 1/(rp2 v)

⇒ slow close encounters do the most “damage”

* Impulse approx: assume one of the forces exerted on a particle acts on for short time but is greater than any other force

http://www.mpia.de/GEMS/gems.htm

• During the course of their lifetimes, galaxies interact with their neighbors. • Such interactions

can alter the morphological appearance of the galaxy

• Notice the different

orientations of the disks at the start.

• Notice how material falls back onto the merger.

The appearance of a galaxy tells a story!

Transformation from spiral to elliptical!

Animation by Volker Springel

• Start with a spiral galaxy

• A similar galaxy, whose disk is more inclined, passes in its orbit close to the first one.

• Notice the tails and the bridge.

• Eventually, the galaxies merge.

Morphological Segregation in the Local Group

Diagram from Grebel 1999

Giant spirals dSph (+dEll) dIrr dIrr/dSph

• Galaxies mainly clustered around the two principal galaxies MW & M31

• Morphological segregation evident

• dE/dSph near large galaxies

• dI at larger distances

Major vs minor interactions

• In addition to the geometry of an encounter, the ratio of the masses of the two galaxies is critical.

• Major: When the mass ratio of the two galaxies is nearly the same

• Minor: When the

“perturber” is much less massive than the “victim”.

Evidence for interactions in the Local Group • Magellanic Stream: The Magellanic Clouds are

embedded in a common envelope of cool hydrogen gas which extends over 180° across the sky

• The most likely explanation is that this gas stream has been pulled out of the smaller galaxies as they passed close to the Milky Way.

Evidence for interactions in the Local Group • Sagittarius Stream: An overdensity of stars at a

common distance can be traced over a very long distance, wrapping around the Milky Way.

Tidal disruption of a small elliptical

David Law: http://www.stsci.edu/~dlaw/research.html#lgroup

The Cartwheel • A member of a of a

group of galaxies

• Distance = 425 Million light years

The Cartwheel • A member

of a of a group of galaxies

• Distance = 425 Million light years

Galaxy surveys: goals & strategies

Discussion for PE #18 Consider what is needed to undertake a survey that is likely to lead to the discovery of: • Thousands of SNe spanning a wide range in

redshift • Discovery of low mass, star forming galaxies at

“cosmic noon” (1 < z < 3) • Discovery of the most distant (very early) galaxies

Galaxy surveys: goals & strategies

Galaxy surveys: goals & strategies

Discussion for PE #18 Consider what is needed to undertake a survey that is likely to lead to the discovery of: • Thousands of SNe spanning a wide range in

redshift • Discovery of low mass, star forming galaxies at

“cosmic noon” (1 < z < 3) • Discovery of the most distant (very early) galaxies

Sloan Digital Sky Survey Original Legacy Survey • Covered large area with multi-color imaging • Spectroscopy of 1/100 galaxies to limiting magnitude

Sloan Digital Sky Survey (II-IV) Added capabilities:

• Enhanced algorithms to handle crowded fields • Infrared spectrometer added (1.5-1.7 µm) for APOGEE • Integral field unit spectrometer

Dark Energy Survey https://www.darkenergysurvey.org/

https://www.darkenergysurvey.org/decam/

Jochen Weller, Recontres de Moriond 2016

Dark Energy Survey

Jochen Weller, Recontres de Moriond 2016

Dark Energy Survey

Jochen Weller, Recontres de Moriond 2016

Dark Energy Survey

Spectroscopic following planned after construction of spectroscopic instrument on 4-m Mayall Telescope at KPNO

Dark Energy Survey

Jochen Weller, Recontres de Moriond 2016

“COSMOS Field”

What decisions go into designing a survey?

Deep Field Surveys

HST Frontier Fields https://apod.nasa.gov/apod/ap170506.html

Abell 370 z=0.375 Use gravitational lensing to probe distant universe.

Discovering dwarf MW companions

Dark Energy Survey

Jochen Weller, Recontres de Moriond 2016

https://astronomynow.com/2015/03/11/new-dwarf-galaxies-discovered-in-orbit-around-the-milky-way/

Dark Energy Survey

Jochen Weller, Recontres de Moriond 2016

http://news.fnal.gov/wp-content/uploads/2015/10/DES-Dwarf-Galaxies-hires.jpg