THE SURPRISINGLY COMPLEX LIVES OF
MASSIVE GALAXIES
RACHEL BEZANSON
H.N. RUSSELL FELLOW
PRINCETON UNIVERSITY/UNIVERSITY OF PITTSBURGH
photo credit: Yuri Beletsky (ESO)
Looking deeper at a dark patch of the sky:
2 million seconds of exposure time!
Hubble XDF: 2 million
seconds of exposure time!
Deepest look at
the Universe
Hubble eXtreme Deep Field
Deepest look at
the Universe
Hubble eXtreme Deep Field
Stars
Deepest look at
the Universe
Hubble eXtreme Deep Field
Blue Galaxies
Deepest look at
the Universe
Hubble eXtreme Deep Field
Red Galaxies
Galaxies take BILLIONS of
years to grow
In terms of human lifetime:
a PhD (6 years) studying an individual galaxy is
like seeing less than the first SECOND after a
baby is born and trying to understand his or her
whole life
A Cosmic Time Machine
Light travels at a constant speed
If we see both stars at the same time. . .
light left further star
first
light left closer
star later
A Cosmic Time Machine
More distant objects = Further back in time
Time
Distance/Redshift
adapted from NASA, ESA, and A. Feild (STScI)
Studying Galaxy Evolution
Snapshots of similar galaxies at different epochs
adapted from NASA, ESA, and A. Feild (STScI)
High redshift galaxy surveys:
initial goal was to identify
✤
(Credit: NASA; ESA; G. Illingworth, D. Magee, and P. Oesch, R. Bouwens, and the HUDF09 Team)
initially, distant galaxies named
by the method by which they
were selected
✤
LBGs, LAEs, DRGs, . . .
✤
these methods are still
useful at the largest
distances and faintest limits
High redshift galaxy surveys now can provide a
real census of galaxy populations through
cosmic time (over the last ~12 billion years)
*
* (we still use acronyms . . . but usually
they are descriptive of objects, not
selection criteria)
the study of galaxy evolution aims to:
describe how galaxies develop with time and identify the
physical processes responsible for those changes
MY FOCUS:
EVOLUTION OF THE MOST MASSIVE
GALAXIES IN THE UNIVERSE IN THE LAST
~10 BILLION YEARS
Two Basic Types of Galaxies Today
Elliptical
Spiral
Two Basic Types of Galaxies Today
COLOR
Elliptical
Spiral
RED - cooler, older stars Blue - hotter, younger stars
Two Basic Types of Galaxies Today
SHAPE
Elliptical
rounder shapes
Spiral
disky/flattened shape
Two Basic Types of Galaxies Today
MOTION of STARS
Stars move around on random orbits
Stars orbit in a plane
Elliptical
Spiral
Massive Galaxies Today
Color
Red
Blue
~Milky Way
9.5
10
10.5
11
Stellar Mass
11.5
Massive Galaxies Today
Red, old stars -- not
making new ones!
What did these look like
~10 billion years ago?
Elliptical
How to build an elliptical galaxy?
— monolithic collapse scenario —
. . . passive evolution . . .
eventually stars age
and look red and dead
Stars form in a giant
burst of star formation
at high redshift
— (gas-rich major) merger scenario —
. . . passive evolution . . .
stars form in disk galaxies
disk galaxies merge
ordered rotation is
destroyed - elliptical
morphology
eventually stars age
and look red and dead
WHAT DID MASSIVE GALAXIES LOOK
LIKE ~10 BILLION YEARS AGO?
Now
Then?
WHAT DID MASSIVE GALAXIES LOOK
LIKE ~10 BILLION YEARS AGO?
Younger stars
Now
Then?
RECENT EVIDENCE HAS SHOWN THAT
MASSIVE GALAXIES HAVE GOTTEN BIGGER IN
THE PAST 10 BILLION YEARS!
Now
Then!
RECENT EVIDENCE HAS SHOWN THAT
MASSIVE GALAXIES HAVE GOTTEN BIGGER IN
THE PAST 10 BILLION YEARS!
L6
VAN DOKKUM ET AL.
re~11 kpc
re~0.8 kpc
van Dokkum et al., 2008
Tal et al., 2009 (submitted)
Now
Then!
Essentially all easily observable
properties of massive galaxies have
changed in the last 10 billion years
Size
Stellar Populations (Color)
Morphologies (Shapes)
Massive galaxies have grown in size
LARGE
Size [kpc]
FORMING STARS
small
+
+
+
+ + +
++ +
+++
+
++
+ +
++
+
Mass in Stars
less massive
+ RED AND DEAD
Szomoru et al. (2012)
more massive
Size
Stellar Populations (Color)
Morphologies (Shapes)
Massive galaxies where not just
“red & dead” which implies the importance of quenching
0.2 < z < 0.5
1.5 < z < 2.0
•
TODAY: >90% OF MASSIVE GALAXIES ARE RED AND DEAD
•
10 BILLION YEARS AGO: HALF WERE STILL FORMING STARS!
QUENCHING = TURNING OFF STAR-FORMATION
Size
Stellar Populations (Color)
Morphologies (Shapes)
Could the apparent size evolution be
driven by galaxy “quenching”?
?
?
Is size evolution just a
“Progenitor Bias”?
Could the apparent size evolution be
driven by galaxy “quenching”?
?
?
Is size evolution just a
“Progenitor Bias”?
youngest galaxies
are the smallest!
HOW TO MAKE A GALAXY GROW
?
(1) Make stars
(2) Add stars
HOW TO MAKE A GALAXY GROW
(1) Make stars
(2) Add stars
Imaging of early massive galaxies shows
that they were more disk-like in shape
van der Wel et al. (2011)
Size
Stellar Populations (Color)
Morphologies (Shapes)
M ASSIVE G ALAXY C ANNIBALS
Simulation run by Thorsten Naab
THE INSIDES OF NEARBY ELLIPTICAL
GALAXIES LOOK LIKE ENTIRE GALAXIES 10
BILLION YEARS AGO!
Density of Stars in the centers of galaxies are the same
Central
Core
forms
first
Outer
Envelope
grows with
time
Now
Then
M ASSIVE G ALAXY C ANNIBALS ???
M ASSIVE G ALAXY C ANNIBALS
Some extreme examples
have been found!
M ASSIVE G ALAXY C ANNIBALS
M ASSIVE G ALAXY C ANNIBALS
~70% of nearby ellipticals show signs of
interacting with other galaxies
Remaining Question:
How were compact red and dead galaxies formed?
?
•
How are Galaxies “Quenched”?
•
truncation of star-formation
•
morphological transformation - just shrinking or also
destroying rotation?
My Time as an Astronomer
1%
at telescope
collecting data
trying to make sense of data
99%
Sometimes you travel to the top of a
mountain and use a telescope
Sometimes you DON’T end up using the telescope
Chile & Arizona
Space
Imaging of small
samples of galaxies
without distortions
from the Earth’s
atmosphere
Surveys to identify
large numbers of galaxies
Chile & Hawaii
Detailed studies of
small numbers of
galaxies with
LARGE telescopes
VERY LARGE TELESCOPES (VLT)
ON PARANAL IN CHILE
One of the premier observatories in the world!
8.2 meter mirrors
VERY LARGE TELESCOPES (VLT)
ON PARANAL IN CHILE
image credit: ESO
VERY LARGE TELESCOPES (VLT)
ON PARANAL IN CHILE
LEGA-C Survey: Large Early Galaxy Astrophysics Census
128 night spectroscopic survey of massive galaxies at a
lookback time of half the age of the Universe
Spectroscopy of Galaxies
Intensity
Grating, Grism, or
Prism splits light
Wavelength
telescope
Spectroscopy of Galaxies
Shape (Color):
Age of Stars
Intensity
(Flux)
Wavelength
Spectroscopy of Galaxies
Emission lines: light
emitted by ionized gas
Shape (Color):
Age of Stars
Absorption lines: light
absorbed by atoms/molecules mostly in stellar atmospheres
Intensity
(Flux)
Wavelength
Spectroscopy of Distant Galaxies has been limited
Nearby galaxies
Kauffmann+03
Typical
“best-case”
spectrum
of
a
Typical
Typical
spectrum
spectrum
from
from
redshift
redshift
surveys
surveys
distant galaxy
Spectra of Distant Galaxies with the
LEGA-C survey look like nearby Galaxies!
LEGA-C spectra
Nearby galaxies
Kauffmann+03
Typical “best-case”
Typical
Typical
spectrum
spectrum
from
from
redshift
redshift
surveys
surveys
spectrum of a distant galaxy
Observing Room
Observing Room
Telescope Operator
Support Astronomer
(runs the instrument)
Visiting Astronomer(s)
How much ordered motion (Rotation) is in
early non-star-forming galaxies?
?
difference in wavelength =
doppler shift due to rotation!
Intensity
(Flux)
Wavelength
More rotation in early non-star-forming galaxies!
Nearby Galaxies
Distant Galaxies
Rotation/
Random
Motion
Depth of Gravitational Potential Well
Bezanson+in prep
More rotation in early non-star-forming galaxies!
Nearby Galaxies
Rotation/
Random
Motion
Distant Galaxies
To Be Continued!!
Depth of Gravitational Potential Well
Bezanson+in prep
CliffsNotes:
10 billion years ago
Today
CliffsNotes:
10 billion years ago
Today
CliffsNotes:
~12 billion years ago
10 billion years ago
?
?
Today
CliffsNotes:
~12 billion years ago
10 billion years ago
?
?
Today
1. All easily observable properties of massive galaxies have changed in the
last 10 billion years!
2. Stellar dynamics may be the most stable (and fundamental) property
through time - CHOMP and LEGA-C will provide insights into dynamics
since z~1
3. Resolved and unresolved spectroscopy (dynamics and stellar
populations) will help distinguish amongst formation mechanisms of
massive galaxies