The Galaxy Evolution Science Case for a
Large Ground-based Telescope
Betsy Gillespie
December 4, 2002
Grateful acknowledgements to: Arjun Dey’s “Galaxy Formation and Evolution”
The team for the Canadian XLT science case
In particular: Bev Oke
Bob Abraham
Ray Carlberg
Jean-Pierre Veran
Laurent Jolissaint
Galaxy Evolution
• What are the assembly histories of galaxies?
Where and when did the first stars form?
Where and when did the first galaxies form?
How did they come to be the way they are today?
• We must:
– Test hierarchical CDM, origin of the luminosity
function, the morphology-density relation, the Hubble
sequence, the Milky Way
– Relate distant galaxies to local fossil evidence
The Requirements for
Progress
• The star formation and chemical enrichment histories of galaxies
as a function of time:
– Star formation histories (rest-frame optical at least); want both
for a Sloan-sized sample and as a function of position in the
galaxy
– Chemical enrichment (rest-frame UV and optical); want as a
function of position in the galaxy
• Identifying the intrinsic properties, ultimately the masses, of
galaxies at high redshift
– Mass measures from internal dynamics
– Mass measures from strong lensing
The Requirements for
Progress
• Measuring morphologies and the merger rate as a function of time
(to constrain hierarchical models) to z=6:
– Evolution of different morphological types; identification of most
strongly evolving populations at different redshifts
– Pair counts at higher redshift (coupled with an understanding of
pair selection effects and a theoretical understanding of merger
timescales)
• Detecting the first objects in the universe
Wide-field case: large samples
for redshift/abundance surveys
• Need large samples to break into
sub-types (500,000 galaxies
MINIMUM, nearly size of SDSS)
• Want many categories:
– Mass
– Luminosity
– Redshift
– Environment
– Metallicity
– Morphological type
(Lilly et al. 1995)
Sensitivity is vital for a
survey down the luminosity
function
• Arrows show S/N=3 limits
for 10,000 seconds in 0.3arcsec seeing
• 500,000 galaxies in <100
clear nights requires at least
1700 galaxies per exposure
• >=16-meter requires deeper
imaging than HDF to feed a
large survey (JWST!)
RAB
PHOTOMETRIC REDSHIFT
Major advantages with some correction
over the wide field (in the optical)
• Arrows show S/N=3 limits
for 10,000 seconds in 0.5arcsec seeing
If seeing degrades to 0.5
arcsec, sensitivity worse
by 0.5 magnitudes for unresolved
sources
RAB
Requires 2.4 x longer
exposures
PHOTOMETRIC REDSHIFT
Near-IR case: for chemical abundances,
star formation histories
weak
absorption
Lines in the optical
and near-infrared
[OII] to z = 6
L/M
Ha to z = 3
optical
H
J
K
•Few strong lines in optical
between redshifts of
about 1 to 3
•NEED near-IR
Plot from Oke & Barton (2000)
The importance of the NIR: sensitivity
as a function z in Ha and [OII]
• At z < 1.5, [OII] in optical and
Ha in NIR are comparable even
with no dust; no metallicity
effects in using Ha star formation
rate
• Beyond z=1.5, both lines perform
well in NIR; for z=2-3, Ha is best
Globular cluster forming in 1 dynamical
timescale
Sensitivity to unresolved
emission lines, R=3000,
T=10,000 sec
30m [OII]
30m Ha
NGST Ha
NGST [OII]
Internal kinematics: absorption lines
Most distant galaxies
fall below the contiuum
surface brightness limit
(from NOAO GSMT Book)
Emission lines: “Typical galaxies”
at z=1.5
z=0
z=1.5
8m
20-hour exposure
z=1.5
20m
8-meter telescopes only
detect the center!
z=1.5
30m
Assumed psf for these simulations
Image-quality studies at HIA:
• Chris Morbey -- telescope
• Laurent Jolissaint and JeanPierre Veran --- added
atmosphere and AO
Bulk property like Strehl ratio likely
important, but detailed features of
psf not important…
K-band PSF
Major issue for image
quality: how lumpy is star
formation?
Typical ground-based
Resolution for local galaxies
30-meter telescope
diffraction limits
Diameter
(pc)
Star clusters in Antennae
have <Re> = 4 pc
(Whitmore et al. 1999)
redshift
Higher redshifts: z > 3
• Galaxy morphologies
“more challenging” to
recognize
• Large disks may not be in
place, but relative
velocities of lumps will
provide information
about dynamical state
and/or total mass
(z=3 galaxy from
Hubble Deep Field;
HST psf ~
0.1” ~ 770 pc)
Distant Galaxy Morphology: details
help enable merger rate measurement
Simulation
by Bob
Abraham
CFHT
2-hour exp.
z=0.8
z=0.6
20-meter
2-hour exp.
(“seeing” 2x
Diffraction limit)
z=0.8
z=0.6
Detecting the first objects in
the universe
• At z=6-10, Lya is at 0.85 < l < 1.4
mm: regime where a 30-meter is
much more sensitive than JWST
• JWST NIRcam proposal science
case: parameters of first objects,
with Charlot & Fall (1993) Lya
analysis, gives Lya fluxes up to
~10-18.4 erg s-1 cm2 at z=10
Sensitivity to unresolved emission
lines, R=3000, t=10,000 seconds
Progress with 8-10 meter
telescopes
• Large redshift surveys to z=3-4 (not too far down LF and
most not in NIR)
– Most diagnostics will be rest-frame UV (exception is
VIRMOS)
– Will measure unobscured SFR as a function of redshift
• Kinematics of bright or strongly star-forming galaxies to
perhaps z=1.5 (plus occasional shear of Lyman break
galaxy)
Complementarity to JWST, ALMA
• Spectroscopic follow-up to JWST imaging surveys
• Locally, the huge bursts of star formation are dustenshrounded (e.g., ultraluminous infrared galaxies)
– argues for complementary imaging and spectroscopy at
longer wavelengths
Summary of Requirements
• Sensitivity.
– For spectroscopic surveys of huge samples
– For internal dynamics with no systematic problems
– To detect the “first” objects in the universe
• Near-IR capabilities.
– For accurate chemical abundances directly comparable to what we know at
low redshift
– For Ha star formation rates to z=3
– For [OII](3727) star formation rates beyond z=5.
• Wide field.
– Large survey (~106 galaxies) of luminosity function as a function of galaxy type
• Good image quality.
– For better sensitivity
– If star formation in the universe is lumpy on small scales
– For high-redshift morphologies: the diffraction limit of a 30-meter telescope is
nearly 5 times better than a 6.5-meter JWST
Major unresolved issues and
required work
• What will the image quality be? Over what field and
fraction of the sky? Is some wide-field correction feasible?
• What is the expected distribution of emission-line (and UV)
flux from high-redshift galaxies? Is it lumpy? Is it likely to
remain resolved or unresolved? (Both spatially and in
velocity width?)
– If this question cannot be answered, what is the best
strategy to adopt?
• Kinematic simulations “from scratch” for adjustable
parameters.
• Worth pursing issue of comparison with NGST at 2.5 to 4
microns.
REFERENCE
SLIDES
Scalings: Magnitude Limits
Scalings: Exposure Times
Not just a matter of patience! Many studies require large
samples of objects.
Lookback Time
Lambda cosmology:
~9 Gyr to z=1.5
(only 4 Gyr from
z=1.5 to z=6)
Time
H0=70 km/s/Mpc
W0=0.3
WL=0.7
z
“Average” Galaxies at
Intermediate Redshift: z=1
z=0
z=1
8m
10-hour exposure
z=1
20m
z=1
30m
“Average” Galaxies at
Intermediate Redshift: z=1.5
z=0
z=1.5
8m
10-hour exposure
z=1.5
20m
z=1.5
30m
Longer Exposures
still only
see the
center!
z=0
z=1.5
8m
20-hour exposure
z=1.5
20m
z=1.5
30m