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- Observing at higher redshift allows us to look back in time.
- Relation of look-back time to redshift
at redshift z=1, lookback time is between 4.5 and 14.5 Gyr dependending
on h and (note Ned Wright's cosmology calculator).
- Only bright objects can be seen at higher redshift. For LCDM
- z=0.5, m-M=42.3
- z=1.0, m-M=44.1
- z=2.0, m-M=46.0
- z=3.0, m-M=47.1
- look to see if implied star formation history from local
galaxies is observed statistically in galaxies at earlier epoch.
- Also look for evolution of galaxy density: are there some types
of galaxies which were present in past but not now or vice versa?
- Since it's difficult to study individual galaxies at a variety of
and in particular to know which galaxies at one redshift to associate
with which galaxies at another redshift, people often study statistical
properties of population; one main tool has been the galaxy luminosity
- can in principle study more than just distribution
of luminosities, e.g., one can study LFs of different morphological types
and relative numbers of different morphological types, or using spectral
- since we've seen that galaxies are not just a
one-parameter family (e.g. in luminosity), but a multidimensional one
(e.g. in luminosity, size/SB, and/or velocity), one would like to study
galaxy evolution in multiple parameters. Note, however, that studying
size-related quantities requires high spatial resolution because typical
images sizes of galaxies are a few arcsec at z=0.5-1, with scale lengths
less than an arcsec (e.g. HST), and studying velocity related quantities
requires large telescopes (and possibly also high spatial resolution).
- Recall also the potential importance of selection effects. Even just a
simple flux limit on a survey can create the appearance of strong evolution;
at large distances, only intrinsically most luminous objects will be
seen, and it is even possible that these objects will be preferentially
missed in nearby universe if they are intrinsically rare compared
with less luminous objects.
- selection effects (e.g. surface brightness,
see summary LF figure from Cross et al. 2001),
- large scale structure / cosmic variance
- K-corrections (large especially if rest frame is shortward of ,
- cosmological model (from angular size-redshift relation for volume size
and distance-redshift relation for distance modulus),
- method of LF determination.
- Observations often characterized with respect to some simple models:
- no evolution (local LF with k-correction only),
- passive evolution (local LF with k-correction, evolution back in
time of currently observed stars, no star formation),
- luminosity evolution (SF occurs),
- density evolution (objects appear/disappear).
- Historical: redshifts not available, so measurements were made of
galaxy counts as a function of magnitude/color.
- Euclidean expectation is
, less steep given
- Large numbers of faint galaxies are in fact observed
(Koo and Kron Fig 1), and
these are found to be blue
(K and K Fig 2)
- the so-called faint
- Question: what are these galaxies? Are they local,
low-luminosity galaxies or more luminous distant
galaxies? If the latter, to what current type (if any)
are they related, and what do they tell us about evolution
in the corresponding type of galaxies?
- Significant recent progress with completion of several redshift surveys
for field galaxies to intermediate redshift:
- CFRS (I-selected with , 600 galaxies,
ApJ 455, 108 and refs therein),
- Autofib (B-selected, combination of several different studies over wide
range of , 1700 galaxies, MNRAS 280, 235),
- CNOC1, CNOC2 (r band selection, 400 galaxies, ApJ ),
- Hawaii K-band ( K-selected with additional galaxies included based
on I and B brightness, 400 objects, AJ 112, 839),
- DEEP and DEEP2, 40,000 galaxies with
- COMBO-17, 30,000 galaxies with , 17-band photometric redshifts
- Larger local surveys:
- Summary table (somewhat out-of-date!)
- Redshift survey results
- CFRS 1995.
Luminosity function evolves, with
different evolution when split by color. Little evolution in red galaxies, more in blue
- Similar split in behavior seen if separation made by spectral type (Autofib)
- Similar conclusions reached with
- Galaxies with different color/type (corresponding to morphology,
but only roughly) have different LFs
(CNOC2 Schecter parameters),
also show different degrees of evolution
(CNOC2 LF evolution)..
- Early and intermediate types show mostly luminosity evolution,
late types mostly density evolution
(CNOC2 evolution of parameters).
- Results seem to be fairly robust
(CNOC2 sensitivity to errors).
- Note the evolution refers to LF changes only, and not necessarily to
physical changes in galaxy population (though this may also be the case).
- CNOC2 shows no evidence for faint-end slope changes, but other studies
(e.g. AUTOFIB) do. Results are comparable between CNOC and
- K-band survey from Cowie et al
suggests significant evolution in star forming, lower luminosity, galaxies,
but mostly passive evolution in more massive galaxies, e.g.
result for blue galaxies
((Cowie etal Fig 24)).
Original suggestion of ``downsizing''.
- However, more recent results with significantly larger number of galaxies
and deeper (COMBO-17 and
suggest the picture may be different.
- These studies use bimodality, rather than (K-corrected) color splits
to divide galaxy. They also extend to higher redshift and have larger
- Luminosity function shows
including red galaxies
- All samples show luminosity evolution, but red galaxies now
show number density evolution. Schecter
- Conclusion supported by lack of
evolution of luminosity density
of red galaxies (potentially measured more robustly);
since older galaxies should have higher M/L, implies an increase
in number density
- red galaxies:
- Density evolution for red galaxies is somewhat debated, with some studies
suggesting little or no density evolution, others some. Clearly, this
is important for understanding the degree to which early type galaxies
are formed hierarchically (and if so, when).
- red galaxies cannot have recent star formation. If numbers have evolved,
they must have come from mergers of a visible population, e.g. migration
from the blue sequence to the red sequence
- Migration could occur in multiple ways
- Entirely early merging unlikely: not enough faint red galaxies at higher redshift.
Also, can't get stellar population variations along red sequence as
- Entirely late merging unlikely: can't get variation of structure along
red sequence as previously discussed
- Multiple paths to red sequence suggested? Downsizing of typical ``quenching'' timescale?
- Still some question about Mg- relation. Issues with IMF? (which would
open all sorts of cans of worms!)
- blue galaxies
- Blue galaxies may substantial evolution, either in luminosity or number
(or a combination). K-band results suggest evolution in luminosity,
since the intrinsically brightest galaxies appear only at higher redshift
and there's a smooth trend of brightness with redshift. However, it's
possible that there is some additonal population of bright blue galaxies
at higher redshift which has essentially disappeared by the present.
- Theories for origin of blue galaxies:
- luminosity evolution: number of galaxies same as present with evolving
(declining) SF rate
- bursting dwarfs: some galaxies have intense bursts of SF, then fade rapidly and become
invisible; mechanism suggested by Babul and Rees in which ioniziation by
background light (e.g., from quasars) keeps small clouds ionized until
then rapid burst in SF makes bright objects, but gas is ejected
and SF ceases
- some combination
- Emission line measurements strongly suggest that luminosity evolution
is ocurring, since higher redshift galaxies have much greater fraction
of [OII] emitters
- also evidence, however, that metallicity of higher z galaxies
is lower, and less dust could partly explain strong
evolution of emission line properties.
When samples are taken and imaged at high spatial resolution, one can
look to see what the galaxies actually look like; in particular, one
would like to assess the question as to whether the faint blue galaxies
are progenitors of current galaxies or a special population. Several
- normal Hubble types are recognizable (
Schade et al ApJL, 451, 1 (1996)
Fig 1 (F814W)
Fig 2 (F450W)).
- with good spatial resoution, one can measure the surface
brightness of disk galaxies, and one finds that the SB is higher for
disk galaxies at
(Simard etal fig 5);
this suggests that part of the LF evolution is due to luminosity
evolution of disk galaxies, in which they were brighter in the past;
size evolution is also a possibility. Interpreting this in terms of
luminosity evolution implies higher SF rates in the past.
note that some or all of this may be a selection effect, since higher
SB galaxies preferentially make it into a sample which is defined from
low-res observations with a magnitude cutoff
(Simard et al fig 6); cannot
separate hypothesis of systematic brightening of SB with increasing
redshift from increased spread in SB at larger redshift.
- there is a population which does not appear to have a modern counterpart:
the blue nucleated galaxies
(Schade Fig 4).
These comprise 15-30% of the high redshift sample, and are preferentially
located in objects with peculiar morphology. In general, the frequency
of peculiar morphology is higher at higher redshift. This is also supported
by the HDF observations, to be discussed later. Note some degree of
this might come from observing at shorter wavelengths, but NICMOS
observations suggest that this is not the dominant portion of the story.
- There may also be a population of high SB galaxies that does not exist
at low redshift
(Simard et al Fig 12);
these may be a new population, early-type spirals exhibiting passive
evolution, or just represent galaxies missing from the local sample,
or very possibly, a combination of all of these.
- Can also consider estimates of stellar mass, e.g. by
looking at near-IR light which is less sensitive to details of the
star formation history (Ellis Fig 13).)
- results suggests that stellar mass in large systems is roughly
constant out to z=1, but strong evolving in interacting/peculiar systems
(Ellis Fig 15), suggesting
number evolution as important, because, while stellar mass density
can grow through star formation, it cannot decline.
Interesting to compare evolution of cluster galaxies, because these
may be subject to an additional evolutionary effect, namely one induced
by the cluster environment. From ground, it was noted that some high-z
clusters () have abnormally large fraction of blue galaxies,
the Butcher-Oemler effect. HST helps in that one can get morphologies. One
- most blue galaxies are ``normal'' spirals, though there is a
subset of abnormal-looking galaxies which comprises a larger fraction of
cluster galaxies than at present,
- ellipticals are nearly as abundant in high z clusters as a present,
and increase of spirals appears to come at the expense of S0 galaxies
(Dressler & Smail Fig 2,
- ellipticals show quite uniform colors, suggesting an early epoch of
formation, and SB evolution consistent with passive evolution; fundamental
plane correlations also support this point of view. In many respects,
evolution in clusters is similar to that in the field, except spirals
may be becoming transformed into S0 systems. Note that this requires
a physical distinction between S0 and E galaxies - not necessarily as
indicated by local studies (as discussed previously).
- Increasing blue fraction may just result from hierarchical clustering,
in that groups and clusters form relatively late, so as one goes back in
redshift, they've had less time to inhibit star formation (e.g. via
processes discussed for local clusters).
- While the blue fraction increases with redshift, it is still smaller
than the field blue fraction, at least to a .
Next: High redshift galaxies
Up: AY616 class notes
Previous: Clusters and cluster galaxies