Update of Individual Sections
Modern Cosmological Observations
The Distance to M31:
Stanek and Garnavich
have proposed using "red clump" stars as distance indicators.
The red clump is a grouping of stars in the HR diagram at the
"red" end of the horizontal branch. Such stars are numerous in our
galaxy and hence calibration could come from both parallax as well
as globular clusters. The Hipparchos sample shows a variation
in the mean I-band magnitude of these stars of 0.10 -- 0.15 mag.
The stars are sufficiently bright that HST can detect them as individual
stars in clusters in M31. Preliminary results give a distance modulus
to M31 of (m-M) = 24.47, about 0.15 mag larger than the mean distance
found from several other methods detailed in the book.
Prior to the initiation of the CFA redshift survey, there was
already a compelling data set that was able to provide a cursory
outline of the Stickman and Great Wall structures. It was a serious
oversight on my part, not to mention poor scholarship, for not
initiating the discussion in 3.5.2 with the
Gregory and Thompson (1978) paper which presented the first
wide angle redshift survey, done in a SLICE style, of the A1367-
Coma region. Their Figure 2, reproduced below,
clearly shows the presence of a void (later known as the CFA
bubble) as well as a portion of the Great Wall. Thus, prior to
the initiation of the CFA redshift survey, there already was a strong
hint of the basic structure which would become more well defined
with more redshifts. However, the work of Gregory and Thompson
did show the power of sparse sampling coupled with clever survey
design in revealing the qualitative features of Large Scale Structure
in a hierarchically clustered Universe. In retrospect, historically
this is an important paper and it was a serious omission on my
part not to have included it in the published book.
The 2MASS project has produced its first important
result. An initial analysis of 421 square degrees has detected
20 candidates for stars that are cooler than the coolest hydrogen
burning stars on the main sequence. Subsequent spectroscopy with
the Keck 10-m shows the candidates to all possess the Lithium doublet
feature, indicating that they must be sub-hydrogen burning objects.
This is the first successful detection of a new population of brown
dwarf stars in the solar neighborhood. These stars define a new
spectral type which will likely receive the designation "L".
At the moment, distances
to these newly discovered brown dwarfs are not yet known with great
reliability so their total contribution to the mass in the Solar
Neighborhood can not yet be determined. However, the mere fact that
they were detected in the one survey that was specifically tailored
to find them, indicates that they are likely much more numerous than
previously thought. This has significant implications for the sub-stellar
mass function in the Galactic Disk.
Dr. Neil Reid has kindly provided the following: (words to come)
Clusters of Galaxies: The book discusses how the detection of
strongly virialized clusters at relatively high redshift places severe
constraints on various structure formation scenarios. With the launch
of AXAF in December of 1998
we should be able to detect distant clusters of galaxies if they exist.
However, currently there are already three interesting cases:
- Hattori etal Nature, 388, 146-148 (1997) have
detected an object at z ~ 0.95 which appears to be an X-ray cluster
of galaxies but without much optical emission from the individual
galaxies. This may be an example of a "failed cluster" in which
only the ICM forms inside the cluster potential and not the
individual galaxies. This is an intriguing observation that needs
follow-up and confirmation.
- Stanford etal 1997 report the
detection, from an IR survey, of a z = 1.27 cluster. This is a prime
candidate for AXAF observations to detect the associated X-ray emission,
if there is any
- Tucker etal report the discovery of a kT = 17.4 keV cluster.
This is extremely hot and indicates an extreme potential well depth.
Such objects can only form from very high density peaks in the statistical
density field and should be extremely rare. In some cosmological models,
the space density of these objects would be zero. This is a very
interesting discovery. Most models will have a hard time accommodating
these objects, particularly if they exist at relatively high redshift.
- Donahue et al 1998
have discovered a 12.3 +/- 3 keV cluster at a redshift of z = 0.892.
This discovery was made in the Einstein Medium Sensitivity Survey.
Donahue etal argue that the proabability of discovering this massiv of
clsuter in the EMSS is 10-5 if Gaussian fluctuations in an
Omega = 1 Universe made these structures. To quote Donahue etal:
These observations, along with the fact that these luminosities and temperatures of the high-z clusters all agree with the low-z
LX-TX relation, argue strongly that Omega 0 < 1. Otherwise, the initial perturbations must be non-Gaussian, if these clusters'
temperatures do indeed reflect their gravitational potentials.
The UV Background:
Galaxies which emit a substantial amount of UV luminosity
(for instance due to a high star formation rate), will show a
sharp decrease or cutoff of that luminosity at a wavelength of
912 angstroms. This corresponds to the Lyman limit for the
ionization of hydrogen (e.g. 13.6 eV). Photons with energies in
excess of that will not reach a distant observer but will instead
maintain the ionization of neutral hydrogen. This break
in the distribution of UV emission is known as the Lyman break.
For a galaxy at z = 3, this break would occur at 3648 Angstroms
with respect to an observer at z = 0. Therefore if one did filter
photometry and noticed that some galaxies "disappeared" when
imaged in the ground-based U-filter, then such galaxies would
be good candidates for Lyman break objects at high redshift.
Steidel etal, as documented in the book, have had excellent
success using this technique to discover a significant population
of star forming galaxies at redshifts z = 2.7 -- 3.4. Recently,
Steidel etal (1999) have extended this technique to looking for
B-band "dropouts" which would correspond to Lyman break galaxies
between z = 3.5 -- 4.5. In detecting this population, Steidel
etal can directly compare the UV luminosity densities in
the redshift range z = 2.5 -- 3.5 with those in the range z = 3.5 --
4.5 to search for signs of evolution.
An earlier study of
Hubble Deep Field (HDF) had suggested that the UV luminsoity density
of the Universe peaked at z ~ 3. However, the HDF is just one
field (of angular size 2.7x2.7 arc-minutes)
and hence is subject to statistical fluctuations as well
as just low numbers of objects. The Steidel etal (1999) study
has sampled 828 square arc minutes (about 100 times larger than the HDF)
and therefore it has greater statistical reliability.
The principal result of the Steidel etal work is that the UV
luminsoity density shows no sign of a downturn all the way
out to z = 4.5 . While this result is completely at odds with
respect to the results for the HDF, the Steidel etal sample
is much better and is likely a better reflection of the intrinsic
state of star forming galaxies at these redshifts. Furthermore,
Steidel etal have good spectroscopy which helps to model the
reddening in these systems. They find that a typical Lyman break
galaxy has about 1.6 magnitudes of reddening at 1500 Angstroms.
There is a significant variation around this mean. hence, the
reddening of these galaxies is reasonably high indicating that
there has been significant dust production, even by redshift 4,
in these objects. Because the UV luminosity is so strongly
effected by dust, its imperative that accurate corrections for this effect
be done if one is to gauge the UV luminsoity density, due to
star forming galaxies, at these redshifts.
Altogether, the results of Steidel etal 1999 strongly
suggest that there may well be no peak in the star formation
activity of galaxies as a function of redshift and that
galaxies which have substantial amount of star formation may
well be located at redshifts greater than z=4.5.
To quote Steidel etal:
It is not clear that the beginning of the epoch of
star formation (in galaxies) has yet been identified
Gamma Ray Bursters:
In 1998 considerable work went into identifying the hosts of
gamma ray bursters. In almost all cases, the properties of the
host are consistent with a galaxy in the redshift range z: 1-3.
The case of GRB 970228, studied by Fructher etal, is a good example.
This GRB was observed by HST in the optical and infrared at times
6 months and one year after the original outburst. In both
cases the optical/infrared counterpart was detected and it showed
spatially extended emission. This "galaxy" is also quit high
in surface brightness and has very blue colors. Taken in concert
with other recent data, there is now a strong suggestion that
GRBs preferentially occur in high star formation rate galaxies
located at high redshift. The physics that is operative in such
an environment that would favor the production of GRBs however
remains elusive. Possible, the presence of GRBs in these
kinds of galaxies is a statistical effect. That is, these hosts
are engaged in the production of lots of massive stars and
hence massive star remnants. This would increase the probability of
neutron star-neutron star collisions compared to more modestly
star forming galaxies located at these redshifts.
Faint Blue Galaxies:
While this problem, as detailed in the book, remains
unresolved, there is a considerably amount of activity in
this field. If this book ever sees a second edition, it will
be expanded to include more results and information on galaxy
evolution. There are several groups world wide now engaged
in this kind of research. One of the more productive groups
involves astronomers in the US and Europe who have
formed the "MORPHS" group. They are studying the detailed
properties of galaxies in and out of the cluster environments
at intermediate redshifts (z: 0.3 -- 1.0). In a recent
paper (Poggianti etal) they have summarized their results
obtained to date:
- They find that the cluster environment greatly
suppresses the star formation activity of the members. This
is accomplished through ram pressure stripping of the
available fuel supply. In turn this requires a significant
ICM density and such clusters are usually X-ray luminous.
This means that selection effects may still be quite significant
in constructing samples of cluster and non-cluster galaxies.
- They find that much of the "blue-excess" population in
clusters is related to recent infall of actively star forming
galaxies. They argue that this effect is more pronounced at z ~ 0.4 than at
z =0 because the general star formation rate of galaxies is
higher at these earlier terms.
- The SO population that dominates z =0 clusters likely
has arisen due mostly to passive evolution and fading of
the light from the disk components of spiral galaxies.
The above results are not necessarily striking (much of this
was discussed a dozen years ago by Bothun and Dressler (1986) -
hey, its my update!) - but the data set the MORPHS group has
acquired is absolutely superb and as such removes much of the
ambiguity of previous data sets. Still once must be concerned about
selection effects in all of this and what might drive the infall around
certain clusters at intermediate redshift compared to others.
Shortly after this book was published, this survey commenced in both the
Northern and Southern Hemisphere (The Northern Hemisphere survey started
in June 1997; the Southern Hemisphere in March 1998).
2MASS is a digital sky survey at near-IR wavelengths. Three primary
sources of interest are likely to be detected in this survey:
- Very cool stars and possibly brown dwarf candidates. In
fact these have now been detected .
- Highly obscured active galactic nuclei, some of which could be
at relatively high redshift.
- Possibly new members of the Local Group of galaxies that have
remained undetected due to their low galactic latitude. This
has important implications regarding the dynamics of the Local Group.
This new 21-cm survey has begun its initial phases of data
acquisition. The goal of the survey is to perform a nearly complete
survey at 21-cm of the nearby sky in hopes of providing an improved
estimation of the neutral hydrogen gas mass function in galaxies.
Part of this survey is comparable to the 2MASS survey in that 21-cm
provides another means of detecting galaxies at low galactic
latitude. Indeed some have already been
Another possibility is that the Parkes survey will discover more examples
like Malin 1 or other forms of
gas-rich but optically
low surface brightness galaxies as discussed in Chapter 6.
Potentially this survey can discover possible examples of dark galaxies,
those in which the baryonic gas never was converted to stars. Come
back later to this page to see if any such galaxies have been