Normal Galaxies and Mergers: The Morphological Mix of Galaxies to
z~1 From GOODS ACS Imaging of the Chandra Deep Field-South
R.A. Lucas (STScI), C. Conselice (Caltech), E. Chatzichristou (Yale), T. Dahlen (STScI), D. de Mello (JHU), J.P. Gardner (GSFC), M. Giavalisco
(STScI), N.A. Grogin, A. Hornschemeier (JHU), S. Jogee, A.M. Koekemoer, B. Mobasher, S. Ravindranath (STScI), C.M. Urry (Yale), and
GOODS Team
Abstract
We present morphological CAS (concentration, asymmetry, and clumpiness) parameters out to z~1.2 for galaxies in an area covering and surrounding the CDF-S which we are imaging in 4 bands (B, V, i, z) with the
ACS Wide Field Camera. These parameters will allow us to determine, based on a nearby galaxy calibration, the similarities between each galaxy and the various major types. We will focus in particular on the
morphological mix in the redshift-limited sample, with a special emphasis on the evolution in the fraction of interactions and mergers with redshift. To do this, we will first calculate rest-frame B-band morphologies
for the galaxies brighter than a given magnitude limit. From this, and using a local calibration, the morphological mix (fraction of mergers, late types, and early types) can be computed as a function of redshift. It
should also be possible to track changes in the parameter values themselves as a function of redshift, and this can be used to assess their implications for the morphological mix. Other relations between variables such
as spectral type (from our photometric redshift catalogs) and the degree of asymmetry can also be used to probe questions such as the relationship between starburst galaxies and mergers, for example.
Introduction
The structures and morphologies of galaxies reveal their past and present
formation histories. This includes recent and past merging events and star
formation activity. Early work on the Hubble Deep Fields showed that
this approach towards understanding galaxy evolution is potentially very
powerful. With the ongoing GOODS project, it is now feasible to trace the
evolution of galaxies through their structures out to z~3 on a larger area
than that offered by the Hubble Deep Fields. In this poster, we discuss the
evolution and breakdown of morphological types out to z~1.2.
Galaxies can be divided into several different major types, and a basic
question is whether or not these galaxies and their morphological types all
formed at once, early in the universe, or did some develop later? To
answer this, we want to understand the relative contribution of these types
(which includes ellipticals, spirals, irregulars, and mergers) as a function
of redshift. To measure this evolution, we use the CAS system of
Conselice (2003) where all major morphological types can be identified
through the use of the concentration, asymmetry, and clumpiness
parameters.
In this poster, we investigate how the morphological population changes
from z~0 to z~1.2 using the CAS system calibrated on nearby galaxies.
Our basic aim is to determine how ellipticals and spirals are evolving as a
function of time and how the presence of these Hubble types evolves with
ongoing major mergers.
Figure 1: Rest-frame B-band
asymmetry versus concentration
diagram for the GOODS CDF-S
galaxies with photometric redshifts.
Colors correlate with redshift such
that blue is z < 0.5, green is 0.5 < z <
0.75, and red is z > 0.75. Also plotted
are the positions of labeled nearby
galaxy types.
Method (The CAS System)
Results
In the CAS (concentration, asymmetry, clumpiness) system, all
nearby major galaxy types fall into well defined corners in CAS
morphological space. Early types are those with a high light
concentration, low asymmetry, and low clumpiness values. Early
type spirals have a lower light concentration, higher
asymmetries, and high clumpiness values. Later type disks on
average have even lower concentrations and slightly higher
asymmetries and clumpiness values. Mergers are galaxies which
have high asymmetry values. (See Table 1.)
The basic results of this study are shown in the figures. Figure 1
shows the concentration – asymmetry diagram for the GOODS
CDF-S sample. Plotted are the average values and their 1 sigma
variations for nearby galaxies. Figure 2 shows a similar diagram, in
this case the clumpiness and asymmetry values, where the averages
and 1 sigma values of nearby galaxies are also shown.
In the nearby universe, these three parameters correlate with the
scale of a galaxy, such as mass (concentration), star formation
(clumpiness), and major galaxy mergers (asymmetry) (Conselice
2003, submitted).
Based on Figures 1 and 2, it appears that there is some evolutionary
difference between nearby galaxies and those seen in the GOODS
CDF-S field. In general, there appear to be few classical, smooth,
low-asymmetry ellipticals and more galaxies that appear to have
clumpy structures and higher asymmetries.
We measure the CAS parameters on galaxies brighter than I = 27
which have good (e.g. ODDS > 0.95, or agreement with the
spectroscopic redshift samples K20 and CXO src is error_z/(1+z)
>~0.1, see Mobasher et al. poster) photometric redshifts,
resulting in 935 reliable measurements for galaxies between z~0
and z~1.2. The redshifts we use are photometric ones calculated
by Mobasher et al. using ground-based data.
We use these CAS values to determine the relative fractions of
galaxy types in the GOODS fields. These relative fractions are
shown in Figure 3 for all 935 galaxies with reliable photometric
redshifts, and Figure 3b shows the same fraction of types, but for
systems with Mb < -20. Very similar patterns appear. Disk galaxies
comprise a relatively high fraction of 60% at all redshifts, whereas
ellipticals make up about 20 – 30% of the same, similar to what is
found in the nearby universe. The fraction of mergers, those with
high asymmetries, tends to decline with redshift.
To avoid any strong morphological k-correlations, we use the
rest frame B-band morphologies for these galaxies via
interpolation of the rest-frame B-band CAS values using the
observed filters, F435W (B), F606W (V), F775W (i), and
F850LP (z). This allows us to use them to compare the same
appearance of different galaxies at various redshifts.
Although these results are preliminary, it appears that out to z~1.2,
the gross morphological mix is very similar, and the Hubble
sequence appears to be in place, with perhaps a slight decline in
mergers at lower redshifts and a slight increase in at least the
fraction of disk galaxies, showing that some mergers might be
evolving into Hubble types.
Figure 2: Same as Figure 1, except the
rest-frame B-band asymmetryclumpiness relationship.
Figure 3a: Fraction of different types
as a function of redshift for all 935
galaxies with reliable photometric
redshifts.
Table 1 (Left): This table shows typical
values of Concentration ( C ), Asymmetry (
A ), and Clumpiness ( S ) parameters for
galaxies of various types. Note how
ellipticals have higher concentration values,
and how starbursts tend to have higher
values for asymmetry and clumpiness, for
example.
Figure 4 (Right): This figure depicts the
physical basis for the determination of CAS
parameters as applied to the example of an
individual galaxy.
Figure 3b: Fraction of morphological
types as a function of redshift for
galaxies brighter than Mb = -20.