Structure and Evolution of Bars and Disks in the GOODS Fields
Jogee, S. (STScI), Conselice, C. (CalTech), Ravindranath, S. (STScI), Shlosman, I. (Kentucky),
Knapen, J. H. (Hertfordshire), Mobasher, B. (STScI), Koekemoer, A. M. (STScI), Lucas,
R. A. (STScI), Laine, S. (SIRTF), Hornschemeier, A. (JHU), and the GOODS Team
SUMMARY
• Non-axisymmetric features (such as bars and spirals) as well as general triaxialities
(such as those of the bulge and halo) drive the dynamical and secular evolution of
disk galaxies by exerting gravitational torques which redistribute mass and angular
momentum. While most (> 70 ‰) spirals are barred in the local Universe, early
studies of the HDF (e.g. Abraham et al. 1999) suggest a remarkably low bar fraction
(< 10 ‰) at intermediate redshifts. This result is even more puzzling given that conditions at intermediate redshifts in terms of the rate and strength of interactions as well
as the expected dynamical state of the disk (Jogee et al. 2002a) would appear to favor
bar formation. If the low bar fraction is confirmed, it would imply that the dynamics
and evolution of disks at intermediate redshifts are fundamentally different from
present-day spirals
From z = 0
to z = 1.0
From z = 0
to z =0.5
From z = 0
to z =1.0
.
• A second puzzle exists on disk structure: Cold Dark Matter hierarchical merging scenarios currently produce disks which are substantially smaller than present-day disks
resulting in the so-called the angular momentum ‘catastrophe’.
• The GOODS ACS data provide improved resolution, a larger field of view (10’x16’x 2
fields), a factor of at least 10 improvement in number statistics, and better wavelength
coverage compared to the WFPC2 HDF data used in earlier studies.
• With the goal of shedding light of both the bar fraction and angular momentum
‘catastrophe’ problems, we have started a study based on the GOODSACS BViz
(F435W, F606W,F775V, F850LP) images and numerical modeling of systems
between redshifts of 0.2 to 1.2. The methodoloy and preliminary results are presented.
FIGURE 1
GALAXY FITTING AND NUMERICAL MODELLING
ASSESSING REDSHIFT-DEPENDENT SYSTEMATIC
EFFECTS IN THE DATA
Structural parameters are being fitted in both the local sample of nearby galaxies and the ACS Z-band images
(e.g., see Fig. 2) using GALFIT (Peng et al. 2002) and a customized automated version of the ellipse routine in
the STSDAS package. An example fit is shown in Fig 3.
Information recovered from images of intermediate redshift (0.2--1.2) galaxies will suffer
from cosmological dimming, redshift dilution, loss of spatial resolution, band shifting, and
other redshift-dependent systematic effects. (see Table 1).
TABLE 1
FIGURE 2: GOODS spirals
We gauge the impact of these redshift-dependent systematic effects via Monte Carlo simulations and by artificially redshfting a well-studied sample of 57 nearby barred and
unbarred spirals for which we have B, R, and K-band data. While it is well-known that
NIR images can recover 25--30 % more bars than optical images, the NIR band is redshifted beyond the reddest ACS filter (F850LP or z-band) which only captures rest-frame
R-band at z=0.5 and rest-frame B-band at z=1 (Table 1). Following Concelice (2003,
submitted), we artificially redshift our R and B-band to z=0.5 and z=1 respectively, and
convolve them with the ACS instrumental and filter response to give a simulated ACS
F850LP (z-band) image. Figure 1 and ongoing analysis show several points:
• Extended strong bars such as in NGC 1300 can be recovered even out to z=1 from resframe B-band light (Fig. 1, left panel)
• For systems with bright bars and faint disks, cosmological dimming can cause the outer
disk to be dimmed out of detection, thereby leading to inner features such as weak bars
with spiral patterns of SF to be misidentified as disks as discussed by Jogee et al.
(2002) (Fig. 1, middle panel)
• Due to the loss of spatial resolution, short bars (a < 3--4 kpc), knotty inner spiral arms
are washed out into diffuse-looking inner disks (Fig. 1, right panel)
Violent equal mass mergers tend to produce elliptical-like galaxies via violent relaxation. Conversely, moderate mergers and
tidal interactions can easily excite bars and spiral in a dynamically cold disk. The fraction of spiral galaxies in pairs undergoing moderate interactions appers to increase out to z=0.8 and
this would suggest at first sight that conditions out to z~0.8 are
favorable to the formation of bars and spirals. However, an
alternative possibility needs exploration. In the presence of triaxial and centrally concentrated halos typical of CDM models,
the disk--halo interaction may destabilisethe bar. Figure 4
shows 3 set of models exhibiting varying stability of a stellar
bar embedded in CDM halos of different triaxiality (decreasing
from top [axial ratio 0.95], to middle [0.99], to bottom [1.0, axisymmetric]). The halos are centrally-concentrated with a flat
core of 0.5 kpc, and the bar’s major semimajor axis is 6 kpc.
The x-axis gives the distance along the bar major axis in kpc,
and the y-axis gives the rotational velocity in units local Keplerian velocity. The greyscale to the right shows fraction of chaotic orbits and to the left configuration space volume. The
white shade represents regular orbits comprising the bar,
while darked shades represent increasing fraction of chaotic
orbits. Note that bars embedded within triaxial halos of axial
ratios 0.95 are profoundly unstable on dynamical timescales
(see El-Zant & Shlosman 2002).
FIGURE 3: Ellipse fits
FIGURE 4