21 research outputs found
A Tale of Two Tails: Exploring Stellar Populations in the Tidal Tails of NGC 3256
We have developed an observing program using deep, multiband imaging to probe
the chaotic regions of tidal tails in search of an underlying stellar
population, using NGC 3256's 400 Myr twin tidal tails as a case study. These
tails have different colours of and for NGC 3256W, and and
for NGC 3256E, indicating different stellar populations. These colours
correspond to simple stellar population ages of Myr and
Myr for NGC 3256W and NGC 3256E, respectively, suggesting
NGC 3256W's diffuse light is dominated by stars formed after the interaction,
while light in NGC 3256E is primarily from stars that originated in the host
galaxy. Using a mixed stellar population model, we break our diffuse light into
two populations: one at 10 Gyr, representing stars pulled from the host
galaxies, and a younger component, whose age is determined by fitting the model
to the data. We find similar ages for the young populations of both tails,
( and Myr for NGC 3256W and NGC 3256E,
respectively), but a larger percentage of mass in the 10 Gyr population for NGC
3256E ( vs ). Additionally, we detect 31 star
cluster candidates in NGC 3256W and 19 in NGC 2356E, with median ages of 141
Myr and 91 Myr, respectively. NGC 3256E contains several young (< 10 Myr), low
mass objects with strong nebular emission, indicating a small, recent burst of
star formation.Comment: Accepted for publication in MNRAS. 16 pages, 19 figure
Stellar Hydrodynamics in Radiative Regions
We present an analysis of the response of a radiative region to waves
generated by a convective region of the star; this wave treatment of the
classical problem of ``overshooting'' gives extra mixing relative to the
treatment traditionally used in stellar evolutionary codes. The interface
between convectively stable and unstable regions is dynamic and nonspherical,
so that the nonturbulent material is driven into motion, even in the absence of
``penetrative overshoot.'' These motions may be described by the theory of
nonspherical stellar pulsations, and are related to motion measured by
helioseismology. Multi-dimensional numerical simulations of convective flow
show puzzling features which we explain by this simplified physical model.
Gravity waves generated at the interface are dissipated, resulting in slow
circulation and mixing seen outside the formal convection zone. The approach
may be extended to deal with rotation and composition gradients. Tests of this
description in the stellar evolution code TYCHO produce carbon stars on the
asymptotic giant branch (AGB), an isochrone age for the Hyades and three young
clusters with lithium depletion ages from brown dwarfs, and lithium and
beryllium depletion consistent with observations of the Hyades and Pleiades,
all without tuning parameters. The insight into the different contributions of
rotational and hydrodynamic mixing processes could have important implications
for realistic simulation of supernovae and other questions in stellar
evolution.Comment: 27 pages, 5 figures, accepted to the Astrophysical Journa
An HI Threshold for Star Cluster Formation in Tidal Debris
Super star clusters are young, compact star clusters found in the central
regions of interacting galaxies. Recently, they have also been reported to
preferentially form in certain tidal tails, but not in others. In this paper,
we have used 21 cm HI maps and the Hubble Space Telescope Wide Field Planetary
Camera 2 images of eight tidal tail regions of four merging galaxy pairs to
compare the kiloparsec scale HI distribution with the location of super star
clusters found from the optical images. For most of the tails, we find that
there is an increase in super star cluster density with increasing projected HI
column density, such that the star cluster density is highest when log N(HI) >=
20.6 cm^{-2}, but equal to the background count rate at lower HI column
density. However, for two tails (NGC 4038/39 Pos A and NGC 3921), there is no
significant star cluster population despite the presence of gas at high column
density. This implies that the N(HI) threshold is a necessary but not
sufficient condition for cluster formation. Gas volume density is likely to
provide a more direct criterion for cluster formation, and other factors such
as gas pressure or strength of encounter may also have an influence. Comparison
of HI thresholds needed for formation of different types of stellar structures
await higher resolution HI and optical observations of larger numbers of
interacting galaxies.Comment: 19 pages, 6 figures, 3 tables, accepted for publication in MNRA
From Globular Clusters to Tidal Dwarfs: Structure Formation in the Tidal Tails of Merging Galaxies
Using V and I images obtained with WFPC2/HST, we investigate compact stellar
structures within tidal tails. Six regions of tidal debris in the four classic
``Toomre Sequence'' mergers: NGC 4038/39 (``Antennae''), NGC 3256, NGC 3921,
and NGC 7252 (``Atoms for Peace'') have been studied in order to explore how
the star formation depends upon the local and global physical conditions. These
mergers sample a range of stages in the evolutionary sequence and tails with
and without embedded tidal dwarf galaxies. The six tails are found to contain a
variety of stellar structures, with sizes ranging from those of globular
clusters up to those of dwarf galaxies. From V and I WFPC2 images, we measure
the luminosities and colors of the star clusters. NGC 3256 is found to have a
large population of blue clusters (0.2 < V-I < 0.9), particularly in its
Western tail, similar to those found in the inner region of the merger. In
contrast, NGC 4038/39 has no clusters in the observed region of the tail, only
less luminous point sources likely to be individual stars. A significant
cluster population is clearly associated with the prominent tidal dwarf
candidates in the eastern and western tails of NGC 7252. The cluster-rich
Western tail of NGC 3256 is not distinguished from the others by its dynamical
age or by its total HI mass. However, the mergers that have few clusters in the
tail all have tidal dwarf galaxies, while NGC 3256 does not have prominent
tidal dwarfs. We speculate that star formation in tidal tails may manifest
itself either in small structures like clusters along the tail or in large
structures such as dwarf galaxies, but not in both. Also, NGC 3256 has the
highest star formation rate of the four mergers studied, which may contribute
to the high number of star clusters in its tidal tails.Comment: Accepted to Astronomical Journal. 34 pages including 15 figures and 4
table