Title: A Detailed Evolution of E+A Galaxies into Early Types
Authors: Y. Yang, A.
Zabludoff, D. Zaritsky, and J. Mihos
Authors’ Institutions: Steward
Observatory, University of Arizona and Department of Astronomy, Case Western
Reserve University
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Article Summary:
The
transitional period between gas-rich, star-forming galaxies to gas-poor,
passively evolving galaxies is an important phase of galaxy evolution.
Post-Starburst Galaxies (PSGs) appear to inhabit this
transitional period. To determine
what PSGs become after their young stellar populations fade away, Yang et al.
acquired detailed morphologies of 21 PSGs using high-resolution images from the
Hubble Space Telescope (HST) Advanced
Camera for Surveys (ACS) and Wide-Field Planetary Camera 2 (WFPC2). They used these images to measure the
morphologies, color profiles, scaling relations, and star cluster
characteristics of their PSG sample. Their results suggest that PSGs evolve
into early-type E/SO galaxies and contribute to the building up of the ‘red
sequence’.
PSGs
represent the best candidates for galaxies caught in the midst of the
transformation between late and early types due to their relatively young
stellar population and lack of ongoing star formation, as suggested by their
strong Balmer absorption lines and absence of emission lines such as [O II]
and H-alpha. Since PSGs reside in low-density
environments, the abrupt end to their star formation is likely due to
galaxy-galaxy interactions rather than cluster-specific mechanisms such as ram pressure stripping and strangulation.
This theory is supported by significant fractions of PSGs having merger
features.
The
21 PSGs in the sample were spectroscopically identified from 11,113 galaxy
spectra of the Las Campanas Redshift Survey (LCRS). The 21 PSGs have redshifts
between 0.07 and 0.18. The
high-resolution HST images, such as
those presented in figure 1, enabled small- and large-scale interaction features
to be identified. The morphologies
of PSGs in this study were very diverse, including train wrecks, barred
galaxies, blue cores, and relaxed-looking disky galaxies. Over half of the PSGs had identifiable
tidal or disturbed features. Five
of the galaxies in the sample had interacting, companion galaxies within ~30
kpc. One of the samples even had a
binary PSG system in which both of the galaxies were tidally disturbed. These findings support the idea that
galaxy interactions and mergers trigger the PSG phase. In addition, morphological analysis determined that six of
the PSGs had distinct, compact blue cores and seven of the galaxies had dust
features such as lanes and filamentary structures.
PSGs
tend to be bulge-dominated systems.
The median bulge fraction (B/T), which gives the ratio of the bulge
luminosity to the total luminosity of the galaxy, was 0.59. This is consistent with that of S0
galaxies, with an average B/T of 0.63.
Sérsic profiles, represented by an r1/n profile describing
how the intensity of a galaxy varies with distance from its center, were also
obtained to further investigate PSG bulge characteristics. Disk galaxies and spheroidals generally
have Sérsic indices of n=1 and n=4, respectively, yet most of the PSGs had
indices with n>5 and a couple with indices n>10. This indicates that the luminosity of PSGs
is highly concentrated, potentially due to substructures near their centers
such as bright nuclei, bars, and rings.
To
quantify asymmetric features, this study calculated the concentration index ‘C’
and rotational asymmetry index ‘A’ to be able to place the sample galaxies on a
C-A plane, as shown in figure 2. In
general, PSGs have high concentration indices consistent with those of
spheroids, but considerably larger asymmetry indices than ellipticals due to
structures that arose from the starburst or recent merger. Therefore, PSGs would be
morphologically classified as early-type galaxies once the disruptions and
tidal features dissipate or fade.
The
color morphologies of the PSGs were just as diverse as their structural
morphologies. Radial color
profiles can depend on dust content and spatial distributions, ages, and
metallicities of stellar populations, which in turn depend on the evolutionary
history of the galaxies. For
example, if galaxy-galaxy interactions are responsible for creating a PSG, then
the young stellar population is expected to be concentrated in the center,
yielding a positive color gradient (i.e., redder color with increasing
radius). If mechanisms such as ram
pressure stripping are responsible for producing the PSG, color profiles may be
more uniform.
High-resolution
color distributions in this study indicate that a significant fraction of the PSGs
have positive color gradients and sometimes distinct blue cores. Figure 3 shows that twelve PSGs had a
positive color gradient, five had a negative color gradient (bluer with
increasing radius), and five had flat or mixed color profiles. Of the five with negative color
gradients, three show clear dust signatures, which may mean that the red cores
in these galaxies arise from increasing dust extinction toward the center. Early-type galaxies typically have
slightly negative color profiles. E/S0s
in the local universe have negative color gradients that originate from their
metallicity gradients; their stellar populations become more metal-rich and
redder towards the center. Over
time, the PSGs with positive color gradients may begin to exhibit negative
color gradients if their young stellar populations are more metal-rich than the
underlying old populations and these young stars run through their lifecycle. This
possibly suggests that PSGs are the precursors of E/SO galaxies.
Figure 3. Redshifted radial color profiles of the 20 PSGs
. A positive slope means the
galaxies are bluer towards the center and a negative slope means the galaxies
are redder towards the center.
|
Half of the PSGs with positive color
gradients also had compact, almost stellar-like, blue cores that were distinct
from the other parts of the galaxy.
Though their origins are not fully understood, blue cores are common in
early-type galaxies at higher redshifts (z > 0.5), when field spheroids were
assembling. Therefore, the
blue-core PSGs may be the local analogs to these higher redshift blue-core
spheroids. Three of the six PSGs
with blue cores also had LINER (low-ioniation emission line region) spectral
signatures, indicative of Active Galactic Nuclei (AGN) activity being the potential mechanism for quenching
star formation in these PSGs.
To
further determine if of the data supports the idea that PSGs evolve into
early-type galaxies, this study compared scaling relations between PSG and
early-type galaxies. The
fundamental plane is an empirical relation between the effective radius, the
central velocity dispersion, and the mean surface brightness. PSGs stand apart from E/S0s in the fundamental
plane, which implies that the stellar populations of PSGs are different from
those of E/S0s. On average, the
mass to luminosity ratio (M/L) of PSGs
is 3.8 times smaller than that of E/S0s.
In PSGs, smaller or less massive galaxies appear to have a smaller M/L. This trend arises naturally from merger scenarios, where
low-mass galaxies have higher gas fractions and could produce relatively larger
populations of young stars.
Properties
of newly formed star clusters in the PSGs were analyzed to determine if they
are consistent with those of early-type galaxies. High-resolution images were required to identify star
clusters in the PSG sample. At
least nine of the PSGs had a population of unresolved compact sources. The colors and luminosities of the
young star clusters are consistent with the ages inferred from the PSG spectra
(0.01-1 Gyr), signifying that these clusters likely arose during the
interaction/starburst phase. Though
it is uncertain how many of these clusters will survive to the E/S0 phase, it
is at least possible that the young star clusters in PSGs can evolve into the
globular cluster systems seen in E/S0s.
To
summarize, using high spatial resolution HST
ACS and WFPC2 images to derive morphological properties, color profiles,
scaling relations, and characteristics of young star clusters, this study
suggests that PSGs are caught in the act of transforming from gas-rich
late-type galaxies to gas-poor early-type galaxies. Further investigation of PSGs is critical to better
understanding of the origin of the red sequence of galaxies.
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