131 research outputs found
Globular Cluster Populations in Four Early-Type Poststarburst Galaxies
We present a study of the globular cluster systems of four early-type
poststarburst galaxies using deep g and I-band images from the ACS camera
aboard the Hubble Space Telescope (HST). All the galaxies feature shells
distributed around their main bodies and are thus likely merger remnants. The
color distribution of the globular clusters in all four galaxies shows a broad
peak centered on g-I ~ 1.4, while PGC 6240 and PGC 42871 show a significant
number of globular clusters with g-I ~ 1.0. The latter globular clusters are
interpreted as being of age ~ 500 Myr and likely having been formed in the
merger. The color of the redder peak is consistent with that expected for an
old metal-poor population that is very commonly found around normal galaxies.
However, all galaxies except PGC 10922 contain several globular clusters that
are significantly brighter than the maximum luminosity expected of a single old
metal-poor population. To test for multiple-age populations of overlapping g-I
color, we model the luminosity functions of the globular clusters as composites
of an old metal-poor subpopulation with a range of plausible specific
frequencies and an intermediate-age subpopulation of solar metallicity. We find
that three of the four sample galaxies show evidence for the presence of an
intermediate-age (~ 1 Gyr) globular cluster population, in addition to the old
metal-poor GC population seen in normal early-type galaxies. None of the
galaxies show a significant population of clusters consistent with an old,
metal-rich red cluster population that is typically seen in early-type
galaxies.Comment: 41 pages, 13 figures, accepted for publication in AJ. Some figues
have been downgraded to reduce their size
A Radio Perspective on the Wet Merger Remnant NGC 34
We present VLA observations of the neutral hydrogen and radio continuum of
NGC 34 (= NGC 17 = Mrk 938). This object is an ideal candidate to study the
fate of gas in mergers, since, as shown by an optical study done by Schweizer &
Seitzer (2007), it is a gas-rich ("wet") merger remnant of two disk galaxies of
unequal mass hosting a strong central starburst and a weak AGN. We detect HI
emission from both tidal tails and from nearby galaxies, suggesting that NGC 34
is actually part of a gas-rich group and might have recently interacted with
one of its companions. The kinematics of the gas suggests this remnant is
forming an outer disk of neutral hydrogen from the gas of the northern tail. We
also detect broad HI absorption (514 +/- 21 km/s wide) at both negative and
positive velocities with respect to the systemic velocity. This absorption
could be explained by the motions of the tidal tails or by the presence of a
circumnuclear disk. In addition, we present radio-continuum images that show
both nuclear (62.4 +/- 0.3 mJy) and extra-nuclear emission (26.5 +/- 3.0 mJy).
The extra-nuclear component is very diffuse and in the shape of two radio
lobes, spanning 390 kpc overall. This emission could be a signature of an AGN
that has turned off, or it could originate from a starburst-driven superwind.
We discuss the possible scenarios that explain our observations, and what they
tell us about the location of the gas and the future evolution of NGC 34.Comment: 29 pages, 12 figures, 2 tables. Accepted for publication in the
Astronomical Journal. Figs. 1, 2 & 6 degraded to reduce file size
The Luminosity Functions of Old and Intermediate-Age Globular Clusters in NGC 3610
The WFPC2 Camera on board HST has been used to obtain high-resolution images
of NGC 3610, a dynamically young elliptical galaxy. These observations
supersede shorter, undithered HST observations where an intermediate-age
population of globular clusters was first discovered. The new observations show
the bimodal color distribution of globular clusters more clearly, with peaks at
(V-I)o = 0.95 and 1.17. The luminosity function (LF) of the blue, metal-poor
population of clusters in NGC 3610 turns over, consistent with a Gaussian
distribution with a peak Mv ~= -7.0, similar to old globular-cluster
populations in ellipticals. The red, metal-rich population of clusters has a LF
that is more extended toward both the bright and faint ends, as expected for a
cluster population of inter-mediate age. It is well fit by a power law with an
exponent of alpha = -1.78 +-0.05, or -1.90+-0.07 when corrected for
observational scatter. A Kolmogorov-Smirnov test confirms the significant
difference between the LFs of the red and blue clusters, with a probability of
less than 0.1% that they come from the same population. A comparison with the
Fall & Zhang cluster disruption models shows marginal agreement with the
observed LF, although there are differences in detail. In particular, there is
no clear evidence of the predicted turnover at the faint end. A by-product of
the analysis is the demonstration that, at any given metallicity, the peak of
the LF should remain nearly constant from 1.5 Gyr to 12 Gyr, since the effect
of the disruption of faint clusters is almost perfectly balanced by the fading
of the clusters. This may help explain the apparent universality of the peak of
the globular cluster luminosity function. (Abridged)Comment: 32 pages, LaTeX, 13 PS figures, 1 table; to appear in AJ (July 2002
Keck Spectroscopy of Two Young Globular Clusters in the Merger Remnant NGC 3921
Low-resolution UV-to-visual spectra of two candidate globular clusters in the
merger remnant NGC 3921 are presented. These two clusters of apparent magnitude
V = 22.2 (Mv = -12.5) lie at projected distances of ~5 kpc from the center and
move with halo-type radial velocities relative to the local galaxy background.
Their spectra show strong Balmer absorption lines indicative of main-sequence
turnoffs dominated by A-type stars. Comparisons with model-cluster spectra
computed by Bruzual & Charlot and others yield cluster ages in the range of
200-530 Myr, and metallicities about solar to within a factor of three. Given
their small half-light radii (Reff < 5 pc) and ages corresponding to ~100 core-
crossing times, these clusters are gravitationally bound and, hence, indeed
young globulars. Assuming that they had Chabrier-type initial mass functions,
their estimated current masses are 2.3(+-0.1)x10^6 Msun and 1.5(+-0.1)x10^6
Msun, respectively, or roughly half the mass of omegaCen. Since NGC 3921 itself
shows many signs of being a 0.7(+-0.3) Gyr old protoelliptical, these two young
globulars of roughly solar metallicity and their many counterparts observed
with the Hubble Space Telescope provide supporting evidence that, in the
process of forming elliptical-like remnants, major mergers of gas-rich disks
can also increase the number of metal-rich globular clusters. (Abridged)Comment: 22 pages, 6 figures, accepted for publication in AJ, July 200
Deep Luminosity Functions of Old and Intermediate-Age Globular Clusters in NGC 1316: Evidence for Dynamical Evolution of Second-Generation Globular Clusters
The Advanced Camera for Surveys on board the Hubble Space Telescope has been
used to obtain deep high-resolution images of the giant early-type galaxy NGC
1316 which is an obvious merger remnant. These observations supersede previous,
shallower observations which revealed the presence of a population of
metal-rich globular clusters of intermediate age (~ 3 Gyr). We detect a total
of 1496 cluster candidates, almost 4 times as many as from the previous WFPC2
images. We confirm the bimodality of the color distribution of clusters, even
in V-I, with peak colors 0.93 and 1.06. The large number of detected clusters
allows us to evaluate the globular cluster luminosity functions as a function
of galactocentric radius. We find that the luminosity function of the inner 50%
of the intermediate-age, metal-rich (`red') population of clusters differs
markedly from that of the outer 50%. In particular, the luminosity function of
the inner 50% of the red clusters shows a clear flattening consistent with a
turnover that is about 1.0 mag fainter than the turnover of the blue clusters.
This constitutes the first direct evidence that metal-rich cluster populations
formed during major mergers of gas-rich galaxies can evolve dynamically
(through disruption processes) into the red, metal-rich cluster populations
that are ubiquitous in `normal' giant ellipticals.Comment: Accepted for publication in ApJ Letters; 4 pages in emulateapj style.
3 figure
Evidence for Three Subpopulations of Globular Clusters in the Early-Type Post-Starburst Shell Galaxy AM 0139-655
We present deep HST ACS images of the post-starburt shell galaxy AM 0139-655.
We find evidence for the presence of three distinct globular cluster
subpopulations associated with this galaxy: a centrally concentrated young
population (~ 0.4 Gyr), an intermediate age population (~ 1 Gyr) and an old,
metal-poor population similar to that seen around normal galaxies. The g-I
color distribution of the clusters is bimodal with peaks at 0.85 and 1.35. The
redder peak at g-I=1.35 is consistent with the predicted color for an old
metal-poor population. The clusters associated with the peak at g-I=0.85 are
centrally concentrated and interpreted as a younger and more metal-rich
population. We suggest that these clusters have an age of ~ 0.4 Gyr and solar
metallicity based on a comparison with population synthesis models. The
luminosity function of these "blue" clusters is well represented by a power
law. Interestingly, the brightest shell associated with the galaxy harbors some
of the youngest clusters observed. This seems to indicate that the same merger
event was responsible for the formation of both the shells and the young
clusters. The red part of the color distribution contains several very bright
clusters, which are not expected for an old, metal-poor population.
Furthermore, the luminosity function of the "red" GCs cannot be fit well by
either a single gaussian or a single power law. A composite (gaussian + power
law) fit to the LF of the red clusters yields both a low rms and very plausible
properties for an old population plus an intermediate-age population of GCs.
Hence, we suggest that the red clusters in AM 0139-655 consist of two distinct
GC subpopulations, one being an old, metal-poor population as seen in normal
galaxies and one having formed during a recent dissipative galaxy merger.Comment: 35 pages, 12 figures, accepted for publication in A
The Luminosity Function of Young Star Clusters In "The Antennae" Galaxies (NGC 4038/4039)
The WFPC2 of the HST has been used to obtain high-resolution images of NGC
4038/4039 that go roughly 3 magnitudes deeper in V than previous observations
made during Cycle 2 (-14 < M_V < -6). To first order the luminosity function
(LF) is a power law, with exponent \alpha = -2.12 +/- 0.04. However, after
decoupling the cluster and stellar LFs, which overlap in the range -9 < M_V <
-6, we find an apparent bend in the young cluster LF at approximately M_V =
-10.4. The LF has a power law exponent -2.6 +/- 0.2 in the brightward and -1.7
+/- 0.2 in the faintward. The bend corresponds to a mass ~ 10^5 M_{\odot}, only
slightly lower than the characteristic mass of globular clusters in the Milky
Way (~2x10^5 M_{\odot}). The star clusters of the Antennae appear slightly
resolved, with median effective radii of 4 +/- 1 pc, similar to or perhaps
slightly larger than those of globular clusters in our Galaxy. However, the
radial extents of some of the very young clusters (ages < 10 Myr) are much
larger than those of old globular clusters. A combination of the UBVI colors,
\Halpha morphology, and GHRS spectra enables us to age-date the clusters in
different regions of The Antennae. We find two groups of young star clusters
with ages <~ 20Myr and ~100Myr, as well as an intermediate-age group (~500 Myr)
and a handful of old globular clusters from the progenitor galaxies. Age
estimates derived from GHRS spectroscopy yield 3 +/- 1 Myr for Knot K (just
south of the nucleus of NGC 4038) and 7 +/- 1 Myr for Knot S in the Western
Loop, in good agreement with ages derived from the UBVI colors. Effective
gas-outflow velocities from Knots S and K are estimated to be about 25-30 km/s.
However, the measured widths of the interstellar absorption lines suggest
dispersion velocities of ~400 km/s along the lines of sight to Knots S and K.Comment: 56 pages, 4 tables and 23 figures, texts in AAS style, to be
published in A
Self-assembly and two-dimensional spontaneous resolution of cyano-functionalized [7]helicenes on Cu111
Birds of a feather flock together: STM and DFT studies provide the first example of spontaneous chiral resolution of a helicene on a surface. Racemic 6,13-dicyano[7]helicene forms fully segregated domains of pure enantiomers (2D conglomerate) on Cu(111). The propensity of the system to optimize intermolecular CNâ‹…â‹…â‹…HC(Ar) hydrogen bonding and CNâ‹…â‹…â‹…CN dipolar interactions translates into chiral recognition with preferential assembly of homochiral molecules
Dynamical Evolution of Globular Cluster Systems formed in Galaxy Mergers: Deep HST/ACS Imaging of Old and Intermediate-Age Globular Clusters in NGC 3610
(ABRIDGED) The ACS camera on board the Hubble Space Telescope has been used
to obtain deep images of the giant elliptical galaxy NGC 3610, a
well-established dissipative galaxy merger remnant. These observations
supersede previous WFPC2 images which revealed the presence of a population of
metal-rich globular clusters (GCs) of intermediate age (~1.5-4 Gyr). We detect
a total of 580 GC candidates, 46% more than from the previous WFPC2 images. The
new photometry strengthens the significance of the previously found bimodality
of the color distribution of GCs. Peak colors in V-I are 0.93 +/-0.01 and 1.09
+/- 0.01 for the blue and red subpopulations, respectively. The luminosity
function (LF) of the inner 50% of the metal-rich (`red') population of GCs
differs markedly from that of the outer 50%. In particular, the LF of the inner
50% of the red GCs shows a flattening consistent with a turnover that is about
1.0 mag fainter than the turnover of the blue GC LF. This is consistent with
predictions of recent models of GC disruption for the age range mentioned above
and for metallicities that are consistent with the peak color of the red GCs as
predicted by population synthesis models. We determine the specific frequency
of GCs in NGC 3610 and find a present-day value of S_N = 1.4 +/- 0.6. We
estimate that this value will increase to S_N = 3.8 +/- 1.7 at an age of 10
Gyr, which is consistent with typical S_N values for `normal' ellipticals. Our
findings constitute further evidence in support of the notion that metal-rich
GC populations formed during major mergers involving gas-rich galaxies can
evolve dynamically (through disruption processes) into the red, metal-rich GC
populations that are ubiquitous in `normal' giant ellipticals.Comment: 15 pages, 14 figures, 4 tables. Accepted for publication in The
Astronomical Journal. Figure 6 somewhat degraded to adhere to astro-ph rule
The Cold and Hot Gas Content of Fine-Structure E and S0 Galaxies
We investigate trends of the cold and hot gas content of early-type galaxies
with the presence of optical morphological peculiarities, as measured by the
fine-structure index (Sigma). HI mapping observations from the literature are
used to track the cold-gas content, and archival ROSAT PSPC data are used to
quantify the hot-gas content. We find that E and S0 galaxies with a high
incidence of optical peculiarities are exclusively X-ray underluminous and,
therefore, deficient in hot gas. In contrast, more relaxed galaxies with little
or no signs of optical peculiarities span a wide range of X-ray luminosities.
That is, the X-ray excess anticorrelates with Sigma. There appears to be no
similar trend of cold-gas content with either fine-structure index or X-ray
content. The fact that only apparently relaxed E and S0 galaxies are strong
X-ray emitters is consistent with the hypothesis that after strong disturbances
such as a merger hot-gas halos build up over a time scale of several gigayears.
This is consistent with the expected mass loss from stars.Comment: 12 pages, latex, 5 figures. Accepted for publication in A
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