3,856 research outputs found
A Hubble Space Telescope Snapshot Survey of Dynamically Close Galaxy Pairs in the CNOC2 Redshift Survey
We compare the structural properties of two classes of galaxies at
intermediate redshift: those in dynamically close galaxy pairs, and those which
are isolated. Both samples are selected from the CNOC2 Redshift Survey, and
have redshifts in the range 0.1 < z <0.6. Hubble Space Telescope WFPC2 images
were acquired as part of a snapshot survey, and were used to measure bulge
fraction and asymmetry for these galaxies. We find that paired and isolated
galaxies have identical distributions of bulge fractions. Conversely, we find
that paired galaxies are much more likely to be asymmetric (R_T+R_A >= 0.13)
than isolated galaxies. Assuming that half of these pairs are unlikely to be
close enough to merge, we estimate that 40% +/- 11% of merging galaxies are
asymmetric, compared with 9% +/- 3% of isolated galaxies. The difference is
even more striking for strongly asymmetric (R_T+R_A >= 0.16) galaxies: 25% +/-
8% for merging galaxies versus 1% +/- 1% for isolated galaxies. We find that
strongly asymmetric paired galaxies are very blue, with rest-frame B-R colors
close to 0.80, compared with a mean (B-R)_0 of 1.24 for all paired galaxies. In
addition, asymmetric galaxies in pairs have strong [OII]3727 emission lines. We
conclude that close to half of the galaxy pairs in our sample are in the
process of merging, and that most of these mergers are accompanied by triggered
star formation.Comment: Accepted for publication in the Astronomical Journal. 40 pages,
including 15 figures. For full resolution version, please see
http://www.trentu.ca/physics/dpatton/hstpairs
The Hamburg/ESO R-process Enhanced Star survey (HERES) II. Spectroscopic analysis of the survey sample
We present the results of analysis of ``snapshot'' spectra of 253 metal-poor
halo stars -3.8 < [Fe/H] < -1.5 obtained in the HERES survey. The spectra are
analysed using an automated line profile analysis method based on the
Spectroscopy Made Easy codes of Valenti & Piskunov. Elemental abundances of
moderate precision have been obtained for 22 elements, C, Mg, Al, Ca, Sc, Ti,
V, Cr, Mn, Fe, Co, Ni, Zn, Sr, Y, Zr, Ba, La, Ce, Nd, Sm, and Eu, where
detectable. Among the sample of 253 stars, we find 8 r-II stars and 35 r-I
stars. We also find three stars with strong enhancements of Eu which are
s-process rich. A significant number of new very metal-poor stars are
confirmed: 49 stars with [Fe/H] < -3 and 181 stars with -3 < [Fe/H] < -2. We
find one star with [Fe/H] < -3.5. We find the scatter in the abundance ratios
of Mg, Ca, Sc, Ti, Cr, Fe, Co, and Ni, with respect to Fe and Mg, to be similar
to the estimated relative errors and thus the cosmic scatter to be small,
perhaps even non-existent. The elements C, Sr, Y, Ba and Eu, and perhaps Zr,
show scatter at [Fe/H] < -2.5 significantly larger than can be explained from
the errors in the analysis, implying scatter which is cosmic in origin.
Significant scatter is observed in abundance ratios between light and heavy
neutron-capture elements at low metallicity and low levels of r-process
enrichment. (*** abridged ***)Comment: Accepted for A&A; 31 pages, 2 electronic tables presently available
at http://www.astro.uu.se/~barklem/papers/heres_tables.tar.gz ; Minor
corrections adde
Tidally-Triggered Star Formation in Close Pairs of Galaxies
We analyze new optical spectra of a sample of 502 galaxies in close pairs and
n-tuples, separated by <= 50/h kpc. We extracted the sample objectively from
the CfA2 redshift survey, without regard to the surroundings of the tight
systems. We probe the relationship between star formation and the dynamics of
the systems of galaxies. The equivalent widths of H\alpha (EW(H\alpha) and
other emission lines anti-correlate strongly with pair spatial separation
(\Delta D) and velocity separation. We use the measured EW(H\alpha) and the
starburst models of Leitherer et al. to estimate the time since the most recent
burst of star for- mation began for each galaxy. In the absence of a large
contribution from an old stellar population to the continuum around H\alpha,
the observed \Delta D -- EW(H\alpha) correlation signifies that starbursts with
larger separations on the sky are, on average, older. By matching the dynamical
timescale to the burst timescale, we show that the data support a simple
picture in which a close pass initiates a starburst; EW(H\alpha) decreases with
time as the pair separation increases, accounting for the anti-correlation.
This picture leads to a method for measuring the duration and the initial mass
function of interaction-induced starbursts: our data are compatible with the
starburst and orbit models in many respects, as long as the starburst lasts
longer than \sim10^8 years and the delay between the close pass and the
initiation of the starburst is less than a few \times 10^7 years. If there is
no large contribution from an old stellar population to the continuum around
H\alpha the Miller-Scalo and cutoff (M <= 30 M_\sun) Salpeter initial mass
functions fit the data much better than a standard Salpeter IMF. (Abridged.)Comment: 43 pages, 22 figures, to appear in the ApJ; we correct an error which
had minor effects on numerical values in the pape
Three Different Types of Galaxy Alignment within Dark Matter Halos
Using a large galaxy group catalogue based on the Sloan Digital Sky Survey
Data Release 4 we measure three different types of intrinsic galaxy alignment
within groups: halo alignment between the orientation of the brightest group
galaxies (BGG) and the distribution of its satellite galaxies, radial alignment
between the orientation of a satellite galaxy and the direction towards its
BGG, and direct alignment between the orientation of the BGG and that of its
satellites. In agreement with previous studies we find that satellite galaxies
are preferentially located along the major axis. In addition, on scales r < 0.7
Rvir we find that red satellites are preferentially aligned radially with the
direction to the BGG. The orientations of blue satellites, however, are
perfectly consistent with being isotropic. Finally, on scales r < 0.1 \Rvir, we
find a weak but significant indication for direct alignment between satellites
and BGGs. We briefly discuss the implications for weak lensing measurements.Comment: 4 pages, 4 figures, ApJL accepte
Rotationally Invariant Hamiltonians for Nuclear Spectra Based on Quantum Algebras
The rotational invariance under the usual physical angular momentum of the
SUq(2) Hamiltonian for the description of rotational nuclear spectra is
explicitly proved and a connection of this Hamiltonian to the formalisms of
Amal'sky and Harris is provided. In addition, a new Hamiltonian for rotational
spectra is introduced, based on the construction of irreducible tensor
operators (ITO) under SUq(2) and use of q-deformed tensor products and
q-deformed Clebsch-Gordan coefficients. The rotational invariance of this
SUq(2) ITO Hamiltonian under the usual physical angular momentum is explicitly
proved, a simple closed expression for its energy spectrum (the ``hyperbolic
tangent formula'') is introduced, and its connection to the Harris formalism is
established. Numerical tests in a series of Th isotopes are provided.Comment: 34 pages, LaTe
The dark matter halo shape of edge-on disk galaxies - I. HI observations
This is the first paper of a series in which we will attempt to put
constraints on the flattening of dark halos in disk galaxies. We observe for
this purpose the HI in edge-on galaxies, where it is in principle possible to
measure the force field in the halo vertically and radially from gas layer
flaring and rotation curve decomposition respectively. In this paper, we define
a sample of 8 HI-rich late-type galaxies suitable for this purpose and present
the HI observations.Comment: Accepted for publication by Astronomy & Astrophysics. For a higher
resolution version see
http://www.astro.rug.nl/~vdkruit/jea3/homepage/12565.pd
The CoRoT target HD175726: an active star with weak solar-like oscillations
Context. The CoRoT short runs give us the opportunity to observe a large
variety of late-type stars through their solar-like oscillations. We report
observations of the star HD175726 that lasted for 27 days during the first
short run of the mission. The time series reveals a high-activity signal and
the power spectrum presents an excess due to solar-like oscillations with a low
signal-to-noise ratio. Aims. Our aim is to identify the most efficient tools to
extract as much information as possible from the power density spectrum.
Methods. The most productive method appears to be the autocorrelation of the
time series, calculated as the spectrum of the filtered spectrum. This method
is efficient, very rapid computationally, and will be useful for the analysis
of other targets, observed with CoRoT or with forthcoming missions such as
Kepler and Plato. Results. The mean large separation has been measured to be
97.2+-0.5 microHz, slightly below the expected value determined from solar
scaling laws.We also show strong evidence for variation of the large separation
with frequency. The bolometric mode amplitude is only 1.7+-0.25 ppm for radial
modes, which is 1.7 times less than expected. Due to the low signal-to-noise
ratio, mode identification is not possible for the available data set of
HD175726. Conclusions. This study shows the possibility of extracting a seismic
signal despite a signal-to-noise ratio of only 0.37. The observation of such a
target shows the efficiency of the CoRoT data, and the potential benefit of
longer observing runs.Comment: 8 pages. Accepted in A&
Merging Galaxies in the SDSS EDR
We present a new catalog of merging galaxies obtained through an automated
systematic search routine. The 1479 new pairs of merging galaxies were found in
approximately 462 sq deg of the Sloan Digital Sky Survey Early Data Release
(SDSS EDR; Stoughton et al. 2002) photometric data, and the pair catalog is
complete for galaxies in the magnitude range 16.0 <= g* <= 20.
The selection algorithm, implementing a variation on the original
Karachentsev (1972) criteria, proved to be very efficient and fast. Merging
galaxies were selected such that the inter-galaxy separations were less than
the sum of the component galaxies' radii.
We discuss the characteristics of the sample in terms of completeness, pair
separation, and the Holmberg effect. We also present an online atlas of images
for the SDSS EDR pairs obtained using the corrected frames from the SDSS EDR
database. The atlas images also include the relevant data for each pair member.
This catalog will be useful for conducting studies of the general
characteristics of merging galaxies, their environments, and their component
galaxies. The redshifts for a subset of the interacting and merging galaxies
and the distribution of angular sizes for these systems indicate the SDSS
provides a much deeper sample than almost any other wide-area catalog to date.Comment: 58 pages, which includes 15 figures and 6 tables. Figures 2, 8, 9,
10, 11, 13, and 14 are provided as JPEG files. For online atlas, see
http://home.fnal.gov/~sallam/MergePair/ . Accepted for publication in A
Study of the interacting system NGC 6845
We present optical spectroscopy, B, V, R and I CCD photometry and VLA neutral
hydrogen observations of the interacting quartet NGC 6845, also know as Klemola
30. NGC 6845 A, the dominant component, sports a broad and bright tidal bridge
and a faint tidal tail, which bifurcate. The tidal bridge has a (B-I) color
bluer than that of NGC 6845 A inner disk. Five strong condensations, identified
as HII regions brighter than the brightest in our Galaxy, are found along the
tidal bridge, with the two most luminous located at the bridge tip. Two giant
HII regions, comparable to 30 Dor, are located where the tidal bridge and the
tidal tail joint the disk of NGC 6845 A. Since the age of the HII regions are
3-8 Myr, star formation has been occurring along the tidal bridge and the tidal
arm well after they had begun to be torn apart (>= 100 Myr). Satoh model
fitting to the rotation curve of the A component reveals a kinematical mass of
4.4(+-1.2)E11 M_sun, inside the central 12 kpc (H_0=75 km/s/Mpc). The HI
emission shows two components, a more massive one that belongs to NGC 6845 A,
and a second one associated to NGC 6845 B. We do not detect gas associated to
galaxies C and D. The total amount of HI is 1.4E10 M_sun, five time the HI
content of the Milky Way. The HI kinematics indicates an amount of dark matter
associated to the A component two times higher than the mass inside its central
12 kpc. The group kinematics indicates an M/L ~ 43(+-2) or M/L ~ 66(+-2) (solar
units), according to two different prescriptions for the internal absorption
correction. In spite of this difference, both values furnish similar group mass
(~1E13 M_sun). Although preliminary results on N-Body simulations indicate that
either B or C might well create a tidal feature like the bridge of the A
component, the collision with B appears to be more favourable.Comment: 24 pages, 11 JPEG figures, uses aastex.cls and natbib.sty (included).
To appear in the June/1999 issue of the Astronomical Journa
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