125 research outputs found
Near-IR photometry of disk galaxies: search for nuclear isophotal twist and double bars
We present a near-IR, mainly H band, photometry of 72 nearby disk galaxies.
The main goal of the survey was to search for isophotal twist inside their
nuclear regions. As the twist can be due in some cases to projection effects,
rather than resulting from a dynamical phenomenon, we deproject - under the
simplifying assumption of a 2D geometry - all galaxies whose disk position
angle and inclination are known, the latter not exceeding 75 degrees. We show
the ellipticity, position angle and surface brightness radial profiles, and
discuss how a projection of 2D and 3D bars can distort the isophotes, give an
illusion of a non-existing double bar or mask a real one. We report 15 new
double-barred galaxies and confirm 2 detected previously. We identify 14
additional twists not known before and we also find nuclear triaxial structures
in three SA galaxies. The frequency of Seyferts among galaxies with nuclear
bars or twists is high. As a secondary product, we publish structural
parameters (length and axis ratio) of large-scale bars in order to extend still
scarce data on bars in the near-IR.Comment: 11 pages of text (Astron. & Astroph. LaTeX l-aa macro) with 3
postscript figures, 7 additional pages of non-main-body postscript figures
containing contour and ellipse fitting plots of 72 galaxies; accepted by
Astronomy & Astrophysics Suppl. Se
Size and properties of the narrow-line region in Seyfert-2 galaxies from spatially-resolved optical spectroscopy
While [OIII] narrow-band imaging is commonly used to measure the size of the
narrow-line regions (NLRs) in active galactic nuclei (AGNs), it can be
contaminated by emission from surrounding starbursts. Recently, we have shown
that long-slit spectroscopy provides a valuable alternative approach to probe
the size in terms of AGN photoionisation. Moreover, several parameters of the
NLR can be directly accessed. We here apply the same methods developed and
described for the Seyfert-2 galaxy NGC1386 to study the NLR of five other
Seyfert-2 galaxies by using high-sensitivity spatially-resolved optical
spectroscopy obtained at the VLT and the NTT. We probe the AGN-photoionisation
of the NLR and thus, its ``real'' size using diagnostic line-ratio diagrams.We
derive physical properties of the NLR such as reddening, ionisation parameter,
electron density, and velocity as a function of distance from the nucleus. For
NGC5643, the diagnostic diagrams unveil a similar transition between line
ratios falling in the AGN regime and those typical for HII regions as found for
NGC1386, thus determining the size of the NLR. For the other four objects, all
measured line ratios fall in the AGN regime. In almost all cases, both electron
density and ionisation parameter decrease with radius. Deviations from this
general behaviour (such as a secondary peak) seen in both the ionisation
parameter and electron density can be interpreted as signs of shocks from the
interaction of a radio jet and the NLR gas. In several objects, the gaseous
velocity distribution is characteristic for rotational motion in an (inclined)
emission-line disk in the centre. We compare our results to those of NGC1386
and show that the latter can be considered as prototypical also for this larger
sample. We discuss our findings in detail for each object.Comment: 23 pages, 41 figures, accepted for publication in A&
Testing MOND gravity in the shell galaxy NGC 3923
Context. The elliptical galaxy NGC 3923 is surrounded by numerous stellar
shells that are concentric arcs centered on the galactic core. They are very
likely a result of a minor merger and they consist of stars in nearly radial
orbits. For a given potential, the shell radii at a given time after the merger
can be calculated and compared to observations. The Modified Newtonian Dynamics
(MOND) is a theory that aims to solve the missing mass problem by modifying the
laws of classical dynamics in the limit of small accelerations. Hernquist &
Quinn(1987) claimed that the shell distribution of NGC 3923 contradicted MOND,
but Milgrom(1988) found several substantial insufficiencies in their work.
Aims. We test whether the observed shell distribution in NGC 3923 is
consistent with MOND using the current observational knowledge of the shell
number and positions and of the host galaxy surface brightness profile, which
supersede the data available in the 1980s when the last (and negative) tests of
MOND viability were performed on NGC 3923.
Methods. Using the 3.6 um bandpass image of NGC 3923 from the Spitzer space
telescope we construct the mass profile of the galaxy. The evolution of shell
radii in MOND is then computed using analytical formulae. We use 27 currently
observed shells and allow for their multi-generation formation, unlike the
Hernquist & Quinn one-generation model that used the 18 shells known at the
time.
Results. Our model reproduces the observed shell radii with a maximum
deviation of 5% for 25 out of 27 known shells while keeping a reasonable
formation scenario. A multi-generation nature of the shell system, resulting
from successive passages of the surviving core of the tidally disrupted dwarf
galaxy, is one of key ingredients of our scenario supported by the extreme
shell radial range. The 25 reproduced shells are interpreted as belonging to
three generations.Comment: 8 pages, 3 figures, Accepted for publication in A&
Shell galaxies as laboratories for testing MOND
Tests of MOND in ellipticals are relatively rare because these galaxies often
lack kinematic tracers in the regions where the MOND effects are significant.
Stellar shells observed in many elliptical galaxies offer a promising way to
constrain their gravitational field. Shells appear as glowing arcs around their
host galaxy. They are observed up to ~100 kpc. The stars in axially symmetric
shell systems move in nearly radial orbits. The radial distributions of shell
locations and the spectra of stars in shells can be used to constrain the
gravitational potential of their host galaxy. The symmetrical shell systems,
being especially suitable for these studies, occur in approximately 3% of all
early-type galaxies. Hence the shells substantially increase the number of
ellipticals in which MOND can be tested up to large radii. In this paper, we
review our work on shell galaxies in MOND. We summarize the paper B\'{i}lek et
al. (2013), where we demonstrated the consistency of shell radii in an
elliptical NGC 3923 with MOND, and the work B\'{i}lek et al. (2014), in which
we predicted a giant (~200 kpc), as yet undiscovered shell of NGC 3923. We
explain the shell identification method, which was used in these two papers. We
further describe the expected shape of line profiles in shell spectra in MOND
which is very special due to the direct relation of the gravitational field and
baryonic matter distribution (B\'{i}lek et al., 2014, in preparation).Comment: 21 pages, 8 figures, 1 table, accepted for publication in the special
MOND issue of the Canadian Journal of Physic
Quadruple-peaked Line-of-sight Velocity Distributions in Shell Galaxies
We present an improved study of the expected shape of the line-of-sight
velocity distribution in shell galaxies. We found a simple analytical
expression connecting prominent and in principle observable characteristics of
the line profile and mass-distribution of the galaxy. The prediction was
compared with the results from a test-particle simulation of a radial merger.Comment: 2 pages, 1 figure, to appear in the Proceedings of JENAM 2010,
Symposium 2: "Environment and the formation of galaxies: 30 years later
Evidence for Merger Remnants in Early-Type Host Galaxies of Low-Redshift QSOs
We present results from a pilot HST ACS deep imaging study in broad-band V of
five low-redshift QSO host galaxies classified in the literature as
ellipticals. The aim of our study is to determine whether these early-type
hosts formed at high redshift and have since evolved passively, or whether they
have undergone relatively recent mergers that may be related to the triggering
of the nuclear activity. We perform two-dimensional modeling of the light
distributions to analyze the host galaxies' morphology. We find that, while
each host galaxy is reasonably well fitted by a de Vaucouleurs profile, the
majority of them (4/5) reveal significant fine structure such as shells and
tidal tails. These structures contribute between ~5% and 10% to the total
V-band luminosity of each host galaxy within a region of r ~ 3 r_eff and are
indicative of merger events that occurred between a few hundred Myr and a Gyr
ago. These timescales are comparable to starburst ages in the QSO hosts
previously inferred from Keck spectroscopy. Our results thus support a
consistent scenario in which most of the QSO host galaxies suffered mergers
with accompanying starbursts that likely also triggered the QSO activity in
some way, but we are also left with considerable uncertainty on physical
mechanisms that might have delayed this triggering for several hundred Myr
after the merger.Comment: 22 pages, 4 figures. Accepted for publication in the Astrophysical
Journa
Deep imaging of the shell elliptical galaxy NGC3923 with MegaCam
Context. The elliptical galaxy NGC 3923 is known to be surrounded by a number
of stellar shells, probable remnants of an accreted galaxy. Despite its
uniqueness, the deepest images of its outskirts come from the 1980s. On the
basis of the modified Newtonian dynamics (MOND), it has recently been predicted
that a new shell lies in this region.
Aims. We obtain the deepest image ever of the galaxy, map the tidal features
in it, and search for the predicted shell.
Methods. The image of the galaxy was taken by the MegaCam camera at the
Canada-France-Hawaii Telescope in the g' band. It reached the
surface-brightness limit of 29 mag/arcsec2. In addition, we reanalyzed an
archival HST image of the galaxy.
Results. We detected up to 42 shells in NGC 3923. This is by far the highest
number among all shell galaxies. We present the description of the shells and
other tidal features in the galaxy. A probable progenitor of some of these
features was discovered. The shell system likely originates from two or more
progenitors. The predicted shell was not detected, but the new image revealed
that the prediction was based on incorrect assumptions and poor data.Comment: 14 pages, 2 tables, 19 figures, accepted for publication in A&
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