16,956 research outputs found
The tidally disturbed luminous compact blue galaxy Mkn 1087 and its surroundings
We present new broad-band optical and near-infrared CCD imaging together with
deep optical intermediate-resolution spectroscopy of Mkn 1087 and its
surrounding objects. We analyze the morphology and colors of the stellar
populations of the brightest objects, some of them star-formation areas, as
well as the kinematics, physical conditions and chemical composition of the
ionized gas associated with them. Mkn 1087 does not host an Active Galactic
Nucleus, but it could be a Luminous Compact Blue Galaxy. Although it was
classified as a suspected Wolf-Rayet galaxy, we do not detect the spectral
features of these sort of massive stars. Mkn 1087 shows morphological and
kinematical features that can be explained assuming that it is in interaction
with two nearby galaxies: the bright KPG 103a and a dwarf ()
star-forming companion. We argue that this dwarf companion is not a tidal
object but an external galaxy because of its low metallicity [12+log(O/H) =
8.24] with respect to the one derived for Mkn 1087 [12+log(O/H) = 8.57] and its
kinematics. Some of the non-stellar objects surrounding Mkn 1087 are connected
by bridges of matter with the main body, host star-formation events and show
similar abundances despite their different angular distances. These facts,
together their kinematics, suggest that they are tidal dwarf galaxies formed
from material stripped from Mkn 1087. A bright star-forming region at the south
of Mkn 1087 (knot #7) does not show indications of being a tidal galaxy or the
product of a merging process as suggested in previous works. We argue that Mkn
1087 and its surroundings should be considered a group of galaxies.Comment: Accepted by A&A, 21 pages, 13 figures, 8 table
The interacting pair Mkn 305/306
Direct images and spectra at different slit positions of the interacting system Mkn 305/306 are discussed. Both galaxies show starburst properties due to tidal interaction. The morphology and velocity structure of Mkn 306 reveals the strongest warp of a stellar disk so far known. The galaxies Mkn 305 and Mkn 306 form a double system with 30 arcsec separation and having a common envelope at m sub B greater than or equal to 24.5. Furthermore a small tidal tail west of Mkn 305A, an isophote twist of Mkn 305 and the near identical redshifts of the two galaxies prove that this is a physical pair. Mkn 306 itself was classified as a double nucleus galaxy (Petrosyan et al., 1978). The optical morphology of Mkn 306 has the form of an integral sign which is similar to the radio morphology of strongly warped galaxies (Bottema et al., 1987). But in the optical the warp of the stellar component is normally far weaker than in the radio for the HI-gas. Therefore the authors investigate whether the double nucleus structure of Mkn 306 is real or an artifact due to an extreme warp in the optical. The origin for the warp phenomenon is not clear yet. Direct images of the galaxy system were taken with the Calar Alto 2.2m telescope in the B-band and at La Silla in the r-band using the 2.2m telescope; low dispersion spectra (240 A/mm) as well as high dispersion spectra (56 A/mm) for studying the velocity field were taken with the Calar Alto 3.5m telescope at different position angles. Also a spectrum of Mkn 305 was taken at Calar Alto with the 3.5m telescope covering the whole spectral range (240 A/mm)
The double nucleus galaxies Mkn 423 and Mkn 739
Long slit spectroscopy and imaging of Mkn 423 and Mkn 739 were performed on the 2.2 m and 3.5 m telescopes of the Calar Alto Observatory using both change coupled device (CCD) and photographic detectors. Low and medium resolution spectra (1.8A, 3.5A, 6.0A) together with V images have permitted the demonstration of the merging nature of Mkn 423 and the double structure of the narrow line region (NLR) of its Seyfert component. This last feature has been found also in the NLR of the Seyfert component of Mkn 739, a double system the two components of which are counterrotating
On the origin of highest energy gamma-rays from Mkn 501
The spectra of very high energy gamma-radiation from distant extragalactic
objects suffer significant deformations during the passage of primary
gamma-rays through the intergalactic medium. The recently reported fluxes of
diffuse infrared background radiation indicate that we detect, most probably,
heavily absorbed TeV radiation from Mkn 421 and Mkn 501. This implies that the
absorption-corrected spectrum of Mkn 501 may contain a sharp pile-up which
contradicts to the predictions of the conventional models of TeV blazars, and
thus may leads to the so-called "IR background-TeV gamma-ray crisis". To
overcome this difficulty, in this paper we propose two independent hypotheses
assuming that (i) the TeV radiation from Mkn 501 has a secondary origin, i.e.
it is formed during the development of electron-photon cascades in the
intergalactic medium initiated by primary gamma-rays; (ii) the pile-up in the
source spectrum is a result of comptonization (in deep Klein-Nishina regime) of
ambient optical radiation by an ultrarelativistic conical cold outflow (jet)
with bulk motion Lorentz factor Gamma_0 >= 3 10^7. Within the uncertainties
caused by the limited energy resolution of spectral measurements, the observed
TeV radiation of Mkn 501 formally can be explained by the intergalactic cascade
gamma-rays, assuming however an extremely low intergalactic magnetic field in
the direction to the source at the level of < 10^{-18} G. We also demonstrate
that the "bulk motion comptonization" scenario can quite naturally reproduce
the unusual spectral features in the absorption-corrected TeV spectrum of Mkn
501, and briefly discuss the astrophysical implications of this hypothesis.Comment: 14 pages, accepted by Astronomy & Astrophysics (revised version;
conceptual changes concerning interpretation of the observed spectrum of Mkn
501 as a spectrum of the electron-positron cascade in the intergalactic
medium initiated by primary gamma-rays
Discovery of the Narrow-Line Seyfert 1 galaxy Mkn 335 in an historical low X-ray flux state
We report the discovery of the Narrow-Line Seyfert 1 galaxy Mkn 335 in an
extremely low X-ray state. A comparison of Swift observations obtained in May
and June/July 2007 with all previous X-ray observations between 1971 to 2006
show the AGN to have diminished in flux by a factor of more than 30, the lowest
X-ray flux Mkn 335 has ever been observed in. The Swift observations show an
extremely hard X-ray spectrum at energies above 2 keV. Possible interpretations
include partial covering absorption or X-ray reflection from the disk. In this
letter we consider the partial covering interpretation. The Swift observations
can be well fit by a strong partial covering absorber with varying absorption
column density N_H= 1-4 x 10^{23} cm-2 and a covering fraction f_c=0.9 - 1.
When corrected for intrinsic absorption, the X-ray flux of Mkn 335 varies by
only factors of 4-6. In the UV Mkn 335 shows variability in the order of 0.2
mag. We discuss the similarity of Mkn 335 with the highly variable NLS1
WPVS007, and speculate about a possible link between NLS1 galaxies and
broad-absorption line quasars.Comment: ApJ Letter accepted; 8 pages, 2 figures; The new version has three
more sentences in the introduction and three references added to the
discussio
Near-infrared spectra of Seyfert galaxies and line production mechanisms
New observations are reported of J-band spectra (1.04micron -- 1.4 micron) of
three Seyfert 2 galaxies, Mkn 34, Mkn 78 and NGC 5929. In each case the
spectral range includes the near-infrared lines of [FeII], [PII], HeI and Pa
beta. Each Seyfert galaxy has a known radio jet, and we investigate the
infrared line ratios of the nuclear and extended regions of each galaxy
compared to the radio structure. In Mkn 34 there is a clear indication of an
extranuclear region, probably coincident with a shock induced by the radio jet,
in which [FeII] is considerably enhanced, although the nuclear emission is
almost certainly the result of photoionization by the continuum of the active
nucleus. Similar effects in extranuclear regions are seen in the other objects,
in the case of Mkn 78 confirming recent studies by Ramos Almeida et al. A
possible detection of extranuclear [PII] emission suggests, if real, that
photoionization by the active nucleus is the dominant line excitation mechanism
over the whole source, including the regions coincident with the radio jet.Comment: 8 pages, 9 figures. Accepted by MNRA
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