388 research outputs found
On the Gas Surrounding High Redshift Galaxy Clusters
Francis & Hewett (1993) identified two 10-Mpc scale regions of the high
redshift universe that were seemingly very overdense in neutral hydrogen.
Subsequent observations showed that at least one of these gas-rich regions
enveloped a cluster of galaxies at redshift 2.38. We present improved
observations of the three background QSOs with sightlines passing within a few
Mpc of this cluster of galaxies. All three QSOs show strong neutral hydrogen
absorption at the cluster redshift, suggesting that this cluster (and perhaps
all high redshift clusters) may be surrounded by a ~5 Mpc scale region
containing ~ 10^12 solar masses of neutral gas.
If most high redshift clusters are surrounded by such regions, we show that
the gas must be in the form of many small ( 0.03 cm^-3)
clouds, each of mass < 10^6 solar masses. These clouds are themselves probably
gathered into > 20 kpc sized clumps, which may be galaxy halos or
protogalaxies.
If this gas exists, it will be partially photoionised by the UV background.
We predict the diffuse Ly-alpha flux from this photoionisation, and place
observational limits on its intensity.Comment: 19 pages, 5 figures. Accepted for publication in PAS
The Double Quasar Q2138-431: Lensing by a Dark Galaxy?
We report the discovery of a new gravitational lens candidate Q2138-431AB,
comprising two quasar images at a redshift of 1.641 separated by 4.5 arcsecs.
The spectra of the two images are very similar, and the redshifts agree to
better than 115 km.sec. The two images have magnitudes and
, and in spite of a deep search and image subtraction procedure, no
lensing galaxy has been found with . Modelling of the system
configuration implies that the mass-to-light ratio of any lensing galaxy is
likely to be around , with an absolute lower limit of
for an Einstein-de Sitter universe. We conclude that
the most likely explanation of the observations is gravitational lensing by a
dark galaxy, although it is possible we are seeing a binary quasar.Comment: 17 pages (Latex), 8 postscript figures included, accepted by MNRA
Diagnostics of the structure of AGN's broad line regions with reverberation mapping data: confirmation of the two-component broad line region model
We re-examine the ten Reverberation Mapping (RM) sources with public data
based on the two-component model of the Broad Line Region (BLR). In fitting
their broad H-beta lines, six of them only need one Gaussian component, one of
them has a double-peak profile, one has an irregular profile, and only two of
them need two components, i.e., a Very Broad Gaussian Component (VBGC) and an
Inter-Mediate Gaussian Component (IMGC). The Gaussian components are assumed to
come from two distinct regions in the two-component model; they are Very Broad
Line Region (VBLR) and Inter-Mediate Line region (IMLR). The two sources with a
two-component profile are Mrk 509 and NGC 4051. The time lags of the two
components of both sources satisfy ,
where and are the lags of the two components while
and represent the mean gas velocities of the two regions,
supporting the two-component model of the BLR of Active Galactic Nuclei (AGN).
The fact that most of these ten sources only have the VBGC confirms the
assumption that RM mainly measures the radius of the VBLR; consequently, the
radius obtained from the R-L relationship mainly represent the radius of VBLR.
Moreover, NGC 4051, with a lag of about 5 days in the one component model, is
an outlier on the R-L relationship as shown in Kaspi et al. (2005); however
this problem disappears in our two-component model with lags of about 2 and 6
days for the VBGC and IMGC, respectively.Comment: 7 pages, 5 figures. Accepted for publication in the Special Issue of
Science in China (G) "Astrophysics of Black holes and Related Compact
Objects
Le retour au village : une solution Ă la crise Ă©conomique au Cameroun ?
La crise Ă©conomique en Afrique subsaharienne et les politiques d'ajustement structurel mises en oeuvre pour y remĂ©dier ne laissent pas de provoquer des effets sociaux nĂ©gatifs, surtout dans les villes et au sein de la classe moyenne urbaine. La migration de retour, en direction du village d'origine, en est une consĂ©quence. Dans une enquĂȘte rĂ©alisĂ©e au Cameroun, les auteurs cherchent Ă rĂ©pondre aux questions les plus importantes qui se posent Ă ce sujet : quelle est l'ampleur du phĂ©nomĂšne actuel du retour au village ? Quels sont les problĂšmes rencontrĂ©s par les migrants dans leur tentative de rĂ©insertion dans un milieu qu'ils ont depuis longtemps quittĂ© ? Comment ceux qui ont rĂ©sidĂ© en ville perçoivent-ils les conditions de vie qui y prĂ©valent par rapport Ă celles du village ? Qu'a retirĂ© le migrant de son dĂ©placement Ă l'extĂ©rieur ? Le migrant de retour envisage-t'il de rester au village ou au contraire de repartir Ă la premiĂšre occasion ? L'analyse conduit finalement Ă une typologie du migrant de retour. Enfin, est-il raisonnable de fonder une politique de dĂ©sengorgement des grands centres urbains, oĂč se posent les problĂšmes sociaux et politiques les plus aigus, sur la migration de retour ? (RĂ©sumĂ© d'auteur
Molecular lines as tracers of Compton-thick AGN ?
Recently, Papadopoulos et al., 2010 using sub-mm CO molecular line
observations of nearby ultra-luminous IRAS galaxies, (U)LIRGs, have found that
exceptionally large gas column densities (N_H > 10^25 cm-2) can be present
across some of the very dense gaseous disks that are typically found in these
objects. They also proposed a diagnostic for finding such sources using CO and
HCN molecular lines. Given that such high column densities are expected to
absorb any X-ray luminous AGN, yielding Compton-thick sources, we set out
toexplore whether this can be discerned using X-ray observations. More
specifically we examine X-ray spectral observations of 14 sources in their
sample, using public Chandra observations (0.5-10 keV) for eleven sources as
well as BeppoSAX results (2-100 keV) from the literature for another three
sources. Our goal is to find candidate Compton-thick AGN and to check whether
the molecular line selection criterion is successful in selecting such systems.
X-ray spectroscopy reveals four candidate Compton-thick AGN of which half fall
within the high obscuration region in the molecular line ratio diagnostics. Of
the remaining five sources falling into the `high dust obscuration' box, one
(Mrk273) is highly obscured (N_H ~4x10^23 cm-2) while in the other four the
X-ray emission is most probably associated with star-forming processes rather
than an AGN on the basis of their X-ray and mid-infrared properties. Overall,
we argue that although this method as expected cannot recover all Compton-thick
AGN, there are no examples of X-ray luminous AGN inside that region that have
low obscuration, suggesting that this method is efficient in finding heavily
obscured AGN in dust-enshrouded star-forming galaxies. The above results bear
important implications for future joint ALMA and X-ray observations for the
detection of Compton-thick AGN.Comment: To appear in A&A Letter
Constraining the Cosmological Density of Compact Objects with the Long-Term Variability of Quasars
By comparing the results from numerical microlensing simulations to the
observed long-term variability of quasars, strong upper limits on the
cosmological density of compact objects in the 0.0001-1 solar mass range may in
principle be imposed. Here, this method is generalized from the Einstein-de
Sitter universe to the currently favored Omega_M=0.3, Omega_Lambda=0.7
cosmology and applied to the latest observational samples. We show that the use
of high-redshift quasars from variability-selected samples has the potential to
substantially improve current constraints on compact objects in this mass
range. We also investigate to what extent the upper limits on such hypothetical
dark matter populations are affected by assumptions concerning the size of the
optical continuum-emitting region of quasars and the velocity dispersion of
compact objects. We find that mainly due to uncertainties in the typical value
of the source size, cosmologically significant populations of compact objects
cannot safely be ruled out with this method at the present time.Comment: 10 pages, 7 figures, accepted for publication in A&
Asymmetrical structure of ionization and kinematics in the Seyfert galaxy NGC 5033
We present integral field spectroscopy of NGC 5033, a low luminosity Seyfert
galaxy. The observations were made with INTEGRAL, a fiber based system
operating at the WHT. The intensity map of the H emission line
represents a spiral or ring-like pattern of HII regions. On the contrary, the
[OIII] intensity map morphology is markedly anisotropic. The strong
morphological differences imply that the [OIII] emitters represent highly
ionized gas illuminated by the central source. The [OIII] map morphology is
compatible with a biconical structure of ionization induced by strong
extinction in the galaxy disc that also obscures half of the spheroidal stellar
bulge. We identify the spectrum corresponding to the Seyfert 1 nucleus from the
presence of H broad emission lines. This spectrum is located in a region
where strong extinction is expected but exhibits the bluest spectral energy
distribution. The Seyfert 1 nucleus seems to be offcenter with respect to the
stellar rotation center. This result has been also found in other Seyfert
galaxies and interpreted in terms of a past merger. The offcentering could
indicate the presence of nonsymmetric departures in the gravitational potential
which could be fueling the active nucleus. The kinematics of the [OIII]
emitters show important deviations at a kpc scale with respect to the stellar
velocity field and show features related to the asymmetrical morphology of the
high ionization region.Comment: 9 pages, accepted for publication in Astronomy and Astrophysics.
Figures 1 and 7 are attached as .gif file
On Uniqueness of Boundary Blow-up Solutions of a Class of Nonlinear Elliptic Equations
We study boundary blow-up solutions of semilinear elliptic equations
with , or with , where is a second order
elliptic operator with measurable coefficients. Several uniqueness theorems and
an existence theorem are obtained.Comment: To appear in Comm. Partial Differential Equations; 10 page
Enhanced Star Formation in Narrow Line Seyfert 1 AGN revealed by Spitzer
We present new low resolution Spitzer mid-infrared spectroscopy of a sample
of 20 ROSAT selected local Narrow Line Seyfert 1 galaxies (NLS1s). We detect
strong AGN continuum in all and clear PAH emission in 70% of the sources. The
6.2 micron PAH luminosity spans three orders of magnitudes, from ~10^(39) erg/s
to ~10^(42) erg/s providing strong evidence for intense ongoing star formation
in the circumnuclear regions of these sources. Using the IRS/Spitzer archive we
gather a large number of additional NLS1s and their broad line counterparts
(BLS1s) and constructed NLS1 and BLS1 sub-samples to compare them in various
ways. The comparison shows a clear separation according to FWHM(H_beta) such
that objects with narrower broad H_beta lines are the strongest PAH emitters.
We test this division in various ways trying to remove biases due to luminosity
and aperture size. Specifically, we find that star formation activity around
NLS1 AGN is larger than around BLS1 of the same AGN luminosity. The above
result seems to hold over the entire range of distance and luminosity. Moreover
the star formation rate is higher in low black hole mass and high L/L_Edd
systems indicating that black hole growth and star formation are occurring
simultaneously.Comment: 30 pages, 11 figures, 4 tables. Now accepted in MNRA
Expression of NM23 in human melanoma progression and metastasis.
NM23 is a putative metastasis-suppressor gene for some human cancers. Here we have studied NM23 expression during melanoma progression using Northern blotting and immunocytochemistry. There was no significant difference in the average amounts of NM23 mRNA between cell lines derived from metastatic and primary melanomas. The level of NM23 mRNA was also determined for three pairs of poorly metastatic parental (P) and their highly metastatic variant (M) cell lines; the ratios for M/P were 1.2, 0.98 and 0.80. Next we used immunocytochemistry to study NM23 protein in normal skin, benign naevi and primary and metastatic melanomas. Melanocytes in all normal skin and benign samples were positive for NM23; however most primary melanomas (7/11) were not stained by the antibody. All metastatic melanoma samples (5/5) were positively stained. Findings were similar with an antiserum reactive with both forms of NM23 (H1 and H2), and with an antibody specific for NM23-H1. No relationship was apparent between NM23 immunoreactivity in primary tumours and their aggressiveness or prognosis. Hence, in contrast to the situation described for murine melanoma, the amount of NM23 mRNA or protein in human melanoma did not correlate inversely with metastasis
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