126 research outputs found
Magnetic order and spin dynamics across a ferromagnetic quantum critical point: SR investigations of YbNi(PAs)
In the quasi-1D heavy-fermion system YbNi(PAs) the
presence of a ferromagnetic (FM) quantum critical point (QCP) at with unconventional quantum critical exponents in the thermodynamic
properties has been recently reported. Here, we present muon-spin relaxation
(SR) experiments on polycrystals of this series to study the magnetic
order and the low energy 4-electronic spin dynamics across the FM QCP. The
zero field SR measurements on pure YbNi(P proved static long
range magnetic order and suggested a strongly reduced ordered Yb moment of
about 0.04. With increasing As substitution the ordered moment is
reduced by half at and to less than 0.005 at . The
dynamic behavior in the SR response show that magnetism remains
homogeneous upon As substitution, without evidence for disorder effect. In the
paramagnetic state across the FM QCP the dynamic muon-spin relaxation rate
follows 1/ with . The critical fluctuations are very slow and are even becoming slower
when approaching the QCP.Comment: 6 pages, 4 figure
Fast recovery of the stripe magnetic order by Mn/Fe substitution in F-doped LaFeAsO superconductors
As Nuclear Magnetic (NMR) and Quadrupolar (NQR) Resonance were used,
together with M\"{o}ssbauer spectroscopy, to investigate the magnetic state
induced by Mn for Fe substitutions in F-doped LaFeMnAsO
superconductors. The results show that % of Mn doping is enough to
suppress the superconducting transition temperature from 27 K to zero and
to recover the magnetic structure observed in the parent undoped LaFeAsO. Also
the tetragonal to orthorhombic transition of the parent compound is recovered
by introducing Mn, as evidenced by a sharp drop of the NQR frequency. The NQR
spectra also show that a charge localization process is at play in the system.
Theoretical calculations using a realistic five-band model show that
correlation-enhanced RKKY exchange interactions between nearby Mn ions
stabilize the observed magnetic order, dominated by and
ordering vectors. These results give compelling evidence that
F-doped LaFeAsO is a strongly correlated electron system at the verge of an
electronic instability.Comment: 5 pages, 5 figures and 4 pages of supplemental materia
HI Observations of the Stephan's Quintet
Using the VLA, we have made spectral-line and continuum observations of the
neutral hydrogen in the direction of the compact group of galaxies Stephan's
Quintet. The high-velocity clouds between 5600 and 6600 km/s, the disk of the
foreground galaxy, NGC 7320, at 800 km/s, the extended continuum ridge near the
center of the group, and 3 faint dwarf-like galaxies in the surrounding field
were imaged with C, CS, and D arrays. Four of the HI clouds previously detected
are confirmed. The two largest HI features are coincident with and concentrated
mainly along separate large tidal tails that extend eastward. The most diffuse
of the four clouds is resolved into two clumps, one coincide with tidal
features south of NGC 7318a and the other devoid of any detectable stellar or
Halfa sources. The two compact clouds, along the same line of sight, have peak
emission at luminous infrared and bright Halfa sources probably indicative of
star-forming activity. The total amount of HI detected at high redshifts is ~
10**10Msol. As in previous HI studies of the group, no detectable emission was
measured at the positions of any high-redshift galaxies so that any HI still
bound to their disks must be less than 2.4 x 10**7Msol.Comment: Accepted for publication in AJ and tentatively scheduled for the May
2002 issue. High-resolution ps figures are available at:
http://www.iaa.es/~lourdes/bw/bw-paper.tar.g
Cepheid Calibration of the Peak Brightness of SNe Ia -- IX. SN 1989B in NGC 3627
(Abridged) Repeated imaging observations have been made of NGC 3627 with the
HST in 1997/98, over an interval of 58 days. Images were obtained on 12 epochs
in the F555W band and on five epochs in the F814W band. The galaxy hosted the
prototypical, `Branch normal', type Ia supernova SN 1989B. A total of 83
variables have been found, of which 68 are definite Cepheid variables with
periods ranging from 75 days to 3.85 days. The de-reddened distance modulus is
determined to be (m-M)_0= 30.22+/-0.12 (internal uncertainty) using a subset of
the Cepheid data whose reddening and error parameters are secure.
The photometric data of Wells et al. (1994), combined with the Cepheid data
for NGC 3627 give M_B(max)= -19.36+/-0.18 and M_V(max)= -19.34+/-0.16 for SN
1989B. Combined with the previous six calibrations in this program, plus two
additional calibrations determined by others gives the mean absolute magnitudes
at maximum of = -19.48+/-0.07 and = -19.48 +/-0.07 for `Branch
normal' SNe Ia at this interim stage in the calibration program.
The second parameter correlations of M(max) of blue SNe Ia with decay rate,
color at maximum, and Hubble type are re-investigated. The dependence of
on decay rate is non-linear, showing a minimum for decay rates between
1.0< Delta m_15 <1.6. Magnitudes corrected for decay rate show no dependence on
Hubble type, but a dependence on color remains. Correcting both the fiducial
sample of 34 SNe Ia with decay-rate data and the current 8 calibrating SNe Ia
for the correlation with decay rate as well as color gives H_0= 60+/-2
(internal) km/s/Mpc, in both B and V. The same value to within 4% is obtained
if only the SNe Ia in spirals (without second parameter corrections) are
considered.Comment: 32 pages (with 7 tables and 14 figures) LaTeX, uses emulateapj.sty; a
full-resolution version with complete figs. 4 and 5 is available at
http://www.astro.unibas.ch/cosmology/papers.html ; accepted for publication
in Ap
Nearby Optical Galaxies: Selection of the Sample and Identification of Groups
In this paper we describe the Nearby Optical Galaxy (NOG) sample, which is a
complete, distance-limited (6000 km/s) and magnitude-limited
(B14) sample of 7000 optical galaxies. The sample covers 2/3 (8.27
sr) of the sky () and appears to have a good completeness in
redshift (98%). We select the sample on the basis of homogenized corrected
total blue magnitudes in order to minimize systematic effects in galaxy
sampling.
We identify the groups in this sample by means of both the hierarchical and
the percolation {\it friends of friends} methods. The resulting catalogs of
loose groups appear to be similar and are among the largest catalogs of groups
presently available. Most of the NOG galaxies (60%) are found to be
members of galaxy pairs (580 pairs for a total of 15% of objects)
or groups with at least three members (500 groups for a total of
45% of objects). About 40% of galaxies are left ungrouped (field
galaxies). We illustrate the main features of the NOG galaxy distribution.
Compared to previous optical and IRAS galaxy samples, the NOG provides a denser
sampling of the galaxy distribution in the nearby universe. Given its large sky
coverage, the identification of groups, and its high-density sampling, the NOG
is suited for the analysis of the galaxy density field of the nearby universe,
especially on small scales.Comment: 47 pages including 6 figures. Accepted for publication in Ap
Galaxy Distances in the Nearby Universe: Corrections For Peculiar Motions
By correcting the redshift--dependent distances for peculiar motions through
a number of peculiar velocity field models, we recover the true distances of a
wide, all-sky sample of nearby galaxies (~ 6400 galaxies with velocities
cz<5500 km/s), which is complete up to the blue magnitude B=14 mag. Relying on
catalogs of galaxy groups, we treat ~2700 objects as members of galaxy groups
and the remaining objects as field galaxies.
We model the peculiar velocity field using: i) a cluster dipole
reconstruction scheme; ii) a multi--attractor model fitted to the Mark II and
Mark III catalogs of galaxy peculiar velocities. According to Mark III data the
Great Attractor has a smaller influence on local dynamics than previously
believed, whereas the Perseus-Pisces and Shapley superclusters acquire a
specific dynamical role. Remarkably, the Shapley structure, which is found to
account for nearly half the peculiar motion of the Local Group, is placed by
Mark III data closer to the zone of avoidance with respect to its optical
position.
Our multi--attractor model based on Mark III data favors a cosmological
density parameter Omega ~ 0.5 (irrespective of a biasing factor of order
unity). Differences among distance estimates are less pronounced in the ~ 2000
- 4000 km/s distance range than at larger or smaller distances. In the last
regions these differences have a serious impact on the 3D maps of the galaxy
distribution and on the local galaxy density --- on small scales.Comment: 24 pages including (9 eps figures and 7 tables). Figures 1,2,3,4 are
available only upon request. Accepted by Ap
The Discovery of New Galaxy Members in the NGC 5044 and NGC 1052 Groups
We present the results of neutral hydrogen (HI) observations of the NGC 5044
and NGC 1052 groups, as part of a GEMS (Group Evolution Multiwavelength Study)
investigation into the formation and evolution of galaxies in nearby groups.
Two new group members have been discovered during a wide-field HI imaging
survey conducted using the ATNF Parkes telescope. These results, as well as
those from followup HI synthesis and optical imaging, are presented here.
J1320-1427, a new member of the NGC 5044 Group, has an HI mass of
M_HI=1.05e9Msun and M_HI/L_B=1.65 Msun/Lsun, with a radial velocity of
v=2750km/s. The optical galaxy is characterised by two regions of star
formation, surrounded by an extended, diffuse halo. J0249-0806, the new member
of the NGC 1052 Group, has M_HI=5.4e8Msun, M_HI/L_R=1.13 Msun/Lsun and
v=1450km/s. The optical image reveals a low surface brightness galaxy. We
interpret both of these galaxies as irregular type, with J0249-0806 possibly
undergoing first infall into the NGC 1052 group.Comment: Accepted for publication in MNRAS. 21 pages, 13 figures. Also
available with high-resolution figures at
http://www.astro.livjm.ac.uk/~npfm/Papers/mmb04.ps.g
Merging Binary Clusters
We study three prominent bi-modal X-ray clusters: A3528, A1750 and A3395.
Using observations taken with ROSAT and ASCA, we analyze the temperature and
surface brightness distributions. We also analyze the velocity distributions of
the three clusters using new measurements supplemented with previously
published data. We examine both the overall cluster properties, as well as the
two sub-cluster elements in each. These results are then applied to the
determination of the overall cluster masses, and demonstrate excellent
consistency between the various methods used. While the characteristic
parameters of the sub-clusters are typical of isolated objects, our temperature
results for the regions between the two sub-clusters clearly confirm the
presence of merger activity. These three clusters represent a progression of
equal-sized sub-cluster mergers, starting from initial contact to immediately
before first core passage.Comment: 12 pages, 3 full page figures in color, accepted for publication in
the Astrophysical Journa
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