14 research outputs found
Resolution of the Compact Radio Continuum Sources in Arp220
We present 2 cm and 3.6 cm wavelength very long baseline interferometry
images of the compact radio continuum sources in the nearby ultra-luminous
infrared galaxy Arp220. Based on their radio spectra and variability
properties, we confirm these sources to be a mixture of supernovae (SNe) and
supernova remnants (SNRs). Of the 17 detected sources we resolve 7 at both
wavelengths. The SNe generally only have upper size limits. In contrast all the
SNRs are resolved with diameters {\geq} 0.27 pc. This size limit is consistent
with them having just entered their Sedov phase while embedded in an
interstellar medium (ISM) of density 10^4 cm^{-3} . These objects lie on the
diameter-luminosity correlation for SNRs (and so also on the diameter-surface
brightness relation) and extend these correlations to very small sources. The
data are consistent with the relation L {\propto} D^{-9/4}. Revised
equipartition arguments adjusted to a magnetic field to relativistic particle
energy density ratio of 1% combined with a reasonable synchrotron-emitting
volume filling factor of 10% give estimated magnetic field strengths in the SNR
shells of ~ 15-50 mG. The SNR shell magnetic fields are unlikely to come from
compression of ambient ISM fields and must instead be internally generated. We
set an upper limit of 7 mG for the ISM magnetic field. The estimated energy in
relativistic particles, 2%-20% of the explosion kinetic energy, is consistent
with estimates from models that fit the IR-radio correlation in compact
starburst galaxies.Comment: 16 pages, 5 figure
Multiple populations in globular clusters. Lessons learned from the Milky Way globular clusters
Recent progress in studies of globular clusters has shown that they are not
simple stellar populations, being rather made of multiple generations. Evidence
stems both from photometry and spectroscopy. A new paradigm is then arising for
the formation of massive star clusters, which includes several episodes of star
formation. While this provides an explanation for several features of globular
clusters, including the second parameter problem, it also opens new
perspectives about the relation between globular clusters and the halo of our
Galaxy, and by extension of all populations with a high specific frequency of
globular clusters, such as, e.g., giant elliptical galaxies. We review progress
in this area, focusing on the most recent studies. Several points remain to be
properly understood, in particular those concerning the nature of the polluters
producing the abundance pattern in the clusters and the typical timescale, the
range of cluster masses where this phenomenon is active, and the relation
between globular clusters and other satellites of our Galaxy.Comment: In press (The Astronomy and Astrophysics Review
Radio properties of young SNe/SNRs in Arp220
Radio observations of young radio supernovae (SNe) and supernova remnants (SNRs) can provide a unique window on the stellar/ISM properties of starburst galaxies. Such observations can potentially constrain issues of cosmological importance such as whether stellar IMFs are radically different in extreme star-forming environments. Recently published observations of the nearest ultra-luminous infra-red galaxy Arp 220 have revealed the radio spectra of a group of SNe/SNR. About half of the sources detected at high frequency have spectral and variability properties consistent with young Type IIn supernovae interacting with their pre-explosion stellar winds. However the high rate of appearance of these sources implies that an unusually large fraction of core-collapse supernovae are highly luminous, which might be at least partly explained by a top heavy IMF. The other half of the compact sources found in Arp 220 were interpreted as SNRs interacting with a dense (104 to 105 cmâ3) ISM. In this paper we report on new more sensitive VLBI observations at wavelengths of 2 cm and 3.6 cm. We find that the spectral evolution of most of the suspected SNe sources is consistent with them being of Type IIn. However two rapidly dimming objects may instead be of Type Ib/c or IIb. Most of the long-lived candidate SNR sources show small or undetectable flux density variations, however one almost doubled its 3.6 cm intensity in 11 months. Another source also shows some variability and a complex spectrum. These two objects plus another with a flat spectrum up to 2 cm are the best candidates for an AGN core, though the data does not yet require this interpretation. At least three sources show signs of resolution with diameters in the range 0.1 to 0.2 pc. These sizes put them sightly above, but still consistent with, the size-luminosity correlation for SNRs. The SKA will have sufficient sensitivity to detect the emission from Arp220-like compact sources out to cosmological distances (i.e. up to zâ 0.5). However the SKA needs global baselines both to separate out the discrete sources from more extended radio emission and to resolve them apart
FGF-2 Stimulation of RANK Ligand Expression in Pagetâs Disease of Bone
Receptor activator for nuclear factor-ÎșB ligand (RANKL), a critical osteoclastogenic factor expressed in marrow stromal/preosteoblast cells is up-regulated in Pagetâs disease of bone (PDB). We previously demonstrated that heat-shock factor-2 (HSF-2) is a downstream target of fibroblast growth factor-2 (FGF-2) signaling to induce RANKL expression in bone marrow stromal/preosteoblast cells. In this study, we identified a 2.5-fold increase in serum FGF-2 levels in patients (n = 8) with PDB compared with normal subjects (n = 10). We showed that HSF-2 co-immunoprecipitates with heat-shock protein-27 (HSP-27) and that FGF-2 stimulation significantly increased phospho-HSP-27 levels in marrow stromal cells. Confocal microscopy revealed HSF-2 colocalization with HSP-27 in unstimulated cells and HSF-2 nuclear translocation upon FGF-2 stimulation. We further show that FGF-2 stimulation significantly increased the levels of phosphorylated signal transducers and activators of the transcription (p-STAT-1) in these cells. Western blot analysis confirmed that small interfering RNA suppression of STAT-1 significantly decreased (3.2-fold) RANKL expression and promoter activity in FGF-2-stimulated cells. Chromatin immunoprecipitation assay revealed STAT-1 binding to a putative motif located far upstream (â8 kb) in the hRANKL gene promoter region. These results suggest STAT-1 is a downstream effector of FGF-2 signaling and that elevated levels of FGF-2 stimulates RANKL expression in PDB