4,259 research outputs found
A Blue Tilt in the Globular Cluster System of the Milky Way-like Galaxy NGC 5170
Here we present HST/ACS imaging, in the B and I bands, of the edge-on Sb/Sc
galaxy NGC 5170. Excluding the central disk region region, we detect a 142
objects with colours and sizes typical of globular clusters (GCs). Our main
result is the discovery of a `blue tilt' (a mass-metallicity relation), at the
3sigma level, in the metal-poor GC subpopulation of this Milky Way like galaxy.
The tilt is consistent with that seen in massive elliptical galaxies and with
the self enrichment model of Bailin & Harris. For a linear mass-metallicity
relation, the tilt has the form Z ~ L^{0.42 +/- 0.13}. We derive a total GC
system population of 600 +/- 100, making it much richer than the Milky Way.
However when this number is normalised by the host galaxy luminosity or stellar
mass it is similar to that of M31. Finally, we report the presence of a
potential Ultra Compact Dwarf of size ~ 6 pc and luminosity M_I ~ -12.5,
assuming it is physically associated with NGC 5170.Comment: Accepted for publication in MNRAS, 11 pages, 10 figure
First Constraints on the Photon Coupling of Axion-like Particles from Multimessenger Studies of the Neutron Star Merger GW170817
We use multimessenger observations of the neutron star merger event GW170817
to derive new constraints on axion-like particles (ALPs) coupling to photons.
ALPs are produced via Primakoff and photon coalescence processes in the merger,
escape the remnant and decay back into two photons, giving rise to a photon
signal approximately along the line-of-sight to the merger. We analyze the
spectral and temporal information of the ALP-induced photon signal, and use the
Fermi-LAT observations of GW170817 to derive our new ALP constraints. We also
show the improved prospects with future MeV gamma-ray missions, taking the
spectral and temporal coverage of AMEGO-X as an example.Comment: 8+8 pages, 4+6 figure
Transient Gamma Ray Spectrometer Measurements of Gamma-Ray Lines from Novae. II. Constraining the Galactic Nova Rate from a Survey of the Southern Sky during 1995-1997
The good energy resolution (3--4 keV FWHM) of the Transient Gamma Ray
Spectrometer (TGRS) on board the WIND spacecraft makes it sensitive to
Doppler-shifted outbursts of 511 keV electron-positron annihilation radiation,
the reason being that the Doppler shift causes the cosmic line to be slightly
offset from a strong instrumental background 511 keV line at rest, which is
ubiquitous in space environments. Such a cosmic line (blueshifted) is predicted
to arise in classical novae due to the annihilation of positrons from
-decay on a timescale of a few hours in an expanding envelope. A further
advantage of TGRS - its broad field of view, containing the entire southern
ecliptic hemisphere - has enabled us to make a virtually complete and unbiased
3-year search for classical novae at distances up to ~1 kpc. We present
negative results of this search, and estimate its implications for the
highly-uncertain Galactic classical nova rate and for future space missions.Comment: 22 pp. + 3 fig
SPI observations of the diffuse 60Fe emission in the Galaxy
Gamma-ray line emission from radioactive decay of 60Fe provides constraints
on nucleosynthesis in massive stars and supernovae. The spectrometer SPI on
board INTEGRAL has accumulated nearly three years of data on gamma-ray emission
from the Galactic plane. We have analyzed these data with suitable
instrumental-background models and sky distributions to produce high-resolution
spectra of Galactic emission. We detect the gamma-ray lines from 60Fe decay at
1173 and 1333 keV, obtaining an improvement over our earlier measurement of
both lines with now 4.9 sigma significance for the combination of the two
lines. The average flux per line is (4.4 \pm 0.9) \times 10^{-5} ph cm^{-2}
s^{-1} rad^{-1} for the inner Galaxy region. Deriving the Galactic 26Al
gamma-ray line flux with using the same set of observations and analysis
method, we determine the flux ratio of 60Fe/26Al gamma-rays as 0.148 \pm 0.06.
The current theoretical predictions are still consistent with our result.Comment: 10 pages, 7 figures, 2 tables, A&A in pres
Deriving Metallicities From the Integrated Spectra of Extragalactic Globular Clusters Using the Near-Infrared Calcium Triplet
The Ca triplet (CaT) feature in the near-infrared has been employed as a
metallicity indicator for individual stars as well as integrated light of
Galactic globular clusters (GCs) and galaxies with varying degrees of success,
and sometimes puzzling results. Using the DEIMOS multi-object spectrograph on
Keck we obtain a sample of 144 integrated light spectra of GCs around the
brightest group galaxy NGC 1407 to test whether the CaT index can be used as a
metallicity indicator for extragalactic GCs. Different sets of single stellar
population models make different predictions for the behavior of the CaT as a
function of metallicity. In this work, the metallicities of the GCs around NGC
1407 are obtained from CaT index values using an empirical conversion. The
measured CaT/metallicity distributions show unexpected features, the most
remarkable being that the brightest red and blue GCs have similar CaT values
despite their large difference in mean color. Suggested explanations for this
behavior in the NGC 1407 GC system are: 1) the CaT may be affected by a
population of hot blue stars, 2) the CaT may saturate earlier than predicted by
the models, and/or 3) color may not trace metallicity linearly. Until these
possibilities are understood, the use of the CaT as a metallicity indicator for
the integrated spectra of extragalactic GCs will remain problematic.Comment: 14 pages, 18 figures, accepted for publication in A
SPI observations of positron annihilation radiation from the 4th galactic quadrant: sky distribution
During its first year in orbit the INTEGRAL observatory performed deep
exposures of the Galactic Center region and scanning observations of the
Galactic plane. We report on the status of our analysis of the positron
annihilation radiation from the 4th Galactic quadrant with the spectrometer
SPI, focusing on the sky distribution of the 511 keV line emission. The
analysis methods are described; current constraints and limits on the Galactic
bulge emission and the bulge-to-disk ratio are presented.Comment: 4 pages, 2 figures, accepted for publication in the proceedings of
the 5th INTEGRAL worksho
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