28 research outputs found
Molecular clouds in the center of M81
We investigate the molecular gas content and the excitation and fragmentation
properties in the central region of the spiral galaxy Messier 81 in both the
^{12}CO(1-0) and ^{12}CO(2-1) transitions. We have recently observed the two
transitions of CO in the M~81 center with A, B, and HERA receivers of the IRAM
30-m telescope. We find no CO emission in the inner 300 pc and a weak
molecular gas clump structure at a distance of around 460 pc from the nucleus.
Observations of the first two CO transitions allowed us to compute the line
ratio, and the average I_{21}/I_{10} ratio is 0.68 for the M~81 center. This
low value, atypical both of the galactic nuclei of spiral galaxies and of
interacting systems, is probably associated to diffuse gas with molecular
hydrogen density that is not high enough to excite the CO molecules. After
analyzing the clumping properties of the molecular gas in detail, we identify
very massive giant molecular associations (GMAs) in CO(2-1) emission with
masses of 10 M and diameters of 250 pc. The deduced
N(H_{2})/I_{CO} ratio for the individually resolved GMAs, assumed to be
virialized, is a factor of 15 higher than the \textit{standard} Galactic
value, showing - as suspected - that the X ratio departs significantly from the
mean for galaxies with an unusual physics of the molecular gas.Comment: 12 pages, 7 figures, 6 tables. Accepted for pubblication in A&
An Overview of the 2014 ALMA Long Baseline Campaign
A major goal of the Atacama Large Millimeter/submillimeter Array (ALMA) is to
make accurate images with resolutions of tens of milliarcseconds, which at
submillimeter (submm) wavelengths requires baselines up to ~15 km. To develop
and test this capability, a Long Baseline Campaign (LBC) was carried out from
September to late November 2014, culminating in end-to-end observations,
calibrations, and imaging of selected Science Verification (SV) targets. This
paper presents an overview of the campaign and its main results, including an
investigation of the short-term coherence properties and systematic phase
errors over the long baselines at the ALMA site, a summary of the SV targets
and observations, and recommendations for science observing strategies at long
baselines. Deep ALMA images of the quasar 3C138 at 97 and 241 GHz are also
compared to VLA 43 GHz results, demonstrating an agreement at a level of a few
percent. As a result of the extensive program of LBC testing, the highly
successful SV imaging at long baselines achieved angular resolutions as fine as
19 mas at ~350 GHz. Observing with ALMA on baselines of up to 15 km is now
possible, and opens up new parameter space for submm astronomy.Comment: 11 pages, 7 figures, 2 tables; accepted for publication in the
Astrophysical Journal Letters; this version with small changes to
affiliation
Multi-messenger observations of a binary neutron star merger
On 2017 August 17 a binary neutron star coalescence candidate (later designated GW170817) with merger time 12:41:04 UTC was observed through gravitational waves by the Advanced LIGO and Advanced Virgo detectors. The Fermi Gamma-ray Burst Monitor independently detected a gamma-ray burst (GRB 170817A) with a time delay of ~1.7 s with respect to the merger time. From the gravitational-wave signal, the source was initially localized to a sky region of 31 deg2 at a luminosity distance of 40+8-8 Mpc and with component masses consistent with neutron stars. The component masses were later measured to be in the range 0.86 to 2.26 Mo. An extensive observing campaign was launched across the electromagnetic spectrum leading to the discovery of a bright optical transient (SSS17a, now with the IAU identification of AT 2017gfo) in NGC 4993 (at ~40 Mpc) less than 11 hours after the merger by the One- Meter, Two Hemisphere (1M2H) team using the 1 m Swope Telescope. The optical transient was independently detected by multiple teams within an hour. Subsequent observations targeted the object and its environment. Early ultraviolet observations revealed a blue transient that faded within 48 hours. Optical and infrared observations showed a redward evolution over ~10 days. Following early non-detections, X-ray and radio emission were discovered at the transientâs position ~9 and ~16 days, respectively, after the merger. Both the X-ray and radio emission likely arise from a physical process that is distinct from the one that generates the UV/optical/near-infrared emission. No ultra-high-energy gamma-rays and no neutrino candidates consistent with the source were found in follow-up searches. These observations support the hypothesis that GW170817 was produced by the merger of two neutron stars in NGC4993 followed by a short gamma-ray burst (GRB 170817A) and a kilonova/macronova powered by the radioactive decay of r-process nuclei synthesized in the ejecta
The 2014 ALMA Long Baseline Campaign: An Overview
A major goal of the Atacama Large Millimeter/submillimeter Array (ALMA) is to make accurate images with resolutions of tens of milliarcseconds, which at submillimeter (submm) wavelengths requires baselines up to ~15 km. To develop and test this capability, a Long Baseline Campaign (LBC) was carried out from September to late November 2014, culminating in end-to-end observations, calibrations, and imaging of selected Science Verification (SV) targets. This paper presents an overview of the campaign and its main results, including an investigation of the short-term coherence properties and systematic phase errors over the long baselines at the ALMA site, a summary of the SV targets and observations, and recommendations for science observing strategies at long baselines. Deep ALMA images of the quasar 3C138 at 97 and 241 GHz are also compared to VLA 43 GHz results, demonstrating an agreement at a level of a few percent. As a result of the extensive program of LBC testing, the highly successful SV imaging at long baselines achieved angular resolutions as fine as 19 mas at ~350 GHz. Observing with ALMA on baselines of up to 15 km is now possible, and opens up new parameter space for submm astronomy
The Wide-field Spectroscopic Telescope (WST) Science White Paper
The Wide-field Spectroscopic Telescope (WST) is proposed as a new facility dedicated to the efficient delivery of spectroscopic surveys. This white paper summarises the initial concept as well as the corresponding science cases. WST will feature simultaneous operation of a large field-of-view (3 sq. degree), a high multiplex (20,000) multi-object spectrograph (MOS) and a giant 3x3 sq. arcmin integral field spectrograph (IFS). In scientific capability these requirements place WST far ahead of existing and planned facilities. Given the current investment in deep imaging surveys and noting the diagnostic power of
spectroscopy, WST will fill a crucial gap in astronomical capability and work synergistically with future ground and space-based facilities. This white paper shows that WST can address outstanding scientific questions in the areas of cosmology; galaxy assembly, evolution, and enrichment, including our own Milky Way; origin of stars and planets; time domain and multi-messenger astrophysics. WST's uniquely rich dataset will deliver unforeseen discoveries in many of these areas. The WST Science Team (already including more than 500 scientists worldwide) is open to the all astronomical community. To register in the WST Science Team please visit https://www.wstelescope.com/for-scientists/participat
Multi-messenger Observations of a Binary Neutron Star Merger
On 2017 August 17 a binary neutron star coalescence candidate (later
designated GW170817) with merger time 12:41:04 UTC was observed through
gravitational waves by the Advanced LIGO and Advanced Virgo detectors.
The Fermi Gamma-ray Burst Monitor independently detected a gamma-ray
burst (GRB 170817A) with a time delay of ⌠1.7 {{s}} with respect to
the merger time. From the gravitational-wave signal, the source was
initially localized to a sky region of 31 deg2 at a
luminosity distance of {40}-8+8 Mpc and with
component masses consistent with neutron stars. The component masses
were later measured to be in the range 0.86 to 2.26 {M}ÈŻ
. An extensive observing campaign was launched across the
electromagnetic spectrum leading to the discovery of a bright optical
transient (SSS17a, now with the IAU identification of AT 2017gfo) in NGC
4993 (at ⌠40 {{Mpc}}) less than 11 hours after the merger by the
One-Meter, Two Hemisphere (1M2H) team using the 1 m Swope Telescope. The
optical transient was independently detected by multiple teams within an
hour. Subsequent observations targeted the object and its environment.
Early ultraviolet observations revealed a blue transient that faded
within 48 hours. Optical and infrared observations showed a redward
evolution over âŒ10 days. Following early non-detections, X-ray and
radio emission were discovered at the transientâs position ⌠9
and ⌠16 days, respectively, after the merger. Both the X-ray and
radio emission likely arise from a physical process that is distinct
from the one that generates the UV/optical/near-infrared emission. No
ultra-high-energy gamma-rays and no neutrino candidates consistent with
the source were found in follow-up searches. These observations support
the hypothesis that GW170817 was produced by the merger of two neutron
stars in NGC 4993 followed by a short gamma-ray burst (GRB 170817A) and
a kilonova/macronova powered by the radioactive decay of r-process
nuclei synthesized in the ejecta.</p