arXiv subjects

Y. Hagiwara

Publications and source records attributed to Y. Hagiwara.

14 recordsLinked to original sources

arXiv2017

White Paper on East Asian Vision for mm/submm VLBI: Toward Black Hole Astrophysics down to Angular Resolution of 1~R$_{S}$

This White Paper details the intentions and plans of the East Asian Very Long Baseline Interferometry (VLBI) community for pushing the frontiers of millimeter/submillimeter VLBI. To this end, we shall endeavor to actively promote coordinated efforts in the East Asia region. Our goal is to establish firm collaborations among the East Asia VLBI community in partnership with related institutes in North America and Europe and to expand existing global mm/submm VLBI arrays for (a) exploring the vicinity of black holes with an ultimate angular resolution down to 1~R$_{S}$ (Schwarzschild radius) and (b) investigating the dynamics of circumstellar gas in star-forming regions and late-type stars, and circumnuclear gas around active galactic nuclei (AGNs). In the first half of this White Paper, we highlight scientific accomplishments of the East Asia (EA) VLBI community. Various VLBI research results on M87, Sgr A*, blazars, narrow-line Seyfert~1 galaxies, and compact symmetric objects are described, and future visions of our VLBI science are briefly presented. Maser science of star formation, stellar evolution, and physics of accretion disks around AGNs are also discussed. A new vision for conducting multi-transition maser studies using mm/submm VLBI together with the Atacama Large Millimeter/Submillimeter Array (ALMA) is described. In the second half of this White Paper, we describe the EA community's vision for using mm/submm VLBI arrays in the framework or extended version of the Event Horizon Telescope (EHT) and the Global Millimeter VLBI Array (GMVA). The accomplishment of the aforementioned goal will maximize the overall scientific outcomes of mm/submm VLBI in the world.

astro-ph.HE↗

arXiv2013

Active Galactic Nucleus and Extended Starbursts in a Mid-stage Merger VV114

High resolution (0.4 arcsec) Atacama Large Millimeter/submillimeter Array (ALMA) Cycle 0 observations of HCO+(4-3) and HCN(4-3) toward a mid-stage infrared bright merger VV114 have revealed compact nuclear (<200 pc) and extended (3 - 4 kpc) dense gas distribution across the eastern part of the galaxy pair. We find a significant enhancement of HCN(4-3) emission in an unresolved compact and broad (290km/s) component found in the eastern nucleus of VV114, and we suggest dense gas associated with the surrounding material around an Active Galactic Nucleus (AGN), with a mass upper limit of < 4 x 10^8 Msun. The extended dense gas is distributed along a filamentary structure with resolved dense gas concentrations (230pc; 10^6 Msun) separated by a mean projected distance of 600 pc, many of which are generally consistent with the location of star formation traced in Pa alpha emission. Radiative transfer calculations suggest moderately dense (10^5 - 10^6 cm^-3) gas averaged over the entire emission region. These new ALMA observations demonstrate the strength of the dense gas tracers in identifying both the AGN and star formation activity in a galaxy merger, even in the most dust enshrouded environments in the local universe.

astro-ph.GA↗

arXiv2013

ATCA survey of water masers in the Large Magellanic Cloud

We have analysed archival data taken with the Australia Telescope Compact Array (ATCA) during 2001--2003 and detected nine new interstellar and circumstellar water masers in the LMC. This takes the total number of star formation water masers in the LMC to 23, spread over 14 different star forming regions and three evolved stars. Three water maser sources (N105a/MC23, N113/MC24, N157a/MC74) have been detected in all the previous observations that targeted these sites, although all show significant variability on timescales of decades. The total number of independent water maser sources now known in the LMC means that through very long baseline interferometry astrometric measurements it will be possible to construct a more precise model of the galactic rotation of the LMC and its orbital motion around the Milky Way Galaxy.

astro-ph.GA↗

arXiv2011

VLBI Observations of NGC6240: resolving the double nuclei and radio supernovae

The European VLBI Network (EVN) has been used at two epochs in 2003 and 2009 to obtain multi-frequency high-resolution images of the merger galaxy NGC6240 in order to study the radio properties of all compact high-brightness components in the galaxy. Our observations at milli-arcsecond resolution detected the northern and southern nuclei and two radio components, which we interpret as long-lived luminous supernovae associated with the circum-nuclear starburst activity at the southern nucleus. The new VLBI data support the presence of an active galactic nucleus (AGN) together with starburst activity at the southern nucleus and provides some evidence for an AGN at the northern nucleus. The two nuclei both display an inverted spectrum at lower GHz frequencies. The spectrum of the southern nucleus indicates thermal free-free absorption on parsec scales, consistent with the presence of an AGN.

astro-ph.CO↗

arXiv2009

The distance to a star forming region in the Outer arm of the Galaxy

We performed astrometric observations with the VLBA of WB89-437, an H2O maser source in the Outer spiral arm of the Galaxy. We measure an annual parallax of 0.167 +/- 0.006 mas, corresponding to a heliocentric distance of 6.0 +/- 0.2 kpc or a Galactocentric distance of 13.4 +/- 0.2 kpc. This value for the heliocentric distance is considerably smaller than the kinematic distance of 8.6 kpc. This confirms the presence of a faint Outer arm toward l = 135 degrees. We also measured the full space motion of the object and find a large peculiar motion of ~20 km/s toward the Galactic center. This peculiar motion explains the large error in the kinematic distance estimate. We also find that WB89-437 has the same rotation speed as the LSR, providing more evidence for a flat rotation curve and thus the presence of dark matter in the outer Galaxy.

astro-ph.GA↗

arXiv2006

Water maser motions in W3(OH) and a determination of its distance

We report phase-referencing VLBA observations of H2O masers near the star-forming region W3(OH) to measure their parallax and absolute proper motions. The measured annual parallax is 0.489 +/- 0.017 milli-arcseconds (2.04 +/- 0.07 kpc), where the error is dominated by a systematic atmospheric contribution. This distance is consistent with photometric distances from previous observations and with the distance determined from CH3OH maser astrometry presented in a related paper. We also find that the source driving the H2O outflow, the ``TW-object'', moves with a 3-dimensional velocity of > 7 km/s relative to the ultracompact HII region W3(OH).

astro-ph↗

arXiv2004

Distance of W3(OH) by VLBI annual parallax measurement

The most powerful tool for measuring distances within our Galaxy is the annual parallax. We carried out phase-referencing VLBI observations of H$_{2}$O masers in the star forming region W3(OH) with respect to the extragalactic continuum source ICRF 0244+624 to measure their absolute proper motions. The measured annual parallax is 0.484 $\pm$ 0.004 milli-arcseconds which corresponds to a distance of 2.07^{+0.01}_{-0.02}$ kpc from the sun. This distance is consistent with photometric and kinematic distances from previous observations.

astro-ph↗

arXiv2004

H2O megamasers: Accretion disks, jet interaction, outflows or massive star formation?

The 25 years following the serendipitous discovery of megamasers have seen tremendous progress in the study of luminous extragalactic H$_2$O emission. Single-dish monitoring and high resolution interferometry have been used to identify sites of massive star formation, to study the interaction of nuclear jets with dense molecular gas and to investigate the circumnuclear environment of active galactic nuclei (AGN). Accretion disks with radii of 0.1--3 pc were mapped and masses of nuclear engines of order 10$^{6}$--10$^{8}$ M$_{\odot}$ were determined. So far, $\sim$50 extragalactic H$_2$O maser sources have been detected, but few have been studied in detail.

astro-ph↗

arXiv2004

VLBI imaging of OH absorption: The puzzle of the nuclear region of NGC 3079

Broad hydroxyl (OH) absorption-lines in the 1667 MHz and 1665 MHz transition towards the central region of NGC 3079 have been observed at high resolution with the European VLBI Network (EVN). Velocity fields of two OH absorption components were resolved across the unresolved nuclear radio continuum of ~10 parsecs. The velocity field of the OH absorption close to the systemic velocity shows rotation in nearly the same sense as the edge-on galactic-scale molecular disk probed by CO(1-0) emission. The velocity field of the blue-shifted OH absorption displays a gradient in almost the opposite direction. The blue-shifted velocity field represents a non-rotational component, which may trace an outflow from the nucleus, or material driven and shocked by the kiloparsec-scale superbubble. This OH absorption component traces a structure that does not support a counter-rotating disk suggested on the basis of the neutral hydrogen absorption.

astro-ph↗

arXiv2003

Dense gas in nearby galaxies: XV. Hot ammonia in NGC253, Maffei2 and IC342

The detection of NH3 inversion lines up to the (J,K)=(6,6) level is reported toward the central regions of the nearby galaxies NGC253, Maffei2, and IC342. The observed lines are up to 406K (for (J,K)=(6,6)) and 848K (for the (9,9) transition) above the ground state and reveal a warm (T_kin= 100 - 140 K) molecular component toward all galaxies studied. The tentatively detected (J,K)=(9,9) line is evidence for an even warmer (>400K) component toward IC342. Toward NGC253, IC342 and Maffei2 the global beam averaged NH3 abundances are 1-2 10^-8, while the abundance relative to warm H2 is around 10^-7. The temperatures and NH3 abundances are similar to values found for the Galactic central region. C-shocks produced in cloud-cloud collisions can explain kinetic temperatures and chemical abundances. In the central region of M82, however, the NH3 emitting gas component is comparatively cool (~ 30K). It must be dense (to provide sufficient NH3 excitation) and well shielded from dissociating photons and comprises only a small fraction of the molecular gas mass in M82. An important molecular component, which is warm and tenuous and characterized by a low ammonia abundance, can be seen mainly in CO. Photon dominated regions (PDRs) can explain both the high fraction of warm H_2 in M82 and the observed chemical abundances.

astro-ph↗

arXiv2002

Red-shifted H2O emission in NGC 3079: more evidence for a pc-scale circumnuclear torus?

Using the Effelsberg 100-m telescope, sensitive measurements of the H2O megamaser in NGC 3079 are presented. During 2000 - 2001, `high velocity' features are seen that are red-shifted up to 225 km/s with respect to the systemic velocity of the galaxy (about 1120 km/s). Symmetrically bracketing the systemic velocity, the H2O emission covers a velocity range of 450 km/s with only one potential narrow gap (20 km/s) near the systemic velocity itself. Velocity drifts of individual components are not convincingly detected. It is shown that the presence of red-shifted emission and the absence of detectable velocity drifts are not inconsistent with the existence of a rotating circumnuclear maser disk at the very center of the galaxy. Significant differences in the overall line profile compared to NGC 4258 and a complex morphology of the radio continuum leave, however, space for scepticism.

astro-ph↗

arXiv2001

A search for extragalactic H2O maser emission towards IRAS galaxies - Detection of a maser from an infrared-luminous merger, NGC6240

We report the result of an on-going survey for 22 GHz H2O maser emission towards infrared luminous galaxies. The observed galaxies were selected primarily from the IRAS bright galaxy sample. The survey has resulted in the detection of one new maser. The new maser was discovered towards the [U]LIRG/merger galaxy NGC6240, which contains a LINER nucleus. This is the first detection of an H2O maser towards this class of galaxy, they are traditionally associated with OH megamaser sources. The detected maser emission is highly redshifted (~ 260-300 km/s) with respect to the adopted systemic velocity of the galaxy, and we identified no other significant emission at velocities < +- 500 km/s relative to the systemic velocity. The presence of high-velocity maser emission implies the possible existence of a rotating maser disk formed in the merging process. The large maser luminosity (~ 40 Lo) suggests that an active galactic nucleus could be the energy source that gives rise to the water emission. Alternatively, the maser emission could be associated with the previously observed double radio source in the centre of the galaxy. Interferometric observations with high angular resolution will be able to clarify the origin of the new maser.

astro-ph↗

arXiv2001

A Flaring Megamaser in Mrk 348

We report new observations of the H2O megamaser in the Seyfert 2 galaxy Mrk 348. Following our initial detection in 2000 March using the Effelsberg 100 m telescope, re-analysis of previous data on this source indicates that the maser was present but only marginally detectable in late 1997. Monitoring through late 2000 shows that the maser has again decreased to its original level. The H2O line is redshifted by ~130 km/s with respect to the systemic velocity, is extremely broad, with a FWHM of 130 km/s, and has no detectable high velocity components within 1500 km/s on either side of the strong line. Followup VLBA observations show that the maser emission emanates entirely from a region >0.25 pc in extent, toward the base of the radio jet.

astro-ph↗

arXiv2000

Dense Gas in Nearby Galaxies: XIV. Detection of hot ammonia in Maffei 2

The (J,K) = (1,1), (2,2), (3,3), and (4,4) inversion lines of ammonia (NH3) have been detected toward the nuclear 40'' sized bar of the nearby spiral galaxy Maffei 2. The relative intensities of the ammonia lines are characterized by a rotational temperature of 85 K. This is higher than rotational temperatures measured toward IC342 and most Galactic Center clouds, implying kinetic temperatures of at least 100 K. Since the kinetic temperature of the gas is larger than that of the dust, NH3 is tracing a particularly dense warm gas component that is heated by young massive stars, cloud-cloud collisions, or ion-slip heating in the nuclear starburst. The gas north of the nucleus (V(LSR} = -80 km/s) is more highly excited than the gas further south (+6 km/s). This asymmetry might be related to a pronounced morphological distortion that is observed in the north-eastern part of the galaxy.

astro-ph↗