arXiv subjects

Yuhei Iwata

Publications and source records attributed to Yuhei Iwata.

25 records · Page 2Linked to original sources

arXiv2018

Detection of Another Molecular Bubble in the Galactic Center

The $l=-1.2^{\circ}$ region in the Galactic center has a high CO $J$=3-2/$J$=1-0 intensity ratio and extremely broad velocity width. This paper reports the detection of five expanding shells in the $l=-1.2^{\circ}$ region based on the CO $J$=1-0, $^{13}$CO $J$=1-0, CO $J$=3-2, and SiO $J$=8-7 line data sets obtained with the Nobeyama Radio Observatory 45 m telescope and James Clerk Maxwell Telescope. The kinetic energy and expansion time of the expanding shells are estimated to be $10^{48.3-50.8}$ erg and $10^{4.7-5.0}$ yr, respectively. The origin of these expanding shells is discussed. The total kinetic energy of $10^{51}$ erg and the typical expansion time of ${\sim}10^5$ yr correspond to multiple supernova explosions at a rate of $10^{-5}$-$10^{-4}$ yr$^{-1}$. This indicates that the $l=-1.2^{\circ}$ region may be a molecular bubble associated with an embedded massive star cluster, although the absence of an infrared counterpart makes this interpretation somewhat controversial. The expansion time of the shells increases as the Galactic longitude decreases, suggesting that the massive star cluster is moving from Galactic west to east with respect to the interacting molecular gas. We propose a model wherein the cluster is moving along the innermost x1 orbit and the interacting gas collides with it from the Galactic eastern side.

astro-ph.GA↗

arXiv2017

Millimetre-wave Emission from an Intermediate-Mass Black Hole Candidate in the Milky Way

It is widely accepted that black holes (BHs) with masses greater than a million solar masses (Msun) lurk at the centres of massive galaxies. The origins of such `supermassive' black holes (SMBHs) remain unknown (Djorgovski et al. 1999), while those of stellar-mass BHs are well-understood. One possible scenario is that intermediate-mass black holes (IMBHs), which are formed by the runaway coalescence of stars in young compact star clusters (Portagies Zwart et al. 1999), merge at the centre of a galaxy to form an SMBH (Ebisuzaki et al. 2001). Although many candidates for IMBHs have been proposed to date, none of them are accepted as definitive. Recently we discovered a peculiar molecular cloud, CO-0.40-0.22, with an extremely broad velocity width near the centre of our Milky Way galaxy. Based on the careful analysis of gas kinematics, we concluded that a compact object with a mass of ~1E5 Msun is lurking in this cloud (Oka et al. 2016). Here we report the detection of a point-like continuum source as well as a compact gas clump near the center of CO-0.40-0.22. This point-like continuum source (CO-0.40-0.22*) has a wide-band spectrum consistent with 1/500 of the Galactic SMBH (Sgr A*) in luminosity. Numerical simulations around a point-like massive object reproduce the kinematics of dense molecular gas well, which suggests that CO-0.40-0.22* is the most promising candidate for an intermediate-mass black hole.

astro-ph.GA↗

arXiv2017

Discovery of Two Small High-Velocity Compact Clouds in the Central 10 Parsecs of Our Galaxy

We discovered two small high-velocity compact clouds (HVCCs) in HCN $J=4-3$ and $J=3-2$ maps of the central 20 pc of our Galaxy. Both HVCCs have broad velocity widths ($ΔV \gtrsim 40$ km s$^{-1}$) and compact sizes ($d\sim 1$ pc), and originate from the dense molecular clouds in the position-velocity space. One of them has a faint counterpart in a Paschen-$α$ image. Their spatial structure, kinematics, and absence of luminous stellar object are compatible with the notion that each of the small HVCCs is driven by the plunge of an invisible compact object into a molecular cloud. Such objects are most likely inactive, isolated black holes.

astro-ph.GA↗

arXiv2017

Distance to the Low-velocity Cloud in the Direction of the High-velocity Compact Cloud CO--0.40--0.22

CO--0.40--0.22 is a peculiar molecular cloud that is compact and has an extraordinary broad velocity width. It is found in the central molecular zone of our Galaxy. In this direction, there is another cloud with an H$_2$O maser spot at a lower velocity. Collision with this low-velocity cloud could be responsible for the broad velocity width of CO--0.40--0.22. We performed phase-referencing VLBI astrometry with VERA and detected the annual parallax of the H$_2$O maser spot in the low-velocity cloud to be $0.33 \pm 0.14$ mas, which corresponds to a distance of $3.07^{+2.22}_{-0.91}$ kpc from the Sun. This implies that the low-velocity cloud is located in the Galactic disk on the near side of the central molecular zone.

astro-ph.GA↗

arXiv2016

Kinematics of Ultra-High-Velocity Gas in the Expanding Molecular Shell adjacent to the W44 Supernova Remnant

We mapped the ultra-high-velocity feature (the "Bullet") detected in the expanding molecular shell associated with the W44 supernova remnant using the Nobeyama Radio Observatory 45-m telescope and the ASTE 10-m telescope. The Bullet clearly appears in the CO ${\it J}$=1-0, CO ${\it J}$=3-2, CO ${\it J}$=4-3, and HCO$^+$ ${\it J}$=1-0 maps with a compact appearance ($0.5\times 0.8$ pc$^2$) and an extremely broad velocity width ($ΔV\!\simeq\!100 \ \rm km \, s ^{-1}$). The line intensities indicate that the Bullet has a higher density and temperature than those in the expanding molecular shell. The kinetic energy of the Bullet amounts to $10^{48.0} \ {\rm erg}$ which is approximately 1.5 orders of magnitude greater than the kinetic energy shared to the small solid angle of it. Two possible formation scenarios with an inactive isolated black hole (BH) are presented.

astro-ph.GA↗

arXiv2012

Chiral multicritical points driven by isospin density in the Ginzburg-Landau approach

We study how a chiral tricritical point (TCP) on QCD phase diagram is affected by the imbalance of up and down quark densities (isospin density), using the generalized Ginzburg-Landau (GL) approach. The resulting phase diagram near TCP shows a rich fine structure which includes inhomogeneities of both the chiral and the charged pion condensations. It turns out that the TCP splits into multicritical points.

hep-ph↗

arXiv2012

Splitting of the chiral critical point and realization of solitonic pion condensate driven by isospin density

We study the influence of the isospin asymmetry on the phase structure of strongly interacting quark matter near the tricritical point (TCP) using a generalized Ginzburg-Landau approach. The effect has proven to be so drastic, not only bringing about the shift of the location of TCP, but resulting in a rich fine structure at the vicinity of TCP. In particular, we find that an arbitrary small perturbation due to isospin density lifts the degeneracy of TCP making it split into four independent multicritical points. Accordingly, the homogeneous pion condensate and its solitonic counterpart come to occupy large domains in the Ginzburg-Landau coupling space.

hep-ph↗