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Yoshiaki Sofue - One of the best experts on this subject based on the ideXlab platform.
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Three-Dimensional Distribution of the ISM in the Milky Way Galaxy: IV. 3D Molecular Fraction and Galactic-Scale HI-to-H$_2$ Transition
Publications of the Astronomical Society of Japan, 2016Co-Authors: Yoshiaki Sofue, Hiroyuki NakanishiAbstract:Three dimensional (3D) distribution of the volume-density molecular fraction, defined by fmol_rho=rho_H2/(rho_HI + rho_H2), is studied in the Milky Way Galaxy. The molecular front appears at galacto-centric distance of R \sim 8 kpc, where the phase transition from atomic to molecular hydrogen occurs suddenly with fmol_rho dropping from \sim 0.8 to 0.2 within a radial interval as narrow as \sim 0.5 kpc. The front in fmol_rho is much sharper than that for surface density molecular fraction. The front also appears in the vertical direction with a full width of the high-fmol_rho disk to be \sim 100 pc. The radial and vertical fmol_rho profiles, particularly the front behaviors, are fitted by theoretical curves calculated using the observed density profile and assumed radiation field and metallicity with exponential gradients. The molecular fraction was found to be enhanced along the Perseus and some other spiral arms. The fmol_rho arms imply that the molecular clouds are produced from HI gas in the spiral arms and are dissociated in the interarm region. We also show that there is a threshold HI density over which the gas is transformed into molecules.
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three dimensional distribution of the ism in the Milky Way Galaxy iii the total neutral gas disk
Publications of the Astronomical Society of Japan, 2016Co-Authors: Hiroyuki Nakanishi, Yoshiaki SofueAbstract:We present newly obtained three-dimensional gaseous maps of the Milky Way Galaxy; HI, H$_2$ and total-gas (HI plus H$_2$) maps, which were derived from the HI and $^{12}$CO($J=1$--0) survey data and rotation curves based on the kinematic distance. The HI and H$_2$ face-on maps show that the HI disk is extended to the radius of 15--20 kpc and its outskirt is asymmetric to the Galactic center, while most of the H$_2$ gas is distributed inside the solar circle. The total gas mass within radius 30 kpc amounts to $8.0\times 10^9$ M$_\odot$, 89\% and 11\% of which are HI and H$_2$, {respectively}. The vertical slices show that the outer HI disk is strongly warped and the inner HI and H$_2$ disks are corrugated. The total gas map is advantageous to trace spiral structure from the inner to outer disk. Spiral structures such as the Norma-Cygnus, the Perseus, the Sagittarius-Carina, the Scutum-Crux, and the Orion arms are more clearly traced in the total gas map than ever. All the spiral arms are well explained with logarithmic spiral arms with pitch angle of $11\degree$ -- $15\degree$. The molecular fraction to the total gas is high near the Galactic center and decreases with the Galactocentric distance. The molecular fraction also locally enhanced at the spiral arms compared with the inter-arm regions.
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Grand Rotation Curve and Dark-Matter Halo in the Milky Way Galaxy
Publications of the Astronomical Society of Japan, 2012Co-Authors: Yoshiaki SofueAbstract:A grand rotation curve of the Milky Way Galaxy is constructed, which covers a wide range of radius from the Galactic Center to ~1 Mpc, and is deconvolved into bulge, disk and halo components by least-squares fitting. We determined the scale radii and masses of the bulge and disk to be M_b=(1.652 +/-0.083)x10^10 Msun, a_b=(0.522 +/- 0.037) kpc, M_d=(3.41 +/- 0.41)x10^10 Msun and a_d=(3.19 +/- 0.3) kpc. The dark halo was fitted by the Navaro-Frenk-White (NFW) density profile, rho=rho_0/[(R/h) (1+R/h)^2], and the fit yielded h=12.5 +/- 0.9 kpc and rho_0=(1.06 +/- 0.14)x10^-2 Msun pc^-3. The local dark matter density near the Sun at R_0=8 kpc is estimated to be rho_0_sun=(6.12 +/-0.80)x10^-3 Msun pc^-3 = 0.235 +/- 0.030 GeV cm^-3. The total mass inside the gravitational boundary of the Galaxy at R~ 385 kpc, a half distance to M31, is estimated to be M_h ~ (7.03 +/- 1.01)x10^11 Msun. This leads to the stellar baryon fraction of (M_b+M_d)/(M_b+M_d+M_h)=0.072 +/-0.018. Considering expected baryon fraction in the Local Group, we suggest that baryons in the form of hot gas are filling the dark halo with temperature of ~ 10^6 K and emission measure ~ 10^-5 pc cm^-6. Such hot halo gas may share a small fraction of the observed X-ray background emission.
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Three-Dimensional HI and H2 Gas Maps of the Milky Way Galaxy
Mapping the Galaxy and Nearby Galaxies, 2008Co-Authors: Hiroyuki Nakanishi, Yoshiaki SofueAbstract:We created three-dimensional HI and H2 gas maps of the Milky Way Galaxy using the latest HI and CO survey data [1, 2, 3, 4], and rotation curves [5, 6], based on the kinematic distance. The total gas map (HI plus H2) revealed clear logarithmic spiral arms; the Outer arm, Perseus arm, the Orion arm, the Sagittarius-Carina arm, the Scutum-Crux arm, and the Norma arm. The Norma and the Outer arms can be identified as the same spiral arm considering pitch angle of 11 – 15◦. Plotting HI or H2 gas densities against the total gas density, we found following characteristics of the HI and H2 phases. In the case of small total density (n 0.4 cm−3. There seems to exist HI saturation density.
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three dimensional distribution of the ism in the Milky Way Galaxy ii the molecular gas disk
Publications of the Astronomical Society of Japan, 2006Co-Authors: Hiroyuki Nakanishi, Yoshiaki SofueAbstract:We created a three-dimensional distribution map of molecular gas throughout the Milky Way Galaxy using the latest 12 CO (J =1–0) survey data cube and the rotation curve based on the kinematic distance. The radial distribution of the molecular gas shows a central peak and a second peak around 0.5 R0 (R0: the solar Galactocentric distance). The thickness of the molecular disk slightly increases from 48pc to 160pc with the Galactocentric distance within a radius range of 0–11kpc. We were able to trace the Outer, the Perseus, the Sagittarius–Carina, the Scutum–Crux, and the Norma arms as logarithmic spiral arms with pitch angles of 11 ◦ –15 ◦ . Considering that the pitch angles of the spiral arms are within this range, the Norma and the Outer arms seem to be identified as the same spiral arm. We could also trace a midplane displacement, whose amplitude is nearly constant inside a 10kpc radius and increases beyond this radius. The ridges of the midplane displacement form the leading spiral arms.
Hiroyuki Nakanishi - One of the best experts on this subject based on the ideXlab platform.
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Three-Dimensional Distribution of the ISM in the Milky Way Galaxy: IV. 3D Molecular Fraction and Galactic-Scale HI-to-H$_2$ Transition
Publications of the Astronomical Society of Japan, 2016Co-Authors: Yoshiaki Sofue, Hiroyuki NakanishiAbstract:Three dimensional (3D) distribution of the volume-density molecular fraction, defined by fmol_rho=rho_H2/(rho_HI + rho_H2), is studied in the Milky Way Galaxy. The molecular front appears at galacto-centric distance of R \sim 8 kpc, where the phase transition from atomic to molecular hydrogen occurs suddenly with fmol_rho dropping from \sim 0.8 to 0.2 within a radial interval as narrow as \sim 0.5 kpc. The front in fmol_rho is much sharper than that for surface density molecular fraction. The front also appears in the vertical direction with a full width of the high-fmol_rho disk to be \sim 100 pc. The radial and vertical fmol_rho profiles, particularly the front behaviors, are fitted by theoretical curves calculated using the observed density profile and assumed radiation field and metallicity with exponential gradients. The molecular fraction was found to be enhanced along the Perseus and some other spiral arms. The fmol_rho arms imply that the molecular clouds are produced from HI gas in the spiral arms and are dissociated in the interarm region. We also show that there is a threshold HI density over which the gas is transformed into molecules.
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three dimensional distribution of the ism in the Milky Way Galaxy iii the total neutral gas disk
Publications of the Astronomical Society of Japan, 2016Co-Authors: Hiroyuki Nakanishi, Yoshiaki SofueAbstract:We present newly obtained three-dimensional gaseous maps of the Milky Way Galaxy; HI, H$_2$ and total-gas (HI plus H$_2$) maps, which were derived from the HI and $^{12}$CO($J=1$--0) survey data and rotation curves based on the kinematic distance. The HI and H$_2$ face-on maps show that the HI disk is extended to the radius of 15--20 kpc and its outskirt is asymmetric to the Galactic center, while most of the H$_2$ gas is distributed inside the solar circle. The total gas mass within radius 30 kpc amounts to $8.0\times 10^9$ M$_\odot$, 89\% and 11\% of which are HI and H$_2$, {respectively}. The vertical slices show that the outer HI disk is strongly warped and the inner HI and H$_2$ disks are corrugated. The total gas map is advantageous to trace spiral structure from the inner to outer disk. Spiral structures such as the Norma-Cygnus, the Perseus, the Sagittarius-Carina, the Scutum-Crux, and the Orion arms are more clearly traced in the total gas map than ever. All the spiral arms are well explained with logarithmic spiral arms with pitch angle of $11\degree$ -- $15\degree$. The molecular fraction to the total gas is high near the Galactic center and decreases with the Galactocentric distance. The molecular fraction also locally enhanced at the spiral arms compared with the inter-arm regions.
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Three-Dimensional HI and H2 Gas Maps of the Milky Way Galaxy
Mapping the Galaxy and Nearby Galaxies, 2008Co-Authors: Hiroyuki Nakanishi, Yoshiaki SofueAbstract:We created three-dimensional HI and H2 gas maps of the Milky Way Galaxy using the latest HI and CO survey data [1, 2, 3, 4], and rotation curves [5, 6], based on the kinematic distance. The total gas map (HI plus H2) revealed clear logarithmic spiral arms; the Outer arm, Perseus arm, the Orion arm, the Sagittarius-Carina arm, the Scutum-Crux arm, and the Norma arm. The Norma and the Outer arms can be identified as the same spiral arm considering pitch angle of 11 – 15◦. Plotting HI or H2 gas densities against the total gas density, we found following characteristics of the HI and H2 phases. In the case of small total density (n 0.4 cm−3. There seems to exist HI saturation density.
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three dimensional distribution of the ism in the Milky Way Galaxy ii the molecular gas disk
Publications of the Astronomical Society of Japan, 2006Co-Authors: Hiroyuki Nakanishi, Yoshiaki SofueAbstract:We created a three-dimensional distribution map of molecular gas throughout the Milky Way Galaxy using the latest 12 CO (J =1–0) survey data cube and the rotation curve based on the kinematic distance. The radial distribution of the molecular gas shows a central peak and a second peak around 0.5 R0 (R0: the solar Galactocentric distance). The thickness of the molecular disk slightly increases from 48pc to 160pc with the Galactocentric distance within a radius range of 0–11kpc. We were able to trace the Outer, the Perseus, the Sagittarius–Carina, the Scutum–Crux, and the Norma arms as logarithmic spiral arms with pitch angles of 11 ◦ –15 ◦ . Considering that the pitch angles of the spiral arms are within this range, the Norma and the Outer arms seem to be identified as the same spiral arm. We could also trace a midplane displacement, whose amplitude is nearly constant inside a 10kpc radius and increases beyond this radius. The ridges of the midplane displacement form the leading spiral arms.
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Three-Dimensional Distribution of the ISM in the Milky Way Galaxy: II. The Molecular Gas Disk
Publications of the Astronomical Society of Japan, 2006Co-Authors: Hiroyuki Nakanishi, Yoshiaki SofueAbstract:We created a three-dimensional distribution map of molecular gas throughout the Milky Way Galaxy using the latest $^{12}$CO($J=$1--0) survey data cube and rotation curve based on the kinematic distance. The radial distribution of the molecular gas shows a central peak and a second peak around 0.5 $R_0$. The thickness of the molecular disk slightly increases from 48 pc to 160 pc with the galactocentric distance within a radius range of 0--11 kpc. We were able to trace the Outer, the Perseus, the Sagittarius-Carina, the Scutum-Crux, and the Norma arms as logarithmic spiral arms with pitch angles of $11\arcdeg - 15\arcdeg$. Considering that the pitch angles of the spiral arms are within this range, the Norma and the Outer arms {seem to} be identified as the same spiral arm. We could also trace a midplane displacement, whose amplitude is nearly constant inside a 10 kpc radius and increases beyond this radius. The ridges of midplane displacement form leading spiral arms.
Gerard Gilmore - One of the best experts on this subject based on the ideXlab platform.
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Gaia: 3-dimensional census of the Milky Way Galaxy
Contemporary Physics, 2018Co-Authors: Gerard GilmoreAbstract:Astrometry from space has unique advantages over ground-based observations: the all-sky coverage, relatively stable, and temperature and gravity invariant operating environment delivers precision, accuracy and sample volume several orders of magnitude greater than ground-based results. Even more importantly, absolute astrometry is possible. The European Space Agency Cornerstone mission Gaia is delivering that promise. Gaia provides 5-D phase space measurements - 3 spatial coordinates and two space motions in the plane of the sky - for a representative sample of the Milky Way's stellar populations, including over two billion stars, being about one percent of the stars over about 50 percent of the radius. Full 6-D phase space data is delivered from Gaia's line-of-sight (radial) velocities for the 300 million brightest stars. These data make substantial contributions to astrophysics and fundamental physics on scales from the Solar System to cosmology. A knowledge revolution is underWay.
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gaia 3 dimensional census of the Milky Way Galaxy
Contemporary Physics, 2018Co-Authors: Gerard GilmoreAbstract:Astrometry from space has unique advantages over ground-based observations: the all-sky coverage, relatively stable, and temperature and gravity invariant, operating environment delivers precision,...
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Infrared stellar populations in the central parts of the Milky Way Galaxy
Monthly Notices of the Royal Astronomical Society, 2003Co-Authors: Jacco Th Van Loon, Gerard Gilmore, A Omont, J A D L Blommaert, I S Glass, M Messineo, F Schuller, M Schultheis, Issei Yamamura, Hongsheng ZhaoAbstract:Near- and mid-IR survey data from DENIS and ISOGAL are used to investigate the structure and formation history of the inner 10 ◦ (1.4 kpc) of the Milky Way Galaxy. Synthetic bolometric corrections and extinction coefficients in the near- and mid-infrared (mid-IR) are derived for stars of different spectral types, to allow the transformation of theoretical isochrones into observable colour‐magnitude diagrams. The observed IR colour‐magnitude diagrams are used to derive the extinction, metallicity and age for individual stars. The inner Galaxy is dominated by an old population (7 Gyr). In addition, an intermediate-age population (∼200 Myr‐7 Gyr) is detected, which is consistent with the presence of a few hundred asymptotic giant branch stars with heavy mass loss. Furthermore, young stars (200 Myr) are found across the inner bulge. The metallicities of these stellar population components are discussed. These results can be interpreted in terms of an early epoch of intense star formation and chemical enrichment that shaped the bulk of the bulge and nucleus, and a more continuous star formation history that gradually shaped the disc from the accretion of subsolar metallicity gas from the halo. A possible increase in star formation ∼200 Myr ago might have been triggered by a minor merger. Ever since the formation of the first stars, mechanisms have been at play that mix the populations from the nucleus, bulge and disc. Luminosity functions across the inner Galactic plane indicate the presence of an inclined (bar) structure at 1 kpc from the Galactic Centre, near the inner Lindblad resonance. The innermost part of the bulge, within ∼1 kpc from the Galactic Centre, seems azimuthally symmetric.
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infrared stellar populations in the central parts of the Milky Way Galaxy
arXiv: Astrophysics, 2002Co-Authors: Jacco Th Van Loon, Gerard Gilmore, A Omont, J A D L Blommaert, I S Glass, M Messineo, F Schuller, M Schultheis, Issei Yamamura, Hongsheng ZhaoAbstract:Near- and mid-IR survey data from DENIS and ISOGAL are used to investigate the structure and formation history of the inner 10 degree (1.4 kpc) of the Milky Way Galaxy. Synthetic bolometric corrections and extinction coefficients in the near- and mid-IR are derived for stars of different spectral types, to allow the transformation of theoretical isochrones into observable colour-magnitude diagrams. The observed IR colour-magnitude diagrams are used to derive the extinction, metallicity and age for individual stars. The inner Galaxy is dominated by an old population (> 7 Gyr). In addition, an intermediate-age population (200 Myr to 7 Gyr) is detected, which is consistent with the presence of a few hundred Asymptotic Giant Branch stars with heavy mass loss. Furthermore, young stars ( 1 kpc from the galactic centre, near the inner Lindblad resonance. The innermost part of the Bulge, within about 1 kpc from the galactic centre, seems azimuthally symmetric.
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Deciphering The Last Major Invasion of the Milky Way Galaxy
The Astrophysical Journal, 2002Co-Authors: Gerard Gilmore, John E. NorrisAbstract:jen@mso.anu.edu.auABSTRACTWe present first results from a spectroscopic survey of ∼ 2000 F/G stars 0.5–5kpcfrom the Galactic plane, obtained with the 2dF facility on the AAT. These data showthe mean rotation velocity of the thick disk about the Galactic center a few kpc from theplane is very different than expectation, being ∼ 100km/s, rather than the predicted∼ 180km/s. We propose that our sample is dominated by stars from a disruptedsatellite which merged with the disk of the Milky Way Galaxy some 10-12Gyr ago. Wedo not find evidence for the many substantial mergers expected in hierarchical clusteringtheories. We find yet more evidence that the stellar halo retains kinematic substructure,indicative of minor mergers.
Jacco Th Van Loon - One of the best experts on this subject based on the ideXlab platform.
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Infrared stellar populations in the central parts of the Milky Way Galaxy
Monthly Notices of the Royal Astronomical Society, 2003Co-Authors: Jacco Th Van Loon, Gerard Gilmore, A Omont, J A D L Blommaert, I S Glass, M Messineo, F Schuller, M Schultheis, Issei Yamamura, Hongsheng ZhaoAbstract:Near- and mid-IR survey data from DENIS and ISOGAL are used to investigate the structure and formation history of the inner 10 ◦ (1.4 kpc) of the Milky Way Galaxy. Synthetic bolometric corrections and extinction coefficients in the near- and mid-infrared (mid-IR) are derived for stars of different spectral types, to allow the transformation of theoretical isochrones into observable colour‐magnitude diagrams. The observed IR colour‐magnitude diagrams are used to derive the extinction, metallicity and age for individual stars. The inner Galaxy is dominated by an old population (7 Gyr). In addition, an intermediate-age population (∼200 Myr‐7 Gyr) is detected, which is consistent with the presence of a few hundred asymptotic giant branch stars with heavy mass loss. Furthermore, young stars (200 Myr) are found across the inner bulge. The metallicities of these stellar population components are discussed. These results can be interpreted in terms of an early epoch of intense star formation and chemical enrichment that shaped the bulk of the bulge and nucleus, and a more continuous star formation history that gradually shaped the disc from the accretion of subsolar metallicity gas from the halo. A possible increase in star formation ∼200 Myr ago might have been triggered by a minor merger. Ever since the formation of the first stars, mechanisms have been at play that mix the populations from the nucleus, bulge and disc. Luminosity functions across the inner Galactic plane indicate the presence of an inclined (bar) structure at 1 kpc from the Galactic Centre, near the inner Lindblad resonance. The innermost part of the bulge, within ∼1 kpc from the Galactic Centre, seems azimuthally symmetric.
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Infrared stellar populations in the central parts of the Milky Way Galaxy
Monthly Notices of the Royal Astronomical Society, 2003Co-Authors: Jacco Th Van Loon, A Omont, J A D L Blommaert, I S Glass, M Messineo, F Schuller, M Schultheis, Issei Yamamura, G. F. Gilmore, H. S. ZhaoAbstract:Near- and mid-IR survey data from DENIS and ISOGAL are used to investigate the structure and formation history of the inner 10 degree (1.4 kpc) of the Milky Way Galaxy. Synthetic bolometric corrections and extinction coefficients in the near- and mid-IR are derived for stars of different spectral types, to allow the transformation of theoretical isochrones into observable colour-magnitude diagrams. The observed IR colour-magnitude diagrams are used to derive the extinction, metallicity and age for individual stars. The inner Galaxy is dominated by an old population (> 7 Gyr). In addition, an intermediate-age population (200 Myr to 7 Gyr) is detected, which is consistent with the presence of a few hundred Asymptotic Giant Branch stars with heavy mass loss. Furthermore, young stars (< 200 Myr) are found across the inner Bulge. The metallicities of these stellar population components are discussed. These results can be interpreted in terms of an early epoch of intense star formation and chemical enrichment which shaped the bulk of the Bulge and nucleus, and a more continuous star formation history which gradually shaped the disk from the accretion of sub-solar metallicity gas from the halo. A possible increase in star formation about 200 Myr ago might have been triggered by a minor merger. Ever since the formation of the first stars, mechanisms have been at play that mix the populations from the nucleus, Bulge and disk. Luminosity functions across the inner galactic plane indicate the presence of an inclined (bar) structure at > 1 kpc from the galactic centre, near the inner Lindblad resonance. The innermost part of the Bulge, within about 1 kpc from the galactic centre, seems azimuthally symmetric.
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infrared stellar populations in the central parts of the Milky Way Galaxy
arXiv: Astrophysics, 2002Co-Authors: Jacco Th Van Loon, Gerard Gilmore, A Omont, J A D L Blommaert, I S Glass, M Messineo, F Schuller, M Schultheis, Issei Yamamura, Hongsheng ZhaoAbstract:Near- and mid-IR survey data from DENIS and ISOGAL are used to investigate the structure and formation history of the inner 10 degree (1.4 kpc) of the Milky Way Galaxy. Synthetic bolometric corrections and extinction coefficients in the near- and mid-IR are derived for stars of different spectral types, to allow the transformation of theoretical isochrones into observable colour-magnitude diagrams. The observed IR colour-magnitude diagrams are used to derive the extinction, metallicity and age for individual stars. The inner Galaxy is dominated by an old population (> 7 Gyr). In addition, an intermediate-age population (200 Myr to 7 Gyr) is detected, which is consistent with the presence of a few hundred Asymptotic Giant Branch stars with heavy mass loss. Furthermore, young stars ( 1 kpc from the galactic centre, near the inner Lindblad resonance. The innermost part of the Bulge, within about 1 kpc from the galactic centre, seems azimuthally symmetric.
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Structure and Evolution of the Inner Milky Way Galaxy
The Evolution of Galaxies, 2001Co-Authors: Jacco Th Van LoonAbstract:The DENIS/ISOGAL near/mid-IR survey of the Milky Way for the first time probes stellar populations in the innermost obscured regions of our Galaxy. Ages, metallicities and extinction-corrected luminosities are derived for these stars individually. An old metal-rich population dominates in the inner galactic Bulge, but there are also indications for the presence of a younger population. The inner Bulge has a tri-axial shape, as traced by depth effects on the observed luminosity distributions.
W B Shi - One of the best experts on this subject based on the ideXlab platform.
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the spiral structure of our Milky Way Galaxy
Astronomy and Astrophysics, 2009Co-Authors: L G Hou, J L Han, W B ShiAbstract:Context. The spiral structure of our Milky Way Galaxy is not yet known. HII regions and giant molecular clouds are the most prominent spiral tracers. Models with 2−4 arms have been proposed to outline the structure of our Galaxy. Aims. Recently, new data of spiral tracers covering a larger region of the Galactic disk have been published. We wish to outline the spiral structure of the Milky Way using all tracer data. Methods. We collected the spiral tracer data of our Milky Way from the literature, namely, HII regions and giant molecular clouds (GMCs). With weighting factors based on the excitation parameters of HII regions or the masses of GMCs, we fitted the distribution of these tracers with models of two, three, four spiral-arms or polynomial spiral arms. The distances of tracers, if not available from stellar or direct measurements, were estimated kinetically from the standard rotation curve of Brand & Blitz (1993, A&A, 275, 67) with R0 = 8.5 kpc, and Θ0 = 220 km s −1 or the newly fitted rotation curves with R0 = 8.0 kpc and Θ0 = 220 km s −1 or R0 = 8.4 kpc and Θ0 = 254 km s −1 . Results. We found that the two-arm logarithmic model cannot fit the data in many regions. The three- and the four-arm logarithmic models are able to connect most tracers. However, at least two observed tangential directions cannot be matched by the three- or four-arm model. We composed a polynomial spiral arm model, which can not only fit the tracer distribution but also match observed tangential directions. Using new rotation curves with R0 = 8.0 kpc and Θ0 = 220 km s −1 and R0 = 8.4 kpc and Θ0 = 254 km s −1 for the estimation of kinematic distances, we found that the distribution of HII regions and GMCs can fit the models well, although the results do not change significantly compared to the parameters with the standard R0 and Θ0.
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the spiral structure of our Milky Way Galaxy
arXiv: Astrophysics of Galaxies, 2009Co-Authors: L G Hou, J L Han, W B ShiAbstract:The spiral structure of our Milky Way Galaxy is not yet known. HII regions and giant molecular clouds are the most prominent spiral tracers. We collected the spiral tracer data of our Milky Way from the literature, namely, HII regions and giant molecular clouds (GMCs). With weighting factors based on the excitation parameters of HII regions or the masses of GMCs, we fitted the distribution of these tracers with models of two, three, four spiral-arms or polynomial spiral arms. The distances of tracers, if not available from stellar or direct measurements, were estimated kinetically from the standard rotation curve of Brand & Blitz (1993) with $R_0$=8.5 kpc, and $\Theta_0$=220 km s$^{-1}$ or the newly fitted rotation curves with $R_0$=8.0 kpc and $\Theta_0$=220 km s$^{-1}$ or $R_0$=8.4 kpc and $\Theta_0$=254 km s$^{-1}$. We found that the two-arm logarithmic model cannot fit the data in many regions. The three- and the four-arm logarithmic models are able to connect most tracers. However, at least two observed tangential directions cannot be matched by the three- or four-arm model. We composed a polynomial spiral arm model, which can not only fit the tracer distribution but also match observed tangential directions. Using new rotation curves with $R_0$=8.0 kpc and $\Theta_0$=220 km s$^{-1}$ and $R_0$=8.4 kpc and $\Theta_0$=254 km s$^{-1}$ for the estimation of kinematic distances, we found that the distribution of HII regions and GMCs can fit the models well, although the results do not change significantly compared to the parameters with the standard $R_0$ and $\Theta_0$.