arXiv subjects

Hiroyuki Nakanishi

Publications and source records attributed to Hiroyuki Nakanishi.

61 records · Page 4Linked to original sources

arXiv2006

ASTE observations of nearby galaxies: A tight correlation between CO(J=3-2) emission and Halpha

Star formation rates (SFRs) obtained via extinction corrected H alpha are compared to dense gas as traced by CO(J=3-2) emission at the centers of nearby galaxies, observed with the ASTE telescope. It is found that, although many of the observed positions are dusty and therefore heavily absorbed at H alpha, the SFR shows a striking correlation with dense gas in the form of the Schmidt law with an index 1.0. The correlation is also compared between gas traced by CO(J=1-0) and application of H alpha extinction correction. We find that dense gas produces a far better correlation with SFR in view of surface density values.

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arXiv2005

Virgo High-Resolution CO Survey :V. Circumnuclear Elliptical Ring in NGC 4569

We present high-resolution ($1\farcs8$ -- $4\farcs5$) CO data of the Virgo spiral galaxy NGC 4569, obtained using the Nobeyama Millimeter Array. We found that the molecular gas is highly concentrated in the circumnuclear region with two off-center peaks. A CO image with the highest angular resolution of $2\farcs0 \times 1\farcs8$ shows that six blobs likely form a circumnuclear elliptical ring (CER) with a semi-major axis radius of 720 pc. The CER shows a strongly twisted velocity field, and the position--velocity (PV) diagram shows significant forbidden velocity components. These kinetic features are understood as being non-circular motion due to a bar-potential. We found that the CER coincides with the H$α$ bright central core and that the mass ratio of the molecular gas to the dynamical mass is about 18%. These results support a gaseous inflow scenario induced by a weak bar potential and self-gravity of the gas.

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arXiv2005

The Schmidt Law at High Molecular Densities

We have combined Halpha and recent high resolution CO(J=1-0) data to consider the quantitative relation between gas mass and star formation rate, or the so-called Schmidt law in nearby spiral galaxies at regions of high molecular density. The relation between gas quantity and star formation rate has not been previously studied for high density regions, but using high resolution CO data obtained at the NMA(Nobeyama Millimeter Array), we have found that the Schmidt law is valid at densities as high as $10^3 \mathrm{M_\odot} \mathrm{pc}^{-2}$ for the sample spiral galaxies, which is an order of magnitude denser than what has been known to be the maximum density at which the empirical law holds for non-starburst galaxies. Furthermore, we obtain a Schmidt law index of $N=1.33\pm0.09$ and roughly constant star formation efficiency over the entire disk, even within the several hundred parsecs of the nucleus. These results imply that the physics of star formation does not change in the central regions of spiral galaxies. Comparisons with starburst galaxies are also given. We find a possible discontinuity in the Schmidt law between normal and starburst galaxies.

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arXiv2005

A Method For Determining Two-Dimensional Orbits In Galactic Disks

We propose a method for determining the two-dimensional orbits of interstellar matter (ISM) in galactic disks using observed line-of-sight velocity fields. Although four components, consisting of two of position and two of velocity, are necessary to determine two-dimensional orbits, the velocity perpendicular to the line of sight can hardly be determined from observations. We show that the ISM orbits can be approximately determined by assuming that (1) the angular momentum is conserved along each orbit, (2) all orbits are closed in a single rotating frame, and (3) the system is symmetric through the origin. Since the angular momentum can be estimated on the major axis of a disk, a velocity perpendicular to the line of sight can be determined according to the conservation of angular momentum, and thus two components of velocity are obtained. The ISM orbits can be calculated by solving differential equations given by an obtained two-dimensional velocity field when a certain pattern speed is given. The typical deviation from the correct orbit is less than the observational resolution according to our test calculations for modeled velocity fields. We applied this method to the observed data of NGC 4569 and obtained possible orbits for this galaxy. Furthermore, we calculated the mass distribution using our result.

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arXiv2004

Offsets between H-alpha and CO arms of a spiral galaxy NGC 4254: A New Method for Determining the Pattern Speed of Spiral Galaxies

We examined offsets between HII regions and molecular clouds belonging to spiral arms of a late type spiral galaxy NGC 4254 (M99). We used a high resolution CO(1-0) image obtained by Nobeyama Millimeter Array (NMA) and an H-alpha image. We derived angular offsets (theta) in the galactic disk, and found that these offsets show a linear dependence on the angular rotation velocity of gas (Omega_G). This linear relation can be expressed by an equation: theta =(Omega_G - Omega_P) * t_{H-alpha}, where Omega_P and t_{H-alpha} are constant. Here, Omega_P corresponds to the pattern speed of spiral arms and t_{H-alpha} is interpreted as the timescale between the peak compression of the molecular gas in spiral arms and the peak of massive star formation. We may thus determine Omega_P and t_{H-alpha} simultaneously by fitting a line to our theta - Omega_G plot, if we assumed they are constant. From our plot, we obtained t_{H-alpha} =4.8 (+/- 1.2) Myr and Omega_P = 26 (+10/-6) km/s/kpc, which are consistent with previous studies. We suggest that this theta - Omega_G plot can be a new tool to determine the pattern speed and the typical timescale needed for star formations.

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arXiv2003

Three-Dimensional Distribution of the ISM in the Milky Way Galaxy: I. The HI Disk

We derived the three-dimensional distribution of HI gas in the Milky Way Galaxy using the latest HI survey data cubes and rotation curves. The distance of the HI gas was determined by the kinematic distance using a rotation curve. We solved the near-far problem in the inner Galaxy by a fitting method which involves introducing a model of vertical HI distribution. In our resultant maps we could trace three prominent arms: the Sagittarius-Carina arm, the Perseus arm, and the Outer arm. These three arms were found to be logarithmic spiral arms. The pitch angles of the Sagittarius-Carina, Perseus, and Outer arms were estimated to be about $11\arcdeg$, $18\arcdeg$, and $7\arcdeg$, respectively. The Sun is located in a region rich in HI gas between the Sagittarius-Carina arm and the Perseus arm. The HI disk shows large and asymmetric warping in the outer disk: the HI disk goes up to about 1.5 kpc above the Galactic plane in the northern hemisphere, and down to about 1 kpc in the southern hemisphere, which means asymmetric warping. The inner HI disk is also found to be tilting. The radius of the HI disk is about 17 kpc and the HI mass within this radius is estimated to be $2.5 \times 10^9$ M$_\odot$, which corresponds to 1.5 % of the dynamical mass predicted from the rotation curve. We also found that the HI outskirt is largely swelling in the fourth quadrant, and hence the Galaxy is significantly lopsided. The scale-height of the HI layer increases with the radius, and is correlated with the HI volume density at the centroid of the HI layer.

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arXiv2003

The Virgo High-Resolution CO Survey. III. NGC 4254

We present high-angular-resolution (1".5 - 5") interferometer observations of the \co emission in the central region of the SA(s)c galaxy NGC 4254. The observations were obtained using the Nobeyama Millimeter-wave Array (NMA) during the course of a long-term CO line survey of Virgo spirals. We present the spectra, channel maps, integrated intensity distributions, velocity fields, position--velocity diagrams, and compare the data with various optical images. The rotation velocity is already finite at the nucleus, or at least it rises steeply to 80 km/s within the central 1", indicating the existence of a massive core of 10^8 Msun within 1" (80 pc) radius. The CO intensity maps show that the inner disk has well-developed multiple spiral arms, winding out from a bar-shaped elongated molecular complex. In addition to the bisymmetric spiral arms, an asymmetric tightly wound arm with high molecular gas density is found to wind out from the molecular bar. The molecular spiral arms, particularly the tightly wound arm, well traces optical dark lanes, and are associated with H$α$ arms having many H {\sc ii} regions. The inner asymmetric spiral structures can be explained by ram-pressure distortion of inter-arm low density regions of the inner disk by the intra-cluster gas wind, and is indeed reproduced by a hydrodynamical simulation.

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