Rie Miura E. | National Astronomical Observatory of Japan (original) (raw)

Papers by Rie Miura E.

Research paper thumbnail of Enhancement of CO(3-2)/CO(1-0) Ratios and Star Formation Efficiencies in Supergiant HII Regions

ApJ, 2014

We present evidence that super giant HII regions (GHRs) and other disk regions of the nearby spir... more We present evidence that super giant HII regions (GHRs) and other disk regions of the nearby spiral galaxy, M33, occupy distinct locations in the correlation between molecular gas, SigmarmH2\Sigma_{\rm H_2}SigmarmH2, and the star formation rate surface density, SigmarmSFR\Sigma_{\rm SFR}SigmarmSFR. This result is based on wide field and high sensitivity CO(3-2) observations at 100 pc resolution. Star formation efficiencies (SFE), defined as SigmarmSFR\Sigma_{\rm SFR}SigmarmSFR/$\Sigma_{\rm H_2}$, in GHRs are found to be about 1 dex higher than in other disk regions. The CO(3-2)/CO(1-0) integrated intensity ratio is also higher than the average over the disk. Such high SFE and CO(3-2)/CO(1-0) can reach the values found in starburst galaxies, which suggests that GHRs may be the elements building up a larger scale starburst region. Three possible contributions to high SFEs in GHR are investigated: (1) the ICOI_{CO}ICO-$N({\rm H_2})$ conversion factor, (2) the dense gas fraction traced by CO(3-2)/CO(1-0), and (3) the initial mass function (IMF). We conclude that these starburst-like properties in GHRs can be interpreted by a combination of both a top-heavy IMF and a high dense gas fraction, but not by changes in the ICOI_{CO}ICO-$N({\rm H_2})$ conversion factor.

Research paper thumbnail of Temperature Variations of Cold Dust in the Triangulum Galaxy M 33

Publications of The Astronomical Society of Japan, 2011

We present wide-field 1.1 mm continuum imaging of the nearby spiral galaxy M 33, conducted with t... more We present wide-field 1.1 mm continuum imaging of the nearby spiral galaxy M 33, conducted with the AzTEC bolometer camera on ASTE. We show that the 1.1 mm flux traces the distribution of dust with T ~20 K. Combined with far-infrared imaging at 160um, we derive the dust temperature distribution out to a galactic radius of ~7 kpc with a spatial resolution of ~100 parsecs. Although the 1.1 mm flux is observed predominantly near star forming regions, we find a smooth radial temperature gradient declining from ~20 K to ~13 K, consistent with recent results from the Herschel satellite. Further comparison of individual regions show a strong correlation between the cold dust temperature and the Ks band brightness, but not with the ionizing flux. The observed results imply that the dominant heating source of cold dust at few hundred parsec scales are due to the non-OB stars, even when associated with star forming regions.

Research paper thumbnail of NRO M 33 All-Disk Survey of Giant Molecular Clouds (NRO MAGiC). II Dense Gas Formation

Research paper thumbnail of Giant Molecular Cloud Evolutions in the Nearby Spiral Galaxy M33

Astrophysical Journal

We present a Giant Molecular Cloud (GMC) catalog toward M33, containing 71 GMCs in total, based o... more We present a Giant Molecular Cloud (GMC) catalog toward M33, containing 71 GMCs in total, based on wide field and high sensitivity CO(J=3-2) observations with a spatial resolution of 100 pc using the ASTE 10 m telescope. Employing archival optical data, we identify 75 young stellar groups (YSGs) from the excess of the surface stellar density, and estimate their ages by comparing with stellar evolution models. A spatial comparison among the GMCs, YSGs, and HII regions enable us to classify GMCs into four categories: Type A showing no sign of massive star formation (SF), Type B being associated only with HII regions, Type C with both HII regions and <10 Myr-old YSGs and Type-D with both HII regions and 10–30 Myr YSGs. Out of 65 GMCs (discarding those at the edges of the observed fields), 1 (1%), 13 (20%), 29 (45%), and 22 (34%) are Types A, B, C, and D, respectively. We interpret these categories as stages in a GMC evolutionary sequence. Assuming that the timescale for each evolutionary stage is proportional to the number of GMCs, the lifetime of a GMC with a mass >10^5 Mo is estimated to be 20–40 Myr. In addition, we find that the dense gas fraction as traced by the CO(J=3-2)/CO(J=1-0) ratio is enhanced around SF regions. This confirms a scenario where dense gas is preferentially formed around previously generated stars, and will be the fuel for the next stellar generation. In this way, massive SF gradually propagates in a GMC until gas is exhausted.

Research paper thumbnail of 13CO (J= 1-0) On-the-fly Mapping of the Giant HII Region NGC 604: Variation in Molecular Gas Density and Temperature due to Sequential Star Formation

Arxiv preprint arXiv: …, Jan 1, 2011

We present 13 CO(J = 1 − 0) line emission observations with the Nobeyama 45m telescope toward the... more We present 13 CO(J = 1 − 0) line emission observations with the Nobeyama 45m telescope toward the giant H II region NGC 604 in the spiral galaxy M 33. We detected 13 CO(J = 1 − 0) line emission in 3 major giant molecular clouds (GMCs) labeled as GMC-A, B, and C beginning at the north. We derived two line intensity ratios, 13 CO(J = 1−0)/ 12 CO(J = 1−0), R 13/12 , and 12 CO(J = 3−2)/ 12 CO(J = 1−0), R 31 , for each GMC at an angular resolution of 25 ′′ (100 pc). Averaged values of R 13/12 and R 31 are 0.06 and 0.31 within the whole GMC-A, 0.11 and 0.67 within the whole GMC-B, and 0.05 and 0.36 within the whole GMC-C, respectively. In addition, we obtained R 13/12 = 0.09±0.02 and R 31 = 0.76±0.06 at the 12 CO(J = 1−0) peak position of the GMC-B. Under the Large Velocity Gradient approximation, we determined gas density of 2.8 ×10 3 cm −3 and kinetic temperature of 33 +9 −5 K at the 12 CO(J = 1 − 0) peak position of the GMC-B. Moreover, we determined 2.5 ×10 3 cm −3 and 25±2 K as averaged values within the whole GMC-B. We concluded that dense molecular gas is formed everywhere in the GMC-B because derived gas density not only at the peak position of the GMC but also averaged over the whole GMC exceeds 10 3 cm −3 . On the other hand, kinetic temperature averaged over the whole GMC-B, 25 K, is significantly lower than that at the peak position, 33 K. This is because H II regions are lopsided to the northern part of the GMC-B, thus OB stars can heat only the 1 northern part, including the 12 CO(J = 1 − 0) peak position, of this GMC.

Research paper thumbnail of Dense Clouds and Star Formation on Spiral Arm in M33-deep CO and HCN Observation in NGC604

Mapping the Galaxy …, Jan 1, 2008

Rie Miura1,2, Yoichi Tamura1,2, Sachiko K. Okumura2, Ryohei Kawabe2, Tomoka Tosaki2, Nario Kuno2,... more Rie Miura1,2, Yoichi Tamura1,2, Sachiko K. Okumura2, Ryohei Kawabe2, Tomoka Tosaki2, Nario Kuno2, Kouichiro Nakanishi2, Seiichi Sakamoto2, and Takashi Hasegawa3 ... 1 the University of Tokyo 2 National Astronomical Observatory of Japan 3 Gunma Astronomical Observatory

Research paper thumbnail of Breakdown of Kennicutt–Schmidt Law at Giant Molecular Cloud Scales in M33

The Astrophysical …, Jan 1, 2010

We have mapped the northern area (30 × 20 ) of a Local Group spiral galaxy M33 in 12 CO(J = 1-0) ... more We have mapped the northern area (30 × 20 ) of a Local Group spiral galaxy M33 in 12 CO(J = 1-0) line with the 45 m telescope at the Nobeyama Radio Observatory. Along with Hα and Spitzer 24 μm data, we have investigated the relationship between the surface density of molecular gas mass and that of star formation rate (SFR) in an external galaxy (Kennicutt-Schmidt law) with the highest spatial resolution (∼80 pc) to date, which is comparable to scales of giant molecular clouds (GMCs). At positions where CO is significantly detected, the SFR surface density exhibits a wide range of over four orders of magnitude, from Σ SFR 10 −10 to ∼10 −6 M yr −1 pc −2 , whereas the Σ H 2 values are mostly within 10-40 M pc −2 . The surface density of gas and that of SFR correlate well at an ∼1 kpc resolution, but the correlation becomes looser with higher resolution and breaks down at GMC scales. The scatter of the Σ SFR -Σ H 2 relationship in the ∼80 pc resolution results from the variety of star-forming activity among GMCs, which is attributed to the various evolutionary stages of GMCs and to the drift of young clusters from their parent GMCs. This result shows that the Kennicutt-Schmidt law is valid only in scales larger than that of GMCs, when we average the spatial offset between GMCs and star-forming regions, and their various evolutionary stages.

Research paper thumbnail of ASTE CO (3--2) Observations of the Southern Barred Spiral Galaxy NGC 986: a Large Gaseous Bar Filled with Dense Molecular Medium

Arxiv preprint arXiv: …, Jan 1, 2008

We present CO(3-2) emission observations toward the 3 ′ × 3 ′ (or 20 × 20 kpc at a distance of 23... more We present CO(3-2) emission observations toward the 3 ′ × 3 ′ (or 20 × 20 kpc at a distance of 23 Mpc) region of the southern barred spiral galaxy NGC 986 using the Atacama Submillimeter Telescope Experiment (ASTE). This effort is a part of our on-going extragalactic CO(3-2) imaging project ADIoS (ASTE Dense gas Imaging of Spiral galaxies). Our CO(3-2) image revealed the presence of a large (the major axis is 14 kpc in total length) gaseous bar filled with dense molecular medium along the dark lanes observed in optical images. This is the largest "dense-gas rich bar" known to date. The dense gas bar discovered in NGC 986 could be a huge reservoir of possible "fuel" for future starbursts in the central region, and we suggest that the star formation in the central region of NGC 986 could still be in a growing phase. We 1 found a good spatial coincidence between the overall distributions of dense molecular gas traced by CO(3-2) and the massive star formation depicted by Hα. The global CO(3-2) luminosity L ′ CO(3−2) of NGC 986 was determined to be (5.4 ± 1.1) × 10 8 K km s −1 pc 2 . The CO(3-2)/CO(1-0) integrated intensity ratio was found to be 0.60 ± 0.13 at a spatial resolution of 44 ′′ or 5 kpc, and a CO(3-2)/CO(2-1) ratio was 0.67 ± 0.14 at a beam size of ∼25 ′′ or ∼2.8 kpc. These line ratios suggest moderate excitation conditions of CO lines (n H 2 ∼ 10 3−4 cm −3 ) in the central a few kpc region of NGC 986.

Research paper thumbnail of Breakdown of Kennicutt-Schmidt Law at GMC Scales in M33

Arxiv preprint arXiv: …, Jan 1, 2010

We have mapped the northern area (30 ′ × 20 ′ ) of a local group spiral galaxy M33 in 12 CO(J = 1... more We have mapped the northern area (30 ′ × 20 ′ ) of a local group spiral galaxy M33 in 12 CO(J = 1-0) line with the 45-m telescope at the Nobeyama Radio Observatory. Along with Hα and Spitzer 24µm data, we have investigated the relationship between the surface density of molecular gas mass and that of star formation rate (SFR) in an external galaxy (Kennicutt-Schmidt law) with the highest spatial resolution (∼ 80 pc) to date, which is comparable to scales of giant molecular clouds (GMCs). At positions where CO is significantly detected, the SFR surface density exhibits a wide range of over four orders of magnitude, from Σ SFR 10 −10 to ∼ 10 −6 M ⊙ yr −1 pc −2 , whereas the Σ H2 values are mostly within 10-40 M ⊙ pc −2 . The surface density of gas and that of SFR correlate well at a ∼1-kpc resolution, but the correlation becomes looser with higher resolution and breaks down at GMC scales. The scatter of the Σ SFR -Σ H2 relationship in the ∼ 80-pc resolution results from the variety of star forming activity among GMCs, which is attributed to the various evolutionary stages of GMCs and to the drift of young clusters from their parent GMCs. This result shows that the Kennicutt-Schmidt law is valid only in scales larger than that of GMCs, when we average the spatial offset between GMCs and star forming regions, and their various evolutionary stages.

Research paper thumbnail of Aperture Synthesis Observations of CO, HCN, and 89 GHz Continuum Emission Toward NGC 604 in M33: Sequential Star Formation Induced by a Supergiant H II …

The Astrophysical …, Jan 1, 2010

We present the results from new Nobeyama Millimeter Array observations of CO(1-0), HCN(1-0), and ... more We present the results from new Nobeyama Millimeter Array observations of CO(1-0), HCN(1-0), and 89-GHz continuum emissions toward NGC 604, known as the supergiant H ii region in a nearby galaxy M 33. Our high spatial resolution images (4. ′′ 2 × 2. ′′ 6, corresponding to 17 pc × 11 pc physical size) of CO emission allowed us to uncover ten individual molecular clouds that have masses of (0.8 -7.4) ×10 5 M ⊙ and sizes of 5 -29 pc, comparable to those of typical Galactic giant molecular clouds (GMCs). Moreover, we detected for the first time HCN emission in the two most massive clouds and 89 GHz continuum emission at the rims of the "Hα shells". The HCN and 89 GHz continuum emission show offset from the CO peak and are distributed to the direction of the central cluster. Three out of -2ten CO clouds are well correlated with the Hα shells both in spatial and velocity domains, implying an interaction between molecular gas and the expanding H ii region. The CO clouds show varieties in star formation efficiencies (SFEs), which are estimated from the 89-GHz and combination of Hα and Spitzer 24-µm data. Furthermore, we found that the SFEs decrease with increasing projected distance measured from the heart of the central OB star cluster in NGC 604, suggesting the radial changes in evolutionary stages of the molecular clouds in course of stellar cluster formation. Our results provide further support to the picture of sequential star formation in NGC604 initially proposed by with the higher spatially resolved molecular clouds, in which an isotropic expansion of the H ii region pushes gases outward and accumulates them to consecutively form dense molecular clouds, and then induces massive star formations.

Research paper thumbnail of NRO M33 All Disk Survey of Giant Molecular Clouds (NRO MAGiC): I. HI to H_2 Transition

Arxiv preprint arXiv: …, Jan 1, 2011

We present the results of the Nobeyama Radio Observatory (NRO) M33 All Disk (30 ′ × 30 ′ , or 7.3... more We present the results of the Nobeyama Radio Observatory (NRO) M33 All Disk (30 ′ × 30 ′ , or 7.3 kpc × 7.3 kpc) Survey of Giant Molecular Clouds (NRO MAGiC) based on 12 CO (J = 1-0) observations using the NRO 45-m telescope. The spatial resolution of the resultant map is 19 ′′ .3, corresponding to 81 pc, which is sufficient to identify each Giant Molecular Cloud (GMC) in the disk. We found clumpy structures with a typical spatial scale of ∼100 pc, corresponding to GMCs, and no diffuse, smoothly distributed component of molecular gas at this sensitivity. The overall distribution of molecular gas roughly agrees with that of Hi. However, closer inspection of the CO and Hi maps suggests that not every CO emission is associated with local Hi peaks, particularly in the inner portion of the disk (r < 2 kpc), although most 1 of CO emission is located at the local Hi peaks in the outer radii. We found that most uncovered GMCs are accompanied by massive star-forming regions, although the star formation rates (SFRs) vary widely from cloud to cloud. The azimuthally averaged Hi gas surface density exhibits a flat radial distribution. However, the CO radial distribution shows a significant enhancement within the central 1-2 kpc region, which is very similar to that of the SFR. We obtained a map of the molecular fraction, f mol = Σ H 2 /(Σ Hi + Σ H 2 ), at a 100-pc resolution. This is the first f mol map covering an entire galaxy with a GMC-scale resolution. We find that f mol tends to be high near the center. The correlation between f mol and gas surface density shows two distinct sequences. The presence of two correlation sequences can be explained by differences in metallicity, i.e., higher (∼2-fold) metallicity in the central region (r < 1.5 kpc) than in the outer parts. Alternatively, differences in scale height can also account for the two sequences, i.e., increased scale height toward the outer disk.

Research paper thumbnail of Enhancement of CO(3-2)/CO(1-0) Ratios and Star Formation Efficiencies in Supergiant HII Regions

ApJ, 2014

We present evidence that super giant HII regions (GHRs) and other disk regions of the nearby spir... more We present evidence that super giant HII regions (GHRs) and other disk regions of the nearby spiral galaxy, M33, occupy distinct locations in the correlation between molecular gas, SigmarmH2\Sigma_{\rm H_2}SigmarmH2, and the star formation rate surface density, SigmarmSFR\Sigma_{\rm SFR}SigmarmSFR. This result is based on wide field and high sensitivity CO(3-2) observations at 100 pc resolution. Star formation efficiencies (SFE), defined as SigmarmSFR\Sigma_{\rm SFR}SigmarmSFR/$\Sigma_{\rm H_2}$, in GHRs are found to be about 1 dex higher than in other disk regions. The CO(3-2)/CO(1-0) integrated intensity ratio is also higher than the average over the disk. Such high SFE and CO(3-2)/CO(1-0) can reach the values found in starburst galaxies, which suggests that GHRs may be the elements building up a larger scale starburst region. Three possible contributions to high SFEs in GHR are investigated: (1) the ICOI_{CO}ICO-$N({\rm H_2})$ conversion factor, (2) the dense gas fraction traced by CO(3-2)/CO(1-0), and (3) the initial mass function (IMF). We conclude that these starburst-like properties in GHRs can be interpreted by a combination of both a top-heavy IMF and a high dense gas fraction, but not by changes in the ICOI_{CO}ICO-$N({\rm H_2})$ conversion factor.

Research paper thumbnail of Temperature Variations of Cold Dust in the Triangulum Galaxy M 33

Publications of The Astronomical Society of Japan, 2011

We present wide-field 1.1 mm continuum imaging of the nearby spiral galaxy M 33, conducted with t... more We present wide-field 1.1 mm continuum imaging of the nearby spiral galaxy M 33, conducted with the AzTEC bolometer camera on ASTE. We show that the 1.1 mm flux traces the distribution of dust with T ~20 K. Combined with far-infrared imaging at 160um, we derive the dust temperature distribution out to a galactic radius of ~7 kpc with a spatial resolution of ~100 parsecs. Although the 1.1 mm flux is observed predominantly near star forming regions, we find a smooth radial temperature gradient declining from ~20 K to ~13 K, consistent with recent results from the Herschel satellite. Further comparison of individual regions show a strong correlation between the cold dust temperature and the Ks band brightness, but not with the ionizing flux. The observed results imply that the dominant heating source of cold dust at few hundred parsec scales are due to the non-OB stars, even when associated with star forming regions.

Research paper thumbnail of NRO M 33 All-Disk Survey of Giant Molecular Clouds (NRO MAGiC). II Dense Gas Formation

Research paper thumbnail of Giant Molecular Cloud Evolutions in the Nearby Spiral Galaxy M33

Astrophysical Journal

We present a Giant Molecular Cloud (GMC) catalog toward M33, containing 71 GMCs in total, based o... more We present a Giant Molecular Cloud (GMC) catalog toward M33, containing 71 GMCs in total, based on wide field and high sensitivity CO(J=3-2) observations with a spatial resolution of 100 pc using the ASTE 10 m telescope. Employing archival optical data, we identify 75 young stellar groups (YSGs) from the excess of the surface stellar density, and estimate their ages by comparing with stellar evolution models. A spatial comparison among the GMCs, YSGs, and HII regions enable us to classify GMCs into four categories: Type A showing no sign of massive star formation (SF), Type B being associated only with HII regions, Type C with both HII regions and <10 Myr-old YSGs and Type-D with both HII regions and 10–30 Myr YSGs. Out of 65 GMCs (discarding those at the edges of the observed fields), 1 (1%), 13 (20%), 29 (45%), and 22 (34%) are Types A, B, C, and D, respectively. We interpret these categories as stages in a GMC evolutionary sequence. Assuming that the timescale for each evolutionary stage is proportional to the number of GMCs, the lifetime of a GMC with a mass >10^5 Mo is estimated to be 20–40 Myr. In addition, we find that the dense gas fraction as traced by the CO(J=3-2)/CO(J=1-0) ratio is enhanced around SF regions. This confirms a scenario where dense gas is preferentially formed around previously generated stars, and will be the fuel for the next stellar generation. In this way, massive SF gradually propagates in a GMC until gas is exhausted.

Research paper thumbnail of 13CO (J= 1-0) On-the-fly Mapping of the Giant HII Region NGC 604: Variation in Molecular Gas Density and Temperature due to Sequential Star Formation

Arxiv preprint arXiv: …, Jan 1, 2011

We present 13 CO(J = 1 − 0) line emission observations with the Nobeyama 45m telescope toward the... more We present 13 CO(J = 1 − 0) line emission observations with the Nobeyama 45m telescope toward the giant H II region NGC 604 in the spiral galaxy M 33. We detected 13 CO(J = 1 − 0) line emission in 3 major giant molecular clouds (GMCs) labeled as GMC-A, B, and C beginning at the north. We derived two line intensity ratios, 13 CO(J = 1−0)/ 12 CO(J = 1−0), R 13/12 , and 12 CO(J = 3−2)/ 12 CO(J = 1−0), R 31 , for each GMC at an angular resolution of 25 ′′ (100 pc). Averaged values of R 13/12 and R 31 are 0.06 and 0.31 within the whole GMC-A, 0.11 and 0.67 within the whole GMC-B, and 0.05 and 0.36 within the whole GMC-C, respectively. In addition, we obtained R 13/12 = 0.09±0.02 and R 31 = 0.76±0.06 at the 12 CO(J = 1−0) peak position of the GMC-B. Under the Large Velocity Gradient approximation, we determined gas density of 2.8 ×10 3 cm −3 and kinetic temperature of 33 +9 −5 K at the 12 CO(J = 1 − 0) peak position of the GMC-B. Moreover, we determined 2.5 ×10 3 cm −3 and 25±2 K as averaged values within the whole GMC-B. We concluded that dense molecular gas is formed everywhere in the GMC-B because derived gas density not only at the peak position of the GMC but also averaged over the whole GMC exceeds 10 3 cm −3 . On the other hand, kinetic temperature averaged over the whole GMC-B, 25 K, is significantly lower than that at the peak position, 33 K. This is because H II regions are lopsided to the northern part of the GMC-B, thus OB stars can heat only the 1 northern part, including the 12 CO(J = 1 − 0) peak position, of this GMC.

Research paper thumbnail of Dense Clouds and Star Formation on Spiral Arm in M33-deep CO and HCN Observation in NGC604

Mapping the Galaxy …, Jan 1, 2008

Rie Miura1,2, Yoichi Tamura1,2, Sachiko K. Okumura2, Ryohei Kawabe2, Tomoka Tosaki2, Nario Kuno2,... more Rie Miura1,2, Yoichi Tamura1,2, Sachiko K. Okumura2, Ryohei Kawabe2, Tomoka Tosaki2, Nario Kuno2, Kouichiro Nakanishi2, Seiichi Sakamoto2, and Takashi Hasegawa3 ... 1 the University of Tokyo 2 National Astronomical Observatory of Japan 3 Gunma Astronomical Observatory

Research paper thumbnail of Breakdown of Kennicutt–Schmidt Law at Giant Molecular Cloud Scales in M33

The Astrophysical …, Jan 1, 2010

We have mapped the northern area (30 × 20 ) of a Local Group spiral galaxy M33 in 12 CO(J = 1-0) ... more We have mapped the northern area (30 × 20 ) of a Local Group spiral galaxy M33 in 12 CO(J = 1-0) line with the 45 m telescope at the Nobeyama Radio Observatory. Along with Hα and Spitzer 24 μm data, we have investigated the relationship between the surface density of molecular gas mass and that of star formation rate (SFR) in an external galaxy (Kennicutt-Schmidt law) with the highest spatial resolution (∼80 pc) to date, which is comparable to scales of giant molecular clouds (GMCs). At positions where CO is significantly detected, the SFR surface density exhibits a wide range of over four orders of magnitude, from Σ SFR 10 −10 to ∼10 −6 M yr −1 pc −2 , whereas the Σ H 2 values are mostly within 10-40 M pc −2 . The surface density of gas and that of SFR correlate well at an ∼1 kpc resolution, but the correlation becomes looser with higher resolution and breaks down at GMC scales. The scatter of the Σ SFR -Σ H 2 relationship in the ∼80 pc resolution results from the variety of star-forming activity among GMCs, which is attributed to the various evolutionary stages of GMCs and to the drift of young clusters from their parent GMCs. This result shows that the Kennicutt-Schmidt law is valid only in scales larger than that of GMCs, when we average the spatial offset between GMCs and star-forming regions, and their various evolutionary stages.

Research paper thumbnail of ASTE CO (3--2) Observations of the Southern Barred Spiral Galaxy NGC 986: a Large Gaseous Bar Filled with Dense Molecular Medium

Arxiv preprint arXiv: …, Jan 1, 2008

We present CO(3-2) emission observations toward the 3 ′ × 3 ′ (or 20 × 20 kpc at a distance of 23... more We present CO(3-2) emission observations toward the 3 ′ × 3 ′ (or 20 × 20 kpc at a distance of 23 Mpc) region of the southern barred spiral galaxy NGC 986 using the Atacama Submillimeter Telescope Experiment (ASTE). This effort is a part of our on-going extragalactic CO(3-2) imaging project ADIoS (ASTE Dense gas Imaging of Spiral galaxies). Our CO(3-2) image revealed the presence of a large (the major axis is 14 kpc in total length) gaseous bar filled with dense molecular medium along the dark lanes observed in optical images. This is the largest "dense-gas rich bar" known to date. The dense gas bar discovered in NGC 986 could be a huge reservoir of possible "fuel" for future starbursts in the central region, and we suggest that the star formation in the central region of NGC 986 could still be in a growing phase. We 1 found a good spatial coincidence between the overall distributions of dense molecular gas traced by CO(3-2) and the massive star formation depicted by Hα. The global CO(3-2) luminosity L ′ CO(3−2) of NGC 986 was determined to be (5.4 ± 1.1) × 10 8 K km s −1 pc 2 . The CO(3-2)/CO(1-0) integrated intensity ratio was found to be 0.60 ± 0.13 at a spatial resolution of 44 ′′ or 5 kpc, and a CO(3-2)/CO(2-1) ratio was 0.67 ± 0.14 at a beam size of ∼25 ′′ or ∼2.8 kpc. These line ratios suggest moderate excitation conditions of CO lines (n H 2 ∼ 10 3−4 cm −3 ) in the central a few kpc region of NGC 986.

Research paper thumbnail of Breakdown of Kennicutt-Schmidt Law at GMC Scales in M33

Arxiv preprint arXiv: …, Jan 1, 2010

We have mapped the northern area (30 ′ × 20 ′ ) of a local group spiral galaxy M33 in 12 CO(J = 1... more We have mapped the northern area (30 ′ × 20 ′ ) of a local group spiral galaxy M33 in 12 CO(J = 1-0) line with the 45-m telescope at the Nobeyama Radio Observatory. Along with Hα and Spitzer 24µm data, we have investigated the relationship between the surface density of molecular gas mass and that of star formation rate (SFR) in an external galaxy (Kennicutt-Schmidt law) with the highest spatial resolution (∼ 80 pc) to date, which is comparable to scales of giant molecular clouds (GMCs). At positions where CO is significantly detected, the SFR surface density exhibits a wide range of over four orders of magnitude, from Σ SFR 10 −10 to ∼ 10 −6 M ⊙ yr −1 pc −2 , whereas the Σ H2 values are mostly within 10-40 M ⊙ pc −2 . The surface density of gas and that of SFR correlate well at a ∼1-kpc resolution, but the correlation becomes looser with higher resolution and breaks down at GMC scales. The scatter of the Σ SFR -Σ H2 relationship in the ∼ 80-pc resolution results from the variety of star forming activity among GMCs, which is attributed to the various evolutionary stages of GMCs and to the drift of young clusters from their parent GMCs. This result shows that the Kennicutt-Schmidt law is valid only in scales larger than that of GMCs, when we average the spatial offset between GMCs and star forming regions, and their various evolutionary stages.

Research paper thumbnail of Aperture Synthesis Observations of CO, HCN, and 89 GHz Continuum Emission Toward NGC 604 in M33: Sequential Star Formation Induced by a Supergiant H II …

The Astrophysical …, Jan 1, 2010

We present the results from new Nobeyama Millimeter Array observations of CO(1-0), HCN(1-0), and ... more We present the results from new Nobeyama Millimeter Array observations of CO(1-0), HCN(1-0), and 89-GHz continuum emissions toward NGC 604, known as the supergiant H ii region in a nearby galaxy M 33. Our high spatial resolution images (4. ′′ 2 × 2. ′′ 6, corresponding to 17 pc × 11 pc physical size) of CO emission allowed us to uncover ten individual molecular clouds that have masses of (0.8 -7.4) ×10 5 M ⊙ and sizes of 5 -29 pc, comparable to those of typical Galactic giant molecular clouds (GMCs). Moreover, we detected for the first time HCN emission in the two most massive clouds and 89 GHz continuum emission at the rims of the "Hα shells". The HCN and 89 GHz continuum emission show offset from the CO peak and are distributed to the direction of the central cluster. Three out of -2ten CO clouds are well correlated with the Hα shells both in spatial and velocity domains, implying an interaction between molecular gas and the expanding H ii region. The CO clouds show varieties in star formation efficiencies (SFEs), which are estimated from the 89-GHz and combination of Hα and Spitzer 24-µm data. Furthermore, we found that the SFEs decrease with increasing projected distance measured from the heart of the central OB star cluster in NGC 604, suggesting the radial changes in evolutionary stages of the molecular clouds in course of stellar cluster formation. Our results provide further support to the picture of sequential star formation in NGC604 initially proposed by with the higher spatially resolved molecular clouds, in which an isotropic expansion of the H ii region pushes gases outward and accumulates them to consecutively form dense molecular clouds, and then induces massive star formations.

Research paper thumbnail of NRO M33 All Disk Survey of Giant Molecular Clouds (NRO MAGiC): I. HI to H_2 Transition

Arxiv preprint arXiv: …, Jan 1, 2011

We present the results of the Nobeyama Radio Observatory (NRO) M33 All Disk (30 ′ × 30 ′ , or 7.3... more We present the results of the Nobeyama Radio Observatory (NRO) M33 All Disk (30 ′ × 30 ′ , or 7.3 kpc × 7.3 kpc) Survey of Giant Molecular Clouds (NRO MAGiC) based on 12 CO (J = 1-0) observations using the NRO 45-m telescope. The spatial resolution of the resultant map is 19 ′′ .3, corresponding to 81 pc, which is sufficient to identify each Giant Molecular Cloud (GMC) in the disk. We found clumpy structures with a typical spatial scale of ∼100 pc, corresponding to GMCs, and no diffuse, smoothly distributed component of molecular gas at this sensitivity. The overall distribution of molecular gas roughly agrees with that of Hi. However, closer inspection of the CO and Hi maps suggests that not every CO emission is associated with local Hi peaks, particularly in the inner portion of the disk (r < 2 kpc), although most 1 of CO emission is located at the local Hi peaks in the outer radii. We found that most uncovered GMCs are accompanied by massive star-forming regions, although the star formation rates (SFRs) vary widely from cloud to cloud. The azimuthally averaged Hi gas surface density exhibits a flat radial distribution. However, the CO radial distribution shows a significant enhancement within the central 1-2 kpc region, which is very similar to that of the SFR. We obtained a map of the molecular fraction, f mol = Σ H 2 /(Σ Hi + Σ H 2 ), at a 100-pc resolution. This is the first f mol map covering an entire galaxy with a GMC-scale resolution. We find that f mol tends to be high near the center. The correlation between f mol and gas surface density shows two distinct sequences. The presence of two correlation sequences can be explained by differences in metallicity, i.e., higher (∼2-fold) metallicity in the central region (r < 1.5 kpc) than in the outer parts. Alternatively, differences in scale height can also account for the two sequences, i.e., increased scale height toward the outer disk.