arXiv subjects

Tomoka Tosaki

Publications and source records attributed to Tomoka Tosaki.

At least 37 records · Page 2Linked to original sources

arXiv2018

Gas, Dust, Stars, Star Formation and their Evolution in M33 at Giant Molecular Cloud Scales

We report on a multi parameter analysis of giant molecular clouds (GMCs) in the nearby spiral galaxy M33. A catalog of GMCs identifed in 12CO(J=3-2) was used to compile associated 12CO(J=1-0), dust, stellar mass and star formation rate. Each of the 58 GMCs are categorized by their evolutionary stage. Applying the principal component analysis on these parameters, we construct two principal components PC1 and PC2 which retain 75% of the information in the original dataset. PC1 is interpreted as expressing the total interstellar matter content, and PC2 as the total activity of star formation. Young (<10Myr) GMCs occupy a distinct region in the PC1-PC2 plane, with lower ISM content and star formation activity compared to intermediate age and older clouds. Comparison of average cloud properties in different evolutionary stages imply that GMCs may be heated or grow denser and more massive via aggregation of diffuse material in their first ~10 Myr. The PCA also objectively identified a set of tight relations between ISM and star formation. The ratio of the two CO lines is nearly constant, but weakly modulated by massive star formation. Dust is more strongly correlated with the star formation rate than the CO lines, supporting recent findings that dust may trace molecular gas better than CO. Stellar mass contributes weakly to the star formation rate, reminiscent of an extended form of the Schmidt Kennicutt relation with the molecular gas term substituted by dust.

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arXiv2017

Diverse nuclear star-forming activities in the heart of NGC 253 resolved with ten-pc scale ALMA images

We present an 8 pc x 5 pc resolution view of the central ~ 200 pc region of the nearby starburst galaxy NGC 253, based on ALMA Band 7 (λ ~ 0.85 mm or ν ~ 350 GHz) observations covering 11 GHz. We resolve the nuclear starburst of NGC 253 into eight dusty star-forming clumps, 10 pc in scale, for the first time. These clumps, each of which contains (4-10) x 10^4 M_sun of dust (assuming that the dust temperature is 25 K) and up to 6 x 10^2 massive (O5V) stars, appear to be aligned in two parallel ridges, while they have been blended in previous studies. Despite the similarities in sizes and dust masses of these clumps, their line spectra vary drastically from clump to clump although they are separated by only ~ 10 pc. Specifically, one of the clumps, Clump 1, exhibits line confusion-limited spectra with at least 36 emission lines from 19 molecules (including CH3OH, HNCO, H2CO, CH3CCH, H2CS, and H3O+) and a hydrogen recombination line (H26α), while much fewer kinds of molecular lines are detected in some other clumps where fragile species, such as complex organic molecules and HNCO, completely disappear from their spectra. We demonstrate the existence of hot molecular gas ($T_rot (SO2) = 90 {\pm} 11 K) in the former clump, which suggests that the hot and chemically rich environments are localized within a 10-pc scale star-forming clump.

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arXiv2017

FOREST Unbiased Galactic plane Imaging survey with the Nobeyama 45-m telescope (FUGIN) I: Project Overview and Initial Results

The FOREST Unbiased Galactic plane Imaging survey with the Nobeyama 45-m telescope (FUGIN) project is one of the legacy projects using the new multi-beam FOREST receiver installed on the Nobeyama 45-m telescope. This project aims to investigate the distribution, kinematics, and physical properties of both diffuse and dense molecular gas in the Galaxy at once by observing 12CO, 13CO, and C18O J=1-0 lines simultaneously. The mapping regions are a part of the 1st quadrant (10d < l < 50d, |b| < 1d) and the 3rd quadrant (198d < l <236d, |b| < 1d) of the Galaxy, where spiral arms, bar structure, and the molecular gas ring are included. This survey achieves the highest angular resolution to date (~20") for the Galactic plane survey in the CO J=1-0 lines, which makes it possible to find dense clumps located farther away than the previous surveys. FUGIN will provide us with an invaluable dataset for investigating the physics of the galactic interstellar medium (ISM), particularly the evolution of interstellar gas covering galactic scale structures to the internal structures of giant molecular clouds, such as small filament/clump/core. We present an overview of the FUGIN project, observation plan, and initial results, which reveal wide-field and detailed structures of molecular clouds, such as entangled filaments that have not been obvious in previous surveys, and large-scale kinematics of molecular gas such as spiral arms.

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arXiv2017

Properties of Molecular Gas in Galaxies in Early and Mid Stage of Interaction: II. Molecular Gas Fraction

We have investigated properties of the interstellar medium in interacting galaxies in early and mid-stage using mapping data of 12CO(1-0) and HI.Assuming the standard CO-H2 conversion factor, we found no difference in molecular gas mass, atomic gas mass, and total gas mass (a sum of atomic and molecular gas mass) between interacting galaxies and isolated galaxies. However, interacting galaxies have a higher global molecular gas fraction, global fmol (a ratio of molecular gas mass to total gas mass averaged over a whole galaxy), than isolated galaxies. The distribution of local molecular gas fraction, local fmol (a ratio of the surface density of molecular gas to that of total gas), is different from the distribution in typical isolated galaxies. By a pixel-to-pixel comparison, isolated spiral galaxies show a gradual increase in local fmol along the surface density of total gas until it is saturated at 1.0, while interacting galaxies show no clear relation. We performed pixel-to-pixel theoretical model fittings varying metallicity and external pressure. According to the model fitting, external pressure can explain the trend of local fmol in the interacting galaxies. Assuming a half of the standard CO-H2 conversion factor for interacting galaxies, the results of pixel-to-pixel theoretical model fitting get worse than adopting the standard conversion factor, although global fmol of interacting galaxies becomes the same as in isolated galaxies. We conclude that external pressure occurs due to the shock prevailing over a whole galaxy or due to collisions between giant molecular clouds. The external pressure accelerates an efficient transition from atomic gas to molecular gas. Regarding chemical time-scale, high local fmol can be achieved at the very early stage of interaction even if shock induced by the collision of galaxies ionises interstellar gas.

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arXiv2017

Discovery of a distant molecular cloud in the extreme outer Galaxy with the Nobeyama 45-m telescope

We report the discovery of the molecular cloud whose kinematic distance is the largest in the Galaxy at the present moment, named G213.042$+$0.003, at $l =$ 213.042$^\circ$ and $b =$ 0.003$^\circ$ in the $^{12}$CO($J =$ 1-0) line using the Nobeyama 45-m telescope and a multi-beam receiver BEARS. This molecular cloud is located at the heliocentric distance of 21$_{-7}^{+12}$ kpc and Galactocentric distance of 29$_{-7}^{+12}$ kpc, which are estimated as the kinematic distances with the Galactic parameters obtained by Reid et al. (2014, ApJ, 783, 130). Its major and minor diameters and line width were measured to be 4.0$_{-1.3}^{+2.3}$ pc, 3.0$_{-1.0}^{+1.7}$ pc, and 1.5 km s$^{-1}$, respectively. The cloud mass was estimated to be 2.5$_{-1.4}^{+3.7}$ $\times$ 10$^2$ $M_\odot$ using the CO-to-H$_2$ conversion factor of 5.6 $\times$ 10$^{20}$ cm$^{-2}$ (K km s$^{-1}$)$^{-1}$ obtained in far outer Galaxy.

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arXiv2017

The 1.1 mm Continuum Survey of the Small Magellanic Cloud: Physical Properties and Evolution of the Dust-selected Clouds

The first 1.1 mm continuum survey toward the Small Magellanic Cloud (SMC) was performed using the AzTEC instrument installed on the ASTE 10-m telescope. This survey covered 4.5 deg$^2$ of the SMC with $1σ$ noise levels of $5-12$ mJy beam$^{-1}$, and 44 extended objects were identified. The 1.1 mm extended emission has good spatial correlation with $Herschel$ 160 $\mathrm{μm}$, indicating that the origin of the 1.1 mm extended emission is thermal emission from a cold dust component. We estimated physical properties using the 1.1 mm and filtered $Herschel$ data (100, 160, 250, 350, and 500 $\mathrm{μm}$). The 1.1 mm objects show dust temperatures of $17-45$ K and gas masses of $4\times10^3-3\times10^5~M_\odot$, assuming single-temperature thermal emission from the cold dust with an emissivity index, $β$, of 1.2 and a gas-to-dust ratio of 1000. These physical properties are very similar to those of giant molecular clouds (GMCs) in our galaxy and the Large Magellanic Cloud. The 1.1 mm objects also displayed good spatial correlation with the $Spitzer$ 24 $\mathrm{μm}$ and CO emission, suggesting that the 1.1 mm objects trace the dense gas regions as sites of massive star formation. The dust temperature of the 1.1 mm objects also demonstrated good correlation with the 24 $\mathrm{μm}$ flux connected to massive star formation. This supports the hypothesis that the heating source of the cold dust is mainly local star-formation activity in the 1.1 mm objects. The classification of the 1.1 mm objects based on the existence of star-formation activity reveals the differences in the dust temperature, gas mass, and radius, which reflects the evolution sequence of GMCs.

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arXiv2016

A statistical study of giant molecular clouds traced by $^{13}$CO, C$^{18}$O, CS, and CH$_3$OH in the disk of NGC 1068 based on ALMA observations

We present $1''.4$ (98 pc) resolution ALMA observations of $^{13}$CO($J$=1-0), C$^{18}$O($J$=1-0), CS($J$=2-1) and CH$_3$OH($J_K$=$2_K$-$1_K$) molecular lines in the central $1'$ (4.2 kpc) region of NGC 1068 to study the physical and chemical properties of giant molecular clouds (GMCs) and to test whether these GMC-scale properties are linked to the larger-scale galactic environment. Using the derived $^{13}$CO cube, we have identified 187 high-significance ($> 8σ$) GMCs by employing the CLUMPFIND algorithm. The molecular gas masses of GMCs $M_{\rm ^{13}CO}$, derived from the $^{13}$CO data, range from $1.8 \times 10^4 ~ M_\odot$ to $4.2 \times 10^7 ~M_\odot$. A mass function of GMCs in NGC 1068 has been obtained for the first time at $\sim$100 pc resolution. We find the slope of the mass function $γ= -1.25 \pm 0.07$ for a mass range of $M_{\rm ^{13}CO} \geq 10^5M_\odot$. This is shallower than the GMCs in the disk regions of the Milky Way, M51 and NGC 300. Further, we find that the high mass cut-off of the GMC mass function occurs at $M_{\rm ^{13}CO} \sim 6 \times 10^7 M_\odot$, which is an order of magnitude larger than that in the nuclear bar region of M51, indicating that the more massive clouds dominate the mass budget in NGC 1068. The observed C$^{18}$O)/$^{13}$CO intensity ratios are found to be fairly uniform (0.27 $\pm$ 0.05) among the identified GMCs. In contrast, the CH$_3$OH/$^{13}$CO ratios exhibit striking spatial variation across the disk, with the smallest values around the bar-end ($<$ 0.03), and larger ratios along the spiral arms ($\sim$ 0.1-0.2). We find that GMCs with detectable methanol emission tend to have systematically larger velocity widths than those without methanol emission, suggesting that (relatively weak) shocks are responsible for the enhancement of the CH$_3$OH/$^{13}$CO ratios of GMCs in the disk of NGC 1068.

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arXiv2016

High resolution ALMA observations of dense molecular medium in the central regions of active galaxies

In the central regions of active galaxies, dense molecular medium are exposed to various types of radiation and energy injections, such as UV, X-ray, cosmic ray, and shock dissipation. With the rapid progress of chemical models and implementation of new-generation mm/submm interferometry, we are now able to use molecules as powerful diagnostics of the physical and chemical processes in galaxies. Here we give a brief overview on the recent ALMA results to demonstrate how molecules can reveal underlying physical and chemical processes in galaxies. First, new detections of Galactic molecular absorption systems with elevated HCO/H$^{13}$CO$^+$ column density ratios are reported, indicating that these molecular media are irradiated by intense UV fields. Second, we discuss the spatial distributions of various types of shock tracers including HNCO, CH$_3$OH and SiO in NGC 253 and NGC 1068. Lastly, we provide an overview of proposed diagnostic methods of nuclear energy sources using ALMA, with an emphasis on the synergy with sensitive mid-infrared spectroscopy, which will be implemented by JWST and SPICA to disentangle the complex nature of heavily obscured galaxies across the cosmic time.

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arXiv2015

CO(J=3-2) On-the-fly Mapping of the Nearby Spiral Galaxies NGC 628 and NGC 7793: Spatially-resolved CO(J=3-2) Star-formation Law

We present the results of CO(J=3-2) on-the-fly mappings of two nearby non-barred spiral galaxies NGC 628 and NGC 7793 with the Atacama Submillimeter Telescope Experiment at an effective angular resolution of 25". We successfully obtained global distributions of CO(J=3-2) emission over the entire disks at a sub-kpc resolution for both galaxies. We examined the spatially-resolved (sub-kpc) relationship between CO(J=3-2) luminosities (L'CO(3-2)) and infrared (IR) luminosities (LIR) for NGC 628, NGC 7793, and M 83, and compared it with global luminosities of JCMT Nearby Galaxy Legacy Survey sample. We found a striking linear L'CO(3-2)-LIR correlation over the 4 orders of magnitude, and the correlation is consistent even with that for ultraluminous infrared galaxies and submillimeter selected galaxies. In addition, we examined the spatially-resolved relationship between CO(J=3-2) intensities (ICO(3-2)) and extinction-corrected star formation rates (SFRs) for NGC 628, NGC 7793, and M 83, and compared it with that for GMCs in M 33 and 14 nearby galaxy centers. We found a linear ICO(3-2)-SFR correlation with 1 dex scatter. We conclude that the CO(J=3-2) star formation law (i.e., linear L'CO(3-2)-LIR and ICO(3-2)-SFR correlations) is universally applicable to various types and spatial scales of galaxies, from spatially-resolved nearby galaxy disks to distant IR-luminous galaxies, within 1 dex scatter.

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arXiv2015

ALMA Observations of the Submillimeter Dense Molecular Gas Tracers in the Luminous Type-1 Active Nucleus of NGC 7469

We present ALMA Cycle 1 observations of the central kpc region of the luminous type-1 Seyfert galaxy NGC 7469 with unprecedented high resolution (0.5$"$ $\times$ 0.4$"$ = 165 pc $\times$ 132 pc) at submillimeter wavelengths. Utilizing the wide-bandwidth of ALMA, we simultaneously obtained HCN(4-3), HCO$^+$(4-3), CS(7-6), and partially CO(3-2) line maps, as well as the 860 $μ$m continuum. The region consists of the central $\sim$ 1$"$ component and the surrounding starburst ring with a radius of $\sim$ 1.5$"$-2.5$"$. Several structures connect these components. Except for CO(3-2), these dense gas tracers are significantly concentrated towards the central $\sim$ 1$"$, suggesting their suitability to probe the nuclear regions of galaxies. Their spatial distribution resembles well those of centimeter and mid-infrared continuum emissions, but it is anti-correlated with the optical one, indicating the existence of dust obscured star formation. The integrated intensity ratios of HCN(4-3)/HCO$^+$(4-3) and HCN(4-3)/CS(7-6) are higher at the AGN position than at the starburst ring, which is consistent to our previous findings (submm-HCN enhancement). However, the HCN(4-3)/HCO$^+$(4-3) ratio at the AGN position of NGC 7469 (1.11$\pm$0.06) is almost half of the corresponding value of the low-luminosity type-1 Seyfert galaxy NGC 1097 (2.0$\pm$0.2), despite the more than two orders of magnitude higher X-ray luminosity of NGC 7469. But the ratio is comparable to that of the close vicinity of the AGN of NGC 1068 ($\sim$ 1.5). Based on these results, we speculate that some other heating mechanisms than X-ray (e.g., mechanical heating due to AGN jet) can contribute significantly for shaping the chemical composition in NGC 1097.

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arXiv2014

Dense Molecular Clumps associated with the LMC Supergiant Shells LMC 4 \& LMC 5

We investigate the effects of Supergiant Shells (SGSs) and their interaction on dense molecular clumps by observing the Large Magellanic Cloud (LMC) star forming regions N48 and N49, which are located between two SGSs, LMC 4 and LMC 5. $^{12}$CO ($J$=3-2, 1-0) and $^{13}$CO ($J$=1-0) observations with the ASTE and Mopra telescopes have been carried out towards these regions. A clumpy distribution of dense molecular clumps is revealed with 7 pc spatial resolution. Large velocity gradient analysis shows that the molecular hydrogen densities ($n({\rm H}_2)$) of the clumps are distributed from low to high density ($10^3$-$10^5$ cm$^{-3}$) and their kinetic temperatures ($T_{\rm kin}$) are typically high (greater than $50$ K). These clumps seem to be in the early stages of star formation, as also indicated from the distribution of H$α$, young stellar object candidates, and IR emission. We found that the N48 region is located in the high column density HI envelope at the interface of the two SGSs and the star formation is relatively evolved, whereas the N49 region is associated with LMC 5 alone and the star formation is quiet. The clumps in the N48 region typically show high $n({\rm H}_2)$ and $T_{\rm kin}$, which are as dense and warm as the clumps in LMC massive cluster-forming areas (30 Dor, N159). These results suggest that the large-scale structure of the SGSs, especially the interaction of two SGSs, works efficiently on the formation of dense molecular clumps and stars.

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arXiv2014

A Multi-Transition Study of Molecules toward NGC 1068 based on High-Resolution Imaging Observations with ALMA

We present 0.8-mm band molecular images and spectra obtained with the Atacama Large Millimeter/submillimeter Array (ALMA) toward one of the nearest galaxies with an active galactic nucleus (AGN), NGC 1068. Distributions of CO isotopic species ($^{13}$CO and C$^{18}$O) $\it{J}$ = 3--2, CN $\it{N}$ = 3--2 and CS $\it{J}$ = 7--6 are observed toward the circumnuclear disk (CND) and a part of the starburst ring with an angular resolution of $\sim$1.$^{\prime\prime}$3 $\times$ 1.$^{\prime\prime}$2. The physical properties of these molecules and shock-related molecules such as HNCO, CH$_{3}$CN, SO, and CH$_{3}$OH detected in the 3-mm band were estimated using rotation diagrams under the assumption of local thermodynamic equilibrium. The rotational temperatures of the CO isotopic species and the shock-related molecules in the CND are, respectively, 14--22 K and upper limits of 20--40 K. Although the column densities of the CO isotopic species in the CND are only from one-fifth to one-third of that in the starburst ring, those of the shock-related molecules are enhanced by a factor of 3--10 in the CND. We also discuss the chemistry of each species, and compare the fractional abundances in the CND and starburst ring with those of Galactic sources such as cold cores, hot cores, and shocked molecular clouds in order to study the overall characteristics. We find that the abundances of shock-related molecules are more similar to abundances in hot cores and/or shocked clouds than to cold cores. The CND hosts relatively complex molecules, which are often associated with shocked molecular clouds or hot cores. Because a high X-ray flux can dissociate these molecules, they must also reside in regions shielded from X-rays.

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arXiv2014

Distributions of molecules in the circumnuclear disk and surrounding starburst ring in the Seyfert galaxy NGC 1068 observed with ALMA

Sensitive observations with ALMA allow astronomers to observe the detailed distributions of molecules with relatively weak intensity in nearby galaxies. In particular, we report distributions of several molecular transitions including shock and dust related species ($^{13}$CO $J$ = 1--0, C$^{18}$O $J$ = 1--0, $^{13}$CN $N$ = 1--0, CS $J$ = 2--1, SO $J_N$ = 3$_2$--2$_1$, HNCO $J_{Ka,Kc}$ = 5$_{0,5}$--4$_{0,4}$, HC$_3$N $J$ = 11--10, 12--11, CH$_3$OH $J_K$ = 2$_K$--1$_K$, and CH$_3$CN $J_K$ = 6$_K$--5$_K$) in the nearby Seyfert 2 galaxy NGC 1068 observed with the ALMA early science program. The central $\sim$1 arcmin ($\sim$4.3 kpc) of this galaxy was observed in the 100 GHz region covering $\sim$96--100 GHz and $\sim$108--111 GHz with an angular resolution of $\sim4"\times2"$ (290 pc$\times$140 pc) to study the effects of an active galactic nucleus and its surrounding starburst ring on molecular abundances. Here, we present images and report a classification of molecular distributions into three main categories: (1) Molecules concentrated in the circumnuclear disk (CND) (SO $J_N$ = 3$_2$--2$_1$, HC$_3$N $J$ = 11--10, 12--11, and CH$_3$CN $J_K$ = 6$_K$--5$_K$), (2) Molecules distributed both in the CND and the starburst ring (CS $J$ = 2--1 and CH$_3$OH $J_K$ = 2$_K$--1$_K$), (3) Molecules distributed mainly in the starburst ring ($^{13}$CO $J$ = 1--0 and C$^{18}$O $J$ = 1--0). Since most of the molecules such as HC$_3$N observed in the CND are easily dissociated by UV photons and X-rays, our results indicate that these molecules must be effectively shielded. In the starburst ring, the relative intensity of methanol at each clumpy region is not consistent with those of $^{13}$CO, C$^{18}$O, and CS. This difference is probably caused by the unique formation and destruction mechanisms of CH$_3$OH.

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arXiv2014

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

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, $Σ_{\rm H_2}$, and the star formation rate surface density, $Σ_{\rm SFR}$. This result is based on wide field and high sensitivity CO(3-2) observations at 100 pc resolution. Star formation efficiencies (SFE), defined as $Σ_{\rm SFR}$/$Σ_{\rm H_2}$, 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 $I_{CO}$-$N({\rm H_2})$ 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 $I_{CO}$-$N({\rm H_2})$ conversion factor.

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arXiv2013

Detection of an ultra-bright submillimeter galaxy behind the Small Magellanic Cloud

We report the discovery of a new ultra-bright submillimeter galaxy (SMG) behind the Small Magellanic Cloud (SMC). This SMG is detected as a 43.3+-8.4 mJy point source (MM J01071-7302, hereafter MMJ0107) in the 1.1 mm continuum survey of the SMC by AzTEC on the ASTE telescope. MMJ0107 is also detected in the radio (843 MHz), Herschel/SPIRE, Spitzer MIPS 24 μm, all IRAC bands, Wide-field Infrared Survey Explorer, and near-infrared (J, H, KS). We find an optical (U, B, V) source, which might be the lensing object, at a distance of 1.4 arcsec from near-infrared and IRAC sources. Photometric redshift estimates for the SMG using representative spectral energy distribution templates show the redshifts of 1.4-3.9. We estimate total far-infrared luminosity of (0.3-2.2)x10^14 μ^-1 L_sun and a star formation rate of 5600-39, 000 μ^-1 M_sun yr^-1, where μ is the gravitational magnification factor. This apparent extreme star formation activity is likely explained by a highly magnified gravitational lens system.

astro-ph.CO↗

arXiv2013

Submillimeter ALMA Observations of the Dense Gas in the Low-Luminosity Type-1 Active Nucleus of NGC 1097

We present the first 100 pc scale view of the dense molecular gas in the central ~ 1.3 kpc region of the type-1 Seyfert NGC 1097 traced by HCN (J=4-3) and HCO+ (J=4-3) lines afforded with ALMA band 7. This galaxy shows significant HCN enhancement with respect to HCO+ and CO in the low-J transitions, which seems to be a common characteristic in AGN environments. Using the ALMA data, we study the characteristics of the dense gas around this AGN and search for the mechanism of HCN enhancement. We find a high HCN (J=4-3) to HCO+ (J=4-3) line ratio in the nucleus. The upper limit of the brightness temperature ratio of HCN (v2=1^{1f}, J=4-3) to HCN (J=4-3) is 0.08, which indicates that IR pumping does not significantly affect the pure rotational population in this nucleus. We also find a higher HCN (J=4-3) to CS (J=7-6) line ratio in NGC 1097 than in starburst galaxies, which is more than 12.7 on the brightness temperature scale. Combined from similar observations from other galaxies, we tentatively suggest that this ratio appears to be higher in AGN-host galaxies than in pure starburst ones similar to the widely used HCN to HCO+ ratio. LTE and non-LTE modeling of the observed HCN and HCO+ lines using J=4-3 and 1-0 data from ALMA, and J=3-2 data from SMA, reveals a high HCN to HCO+ abundance ratio (5 < [HCN]/[HCO+] < 20: non-LTE analysis) in the nucleus, and that the high-J lines (J=4-3 and 3-2) are emitted from dense (10^{4.5} < n_H2 [/cc] < 10^6), hot (70 < Tkin [K] < 550) regions. Finally we propose that the high temperature chemistry is more plausible to explain the observed enhanced HCN emission in NGC 1097 than the pure gas phase PDR/XDR chemistry.

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arXiv2012

NRO M33 All-Disk Survey of Giant Molecular Clouds (NRO MAGiC): II. Dense Gas Formation within Giant Molecular Clouds in M33

We report the results of our observations of the 12CO (J=1-0) and 12CO (J=3-2) line emission of 74 major giant molecular clouds (GMCs) within the galactocentric distance of 5.1 kpc in the Local Group galaxy M33. The observations have been conducted as part of the Nobeyama Radio Observatory M33 All-disk survey of Giant Molecular Clouds project (NRO MAGiC). The spatial resolutions are 80 pc for 12CO (J=1-0) and 100 pc for 12CO (J=3-2). We detect 12CO (J=3-2) emission of 65 GMCs successfully. Furthermore, we find that the correlation between the surface density of the star formation rate, which is derived from a linear combination of Halpha and 24um emissions, and the 12CO (J=3-2) integrated intensity still holds at this scale. This result show that the star-forming activity is closely associated with warm and dense gases that are traced with the 12CO (J=3-2) line, even in the scale of GMCs. We also find that the GMCs with a high star-forming activity tend to show a high integrated intensity ratio (R3-2/1-0). Moreover, we also observe a mass-dependent trend of R3-2/1-0 for the GMCs with a low star-forming activity. From these results, we speculate that the R3-2/1-0 values of the GMCs with a low star-forming activity mainly depend on the dense gas fraction and not on the temperature, and therefore, the dense gas fraction increases with the mass of GMCs, at least in the GMCs with a low star-forming activity.

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arXiv2012

GMC Evolutions in the Nearby Spiral Galaxy M33

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.

astro-ph.CO↗