分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： Dense gas is the key for understanding star formation in galaxies. We present high resolution ($\sim3''$) observations of CN 2-1 and CS 5-4 as dense gas tracers toward Arp 299, a mid stage major merger of galaxies, with the Submillimeter Array (SMA). The spatial distribution of CN 2-1 and CS 5-4 are generally consistent with each other, as well as HCN 1-0 in literature. However, different line ratios of CS 5-4 and CN 2-1 are found in A, B, and C regions, with highest value in B. Dense gas fraction decreases from IC 694 (A), to NGC 3690 (B) and the overlap starburst region (C and C$'$), which indicates that circum-nuclear upcoming starburst in A and B will be more efficient than that in the overlap region of Arp 299.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： Dense gas is the key for understanding star formation in galaxies. We present high resolution ($\sim3''$) observations of CN 2-1 and CS 5-4 as dense gas tracers toward Arp 299, a mid stage major merger of galaxies, with the Submillimeter Array (SMA). The spatial distribution of CN 2-1 and CS 5-4 are generally consistent with each other, as well as HCN 1-0 in literature. However, different line ratios of CS 5-4 and CN 2-1 are found in A, B, and C regions, with highest value in B. Dense gas fraction decreases from IC 694 (A), to NGC 3690 (B) and the overlap starburst region (C and C$'$), which indicates that circum-nuclear upcoming starburst in A and B will be more efficient than that in the overlap region of Arp 299.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： Ground-based, high-resolution bolometric (sub)millimeter continuum mapping observations on spatially extended target sources are often subject to significant missing fluxes. This hampers accurate quantitative analyses. Missing flux can be recovered by fusing high-resolution images with observations that preserve extended structures. However, the commonly adopted image fusion approaches do not maintain the simplicity of the beam response function and do not try to elaborate the details of the yielded beam response functions. These make the comparison of the observations at multiple wavelengths not straightforward. We present a new algorithm, J-comb, which combines the high and low-resolution images linearly. By applying a taper function to the low-pass filtered image and combining it with a high-pass filtered image using proper weights, the beam response functions of our combined images are guaranteed to have near-Gaussian shapes. This makes it easy to convolve the observations at multiple wavelengths to share the same beam response functions. Moreover, we introduce a strategy to tackle the specific problem that the imaging at 850 um from the present-date ground-based bolometric instrument and that taken with the Planck satellite do not overlap in the Fourier domain. We benchmarked our method against two other widely-used image combination algorithms, CASA-feather and MIRIAD-immerge, with mock observations of star-forming molecular clouds. We demonstrate that the performance of the J-comb algorithm is superior to those of the other two algorithms. We applied the J-comb algorithm to real observational data of the Orion A star-forming region. We successfully produced dust temperature and column density maps with ~10" angular resolution, unveiling much greater details than the previous results.
分类： 天文学 >> 天文仪器与技术 提交时间： 2024-04-16 合作期刊: 《天文技术与仪器（英文）》
Peng Jiang
Rurong Chen
Hengqian Gan
Jinghai Sun
Boqin Zhu
Hui Li
Weiwei Zhu
Jingwen Wu
Xuelei Chen
Haiyan Zhang
Tao An
摘要：The Five-hundred-meter Aperture Spherical Radio Telescope (FAST) Core Array is a proposed extension of FAST, integrating 24 secondary 40-m antennas implanted within 5 km of the FAST site. This original array design will combine the unprecedented sensitivity of FAST with a high angular resolution (4.3" at a frequency of 1.4 GHz), thereby exceeding the capabilities at similar frequencies of next-generation arrays such as the Square Kilometre Array Phase 1 or the next-generation Very Large Array. This article presents the technical specifications of the FAST Core Array, evaluates its potential relatively to existing radio telescope arrays, and describes its expected scientific prospects. The proposed array will be equipped with technologically advanced backend devices, such as real-time signal processing systems. A phased array feed receiver will be mounted on FAST to improve the survey efficiency of the FAST Core Array, whose broad frequency coverage and large field of view（ FOV） will be essential to study transient cosmic phenomena such as fast radio bursts and gravitational wave events, to conduct surveys and resolve structures in neutral hydrogen galaxies, to monitor or detect pulsars, and to investigate exoplanetary systems. Finally, the FAST Core Array can strengthen China's major role in the global radio astronomy community, owing to a wide range of potential scientific applications from cosmology to exoplanet science.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Jinjin Xie
Gary A. Fuller
Di Li
Longfei Chen
Zhiyuan Ren
Jingwen Wu
Yan Duan
Junzhi Wang
Juan Li
Nicolas Peretto
Tie Liu
Zhiqiang Shen
摘要： We present one of the first Shanghai Tian Ma Radio Telescope (TMRT) K Band observations towards a sample of 26 infrared dark clouds (IRDCs). We observed the (1,1), (2,2), (3,3), and (4,4) transitions of NH$_{3}$ together with CCS (2$_{1}$-1$_{0}$) and HC$_{3}$N $J\,$=2-1, simultaneously. The survey dramatically increases the existing CCS-detected IRDC sample from 8 to 23, enabling a better statistical study of the ratios of carbon-chain molecules (CCM) to N-bearing molecules in IRDCs. With the newly developed hyperfine group ratio (HFGR) method of fitting NH$_{3}$ inversion lines, we found the gas temperature to be between 10 and 18 K. The column density ratios of CCS to NH$_{3}$ for most of the IRDCs are less than 10$^{-2}$, distinguishing IRDCs from low-mass star-forming regions. We carried out chemical evolution simulations based on a three-phase chemical model NAUTILUS. Our measurements of the column density ratios between CCM and NH$_{3}$ are consistent with chemical evolutionary ages of $\lesssim$10$^{5}$ yr in the models. Comparisons of the data and chemical models suggest that CCS, HC$_{3}$N, and NH$_{3}$ are sensitive to the chemical evolutionary stages of the sources.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Jinjin Xie
Jingwen Wu
Gary A. Fuller
Nicolas Peretto
Zhiyuan Ren
Longfei Chen
Yaoting Yan
Guodong Li
Yan Duan
Jifeng Xia
Yongxiong Wang
Di Li
摘要： We investigate the infall properties in a sample of 11 infrared dark clouds (IRDCs) showing blue-asymmetry signatures in HCO$^{+}$ J=1--0 line profiles. We used JCMT to conduct mapping observations in HCO$^{+}$ J=4--3 as well as single-pointing observations in HCO$^{+}$ J =3--2, towards 23 clumps in these IRDCs. We applied the HILL model to fit these observations and derived infall velocities in the range of 0.5-2.7 km s$^{-1}$, with a median value of 1.0 km s$^{-1}$, and obtained mass accretion rates of 0.5-14$\times$10$^{-3}$ Msun yr$^{-1}$. These values are comparable to those found in massive star forming clumps in later evolutionary stages. These IRDC clumps are more likely to form star clusters. HCO$^{+}$ J =3--2 and HCO$^{+}$ J =1--0 were shown to trace infall signatures well in these IRDCs with comparable inferred properties. HCO$^{+}$ J=4--3, on the other hand, exhibits infall signatures only in a few very massive clumps, due to smaller opacties. No obvious correlation for these clumps was found between infall velocity and the NH3/CCS ratio.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Zhiyuan Ren
Lei Zhu
Hui Shi
Nannan Yue
Di Li
Qizhou Zhang
Diego Mardones
Jingwen Wu
Sihan Jiao
Shu Liu
Gan Luo
Jinjin Xie
Chao Zhang
Xuefang Xu
摘要： Filamentary structures are closely associated with star-forming cores, but their detailed physical connections are still not clear. We studied the dense gas in the region of OMC-3 MMS-7 in Orion A molecular cloud using the molecular lines observed with the Atacama Large Millimeter/submillimeter Array (ALMA) and the Submillimeter Array (SMA). The ALMA N$_2$H$^+$ (1-0) emission has revealed three dense filaments intersected at the center, coincident with the central core MMS-7, which has a mass of $3.6\,M_\odot$. The filaments and cores are embedded in a parental clump with total mass of $29\,M_\odot$. The N$_2$H$^+$ velocity field exhibits a noticeable increasing trend along the filaments towards the central core MMS-7 with a scale of $v-v_{\rm lsr} \simeq 1.5$ ${\rm km\, s^{-1}}$ over a spatial range of $\sim$20 arcsec ($8\times 10^3$ AU), corresponding to a gradient of $40\,{\rm km\, s^{-1}}\,{\rm pc}^{-1}$. This feature is most likely to indicate an infall motion towards the center. The derived infall rate ($8\times 10^{-5}\,M_\odot$ year$^{-1}$) and timescale ($3.6\times 10^5$ years) are much lower than that in a spherical free-fall collapse and more consistent with the contraction of filament structures. The filaments also exhibit a possible fragmentation, but it does not seem to largely interrupt the gas structure or the infall motion towards the center. MMS-7 thus provides an example of filamentary infall into an individual prestellar core. The filament contraction could be less intense but more steady than the global spherical collapse, and may help generate an intermediate- or even high-mass star.
分类： 天文学 >> 天文学 提交时间： 2024-03-30
Niankun Yu
Zheng Zheng
Chao-Wei Tsai
Pei Zuo
Sara L. Ellison
David V. Stark
Di Li
Jingwen Wu
Karen L. Masters
Ting Xiao
Yinghui Zheng
Zongnan Li
Kai Zhang
Hongying Chen
Shu Liu
Sihan Jiao
Fanyi Meng
摘要： The atomic-to-molecular gas conversion is a critical step in the baryon cycle of galaxies, which sets the initial conditions for subsequent star formation and influences the multi-phase interstellar medium. We compiled a sample of 94 nearby galaxies with observations of multi-phase gas contents by utilizing public H I, CO, and optical IFU data from the MaNGA survey together with new FAST H I observations. In agreement with previous results, our sample shows that the global molecular-to-atomic gas ratio ($R_{\rm mol} \equiv$ log $M_{\rm H_2}/M_{\rm H\ I}$) is correlated with the global stellar mass surface density $\mu_*$ with a Kendall's $\tau$ coefficient of 0.25 and $p < 10^{-3}$, less tightly but still correlated with stellar mass and NUV$-$ r color, and not related to the specific star formation rate (sSFR). The cold gas distribution and kinematics inferred from the H I and CO global profile asymmetry and shape do not significantly rely on $R_{\rm mol}$. Thanks to the availability of kpc-scale observations of MaNGA, we decompose galaxies into H II, composite, and AGN-dominated regions by using the BPT diagrams. With increasing $R_{\rm mol}$, the fraction of H II regions within 1.5 effective radius decreases slightly; the density distribution in the spatially resolved BPT diagram also changes significantly, suggesting changes in metallicity and ionization states. Galaxies with high $R_{\rm mol}$ tend to have high oxygen abundance, both at one effective radius with a Kendall's $\tau$ coefficient of 0.37 ($p < 10^{-3}$) and their central regions. Among all parameters investigated here, the oxygen abundance at one effective radius has the strongest relation with global $R_{\rm mol}$, but the dependence of gas conversion on gas distribution and galaxy ionization states is weak.
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