分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： The Five-hundred-meter Aperture Spherical radio Telescope (FAST) completed its commissioning and began the Commensal Radio Astronomy FasT Survey (CRAFTS), a multi-year survey to cover 60% of the sky, in 2020. We present predictions for the number of radio-flaring ultracool dwarfs (UCDs) that are likely to be detected by CRAFTS. Based on the observed flaring UCDs from a number of unbiased, targeted radio surveys in the literature, we derive a detection rate of $\ge$3%. Assuming a flat radio spectrum $\nu L _{\nu}\propto \nu^{\beta+1} $ with $\beta$ = -1.0 for UCD flares, we construct a flare luminosity function $d N/d L \propto L^{-1.96 \pm 0.45}$ (here $L=\nu L_\nu$). CRAFTS is found to be sensitive enough for flares from UCDs up to $\sim$180 pc. Considering the Galactic thin disk, we carry out a 3D Monte Carlo simulation of the UCD population, which is then fed to mock CRAFTS observations. We estimate that $\sim$170 flaring UCDs would be detected through transient searches in circular polarization. Though only marginally sensitive to the scale height of UCDs, the results are very sensitive to the assumed spectral index $\beta$. For $\beta$ from 0 to -2.5, the number of expected detections increases dramatically from $\sim$20 to $\sim$3460. We also contemplate the strategies for following up candidates of flaring UCDs, and discuss the implications of survey results for improving our knowledge of UCD behavior at $L$ band and dynamos.
分类： 天文学 >> 天文学 提交时间： 2024-07-15 合作期刊: 《Research in Astronomy and Astrophysics》
摘要： The Submillimeter Wave Astronomy Satellite (SWAS) was the first space telescope capable of high spectral resolution observations of terahertz spectral lines. We have investigated the integration ability of its two receivers and spectrometer during five and a half years of on-orbit operation. The C i , O2, H2O, and 13CO spectra taken toward all observed Galactic sources were analyzed. The present results are based on spectra with a total integration time of up to 2.72 × 104 hr (≃108 s). The noise in the spectra is generally consistent with that expected from the radiometer equation, without any sign of approaching a noise floor. This noise performance reflects the extremely stable performance of the passively cooled front end as well as other relevant components in the SWAS instrument throughout its mission lifetime.
分类： 天文学 >> 天文学 提交时间： 2024-02-28 合作期刊: 《Research in Astronomy and Astrophysics》
摘要： The star-forming clumps in star-bursting dwarf galaxies provide valuable insights into understanding the evolution of dwarf galaxies. In this paper, we focus on five star-bursting dwarf galaxies featuring off-centered clumps in the Mapping Nearby Galaxies at Apache Point Observatory survey. Using the stellar population synthesis software Fitting Analysis using Differential evolution Optimization, we obtain the spatially resolved distribution of the star formation history, which allows us to construct the g-band images of the five galaxies at different ages. These images can help us to probe the evolution of the morphological structures of these galaxies. While images of a stellar population older than 1 Gyr are typically smooth, images of a stellar population younger than 1 Gyr reveal significant clumps, including multiple clumps which appear at different locations and even different ages. To study the evolutionary connections of these five galaxies to other dwarf galaxies before their star-forming clumps appear, we construct the images of the stellar populations older than three age nodes, and define them to be the images of the "host" galaxies. We find that the properties such as the central surface brightness and the effective radii of the hosts of the five galaxies are in between those of dwarf ellipticals (dEs) and dwarf irregulars (dIrrs), with two clearly more similar to dEs and one more similar to dIrrs. Among the five galaxies, 8257-3704 is particularly interesting, as it shows a previous starburst event that is not quite visible from its gri image, but only visible from images of the stellar population at a few hundred million years. The star-forming clump associated with this event may have appeared at around 600 Myr ago and disappeared at around 40 Myr ago.
分类： 天文学 >> 天文学 提交时间： 2024-07-15 合作期刊: 《Research in Astronomy and Astrophysics》
摘要： Recent submillimeter dust thermal emission observations have unveiled a significant number of inter-arm massive molecular clouds in M31. However, the effectiveness of this technique is limited to its sensitivity, making it challenging to study more distant galaxies. This study introduces an alternative approach, utilizing optical extinctions derived from space-based telescopes, with a focus on the forthcoming China Space Station Telescope (CSST). We first demonstrate the capability of this method by constructing dust extinction maps for 17 inter-arm massive molecular clouds in M31 using the Panchromatic Hubble Andromeda Treasury data. Our analysis reveals that inter-arm massive molecular clouds with an optical extinction (AV) greater than 1.6 mag exhibit a notable AV excess, facilitating their identification. The majority of these inter-arm massive molecular clouds show an AV around 1 mag, aligning with measurements from our JCMT data. Further validation using a mock CSST RGB star catalog confirms the method's effectiveness. We show that the derived AV values using CSST z and y photometries align more closely with the input values. Molecular clouds with AV > 1.6 mag can also be identified using the CSST mock data. We thus claim that future CSST observation clouds provide an effective way for the detection of inter-arm massive molecular clouds with significant optical extinction in nearby galaxies.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Han Qu
Zhen Yuan
Amandine Doliva-Dolinsky
Nicolas F. Martin
Xi Kang
Chengliang Wei
Guoliang Li
Yu Luo
Jiang Chang
Chao-Wei Tsai
Zhou Fan
Rodrigo Ibata
摘要： We predict the dwarf galaxy detection limits for the upcoming Chinese Space Station Telescope (CSST) survey that will cover 17,500 deg$^{2}$ of the sky with a wide field of view of 1.1 deg$^2$. The point-source depth reaches 26.3 mag in the $g$ band and 25.9 mag in the $i$ band. Constructing mock survey data based on the designed photometric bands, we estimate the recovery rate of artificial dwarf galaxies from mock point-source photometric catalogues. The detection of these artificial dwarf galaxies is strongly dependent on their distance, magnitude and size, in agreement with searches in current surveys. We expect CSST to enable the detection of dwarf galaxies with $M_V = -3.0$ and $\mu_{250} = 32.0$ mag/arcsec$^2$ (surface-brightness limit for a system of half-light radius $r_{\rm h}$ = 250 pc at 400 kpc, and $M_V = -4.9$ and $\mu_{250} = 30.5$ mag/arcsec$^2$ around the Andromeda galaxy. Beyond the Local Group, the CSST survey will achieve $M_V = -5.8$, and $\mu_{250}$ = 29.7 mag/arcsec$^2$ in the distance range of 1--2 Mpc, opening up an exciting discovery space for faint field dwarf galaxies. With its optical bands, wide survey footprint, and space resolution, CSST will undoubtedly expand our knowledge of low-mass dwarf galaxies to an unprecedented volume.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Lei Zhang
Zhi-Yu Zhang
James. W. Nightingale
Ze-Cheng Zou
Xiaoyue Cao
Chao-Wei Tsai
Chentao Yang
Yong Shi
Junzhi Wang
Dandan Xu
Ling-Rui Lin
Jing Zhou
Ran Li
摘要： The fast growth of supermassive black holes and their feedback to the host galaxies play an important role in regulating the evolution of galaxies, especially in the early Universe. However, due to cosmological dimming and the limited angular resolution of most observations, it is difficult to resolve the feedback from the active galactic nuclei (AGN) to their host galaxies. Gravitational lensing, for its magnification, provides a powerful tool to spatially differentiate emission originated from AGN and host galaxy at high redshifts. Here we report a discovery of a radio lobe in a strongly lensed starburst quasar, H1413+117 or Cloverleaf at redshift $z= 2.56$, based on observational data at optical, sub-millimetre, and radio wavelengths. With both parametric and non-parametric lens models and with reconstructed images on the source plane, we find a differentially lensed, kpc scaled, single-sided radio lobe, located at ${\sim}1.2\,\mathrm{kpc}$ to the north west of the host galaxy on the source plane. From the spectral energy distribution in radio bands, we find that the radio lobe has an energy turning point residing between 1.5 GHz and 8 GHz, indicating an age of 20--50 Myr. This could indicate a feedback switching of Cloverleaf quasar from the jet mode to the quasar mode.
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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