The Clumpy Structure of Five Star-bursting Dwarf Galaxies in the MaNGA Survey

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.

FAST Observations of Four Comets to Search for the Molecular Line Emissions between 1.0 and 1.5 GHz Frequencies

We used the Five-hundred-meter Aperture Spherical radio Telescope(FAST)to search for the molecular emissions in the L-band between 1.0 and 1.5 GHz toward four comets,C/2020 F3(NEOWISE),C/2020 R4(ATLAS),C/2021 A1(Leonard),and 67P/Churyumov-Gerasimenko during or after their perihelion passages.Thousands of molecular transition lines fall in this low-frequency range,many attributed to complex organic or prebiotic molecules.We conducted a blind search for the possible molecular lines in this frequency range in those comets and could not identify clear signals of molecular emissions in the data.Although several molecules have been detected at high frequencies of greater than100 GHz in comets,our results confirm that it is challenging to detect molecular transitions in the L-band frequency ranges.The non-detection of L-band molecular lines in the cometary environment could rule out the possibility of unusually strong lines,which could be caused by the masers or non-LTE effects.Although the line strengths are predicted to be weak,for FAST,using the ultra-wide bandwidth receiver and improving the radio frequency interference environments would enhance the detectability of those molecular transitions at low frequencies in the future.

Long-term Integration Ability of the Submillimeter Wave Astronomy Satellite(SWAS)Spectral Line Receivers

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 CI,O_(2),H_(2)O,and^(13)CO spectra taken toward all observed Galactic sources were analyzed.The present results are based on spectra with a total integration time of up to 2.72×10~4hr(■10~8s).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.

Molecular Oxygen Abundance in Galactic Massive Star Formation Regions Based on SWAS Observations

Molecular oxygen abundance is a key parameter in understanding the chemical network of the interstellar medium.We estimate the molecular oxygen column density and abundance for a sample of Galactic massive star formation regions based on observations from the Submillimiter Wave Astronomy Satellite(SWAS)survey.We obtained an averaged O_(2)spectrum based on this sample using the(SWAS)survey data(O_(2),487.249 GHz,N=3-1,J=3-2).No emission or absorption feature is seen around the supposed central velocity with a total integration time of t_(total)=8.67×10^(3)hr and an rms noise per channel of 1.45 m K.Assuming a kinetic temperature T_(kin)=30 K,we derive the 3σupper limit of the O_(2)column density to be 3.3×10^(15)cm^(-2),close to the lowest values reported in Galactic massive star formation regions in previous studies.The corresponding O_(2)abundance upper limit is6.7×10^(-8),lower than all previous results based on SWAS observations and is close to the lowest reported value in massive star formation regions.On a galactic scale,our statistical results confirm a generally low O_(2)abundance for Galactic massive star formation regions.This abundance is also lower than results reported in extragalactic sources.

Optical Extinctions of Inter-Arm Molecular Clouds in M31:A Pilot Study for the Upcoming CSST Observations

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(A_(V)) greater than 1.6 mag exhibit a notable A_(V) excess,facilitating their identification.The majority of these inter-arm massive molecular clouds show an A_(V) 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 A_(V)values using CSST z and y photometries align more closely with the input values.Molecular clouds with A_(V)> 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.

Identifying Host Galaxies of Extragalactic Radio Emission Structures using Machine Learning

This paper presents an automatic multi-band source cross-identification method based on deep learning to identify the hosts of extragalactic radio emission structures.The aim is to satisfy the increased demand for automatic radio source identification and analysis of large-scale survey data from next-generation radio facilities such as the Square Kilometre Array and the Next Generation Very Large Array.We demonstrate a 97%overall accuracy in distinguishing quasi-stellar objects,galaxies and stars using their optical morphologies plus their corresponding mid-infrared information by training and testing a convolutional neural network on Pan-STARRS imaging and WISE photometry.Compared with an expert-evaluated sample,we show that our approach has 95%accuracy at identifying the hosts of extended radio components.We also find that improving radio core localization,for instance by locating its geodesic center,could further increase the accuracy of locating the hosts of systems with a complex radio structure,such as C-shaped radio galaxies.The framework developed in this work can be used for analyzing data from future large-scale radio surveys.

FAST Observations of an Extremely Active Episode of FRB 20201124A.Ⅱ.Energy Distribution

We report the properties of more than 800 bursts detected from the repeating fast radio burst(FRB)source FRB20201124A with the Five-hundred-meter Aperture Spherical radio Telescope(FAST)during an extremely active episode on UTC 2021 September 25–28 in a series of four papers.In this second paper of the series,we study the energy distribution of 881 bursts(defined as significant signals separated by dips down to the noise level)detected in the first four days of our 19 hr observational campaign spanning 17 days.The event rate initially increased exponentially but the source activity stopped within 24 hr after the 4th day.The detection of 542 bursts in one hour during the fourth day marked the highest event rate detected from one single FRB source so far.The bursts have complex structures in the time-frequency space.We find a double-peak distribution of the waiting time,which can be modeled with two log-normal functions peaking at 51.22 ms and 10.05 s,respectively.Compared with the emission from a previous active episode of the source detected with FAST,the second distribution peak time is smaller,suggesting that this peak is defined by the activity level of the source.We calculate the isotropic energy of the bursts using both a partial bandwidth and a full bandwidth and find that the energy distribution is not significantly changed.We find that an exponentially connected broken-power law function can fit the cumulative burst energy distribution well,with the lower and higher-energy indices being-1.22±0.01 and-4.27±0.23,respectively.Assuming a radio radiative efficiency ofη_(r)=10^(-4),the total isotropic energy of the bursts released during the four days when the source was active is already 3.9×10^(46)erg,exceeding~23%of the available magnetar dipolar magnetic energy.This challenges the magnetar models which invoke an inefficient radio emission(e.g.,synchrotron maser models).

The Potential of Detecting Radio-flaring Ultracool Dwarfs at L band in the FAST Drift-scan Survey

The Five-hundred-meter Aperture Spherical radio Telescope(FAST)completed its commissioning and began the Commensal Radio Astronomy Fas T 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≥3%.Assuming a flat radio spectrumνLν∝ν^(β+1) withβ=-1.0 for UCD flares,we construct a flare luminosity function dN/dL∝L^(-1.96±0.45)(here L=νLν).CRAFTS is found to be sensitive enough for flares from UCDs up to~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~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 indexβ.Forβfrom 0 to-2.5,the number of expected detections increases dramatically from~20 to~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.

FAST Observations of an Extremely Active Episode of FRB 20201124A.Ⅲ.Polarimetry

As the third paper in the multiple-part series,we report the statistical properties of radio bursts detected from the repeating fast radio burst(FRB)source FRB 20201124A with the Five-hundred-meter Aperture Spherical radio Telescope during an extremely active episode between the 25th and 28th of September 2021(UT).We focus on the polarization properties of536 bright bursts with S/N>50.We found that the Faraday rotation measures(RMs)monotonically dropped from-579to-605 rad m^(-2)in the 4 day window.The RM values were compatible with the values(-300 to-900 rad m^(-2))reported 4 months ago.However,the RM evolution rate in the current observation window was at least an order of magnitude smaller than the one(~500 rad m^(-2)day^(-1))previously reported during the rapid RM-variation phase,but is still higher than the one(≤1 rad m^(-2)day^(-1))during the later RM no-evolution phase.The bursts of FRB 20201124A were highly polarized with the total degree of polarization(circular plus linear)greater than 90%for more than 90%of all bursts.The distribution of linear polarization position angles(PAs),degree of linear polarization(L/I)and degree of circular polarization(V/I)can be characterized with unimodal distribution functions.During the observation window,the distributions became wider with time,i.e.,with larger scatter,but the centroids of the distribution functions remained nearly constant.For individual bursts,significant PA variations(confidence level 5σ)were observed in 33%of all bursts.The polarization of single pulses seems to follow certain complex trajectories on the Poincarésphere,which may shed light on the radiation mechanism at the source or the plasma properties along the path of FRB propagation.

Can Turbulent, High-density Gas Form Stars in Molecular Clouds: A Case Study in Ophiuchus

Star formation is governed by the interplay between gravity and turbulence in most of molecular clouds.Recent theoretical works assume that dense gas,whose column density is above a critical value in the column density probability distribution function(N-PDF),where gravity starts to overcome turbulence,becomes star-forming gas and will collapse to form stars.However,these high-density gases will include some very turbulent areas in the clouds.Will these dense but turbulent gases also form stars?We test this scenario in Ophiuchus molecular cloud using N-PDF analysis and find that at least in some regions,the turbulent,dense gas is not forming stars.We identified two isolated high-density structures in Ophiuchus,which are gravitationally unbound and show no sign of star formation.Their high densities may come from turbulence.

Revisiting pulsar velocities using Gaia Data Release 2

Precise measurements of neutron star(NS)velocities provide critical clues in regard to the supernova physics and evolution of binary systems.Based on Gaia Data Release 2(DR2),we selected a sample of 24 young(<3 Myr)pulsars with precise parallax measurements and measured the velocity of their local standard of rest(LSR)and the velocity dispersion among their respective local stellar groups.The median velocity difference between thus calculated LSRs and the Galactic rotation model is~7.6 km s^(-1),small compared to the typical velocity dispersion of~27.5 km s^(-1).For pulsars off the Galactic plane,such differences grow significantly to as large as~40 km s^(-1).More importantly,the velocity dispersion of stars in the local group of low-velocity pulsars can be comparable to their transverse velocities,suggesting that the intrinsic velocities of NS progenitors should be taken into account when we consider their natal kicks and binary evolution.We also examined the double NS system J0737-3039 A/B,and measured its transverse velocity to be 26-13+18 km s^(-1)assuming nearby Gaia sources are representative of their birth environment.This work demonstrated the feasibility and importance of using Gaia data to study the velocity of individual systems and velocity distribution of NSs.

The ALMaQUEST Survey ⅩⅤ:The dependence of the molecular-to-atomic gas ratios on resolved optical diagnostics

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 Ma NGA 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_(mol)≡log M_(H2)/M_(H_(Ⅰ)))is correlated with the global stellar mass surface densityμ_*with a Kendall'sτcoefficient 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_(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_(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_(mol)tend to have high oxygen abundance,both at one effective radius with a Kendall'sτcoefficient 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_(mol).The dependence of gas conversion on gas distribution and galaxy ionization states is weak.In contrast,the observed positive relation between oxygen abundance(μ_(*))and R_(mol)indicates that the gas conversion is efficient in regions of high metallicity(density).