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.

Comparison of NH_(3) and ^(12)CO,^(13)CO,C^(18)O Molecular Lines in the Aquila Rift Cloud Complex

The observations of the Aquila Rift cloud complex at 23.708 and 115.271 GHz made using the Nanshan 26 m radio telescope and the 13.7 m millimeter-wavelength telescope are presented.We find that the CO(1-0)gas distribution is similar to the NH_(3)gas distribution in the Aquila Rift cloud complex.In some diffusion regions characterized by CO,we identified several dense clumps based on the distribution of detected ammonia molecular emission.Through the comparison of spectral line parameters for NH_(3),^(13)CO,and C^(18)O,our study reveals that the line center velocities of the NH_(3),^(13)CO,and C^(18)O lines are comparable and positively correlated,indicating that they originate from the same emission region.No significant correlation was identified for other parameters,including integrated intensity,line widths,main beam brightness temperature,as well as the column densities of NH_(3),^(13)CO,and C^(18)O.The absolute difference in line-center velocities between the^(13)CO and NH_(3)lines is less than both the average line width of NH_(3)and that of^(13)CO.This suggests that there are no significant movements of NH_(3)clumps in relation to their envelopes.The velocity deviation is likely due to turbulent activity within the clumps.

The comparison of H_2CO(1_(10)–1_(11)),C^(18)O(1–0) and the continuum towards molecular clouds

We present large scale observations of C^18O （1-0） towards four massive star forming regions： MON R2, S156, DR17/L906 and M17/M18. The transitions of H2CO （110-111）, C^18O （1-0） and the 6cm continuum are compared in these four regions. Our analysis of the observations and the results of the Non-LTE model shows that the brightness temperature of the formaldehyde absorption line is strongest in a background continuum temperature range of about 3 - 8 K. The excitation of the H2CO absorption line is affected by strong background continuum emission. From a comparison of H2CO and C^18O maps, we found that the extent of H2CO absorption is broader than that of C^18O emission in the four regions. Except for the DR17 region, the maximum in H2CO absorption is located at the same position as the C^18O peak. A good correlation between intensities and widths of H2CO absorption and C^18O emission lines indicates that the H2CO absorption line can trace the dense, warm regions of a molecular cloud. We find that N（H2CO） is well correlated with N（ C^18O） in the four regions and that the average ratio of column densities is （N（H2CO）/N（ClSO）） ～0.03.

Molecular Clouds Associated with HⅡRegions and Candidates within l=106°.65 to 109°.50 and b=-1°.85 to 0°.95

We present a large-scale simultaneous survey of the CO isotopologues(^(12)CO,^(13)CO,and C^(18)O)J=1-0 line emission toward the Galactic plane region of l=106°.65 to 109°.50 and b=-1°.85 to 0°.95 using the Purple Mountain Observatory 13.7 m millimeter-wavelength telescope.Except for the molecular gas in the solar neighborhood,the emission from the molecular gas in this region is concentrated in the velocity range of[-60,-35]km s^(-1).The gas in the region can be divided into four clouds,with mass in the range of~10^(3)-10^(4)M☉.We have identified 25 filaments based on the^(13)CO data.The median excitation temperature,length,line mass,line width,and virial parameter of the filaments are 10.89 K,8.49 pc,146.11 M☉pc^(-1),1.01 km s^(-1),and 3.14,respectively.Among these filaments,eight have virial parameters of less than 2,suggesting that they are gravitationally bound and can lead to star formation.Nineteen HⅡregions or candidates have previously been found in the region and we investigate the relationships between these HⅡregions/candidates and surrounding molecular clouds in detail.Using morphology similarity and radial velocity consistency between HⅡregions/candidates and molecular clouds as evidence for association,and raised temperature and velocity broadening as signatures of interaction,we propose that 12 HⅡregions/candidates are associated with their surrounding molecular clouds.In the case of the HⅡregion of S142,the energy of the HⅡregion is sufficient to maintain the turbulence in the surrounding molecular gas.

Oxygen isotopic ratios toward molecular clouds in the Galactic disk

We present our observations of the J = 1-0 rotation transitions in molecular isotopes C^18O and C^17O toward a sample of molecular clouds with different galactocentric distances, using the Delingha 13.7 m（DLH 13.7 m） telescope, administered by Purple Mountain Observatory, and its 9-beam SIS receiver.Complementary observations toward several sources with large galactocentric distance are obtained with the IRAM 30 m and Mopra 22 m telescopes. C^18O/C^17O abundance ratios reflecting the ^（18）O/^（17）O isotope ratios are obtained from integrated intensity ratios of C^18O and C^17O. We derived the ratio value for 13 sources covering a galactocentric distance range of 3 kpc to 16 kpc. In combination with our mapping results that provide a ratio value of 3.01±0.14 in the Galactic center region, it shows that the abundance ratio tends to increase with galactocentric distance, i.e., it supports a radial gradient along the Galactic disk for the abundance ratio. This is consistent with the inside-out formation scenario of our Galaxy. However, our results may suffer from small samples with large galactocentric distance. Combining our data with multitransition lines of C^18O and C^17O will be helpful for constraining opacities and abundances and further confirming the Galactic radial gradient shown by the isotope ratio ^（18）O/^（17）O.

^(12)CO,^(13)CO and C^(18)O observations along the major axes of nearby bright infrared galaxies

We present simultaneous observations of 12CO,13CO and C18O J=1-0 emission in 11 nearby（cz1000 km s-1） bright infrared galaxies.Both 12CO and 13CO are detected in the centers of all the galaxies,except for 13CO in NGC 3031. We have also detected C18O,CSJ=2-1 and HCO＋J=1-0 emission in the nuclear regions of M82 and M51.These are the first systematical extragalactic detections of 12CO and its isotopes from the PMO 14 m telescope.We have conducted half-beam-spaced mapping of M82 over an area of 4×2.5 and major axis mapping of NGC 3627,NGC 3628,NGC 4631 and M51.The radial distributions of 12CO and 13CO in NGC 3627,NGC 3628 and M51 can be well fitted by an exponential profile. The 12CO/13CO intensity ratio,R,decreases monotonically with the galactocentric radius in all mapped sources.The average R in the center and disk of the galaxies are 9.9±3.0 and 5.6±1.9,respectively,much lower than the peculiar R（～24） found in the center of M82.The intensity ratios of 13CO/C18O,13CO/HCO＋and 13CO/CS（either our or literature data） show little variation with galactocentric radius,in sharp contrast with the greatly varied R.This supports the notion that the observed gradient in R could be the result of the variations of the physical conditions across the disks.The H2 column density derived from C18O shows that the Galactic standard conversion factor（X-factor） overestimates the amount of the molecular gas in M82 by a factor of～2.5.These observations suggest that the X-factor in active star-forming regions（i.e.,nuclear regions） should be lower than that in normal star-forming disks and the gradient in R can be used to trace the variations of the X-factor.

CO observations towards a sample of nearby galaxies

We have simultaneously observed 12CO, laCO and clSo （J = 1 - 0） rotational transitions in the centers of a sample of 58 nearby spiral galaxies using the 13.7-m millimeter-wave telescope administered by Purple Mountain Observatory. Forty-two galaxies were detected in 13CO emission, but there was a null detection for C180 emission with a cr upper limit of 2 mK. The central beam ratios, R, of 12CO and 13CO range mostly from 5 to 13, with an average value of 8.1 ±4.2, which is slightly lower than previous estimates for normal galaxies. Clear correlations are found between ^12CO and ^13CO luminosities. An average X factor of 1.44± 0.84 × 10^20 cm^-2 （K km s-l）-1 is slightly lower than that in the Milky Way.

CO Clouds around SNR G21.8-0.6 and G32.8-0.1

We made the first CO（1-0） mapping to SNR G21.8-0.6 and SNR G32.8-0.1, both associated with OH 1720MHz maser. Based on the morphological correspondence and velocity and position agreement between the radio remnant and the CO clouds, we tentatively identify the clouds that are respectively interacting with the two SNRs.

C^(18)O Observations of the Dark Molecular Cloud L134 and Gas Depletion onto Dust

We map the dark molecular cloud core of L134 in the C^(18)O (J = 1 - 0)emission line using the PMO 13.7m telescope, and present a contour map of integrated intensity ofC^(18)O (J = 1 - 0) emission. The C^(18)O cloud is inside the distribution of extinction A_B, thevisual extinction of blue light, as well as inside the ^(13)CO cloud in the L134 region. Thedepletion factors in this C^(18)O cloud are generally greater than unity, which means there is gasdepletion onto dust. Since only a minimum A_B - 9.7 mag is available, and our observations measureboth undepleted and depleted regions along the line of sight, the depletion factors could verylikely be larger in the central core than the calculated value. So we conclude that depletion doesoccur in the bulk of the C^(18)O cloud through a comparison between the C^(18)O and blue extinctionmaps in the L134 region. There is no direct evidence as yet for star formation in L134, and so coreson the verge of collapse will not be visible in CO and other gas molecules.

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.

The Gas-to-Dust Relation in the Dark Cloud L1523-Observational Evidence for CO Gas Depletion

Correlation between gas and dust column density has been studied for the dark globule L1523. The 13CO（J=1→0） emission is used for tracing the gas, and the IR emissions, for tracing the dust constituent. In order to match the beam resolution between the images, a beam de-convolution algorithm based on the Maximum Correlation Method （MCM） was applied on the Infrared Astronomical Satellite （IRAS） data. The morphology of 13CO column density map shows a close correlation to that of 100μm dust optical depth. The distribution of the optical depth at 100 μm follows that of gas column density more closely than does the flux map at either 60 or 100μm. The ratio of the 13CO column density to the 100μm optical depth shows a decreasing trend with increasing dust optical depth in the central part, indicating possible molecular gas condensation onto dust particles. The excessive decrease in the CO column density in the envelope may most probably be due to the photo-dissociation of CO molecules.

Extragalactic dispersion measures of fast radio bursts

Fast radio bursts show large dispersion measures, much larger than the Galactic dispersion measure foreground. Therefore, they evidently have an extragalac- tic origin. We investigate possible contributions to the dispersion measure from host galaxies. We simulate the spatial distribution of fast radio bursts and calculate the dis- persion measures along the sightlines from fast radio bursts to the edge of host galaxies by using the scaled NE2001 model for thermal electron density distributions. We find that contributions to the dispersion measure of fast radio bursts from the host galaxy follow a skew Gaussian distribution. The peak and the width at half maximum of the dispersion measure distribution increase with the inclination angle of a spiral galaxy, to large values when the inclination angle is over 70~. The largest dispersion measure produced by an edge-on spiral galaxy can reach a few thousand pc cm-3, while the dispersion measures from dwarf galaxies and elliptical galaxies have a maximum of only a few tens of pc cm-3. Notice, however, that additional dispersion measures of tens to hundreds of pc cm-3 can be produced by high density clumps in host galaxies. Simulations that include dispersion measure contributions from the Large Magellanic Cloud and the Andromeda Galaxy are shown as examples to demonstrate how to ex- tract the dispersion measure from the intergalactic medium.

SMA molecular line survey towards the massive star-forming region G10.6-0.4in W31complex

Line surveys of complex molecules with millimeter and sub-millimeter telescopes are important for probing the physical and chemical environments of massive star forming regions(MSFRs).We present a molecular line survey with the Submillimeter Array(SMA) in the frequency ranges of 220.3–222.3 GHz and 230.3–232.3 GHz toward G10.6-0.4, the brightest star forming core in the W31 complex. Ninety-nine transitions from 22 molecular species and their isotopologues are identified. The moment 0 images of typical molecules show a compact core which is concentrated at the continuum peak position. Based on the local thermodynamic equilibrium assumption, the molecular line data are modeled. The rotational temperatures of those molecular species range from 96 to 178 K and their column densities range from 2.0×1014to 3.7×1017cm-2. The observational data suggest that all complex molecules are located in a warm environment. Chemical environments of the molecules are discussed. We compared molecular abundances and gas temperatures in G10.6-0.4 with those in other MSFRs, and found that gas temperatures and fractional abundances of specific molecules in G10.6-0.4 are similar to the typical MSFR W51 North, suggesting that there are similar physical and chemical environments in these two MSFRs.

Dense molecular gas tracers in high mass star formation regions

We report the FCRAO observations that mapped HCN （1-0）, CS （2-1）, HNC （1-0） and HCO＋ （1-0） in ten high-mass star forming cores associated with water masers. We present velocity integrated intensity maps of the four lines for these dense cores, compare their line profiles, and derive physical properties of these cores. We find that these four tracers identify areas with similar properties in these massive dense cores, and in most cases, the emissions of HCN and HCO＋ are stronger than those of HNC and CS. We also use the line ratios of HCO＋/HCN, HNC/HCN and HNC/HCO＋ as the diagnostics to explore the environment of these high-mass star forming regions, and find that most of the cores agree with the model that photodominated regions dominate the radiation field, except for W44, for which the radiation field is similar to an X-ray dominated region.

Study of the filamentary infrared dark cloud G192.76+00.10 in the S254–S258 OB complex

We present results of a high resolution study of the filamentary infrared dark cloud G192.76＋00.10 in the S254-S258 OB complex in several molecular species tracing different physical conditions. These include three isotopologues of carbon monoxide （CO）, ammonia （NH3） and carbon monosulfide （CS）. The aim of this work is to study the general structure and kinematics of the filamen- tary cloud, and its fragmentation and physical parameters. The gas temperature is derived from the NH3 （J, K） = （1, 1）, （2, 2） and 12CO（2-1） lines, and the 13CO（1-0）, 13CO（2-1） emission is used to inves- tigate the overall gas distribution and kinematics. Several dense clumps are identified from the CS（2-1） data. Values of the gas temperature lie in the range 10 - 35 K, and column density N（H2） reaches the value 5.1 ×1022 cm-2. The width of the filament is of order 1 pc. The masses of the dense clumps range from ×30 M⊙ to - 160 M⊙. They appear to be gravitationally unstable. The molecular emission shows a gas dynamical coherence along the filament, The velocity pattern may indicate longitudinal collapse.

Is the filamentary dark cloud GF 6 a star forming region?--Stability analysis and infrared properties

We present the results of mapping observations and stability analyses toward the filamentary dark cloud GF 6. We investigate the internal structures of a typical filamentary dark cloud GF 6 to know whether the filamentary dark cloud will form stars. We perform radio observations with both 12CO（J =1–0） and 13CO（J =1–0） emission lines to examine the mass distribution and its evolutionary status.The 13CO gas column density map shows eight subclumps in the GF 6 region with sizes on a sub-pc scale. The resulting local thermodynamic equilibrium masses of all the subclumps are too low to form stars against the turbulent dissipation. We also investigate the properties of embedded infrared point sources to know whether they are newly formed stars. The infrared properties also indicate that these point sources are not related to star forming activities associated with GF 6. Both radio and infrared properties indicate that the filamentary dark cloud GF 6 is too light to contract gravitationally and will eventually be dissipated away.

A survey of CO and its isotope lines for possible cloud-cloud collision candidates

In the 12CO （J=1-0） survey of 1331 cold 1RAS sources, 214 sources show profiles with multiple peaks and are selected as cloud-cloud collision candidates. In January 2005, 201 sources were detected with 12CO（1-0）, 13CO（1-0）, and c18O（1-0） emissions by the 13.7 m telescope at Purple Mount Observatory. This is the first survey of CO and its isotope lines directed toward possible cloud-cloud collision regions. According to the statistics of the 201 sources in the Galactic distribution, the 201 sources show a similar distribution to the parent sample （1331 cold IRAS sources）. These sources are located over a wide range of Galactocentric distances, and are partly associated with the star forming region. Based on preliminary criteria which describe the spectral properties of the possible cloud-cloud collision region, the 201 sources are classified into four types by the fit of the spectral profiles between the optically thick and thin lines toward each source. The survey is focused on possible cloud-cloud collision regions, and gives some evidence to help us with selecting the target region. We will continue the process of mapping and studying multi-wavelength observations for the selected region in the future.

A Catalog of Molecular Clumps and Cores with Infall Signatures

The research of infall motion is a common means to study molecular cloud dynamics and the early process of star formation. Many works had been done in-depth research on infall. We searched the literature related to infall study of molecular cloud since 1994, summarized the infall sources identified by the authors. A total of 456 infall sources are cataloged. We classify them into high-mass and low-mass sources, in which the high-mass sources are divided into three evolutionary stages: prestellar, protostellar and H II region. We divide the sources into clumps and cores according to their sizes. The H2 column density values range from 1.21 × 10^(21) to 9.75 × 10^(24) cm^(-2), with a median value of 4.17 × 10^(22) cm^(-2). The H_(2) column densities of high-mass and low-mass sources are significantly separated. The median value of infall velocity for high-mass clumps is 1.12 km s^(-1), and the infall velocities of lowmass cores are virtually all less than 0.5 km s^(-1). There is no obvious difference between different stages of evolution. The mass infall rates of low-mass cores are between 10^(-7) and 10^(-4) M⊙yr^(-1), and those of high-mass clumps are between 10^(-4 )and 10-1 M⊙yr^(-1) with only one exception. We do not find that the mass infall rates vary with evolutionary stages.

In search of infall motion in molecular clumpsⅡ:HCO+(1-0)and HCN(1-0)observations toward a sub-sample of infall candidates

Gravitational accretion accumulates the original mass.This process is crucial for us to understand the initial phases of star formation.Using the specific infall profiles in optically thick and thin lines,we searched the clumps with infall motion from the Milky Way Imaging Scroll Painting(MWISP)CO data in previous work.In this study,we selected 133 sources as a sub-sample for further research and identification.The excitation temperatures of these sources are between 7.0 and 38.5 K,while the H2 column densities are between 10^21 and 10^23 cm^-2.We have observed optically thick lines HCO+(1-0)and HCN(1-0)using the DLH 13.7-m telescope,and found 56 sources with a blue profile and no red profile in these two lines,which are likely to have infall motions,with a detection rate of 42%.This suggests that using CO data to restrict the sample can effectively improve the infall detection rate.Among these confirmed infall sources are 43 associated with Class O/I young stellar objects(YSOs),and 13 which are not.These 13 sources are probably associated with the sources in the earlier evolutionary stage.In comparison,the confirmed sources that are associated with Class O/I YSOs have higher excitation temperatures and column densities,while the other sources are colder and have lower column densities.Most infall velocities of the sources that we confirmed are between 10^-1 to 10^0 km s^-1,which is consistent with previous studies.

A cold and diffuse giant molecular filament in the region of l=41°,b=-1°

Data of 12CO/13CO/C180 J=1→0 emission toward the Galactic plane region of l=35°to 450 and b=-5°to+5°are available with the Milky Way Imaging Scroll Painting(MWISP)project.Using the data,we found a giant molecular filament(GMF)around l≈38°42°,b≈-3.5°0°,VLSR≈2740 km s-1,named the GMF MWISP G041-01.At a distance of 1.7 kpc,the GMF is about 160 pc long.With a median excitation temperature about 7.5 K and a median column density about1021 cm-2,this GMF is very cold and very diffuse compared to known GMFs.Using the morphology in the data cube,the GMF is divided into four components among which three show filamentary structure.Masses of the components are 103104 M☉,with a total mass for the whole filament being about 7×104 M☉from the local thermodynamic equilibrium method.13CO cores inside each component are searched.Virial parameters are about 2.5 for these cores and have a power-law index of-0.34 against the mass.The mass fraction of dense cores traced by 13CO to the diffuse clouds traced by 12CO is about 7%for all components of the GMF.We found signatures of possible large scale filament-filament collision in the GMF.