Identification of Galactic H I Counterpart of the Large [O iii] Emission Structure 1.°2 Southeast of M31

Neutral hydrogen (HI) is the fundamental component of the interstellar medium. The Galactic Plane Pulsar Snapshot (GPPS) survey is designed for hunting pulsars by using the Five-hundred-meter Aperture Spherical radio Telescope (FAST) from the visible Galactic plane within $|b| \leq 10^{\circ}$. The survey observations are conducted with the L-band 19-beam receiver in the frequency range of 1.0 $-$ 1.5 GHz, and each pointing has an integration time of 5 minutes. The piggyback spectral data simultaneously recorded during the FAST GPPS survey are great resources for studies on the Galactic HI distribution and ionized gas. We process the piggyback HI data of the FAST GPPS survey in the region of $33^{\circ} \leq l \leq 55^{\circ}$ and $|b| \leq 2^{\circ}$. The rms of the data cube is found to be approximately 40 mK at a velocity resolution of $0.1$ km s$^{-1}$, placing it the most sensitive observations of the Galactic HI by far. The high velocity resolution and high sensitivity of the FAST GPPS HI data enable us to detect weak exquisite HI structures in the interstellar medium. HI absorption line with great details can be obtained against bright continuum sources. The FAST GPPS survey piggyback HI data cube will be released and updated on the web: http://zmtt.bao.ac.cn/MilkyWayFAST/.

The Five-hundred-meter Aperture Spherical radio Telescope (FAST) is the most sensitive radio telescope for pulsar observations. We make polarimetric measurements of a large number of faint and distant pulsars using the FAST. We present the new measurements of Faraday rotation for 134 faint pulsars in the Galactic halo. Significant improvements are also made for some basic pulsar parameters for 15 of them. We analyse the newly determined rotation measures (RMs) for the Galactic magnetic fields by using these 134 halo pulsars, together with previously available RMs for pulsars and extragalactic radio sources and also the newly determined RMs for another 311 faint pulsars which are either newly discovered in the project of the Galactic Plane Pulsar Snapshot (GPPS) survey or previously known pulsars without RMs. The RM tomographic analysis in the first Galactic quadrant gives roughly the same field strength of around 2~$\mu$G for the large-scale toroidal halo magnetic fields. The scale height of the halo magnetic fields is found to be at least 2.7$\pm$0.3~kpc. The RM differentiation of a large number of pulsars in the Galactic disk in the Galactic longitude range of $26^{\circ}<l<90^{\circ}$ gives evidence for the clockwise magnetic fields (viewed from the north Galactic pole) in two interarm regions inside the Scutum arm and between the Scutum and Sagittarius arm, and the clockwise fields in the Local-Perseus interarm region and field reversals in the Perseus arm and beyond.

The first detection and deep follow-up of a Galactic fast radio burst (FRB) phenomenon were reported in three papers published in the journal Nature in November 2020. Interestingly, this FRB is accompanied by a X-ray burst. The observations from multiple space and ground-based telescopes were combined to accomplish this discovery and ascertain its association with a source in the Milky Way. As the name implies, a FRB is a transient bright pulse of radio waves with a burst duration measured in milliseconds. This phenomenon was first discovered in 2007. It is extremely difficult to detect and even more so to determine their position in the sky due to their brief existence. This is the first detection of a FRB with radiation other than radio waves, as well as the first of its kind in the Milky Way. For the first time, these observations have confirmed that the source(s) of FRBs can be a magnetar(s), which so far is the only verified celestial body capable of producing FRBs. It is worth noting that one of the papers was written by a Chinese research team, the co-first authors of which are Lin Lin from Beijing Normal University, Chunfeng Zhang from Peking University, and Pei Wang from the National Astronomical Observatories of China. The observations came from China's “Sky Eye”—the Five-hundred-meter Aperture Spherical radio Telescope (FAST).①①Original source in Chinese: Di Li, Consistency radio bursts in the Milky Way, Bulletin of National Natural Science Foundation of China. 35 (2) (2021) 243-244.

As one of the major components of the interstellar medium, the ionized gas in our Milky Way, especially the low-density diffuse component, has not been extensively observed in the radio band. The Galactic Plane Pulsar Snapshot (GPPS) survey covers the sky area within the Galactic latitude of $\pm10^\circ$ around the Galactic plane visible by the Five-hundred-meter Aperture Spherical radio Telescope (FAST), and the spectral line data are simultaneously recorded during the pulsar survey observations. With an integration time of 5 minutes for each beam, the GPPS survey project provides the most sensitive piggyback spectra for tens of radio recombination lines (RRLs) in the band of 1000$-$1500 MHz for H$n\alpha$, He$n\alpha$, C$n\alpha$, as well as H$n\beta$ and H$n\gamma$. We processed the spectral data of RRLs, and obtained a sensitive averaged H$n\alpha$ RRL map of a sky area of 88 square degrees in the inner Galaxy of 33$^\circ$ $\leqslant l \leqslant$ 55$^\circ$ and $|b| \leqslant$ 2.0$^\circ$. The final spectral data of the H$n\alpha$ RRLs have a spatial resolution of $\sim$3$^\prime$, a spectral resolution of 2.2 km s$^{-1}$, and a typical spectral rms noise of 0.25 mJy beam$^{-1}$ or 6.3 mK in main-beam brightness temperature. The new H$n\alpha$ RRL map shows complex structural features dominated by a number of HII regions and large extended diffuse ionized gas regions. We detect about 94% of the known HII regions and confirm 43 WISE HII regions in the observed sky area. Several large HII regions or star-forming complexes in the distant outer Galaxy are resolved in the map of H$n\alpha$ RRLs. Extended RRL features of the diffuse ionized gas are detected. The RRL data products of the GPPS survey will be published and updated at http://zmtt.bao.ac.cn/MilkyWayFAST/

Typically, neutron stars are discovered by observations at radio, X-ray or gamma-ray wavelengths. Unlike radio pulsar surveys and X-ray observations, optical time-domain surveys can unveil and characterize exciting but less explored non-accreting and/or non-beaming neutron stars in binaries. Here we report the discovery of such a neutron star candidate using the LAMOST spectroscopic survey. The candidate, designated LAMOST J112306.9 + 400736, is in a single-lined spectroscopic binary containing an optically visible M star. The star’s large radial velocity variation and ellipsoidal variations indicate a relatively massive unseen companion. Utilizing follow-up spectroscopy from the Palomar 200 in. telescope and high-precision photometry from the Transiting Exoplanet Survey Satellite, we measure a companion mass of \(1.2{4}_{-0.03}^{+0.03}\,{M}_{\odot }\). Main-sequence stars with this mass are ruled out, leaving a neutron star or a massive white dwarf. Although a massive white dwarf cannot be excluded, the lack of UV excess radiation from the companion supports the neutron star hypothesis. Deep radio observations with the Five-hundred-meter Aperture Spherical radio Telescope (FAST) yielded no detections of either pulsed or persistent emission. J112306.9 + 400736 is not detected in numerous X-ray and gamma-ray surveys, suggesting that the neutron star candidate is not currently accreting and pulsing. Our work exemplifies the capability of discovering compact objects in non-accreting close binaries by synergizing optical time-domain spectroscopy and high-cadence photometry.

Unveiling the history and nature of the Milky Way with galactic surveys and numerical simulations

An updated, binary-coded message has been developed for transmission to extraterrestrial intelligences in the Milky Way galaxy. The proposed message includes basic mathematical and physical concepts to establish a universal means of communication followed by information on the biochemical composition of life on Earth, the Solar System’s time-stamped position in the Milky Way relative to known globular clusters, as well as digitized depictions of the Solar System, and Earth’s surface. The message concludes with digitized images of the human form, along with an invitation for any receiving intelligences to respond. Calculation of the optimal timing during a given calendar year is specified for potential future transmission from both the Five-hundred-meter Aperture Spherical radio Telescope in China and the SETI Institute’s Allen Telescope Array in northern California to a selected region of the Milky Way which has been proposed as the most likely location for life to have developed. These powerful new beacons, the successors to the Arecibo radio telescope which transmitted the 1974 message upon which this expanded communication is in part based, can carry forward Arecibo’s legacy into the 21st century with this equally well-constructed communication from Earth’s technological civilization.

The circum- and intergalactic medium and its connection to the large scale structure in the nearby universe

Context.We report new H Iobservations of fourz ∼ 0.05 VALES galaxies undertaken during the commissioning phase of the Five-hundred-meter Aperture Spherical Radio Telescope (FAST).Aims.FAST is the largest single-dish telescope in the world, with a 500 m aperture and a 19-Beam receiver. Exploiting the unprecedented sensitivity provided by FAST, we aim to study the atomic gas content, via the H I21 cm emission line, in low-zstar formation galaxies taken from the Valparaíso ALMA/APEX Line Emission Survey (VALES). Together with previous Atacama Large Millimeter/submillimeter Array (ALMA) CO(J = 1−0) observations, the H Idata provides crucial information to measure the gas mass and dynamics.Methods.As a pilot H Igalaxy survey, we targeted four local star-forming galaxies atz ∼ 0.05. In particular, one of them has already been detected in H Iby the Arecibo Legacy Fast ALFA survey (ALFALFA), allowing a careful comparison. We use an ON-OFF observing approach that allowed us to reach an rms of 0.7 mJy beam−1at a 1.7 km s−1velocity resolution within only 20 min ON-target integration time.Results.In this Letter, we demonstrate the extraordinary capability of the FAST 19-beam receiver to push the detectability of the H Iemission line of extra-galactic sources. The H Iemission line detected by FAST shows good consistency with the previous Arecibo telescope ALFALFA results. Our observations are put into context with previous multi-wavelength data to reveal the physical properties of these low-zgalaxies. We find that the CO(J = 1−0) and H Iemission line profiles are similar. The dynamical mass estimated from the H Idata is an order of magnitude higher than the baryon mass and the dynamical mass derived from the CO observations, implying that the mass probed by dynamics of H Iis dominated by the dark matter halo. In one case, a target shows an excess of CO(J = 1−0) in the line centre, which can be explained by an enhanced CO(J = 1−0) emission induced by a nuclear starburst showing high-velocity dispersion.

Deuterium in high redshift clouds and baryonic dark matter

We have performed population synthesis calculation on the formation of binaries containing a black hole (BH) and a neutron star (NS) in the Galactic disc. Some of important input parameters, especially for the treatment of common envelope evolution, are updated in the calculation. We have discussed the uncertainties from the star formation rate of the Galaxy and the velocity distribution of NS kicks on the birthrate (${\sim } 0.6\hbox{--}13 \rm \, M\,yr^{-1}$) of BH/NS binaries. From incident BH/NS binaries, by modelling the orbital evolution due to gravitational wave radiation and the NS evolution as radio pulsars, we obtain the distributions of the observable parameters such as the orbital period, eccentricity, and pulse period of the BH/pulsar binaries. We estimate that there may be ∼3–80 BH/pulsar binaries in the Galactic disc and around 10 per cent of them could be detected by the Five-hundred-metre Aperture Spherical radio Telescope.

The Zone of Avoidance (ZoA) is a region of low galactic latitude that is heavily obscured by the Milky Way. Observations with radio telescopes are basically unaffected by dust extinction and can unveil the structure behind it through the Milky Way. One of the scientific goals of the Five-hundred-meter Aperture Spherical radio Telescope (FAST) is to search for the neutral hydrogen and understand the large-scale physics to explore the origin and evolution of the universe. We take the 15,500 IRAS (the Infrared Astronomical Satellite) galaxies from PSCz (“Point Source Catalog”) survey to reconstruct the density field of the local universe, obtain the distribution of the relative density of galaxies in the ZoA region with a redshift z below 0.07, and the number of detectable galaxies with FAST is estimated by using the neutral hydrogen mass function of the ALFA (Arecibo L-band Feed Array) survey. We conclude that FAST can observe more than 2000 ZoA galaxies within a distance of 300 Mpc h70−1 , and present preliminary results of the partial GPPS (the FAST Galactic Plane Pulsar Snapshot survey) data, compared with ALFA ZoA (The Arecibo L-band Feed Array Zone of Avoidance), show that FAST has a higher detection sensitivity to search for H i galaxies in the ZoA area.

Long and skinny molecular filaments running along Galactic spiral arms are known as “bones,” since they make up the skeleton of the Milky Way. However, their origin is still an open question. Here, we compare spectral images of HI taken by the Five-hundred-meter Aperture Spherical radio Telescope (FAST) with archival CO and Herschel dust emission to investigate the conversion from HI to H2 in two typical Galactic bones, CFG028.68-0.28 and CFG047.06+0.26. Sensitive FAST HI images and an improved methodology enabled us to extract HI narrow self-absorption (HINSA) features associated with CO line emission on and off the filaments, revealing the ubiquity of HINSA toward distant clouds for the first time. The derived cold HI abundances, [HI]/[H2], of the two bones range from ∼(0.5 to 44.7) × 10−3, which reveal different degrees of HI–H2 conversion, and are similar to those of nearby, low-mass star-forming clouds, Planck Galactic cold clumps, and a nearby active high-mass star-forming region G176.51+00.20. The HI–H2 conversion has been ongoing for 2.2–13.2 Myr in the bones, a timescale comparable to that of massive star formation therein. Therefore, we are witnessing young giant molecular clouds (GMCs) with rapid massive star formation. Our study paves the way of using HINSA to study cloud formation in Galactic bones and, more generally, in distant GMCs in the FAST era.

We report the discovery of a possible accretion stream toward a Milky Way–type galaxy M106 based on very deep H i imaging data with the Five-hundred-meter Aperture Spherical radio Telescope (FAST). The accretion stream extends for about 130 kpc in projection length and it is similar to the Magellanic stream in many respects. We provide unambiguous evidence based on the stream morphology, kinematics and local star formation activity to show that the H i gas is being accreted onto the disk of M106. Such a long continuous flow of gas provides a unique opportunity to probe the circumgalactic medium (CGM) and reveals how the gas stream traverses the hot halo and CGM, and eventually reaches the galaxy disk. The source of the stream appears to be from M106's satellite galaxy NGC 4288. We argue that the stream of gas could be due to the tidal interaction with NGC 4288, or with a high speed encounter near this system. Close to the position of UGC 7356 the stream bifurcates into two streams. The second stream may be gas tidally stripped from UGC 7356 or due to an interaction with UGC 7356. Our results show that high-sensitivity H i imaging is crucial in revealing low column density accretion features in nearby galaxies.

TreeSPH simulations of galaxy formation in a standard Λ cold dark matter cosmology, including star formation and the effects of energetic stellar feedback processes and of a metagalactic UV field, have been performed, resulting in a mix of realistic disk, lenticular, and elliptical galaxies at redshift z = 0. The disk galaxies are deficient in angular momentum by only about a factor of 2 compared with observed disk galaxies for simulations with fairly strong starbursts in early, protogalactic clouds, leading to "blow-away" of the remaining gas in the clouds. In this respect the present scenario is hence doing almost as well as the warm dark matter (WDM) scenarios discussed by Sommer-Larsen & Dolgov. The surface density profiles of the stellar disks are approximately exponential, and those of the bulges range from exponential to r1/4, as observed. The bulge-to-disk ratios of the disk galaxies are consistent with observations, as are their integrated B-V colors, which have been calculated using stellar population synthesis techniques. Furthermore, the observed I-band Tully-Fisher relation can be matched, provided that the stellar mass-to-light ratio of disk galaxies is M/LI ~ 0.8, similar to what was found by Sommer-Larsen & Dolgov from their WDM simulations and in fair agreement with several recent observational determinations of M/LI for disk galaxies. The elliptical and lenticular galaxies have approximately r1/4 stellar surface density profiles, are dominated by nondisklike kinematics, and are flattened as a result of nonisotropic stellar velocity distributions, again consistent with observations. Hot halo gas is predicted to cool out and be accreted onto the Galactic disk at a rate of 0.5-1 M☉ yr-1 at z = 0, consistent with upper limits deduced from Far Ultraviolet Spectroscopic Explorer observations of O VI. We have analyzed in detail the formation history of two disk galaxies with circular speeds comparable to that of the Milky Way and find gas accretion rates, and hence bolometric X-ray luminosities of the halos, 6-7 times larger at z ~ 1 than at z = 0 for these disk galaxies. More generally, it is found that gas infall rates onto these disks are nearly exponentially declining with time, both for the total disk and for the "solar cylinder." This theoretical result hence supports the exponentially declining gas infall approximation often used in chemical evolution models. The infall timescales deduced are ~5-6 Gyr, comparable to what is adopted in current chemical evolution models to solve the "G dwarf problem." The disk of one of the two galaxies forms "inside-out," the other "outside-in," but in both cases the mean ages of the stars in the outskirts of the disks are ≳6-8 Gyr, fairly consistent with the findings of Ferguson & Johnson for the disk of M31. The amount of hot gas in disk galaxy halos is consistent with observational upper limits. The globular cluster M53 and the LMC are "inserted" in the halos of the two Milky Way-like disk galaxies, and dispersion measures to these objects are calculated. The results are consistent with upper limits from observed dispersion measures to pulsars in these systems. Finally, the results of the simulations indicate that the observed peak in the cosmic star formation rate at redshift z ~ 2 can be reproduced. Depending on the star formation and feedback scenarios, one predicts either a cosmic star formation rate that decreases monotonically with redshift beyond these redshifts or a second peak at z ~ 6-8, corresponding to the putative Population III and interestingly similar to recent estimates of the redshift at which the universe was reionized. These various scenarios should hence be observationally constrainable with upcoming instruments such as the James Webb Space Telescope and the Atacama Large Millimeter Array.