Optical Spectroscopic Survey Research Articles

Detection of High-frequency Pulsation in WR 135: Investigation of Stellar Wind Dynamics

We report the detection of high-frequency pulsations in WR 135 from short-cadence (10 minute) optical photometric and spectroscopic time series surveys. The harmonics up to the sixth order are detected from the integrated photometric flux variations, while the comparatively weaker eighth harmonic is detected from the strengths of the emission lines. We investigate the driving source of the stratified winds of WR 135 using the radiative transfer modeling code, CMFGEN, and find the physical conditions that can explain the propagation of such pulsations. From our study, we find that the optically thick subsonic layers of the atmosphere are close to the Eddington limit and are launched by the Fe opacity. The outer optically thin supersonic winds (τ ross = 0.1–0.01) are launched by the He II and C iv opacities. The stratified winds above the sonic point undergo velocity perturbation that can lead to clumps. In the optically thin supersonic winds, dense clumps of smaller size (f VFF = 0.27–0.3, where f VFF is the volume filling factor) pulsate with higher-order harmonics. The larger clumps ( f VFF = 0.2) oscillate with lower-order harmonics of the pulsation and affect the overall wind variability.

Starspots and Undetected Binary Stars Have Distinct Signatures in Young Stellar Associations

Young stars form in associations, meaning that young stellar associations provide an ideal environment to measure the age of a nominally coeval population. Isochrone fitting, which is the typical method for measuring the age of a coeval population, can be impacted by observational biases that obscure the physical properties of a population. One feature in isochrone fits of star-forming regions is an apparent mass-dependent age gradient, where lower-mass stars appear systematically younger than higher-mass stars. Starspots and stellar multiplicity are proposed mechanisms for producing the mass-dependent age gradient, but the relative importance of starspots versus multiplicity remains unclear. We performed a synthetic red optical low-resolution spectroscopic survey of a simulated analog to a 10 Myr stellar association including mass-dependent multiplicity statistics and age-dependent starspot coverage fractions. We found that undetected starspots alone do not produce an apparent mass-dependent age gradient, but instead uniformly reduce the average measured age of the population. We also found that binaries continue to produce an apparent mass-dependent age gradient and introduce more scatter in the age measurement than spots, but are easily removed from the population as long as there are good distance measurements to each target. We conclude that it is crucial to incorporate treatments of both starspots and undetected stellar multiplicity into isochrone fits of young stellar associations to attain reliable ages.

Evolutionary nature of puffed-up stripped star binaries and their occurrence in stellar populations

The majority of massive stars are formed in multiple systems, and at some point during their life, they interact with their companions via mass transfer. This interaction typically leads to the primary shedding its outer envelope, resulting in the formation of a “stripped star”. Classically, stripped stars are expected to quickly contract to become hot and UV-bright helium stars. Surprisingly, recent optical spectroscopic surveys have unveiled many stripped stars that are much larger and cooler, appearing “puffed up” and overlapping with the Main Sequence (MS) in the Hertzsprung–Russell diagram. Here, we study the evolutionary nature of puffed-up stripped (PS) stars and the duration of this enigmatic phase using the stellar-evolution code MESA. We computed grids of binary models at four metallicities: Solar (Z⊙ = 0.017), Large Magellanic Cloud (LMC, Z = 0.0068), Small Magellanic Cloud (SMC, Z = 0.0034), and Z = 0.1 Z⊙. Contrary to previous assumptions, we find that stripped stars regain thermal equilibrium shortly after the end of mass transfer and maintain it during most of the PS phase. Their further contraction towards hot and compact He stars is determined by the rate at which the residual H-rich envelope is depleted, with the main agents being H-shell burning (dominant for M ≲ 50 M⊙) and mass-loss in winds. The duration of the PS star phase is approximately 10% of the core-He burning lifetime (1% total lifetime) and up to 100 times more than the thermal timescale. It decreases with increasing mass and luminosity and increases with metallicity. We explored several factors influencing PS star lifetimes: orbital period, mass ratio, winds, and semiconvection. We further carried out a simple binary population synthesis estimation, finding that ∼0.5–0.7% of all the stars with log (L/L⊙) > 3.7 may, in fact, be PS stars. Our results indicate that tens to hundreds of PS stars in post-interaction binaries may be hiding in the MS population, disguised as ‘normal’ stars: ∼100 (∼280) in the SMC (LMC) and ∼1500 in the Milky Way. Their true nature may be revealed by spectroscopically measured low surface gravities, high N enrichment, and likely slow rotation rates.

The Lockman–SpReSO project

Context. The Lockman–SpReSO project is an optical spectroscopic survey of 956 far-infrared (FIR) objects within the Lockman Hole field limited by magnitude RC(AB) < 24.5. Fe II and Mg II absorption lines have been detected in 21 out of 456 objects with a determined spectroscopic redshift in the catalogue. The redshifts of these objects are in the range 0.5 ≲ z ≲ 1.44. Aims. The aim of this study is to investigate material ejection from star-forming regions and material infall into galaxies by analysing the Fe II and Mg II absorption lines. Additionally, we explore whether the correlations found in previous studies between these galactic wind velocities, line equivalent widths (EWs), and galaxy properties such as stellar mass (M*), star formation rate (SFR), and specific star formation rate (sSFR) are valid for a sample with FIR-selected objects. The objects analysed span an M* range of 9.89 < log(M*/M⊙) < 11.50 and an SFR range of 1.01 < log(SFR) < 2.70. Methods. We performed measurements of the Mg IIλλ2796, 2803, Mg Iλ2852, Fe IIλλ2374, 82, Fe IIλλ2586, 2600, and Fe IIλ2344 spectral lines present in the spectra of the selected sample to determine the EW and velocity of the flows observed in the star-forming galaxies. Subsequently, we conducted 107 bootstrap simulations using the Spearman’s rank correlation coefficient (ρs) to explore correlations with galaxy properties. Furthermore, we calculated the covering factor, gas density, and optical depth for the measured Fe II doublets. Results. Our analysis reveals strong correlations between the EW of Mg II lines and both M* (ρs = 0.43, 4.5σ) and SFR (ρs = 0.42, 4.4σ). For the Fe II lines, we observed strong correlations between the EW and SFR (ρs ∼ 0.65, > 3.9σ), with a weaker correlation for M* (ρs ∼ 0.35, > 1.9σ). No notable correlations were found between velocity measurements of the Mg II line and M*, SFR, or sSFR of the objects (ρs ∼ 0.1). However, a strong negative correlation was found between the velocity of the Fe II lines and the SFR of the galaxies (ρs ∼ −0.45, ∼3σ). Our results align with those of previous studies, although only FIR-selected objects are investigated here. Finally, we detect a candidate ‘loitering outflow’, a recently discovered subtype of the iron low-ionisation broad absorption line (FeLoBAL) quasars, at a redshift of z = 1.4399, exhibiting emission in C III] and low line velocities (|v|≲200 km s−1).

LATIS: Constraints on the Galaxy–Halo Connection at z ∼ 2.5 from Galaxy–Galaxy and Galaxy–Lyα Clustering

The connection between galaxies and dark matter halos is often quantified using the stellar mass–halo mass (SMHM) relation. Optical and near-infrared imaging surveys have led to a broadly consistent picture of the evolving SMHM relation based on measurements of galaxy abundances and angular correlation functions. Spectroscopic surveys at z ≳ 2 can also constrain the SMHM relation via the galaxy autocorrelation function and through the cross-correlation between galaxies and Lyα absorption measured in transverse sight lines; however, such studies are very few and have produced some unexpected or inconclusive results. We use ∼3000 spectra of z ∼ 2.5 galaxies from the Lyα Tomography IMACS Survey (LATIS) to measure the galaxy–galaxy and galaxy–Lyα correlation functions in four bins of stellar mass spanning 109.2 ≲ M */M ⊙ ≲ 1010.5. Parallel analyses of the MultiDark N-body and ASTRID hydrodynamic cosmological simulations allow us to model the correlation functions, estimate covariance matrices, and infer halo masses. We find that results of the two methods are mutually consistent and broadly accord with standard SMHM relations. This consistency demonstrates that we are able to measure and model Lyα transmission fluctuations δ F in LATIS accurately. We also show that the galaxy–Lyα cross-correlation, a free by-product of optical spectroscopic galaxy surveys at these redshifts, can constrain halo masses with similar precision to galaxy–galaxy clustering.

Supernova Remnants in the Irregular Galaxy NGC 4449

The nearby irregular galaxy NGC 4449 has a star formation rate of ∼0.4M ☉ yr−1 and should host of order 70 supernova remnants (SNRs) younger than 20,000 yr, a typical age for SNRs expanding into an interstellar medium (ISM) with a density of 1 cm−3 to reach the radiative phase. We have carried out an optical imaging and spectroscopic survey in an attempt to identify these SNRs. This task is challenging because diffuse gas with elevated ratios of [S ii]:Hα is omnipresent in NGC 4449, causing confusion when using this common diagnostic for SNRs. Using narrowband interference-filter images, we first identified 49 objects that have elevated [S ii]:Hα ratios compared to nearby H ii regions. Using Gemini-N and GMOS, we then obtained high-resolution spectra of 30 of these SNR candidates, 25 of which have [S ii]:Hα ratios greater than 0.5. Of these, 15 nebulae are almost certainly SNRs, based on a combination of characteristics: higher [O i]:Hα ratios and broader line widths than observed from H ii regions. The remainder are good candidates as well, but need additional confirmation. Surprisingly, despite having superior imaging and spectroscopic data sets to examine, we are unable to confirm most of the candidates suggested by Leonidaki et al. While NGC 4449 is likely an extreme case because of the high surface brightness and elevated [S ii]:Hα ratio of diffuse gas, it highlights the need for sensitive high-resolution optical spectroscopy, or high spatial resolution radio or X-ray observations that can ensure accurate SNR identifications in external galaxies.

VPNEP: Detailed characterization of TESS targets around the Northern Ecliptic Pole

Context.We embarked on a high-resolution optical spectroscopic survey of bright Transiting Exoplanet Survey Satellite (TESS) stars around the Northern Ecliptic Pole (NEP), dubbed the Vatican-Potsdam-NEP (VPNEP) survey.Aims.Our NEP coverage comprises ≈770 square degrees with 1067 stars, of which 352 are bona fide dwarf stars and 715 are giant stars, all cooler than spectral type F0 and brighter thanV= 8m.5. Our aim is to characterize these stars for the benefit of future studies in the community.Methods.We analyzed the spectra via comparisons with synthetic spectra. Particular line profiles were analyzed by means of eigenprofiles, equivalent widths, and relative emission-line fluxes (when applicable).Results.TwoR= 200 000 spectra were obtained for each of the dwarf stars with the Vatican Advanced Technology Telescope (VATT) and the Potsdam Echelle Polarimetric and Spectroscopic Instrument (PEPSI), with typically threeR= 55 000 spectra obtained for the giant stars with STELLA and the STELLA Echelle Spectrograph (SES). Combined withV-band magnitudes,GaiaEDR3 parallaxes, and isochrones from the Padova and Trieste Stellar Evolutionary Code, the spectra can be used to obtain radial velocities, effective temperatures, gravities, rotational and turbulence broadenings, stellar masses and ages, and abundances for 27 chemical elements, as well as isotope ratios for lithium and carbon, line bisector spans, convective blue-shifts (when feasible), and levels of magnetic activity from Hα, Hβ, and the Ca IIinfrared triplet. In this initial paper, we discuss our analysis tools and biases, presenting our first results from a pilot sub-sample of 54 stars (27 bona-fide dwarf stars observed with VATT+PEPSI and 27 bona-fide giant stars observed with STELLA+SES) and making all reduced spectra available to the community. We carried out a follow-up error analysis, including systematic biases and standard deviations based on a joint target sample for both facilities, as well as a comparison with external data sources.

Cold gas mass measurements for the era of large optical spectroscopic surveys

ABSTRACT Gas plays an important role in many processes in galaxy formation and evolution, but quantifying the importance of gas has been hindered by the challenge to measure gas masses for large samples of galaxies. Data sets of direct atomic and molecular gas measurements are sufficient to establish simple scaling relations, but often not large enough to quantify three-parameter relations, or second-order dependences. As an alternative approach, we derive here indirect cold gas measurements from optical emission lines using photoionization models for galaxies in the Sloan Digital Sky Survey (SDSS) main galaxy sample and the PHANGS-MUSE survey. We calibrate the gas surface density measurements using xCOLD GASS and PHANGS-ALMA molecular gas measurements to ensure that our measurements are reliable. We demonstrate the importance of taking into account the scale dependence of the relation between optical depth (τV) and gas surface density (Σgas) and provide a general prescription to estimate Σgas from τV, metallicity, and the dust-to-metal ratio, at any arbitrary physical resolution. To demonstrate that the indirect cold gas masses are accurate enough to quantify the role of gas in galaxy evolution, we study the mass–metallicity relation of SDSS galaxies and show that as a third parameter gas mass is better than star formation rate at reducing the scatter of the relation, as predicted by models and simulations.

A White Dwarf–Main-sequence Binary Unveiled by Time-domain Observations from LAMOST and TESS

We report a single-lined white dwarf–main-sequence binary system, LAMOST J172900.17+652952.8, which is discovered by the Large Sky Area Multi-Object Fibre Spectroscopic Telescope (LAMOST)’s medium-resolution time-domain surveys. The radial-velocity semi-amplitude and orbital period of the optical visible star are measured by using follow-up observations with the Palomar 200 inch telescope and light curves from the Transiting Exoplanet Survey Satellite (TESS). Thus the mass function of the invisible candidate white dwarf is derived, f(M 2) = 0.120 ± 0.003 M ⊙. The mass of the visible star is measured based on a spectral energy distribution fitting, M 1 = . Hence, the mass of its invisible companion is M 2 ≳ 0.63 M ⊙. The companion ought to be a compact object rather than a main-sequence star owing to the mass ratio q = M 2/M 1 ≳ 0.78 and the single-lined spectra. The compact object is likely to be a white dwarf if the inclination angle is not small, i ≳ 40°. By using the Galaxy Evolution Explorer (GALEX) near-UV flux, the effective temperature of the white dwarf candidate is constrained as ≲ 12,000–13,500 K. It is difficult to detect white dwarfs which are outshone by their bright companions via single-epoch optical spectroscopic surveys. Therefore, optical time-domain surveys can play an important role in unveiling invisible white dwarfs and other compact objects in binaries.

A New Method to Reduce Near-Infrared Data of Palomar Observatory

Compared to large optical spectral surveys, near infrared spectra is overwhelmingly outnumbered. Sloan Digital Sky Survey (SDSS), the largest optical spectroscopic survey, has published about 5,789,200 optical spectra in its latest data release DR16, while no large scale near infrared spectra survey has been performed. Typical studies in near infrared are based on limited samples of a few tens to hundreds of near infrared spectra. TripleSpec is one of the workhorses near Infrared (NIR) spectrograph, mounted on the Palomar 200-inch telescope (P200). We have acquired more than 200 new quasar NIR spectra during the past years, compared to the largest sample ever compiled. During the reduction of our latest TripleSpec data, we found that the well accepted data reduction pipeline Spextool (citation: 835) completely ignored one of the five spectral orders (Order 7) of the raw data, probably due to its low throughput. Astronomical observations are always photons starving, we investigate the possibility of incorporation of the photons on Order 7 by developing a new data reduction pipeline. We test the new pipeline on our new quasar NIR data. Compared to the results from Spextool, 1. The pipeline is also featured by its full automaticity, 2. the wavelength coverage extended from 1-2.5 μm to 0.9-2.5 μm, revealing new features such as rest frame Pa ε, [S III] 9531, [C I] 9824, [C I] 9850, [S VIII] 9910, as well as redshifted spectral features for higher redshift targets. 3. The median signal to noise ratio for 0.9-1.1 μm have been elevated by a factor of 2. The pipeline could be easily generalized to accommodate other popular NIR instruments such as IRTF/SPEX, we plan to publish the pipeline and the 200 new quasar NIR spectra to the community. Keck/NIRSPEC.

MALS SALT-NOT Survey of MIR-selected Powerful Radio-bright AGN at 0 < z < 3.5

We present results of an optical spectroscopic survey using SALT and the Nordic Optical Telescope to build a Wide-field Infrared Survey Explorer mid-infrared color-based, dust-unbiased sample of powerful radio-bright (>200 mJy at 1.4 GHz) active galactic nuclei (AGN) for the MeerKAT Absorption Line Survey (MALS). Our sample has 250 AGN (median z = 1.8) showing emission lines, 26 with no emission lines, and 27 without optical counterparts. Overall, our sample is fainter (Δi = 0.6 mag) and redder (Δ(g−i) = 0.2 mag) than radio-selected quasars, and representative of fainter quasar population detected in optical surveys. About 20% of the sources are narrow-line AGN (NLAGN)–65% of these, at z < 0.5 are galaxies without strong nuclear emission, and 10% at z > 1.9, have emission line ratios similar to radio galaxies. The farthest NLAGN in our sample is M1513-2524 (z em = 3.132), and the largest radio source (size ∼330 kpc) is M0909-3133 (z em = 0.884). We discuss in detail 110 AGN at 1.9 < z < 3.5. Despite representing the radio loudest quasars (median R = 3685), their Eddington ratios are similar to the Sloan Digital Sky Survey quasars having lower R. We detect four C iv broad-absorption line (BAL) QSOs, all among AGN with least R, and highest black hole masses and Eddington ratios. The BAL detection rate (%) is consistent with that seen in extremely powerful (L 1.4GHz > 1025 W Hz−1) quasars. Using optical light curves, radio polarization, and γ-ray detections, we identify seven high-probability BL Lacertae objects. We also summarize the full MALS footprint to search for H i 21 cm and OH 18 cm lines at z < 2.

First HETDEX Spectroscopic Determinations of Lyα and UV Luminosity Functions at z = 2–3: Bridging a Gap between Faint AGNs and Bright Galaxies

Abstract We present Lyα and ultraviolet (UV)-continuum luminosity functions (LFs) of galaxies and active galactic nuclei (AGNs) at z = 2.0–3.5 determined by the untargeted optical spectroscopic survey of the Hobby–Eberly Telescope Dark Energy Experiment (HETDEX). We combine deep Subaru imaging with HETDEX spectra resulting in 11.4 deg2 of fiber spectra sky coverage, obtaining 18,320 galaxies spectroscopically identified with Lyα emission, 2126 of which host type 1 AGNs showing broad (FWHM &gt; 1000 km s−1) Lyα emission lines. We derive the Lyα (UV) LF over 2 orders of magnitude covering bright galaxies and AGNs in log L Ly α / [ erg s − 1 ] = 43.3 – 45.5 (−27 &lt; M UV &lt; −20) by the 1/V max estimator. Our results reveal that the bright-end hump of the Lyα LF is composed of type 1 AGNs. In conjunction with previous spectroscopic results at the faint end, we measure a slope of the best-fit Schechter function to be α Sch = − 1.70 − 0.14 + 0.13 , which indicates that α Sch steepens from z = 2–3 toward high redshift. Our UV LF agrees well with previous AGN UV LFs and extends to faint-AGN and bright-galaxy regimes. The number fraction of Lyα-emitting objects (X LAE) increases from M UV * ∼ − 21 to bright magnitude due to the contribution of type 1 AGNs, while previous studies claim that X Lyα decreases from faint magnitudes to M UV * , suggesting a valley in the X Lyα –magnitude relation at M UV * . Comparing our UV LF of type 1 AGNs at z = 2–3 with those at z = 0, we find that the number density of faint (M UV &gt; −21) type 1 AGNs increases from z ∼ 2 to 0, as opposed to the evolution of bright (M UV &lt; −21) type 1 AGNs, suggesting AGN downsizing in the rest-frame UV luminosity.

R-process enhancements of Gaia-Enceladus in GALAH DR3

Context. The dominant site of production of r-process elements remains unclear despite recent observations of a neutron star merger. Observational constraints on the properties of the sites can be obtained by comparing r-process abundances in different environments. The recent Gaia data releases and large samples from high-resolution optical spectroscopic surveys are enabling us to compare r-process element abundances between stars formed in an accreted dwarf galaxy, Gaia-Enceladus, and those formed in the Milky Way. Aims. Our aim is to understand the origin of r-process elements in Gaia-Enceladus. Methods. We first constructed a sample of stars so that our study on Eu abundance is not affected by the detection limit. We then kinematically selected 76 Gaia-Enceladus stars and 81 in situ stars from the Galactic Archaeology with HERMES (GALAH) DR3, of which 47 and 55 stars, respectively, can be used to study Eu reliably. Results. Gaia-Enceladus stars clearly show higher ratios of [Eu/Mg] than in situ stars. High [Eu/Mg] along with low [Mg/Fe] are also seen in relatively massive satellite galaxies such as the LMC, Fornax, and Sagittarius dwarfs. On the other hand, unlike these galaxies, Gaia-Enceladus does not show enhanced [Ba/Eu] or [La/Eu] ratios suggesting a lack of significant s-process contribution. From comparisons with simple chemical evolution models, we show that the high [Eu/Mg] of Gaia-Enceladus can naturally be explained by considering r-process enrichment by neutron-star mergers with delay time distribution that follows a power-law similar to type Ia supernovae but with a shorter minimum delay time.

Characterization of 92 southern TESS candidate planet hosts and a new photometric [Fe/H] relation for cool dwarfs

ABSTRACT We present the results of a medium-resolution optical spectroscopic survey of 92 cool ($3000 \,\mathrm{ K}\lesssim T_{\rm eff} \lesssim 4500\,$K) southern TESS candidate planet hosts, and describe our spectral fitting methodology used to recover stellar parameters. We quantify model deficiencies at predicting optical fluxes, and while our technique works well for Teff, further improvements are needed for [Fe/H]. To this end, we developed an updated photometric [Fe/H] calibration for isolated main-sequence stars built upon a calibration sample of 69 cool dwarfs in binary systems, precise to $\pm 0.19\,$dex, from supersolar to metal poor, over 1.51 &amp;lt; Gaia (BP − RP) &amp;lt; 3.3. Our fitted Teff and R⋆ have median precisions of 0.8 per cent and 1.7 per cent, respectively, and are consistent with our sample of standard stars. We use these to model the transit light curves and determine exoplanet radii for 100 candidate planets to 3.5 per cent precision and see evidence that the planet radius gap is also present for cool dwarfs. Our results are consistent with the sample of confirmed TESS planets, with this survey representing one of the largest uniform analyses of cool TESS candidate planet hosts to date.

Innovations in science and technology with the LAMOST

<p indent=0mm>The LAMOST (Large Sky Area Multi-Object Fiber Spectroscopic Telescope) is a special reflecting Schmidt telescope with an effective aperture of <sc>3.6–4.9 m,</sc> a focal length of <sc>20 m</sc> and a field of view (FOV) of 5 degrees. Its optical system is designed with three major components, namely Ma, Mb, and a focal surface. The correcting mirror Ma <sc>(5.72 m</sc>×<sc>4.40 m)</sc> is made up of 24 hexagonal plane sub-mirrors; the primary mirror Mb <sc>(6.67 m</sc>×<sc>6.05 m)</sc> has 37 hexagonal spherical sub-mirrors. In the process of observation, Ma is capable of synchronously changing its specific aspherical figure to give excellent optical quality. The focal surface has 4000 precisely positioned fibers connected to 16 spectrographs with a distributive parallel-controllable fiber positioning system, thus LAMOST can observe up to 4000 objects simultaneously. LAMOST adopts an innovative active optics technique, achieved by changing its mirror surface continuously to achieve a series different reflecting Schmidt systems. This novel concept and unique design broke through the bottleneck of former astronomical instruments that could not combine a large clear aperture with a wide FOV. The new active optics system creatively developed in LAMOST is used for correcting Ma, which is a combination of the thin deformable mirror active and segmented active optics. The distributive parallel-controllable fiber positioning technique is also an advanced technology which solved the problem of simultaneously and accurately locating 4000 astronomical objects. LAMOST opened up the ability to have a large scale optical fiber spectroscopic sky survey with as many as 4000 optical fibers. The LAMOST project was approved as a national major science project by the Chinese government in April 1997. LAMOST started to be constructed in August 2001. The installation of all LAMOST systems was completed on time in August 2008. The telescope is located at the Xinglong Station of National Astronomical Observatories, Chinese Academy of Sciences (NAOC). After the two years commissioning period starting in 2009, a pilot spectroscopic survey was performed using LAMOST from October 2011 to June 2012. The regular survey of LAMOST began in September 2012. Up to June 2020, LAMOST has obtained more than 17 million spectra of celestial objects, in which the stellar parameters of 7.7 million stars were obtained. The data archive of LAMOST would be an exceedingly valuable resource for a variety of astronomical fields. It would be used as a “digital Galaxy” to understand our Galaxy. As the telescope of the highest spectrum acquiring rate, LAMOST breaks through the “bottleneck” of spectroscopic observation in astronomy, becoming the most powerful spectroscopic survey telescope for researches of wide field and large sample astronomy. LAMOST yields a unique data set of more than 10 million stellar spectra and makes a substantial contribution to the study of stellar astrophysics and the research of the Galaxy, such as stellar populations, chemical composition, kinematics and structure of the disk and the halo, gravitational potential and dark matter distribution in the Galaxy, interstellar dust extinction, extremely metal-poor and hyper-velocity stars, and to ultimately advance our understanding of the assemblage of the Milky Way and other galaxies and the origin of regularity and diversity of their properties.

ALMaQUEST. IV. The ALMA-MaNGA QUEnching and STar Formation (ALMaQUEST) Survey

Abstract The ALMaQUEST (ALMA-MaNGA QUEnching and STar formation) survey is a program with spatially resolved 12CO(1−0) measurements obtained with the Atacama Large Millimeter Array (ALMA) for 46 galaxies selected from the Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) DR15 optical integral-field spectroscopic survey. The aim of the ALMaQUEST survey is to investigate the dependence of star formation activity on the cold molecular gas content at kiloparsec scales in nearby galaxies. The sample consists of galaxies spanning a wide range in specific star formation rate (sSFR), including starburst (SB), main-sequence (MS), and green valley (GV) galaxies. In this paper, we present the sample selection and characteristics of the ALMA observations and showcase some of the key results enabled by the combination of spatially matched stellar populations and gas measurements. Considering the global (aperture-matched) stellar mass, molecular gas mass, and star formation rate of the sample, we find that the sSFR depends on both the star formation efficiency (SFE) and the molecular gas fraction ( ), although the correlation with the latter is slightly weaker. Furthermore, the dependence of sSFR on the molecular gas content (SFE or ) is stronger than that on either the atomic gas fraction or the molecular-to-atomic gas fraction, albeit with the small Hi sample size. On kiloparsec scales, the variations in both SFE and within individual galaxies can be as large as 1–2 dex, thereby demonstrating that the availability of spatially resolved observations is essential to understand the details of both star formation and quenching processes.

The ALMA Spectroscopic Survey in the Hubble Ultra Deep Field: Constraining the Molecular Content at log(M*/M⊙) ∼ 9.5 with CO Stacking of MUSE-detected z ∼ 1.5 Galaxies

Abstract We report molecular gas mass estimates obtained from a stacking analysis of CO line emission in the ALMA Spectroscopic Survey (ASPECS) using the spectroscopic redshifts from the optical integral field spectroscopic survey by the Multi Unit Spectroscopic Explorer (MUSE) of the Hubble Ultra Deep Field (HUDF). Stacking was performed on subsets of the sample of galaxies classified by their stellar mass and position relative to the main-sequence relation (on, above, below). Among all the CO emission lines, from CO(2–1) to CO(6–5), with redshifts accessible via the ASPECS Band 3 and the MUSE data, CO(2–1) provides the strongest constraints on the molecular gas content. We detect CO(2–1) emission in galaxies down to stellar masses of . Below this stellar mass, we present a new constraint on the molecular gas content of main-sequence galaxies by stacking based on the MUSE detections. We find that the molecular gas mass of main-sequence galaxies continuously decreases with stellar mass down to . Assuming a metallicity-based CO–to–H 2 conversion factor, the molecular gas-to-stellar mass ratio from to ∼10.0 does not seem to decrease as fast as for , which is in line with simulations and studies at lower redshift. The inferred molecular gas density of MUSE-selected galaxies at is comparable with the one derived in the HUDF with a different CO selection. Using the MUSE data we recover most of the CO emission in our deep ALMA observations through stacking, demonstrating the synergy between volumetric surveys obtained at different wave bands.

A method for checking high-redshift identification of radio AGNs

ABSTRACT In large-scale optical spectroscopic surveys, there are many objects found to have multiple redshift measurements due to the weakness of their emission lines and the different automatic identification algorithms used. These include some suspicious high-redshift $(z \gtrsim 5)$ active galactic nuclei (AGNs). Here, we present a method for inspecting the high-redshift identification of such sources provided that they are radio-loud and have very long baseline interferometry (VLBI) imaging observations of their milli-arcsec (mas) scale jet structure available at multiple epochs. The method is based on the determination of jet component proper motions, and the fact that the combination of jet physics (the observed maximal values of the bulk Lorentz factor) and cosmology (the time dilation of observed phenomena in the early Universe) constrain the possible values of apparent proper motions. As an example, we present the case of the quasar J2346 + 0705 that was reported with two different redshifts, z1 = 5.063 and z2 = 0.171, in the literature. We measured the apparent proper motions (μ) of three components identified in its radio jet by utilizing VLBI data taken from 2014 to 2018. We obtained μJ1 = 0.334 ± 0.099 mas yr−1, μJ2 = 0.116 ± 0.029 mas yr−1, and μJ3 = 0.060 ± 0.005 mas yr−1. The maximal proper motion is converted to an apparent transverse speed of $\beta _{\rm app} = 41.2\pm 12.2\, c$, if the source is at redshift 5.063. This value exceeds the blazar jet speeds known to date. This and other arguments suggest that J2346 + 0705 is hosted by a low-redshift galaxy. Our method may be applicable for other high-redshift AGN candidates lacking unambiguous spectroscopic redshift determination or having photometric redshift estimates only, but showing prominent radio jets allowing for VLBI measurements of fast jet proper motions.

High-redshift damped Ly α absorbing galaxy model reproducing the N H I − Z distribution

ABSTRACT We investigate how damped Lyman α absorbers (DLAs) at $z$ ∼ 2 − 3, detected in large optical spectroscopic surveys of quasars, trace the population of star-forming galaxies. Building on previous results, we construct a model based on observed and physically motivated scaling relations in order to reproduce the bivariate distributions of metallicity, Z, and H i column density, $N_{\rm H\, \rm{\small {I}}}$. Furthermore, the observed impact parameters for galaxies associated to DLAs are in agreement with the model predictions. The model strongly favours a metallicity gradient, which scales with the luminosity of the host galaxy, with a value of γ* = −0.019 ± 0.008 dex kpc−1 for L* galaxies that gets steeper for fainter galaxies. We find that DLAs trace galaxies over a wide range of galaxy luminosities, however, the bulk of the DLA cross-section arises in galaxies with L ∼ 0.1 L* at $z$ ∼ 2.5 consistent with numerical simulations.

Abundance Ratios in GALAH DR2 and Their Implications for Nucleosynthesis

Abstract Using a sample of 70,924 stars from the second data release of the GALAH optical spectroscopic survey, we construct median sequences of [X/Mg] versus [Mg/H] for 21 elements, separating the high-α/“low-Ia” and low-α/“high-Ia” stellar populations through cuts in [Mg/Fe]. Previous work with the near-IR APOGEE survey has shown that such sequences are nearly independent of location in the Galactic disk, implying that they are determined by stellar nucleosynthesis yields with little sensitivity to other chemical evolution aspects. The separation between the two [X/Mg] sequences indicates the relative importance of prompt and delayed enrichment mechanisms, while the sequences’ slopes indicate metallicity dependence of the yields. GALAH and APOGEE measurements agree for some of their common elements, but differ in sequence separation or metallicity trends for others. GALAH offers access to nine new elements. We infer that about 75% of solar C comes from core-collapse supernovae and 25% from delayed mechanisms. We find core-collapse fractions of 60%–80% for the Fe-peak elements Sc, Ti, Cu, and Zn, with strong metallicity dependence of the core-collapse Cu yield. For the neutron capture elements Y, Ba, and La, we infer large delayed contributions with non-monotonic metallicity dependence. The separation of the [Eu/Mg] sequences implies that at least ∼30% of Eu enrichment is delayed with respect to star formation. We compare our results to predictions of several supernova and asymptotic giant branch yield models; C, Na, K, Mn, and Ca all show discrepancies with models that could make them useful diagnostics of nucleosynthesis physics.