Low Column Density Research Articles

Origin of Weak Mg ii and Higher-ionization Absorption Lines in Outflows from Intermediate-redshift Dwarf Galaxies

Abstract Observations at intermediate redshifts reveal the presence of numerous compact, weak Mg ii absorbers with near to supersolar metallicities, often surrounded by extended regions that produce C iv and/or O vi absorption, in the circumgalactic medium at large impact parameters from luminous galaxies. Their origin and nature remain unclear. We hypothesize that undetected satellite dwarf galaxies are responsible for producing some of these weak Mg ii absorbers. We test our hypothesis using gas dynamical simulations of galactic outflows from a dwarf galaxy with a halo mass of 5 × 109 M ⊙, as might be falling into a larger L* halo at z = 2. We find that thin, filamentary, weak Mg ii absorbers (≲100 pc) are produced in two stages: (1) when shocked core-collapse supernova (SN II)–enriched gas descending in a galactic fountain gets shock compressed by upward flows driven by subsequent SN II and cools (phase 1) and, later, (2) during an outflow driven by Type Ia supernovae that shocks and sweeps up pervasive SN II–enriched gas, which then cools (phase 2). The Mg ii absorbers in our simulations are continuously generated by shocks and cooling with moderate metallicity ∼0.1–0.2 Z ⊙ but low column density <1012 cm−2. They are also surrounded by larger (0.5–1 kpc) C iv absorbers that seem to survive longer. Larger-scale (>1 kpc) C iv and O vi clouds are also produced in both expanding and shocked SN II–enriched gas. Observable ion distributions from our models appear well converged at our standard resolution (12.8 pc). Our simulation highlights the possibility of dwarf galactic outflows producing highly enriched multiphase gas.

A Census of the Extended Neutral Hydrogen around 18 MHONGOOSE Galaxies

Abstract We present the analysis of the diffuse, low column density H i environment of 18 MHONGOOSE galaxies. We obtained deep observations with the Robert C. Byrd Green Bank Telescope and reached down to a 3σ column density detection limit of N HI = 6.3 × 1017 cm−2 over a 20 km s−1 line width. We analyze the environment around these galaxies, with a focus on H i gas that reaches column densities below N HI = 1019 cm−2. We calculate the total amount of H i gas in and around the galaxies, revealing that nearly all of these galaxies contained excess H i outside of their disks. We quantify the amount of diffuse gas in the maps of each galaxy, defined by H i gas with column densities below 1019 cm−2, and find a large spread in percentages of diffuse gas. However, by binning the percentage of diffuse H i into quarters, we find that the bin with the largest number of galaxies is the lowest quartile (0%–25% diffuse H i). We identified several galaxies that may be undergoing gas accretion onto the galaxy disk using multiple methods of analysis, including azimuthally averaging column densities beyond the disk, and identifying structure within our integrated intensity (moment 0) maps. We measured H i mass outside the disks of most of our galaxies, with rising cumulative flux even at large radii. We also find a strong correlation between the fraction of diffuse gas in a galaxy and its baryonic mass, and we test this correlation using both Spearman and Pearson correlation coefficients. We see evidence of a dark matter halo mass threshold of M halo ∼ 1011.1 M ☉ in which galaxies with high fractions of diffuse H i all reside below. It is in this regime that cold-mode accretion should dominate. Finally, we suggest a rotation velocity of v rot ∼ 80 km s−1 as an upper threshold to find diffuse-gas-dominated galaxies.

Pair lines of sight observations of multiphase gas bearing O vi in a galaxy environment

ABSTRACT Using HST/COS spectra of the twin quasar lines of sight Q 0107–025A & Q 0107–025B, we report on the physical properties, chemical abundances, and transverse sizes of a multiphase medium in a galaxy field at z = 0.399. The angular separation between the quasars corresponds to a physical separation of 520 kpc at the absorber redshift. The absorber towards Q 0107–025B is a partial Lyman limit system (pLLS) with $\log N({\mathrm{H}}{\small I})/\hbox{cm$^{-2}$}\approx 16.8$. The H i column density in the absorber along the other sightline is ≈ 2 orders of magnitude lower. The O vi along both sightlines have comparable column densities and broad b-values (b > 30 km s−1) whereas the low ionization lines are considerably narrower. The low ionization gas is inconsistent with the O vi when modelled assuming photoionization from the same phase. In both lines of sight, O vi and the broad H i coinciding, are best explained through collisional ionization in a cooling plasma with solar metallicity. Ionization models infer 1/10th solar metallicity for the pLLS and solar metallicity for the lower column density absorber along the other sightline. Within ± 250 km s−1 and 2 Mpc of projected distance from the sightlines 12 galaxies are identified, of which five are within 500 kpc. The twin sightlines are at normalized impact parameters of ρ ∼ 1.1Rvir, and ρ ∼ 0.8Rvir from a M* ∼ 1010.7 M⊙, L ∼ 0.07L*, and star formation rate (SFR) < 0.1 M⊙ yr−1 galaxy, potentially probing its CGM (circumgalactic medium). The next closest in normalized separation are a dwarf galaxy with M* ∼ 108.7 M⊙, and SFR ∼ 0.06 M⊙ yr−1, and an intermediate mass galaxy with M* ∼ 1010.0 M⊙, and SFR ∼ 3 M⊙ yr−1. Along both sightlines, O vi could be either tracing narrow transition temperature zones at the interface of low ionization gas and the hot halo of nearest galaxy, or a more spread-out warm component that could be gas bound to the circumgalactic halo or the intragroup medium. The latter scenarios lead to a warm gas mass limit of M ≳ 4.5 × 109 M⊙.

Formation of Dust Filaments in the Diffuse Envelopes of Molecular Clouds

Abstract The path to understanding star formation processes begins with the study of the formation of molecular clouds. The outskirts of these clouds are characterized by low column densities that allow the penetration of ultraviolet radiation, resulting in a nonnegligible ionization fraction and the charging of the small dust grains that are mixed with the gas; this diffuse phase is then coupled to the ambient magnetic field. Despite the general assumption that dust and gas are tightly correlated, several observational and theoretical studies have reported variations in the dust-to-gas ratio toward diffuse and cold clouds. In this work, we present the implementation of a new charged particles module for analyzing the dust dynamics in molecular cloud envelopes. We study the evolution of a single population of small charged grains (0.05 μm) in the turbulent, magnetized molecular cloud envelope using this module. We show that variations in the dust-to-gas ratio arise due to the coupling of the grains with the magnetic field, forming elongated dust structures decoupled from the gas. This emphasizes the importance of considering the dynamics of charged dust when simulating the different phases of the interstellar medium, especially for star formation studies.

The Kiloparsec-scale Fe Ka Emission in the Compton-thin Seyfert 2 Galaxy NGC 4388 Resolved by Chandra

Abstract We present imaging and spectral analysis of the combined ∼48 ks Chandra observations of Seyfert 2 galaxy NGC 4388. Compared with previous studies, three prominent extended X-ray structures around the nucleus on a kiloparsec-scale are well imaged, allowing an in-depth spatially resolved study. Both the extended hard continuum (4–6 keV) and the Fe Kα line (6.2–6.7 keV) show similar morphology, consistent with a scenario where the ionizing emission from the nucleus is reprocessed by circumnuclear cold gas, resulting in a weak reflection continuum and an associated neutral Fe Kα line. This has been seen in other Compton-thick active galactic nuclei (AGN), but NGC 4388 is one of the rare cases with a lower column density (N H < 1.25 × 1024 cm−2) along the line of sight. Significant differences in equivalent width of the Fe Kα emission line (up to a factor of 3) are found for the nuclear and extended regions. Such a difference could be ascribed to different column densities or scattering angles with respect to the line of sight, rather than variations in iron abundances. The northeast and west extended structures appear to be aligned with the larger-scale galactic disk and dust lane in the HST V − H color map, and are located at the peak of molecular gas distribution. The morphology of remaining extended features likely traces edges of a known radio jet, indicating that the outflow at the kiloparsec-scale may have compressed the interstellar gas and produced clumps working as the reflector to enhance line emission. In addition, using [O iv] emission as a proxy of the AGN intrinsic luminosity, we find that both of the extended Fe Kα emission and reflection continuum are linearly correlated with the [O iv] luminosity, which indicates a connection between the central AGN and the extended emission.

Radiation Hydrodynamics of Turbulent H ii Regions in Molecular Clouds: A Physical Origin of LyC Leakage and the Associated Lyα Spectra

Abstract We examine Lyman continuum (LyC) leakage through H ii regions regulated by turbulence and radiative feedback in a giant molecular cloud in the context of fully coupled radiation hydrodynamics (RHD). The physical relations of the LyC escape with H i covering fraction, kinematics, ionizing photon production efficiency, and emergent Lyα line profiles are studied using a series of RHD turbulence simulations performed with ramses-rt. The turbulence-regulated mechanism allows ionizing photons to leak out at early times before the onset of supernova feedback. The LyC photons escape through turbulence-generated low column density channels that are evacuated efficiently by radiative feedback via photoheating-induced shocks across the D-type ionization fronts. The Lyα photons funnel through the photoionized channels along the paths of LyC escape, resulting in a diverse Lyα spectral morphology including narrow double-peaked profiles. The Lyα peak separation is controlled by the residual H i column density of the channels, and the line asymmetry correlates with the porosity and multiphase structure of the H ii region. This mechanism through the turbulent H ii regions can naturally reproduce the observed Lyα spectral characteristics of some of the LyC-leaking galaxies. This RHD turbulence origin provides an appealing hypothesis to explain high LyC leakage from very young (∼3 Myr) star-forming galaxies found in the local universe without need of extreme galactic outflows or supernova feedback. We discuss the implications of the turbulent H ii regions on other nebular emission lines and a possible observational test with the Magellanic System and local blue compact dwarf galaxies as analogs of reionization-era systems.

Characterizing the line emission from molecular clouds

Context. The traditional approach to characterize the structure of molecular clouds is to map their line emission. Aims. We aim to test and apply a stratified random sampling technique that can characterize the line emission from molecular clouds more efficiently than mapping. Methods. We sampled the molecular emission from the Perseus cloud using the H2 column density as a proxy. We divided the cloud into ten logarithmically spaced column density bins, and we randomly selected ten positions from each bin. The resulting 100 cloud positions were observed with the IRAM 30 m telescope, covering the 3 mm-wavelength band and parts of the 2 and 1 mm bands. Results. We focus our analysis on the 11 molecular species (plus isotopologs) detected toward most column density bins. In all cases, the line intensity is tightly correlated with the H2 column density. For the CO isotopologs, the trend is relatively flat, while for high-dipole moment species such as HCN, CS, HCO+, and HNC, the trend is approximately linear. To reproduce this behavior, we developed a cloud model in which the gas density increases with column density, and where most species have abundance profiles characterized by an outer photodissociation edge and an inner freeze-out drop. With this model, we determine that the intensity behavior of the high-dipole moment species arises from a combination of excitation effects and molecular freeze out, with some modulation from optical depth. This quasi-linear dependence with the H2 column density makes the gas at low column densities dominate the cloud-integrated emission. It also makes the emission from most high-dipole moment species proportional to the cloud mass inside the photodissociation edge. Conclusions. Stratified random sampling is an efficient technique for characterizing the emission from whole molecular clouds. When applied to Perseus, it shows that despite the complex appearance of the cloud, the molecular emission follows a relatively simple pattern. A comparison with available studies of whole clouds suggests that this emission pattern may be common.

The hot interstellar medium towards 4U 1820-30: a Bayesian analysis

Context. High-ionisation lines in the soft X-ray band are generally associated to either interstellar hot gas along the line of sight or to photoionised gas intrinsic to the source. In the low-mass X-ray binary 4U 1820-30, the nature of these lines is not well understood. Aims. We aim to characterise the ionised gas present along the line of sight towards 4U 1820-30 producing the X-ray absorption lines of Mg XI, Ne IX, Fe XVII, O VII, and O VIII. Methods. We analysed all the observations available for this source in the XMM-Newton and Chandra archives that were taken with the RGS, HETG, and LETG spectrometers. We accurately examined the high-resolution grating spectra using a standard X-ray analysis procedure based on the C-statistic and using Bayesian parameter inference. We tested two physical models which describe a plasma in either collisional ionisation or photoionisation equilibrium. We adopted the Bayesian model comparison to statistically compare the different combinations of models used for the analysis. Results. We find that the lines are consistent with hot gas in the interstellar medium rather than the intrinsic gas of the X-ray binary. Our best-fit model reveals the presence of a collisionally ionised plasma with a temperature of T = (1.98 ± 0.05) × 106 K. The photoionisation model fails to fit the Fe XVII line (which is detected with a significance of 6.5σ) because of the low column density predicted by the model. Moreover, the low inclination of the binary system is likely the reason for the non-detection of ionised gas intrinsic to the source.

Reconciling escape fractions and observed line emission in Lyman-continuum-leaking galaxies

Context. Finding and elucidating the properties of Lyman-continuum(LyC)-emitting galaxies is an important step in improving our understanding of cosmic reionization. Aims. Although the z ∼ 0.3 − 0.4 LyC emitters found recently show strong optical emission lines, no consistent quantitative photoionization model taking into account the escape of ionizing photons and inhomogenous interstellar medium (ISM) geometry of these galaxies has yet been constructed. Furthermore, it is unclear to what extent these emission lines can be used to distinguish LyC emitters. Methods. To address these questions we construct one- and two-zone photoionization models accounting for the observed LyC escape, which we compare to the observed emission line measurements. The main diagnostics used include lines of [O III], [O II], and [O I] plus sulfur lines ([S II], [S III]) and a nitrogen line ([N II]), which probe regions of different ionization in the ISM. Results. We find that single (one-zone) density-bounded photoionization models cannot reproduce the emission lines of the LyC leakers, as pointed out by earlier studies, because they systematically underpredict the lines of species of low ionization potential, such as [O I] and [S II]. Introducing a two-zone model, with differing ionization parameter and a variable covering fraction and where one of the zones is density-bounded, we show that the observed emission line ratios of the LyC emitters are well reproduced. Furthermore, our model yields LyC escape fractions, which are in fair agreement with the observations and independent measurements. The [O I] λ6300 excess, which is observed in some LyC leakers, can be naturally explained in this model, for example by emission from low-ionization and low-filling-factor gas. LyC emitters with a high escape fraction (fesc ≳ 38%) are deficient both in [O I] λ6300 and in [S II] λλ6716,6731. We also confirm that a [S II] λλ6716,6731 deficiency can be used to select LyC emitter candidates, as suggested earlier. Finally, we find indications for a possible dichotomy in terms of escape mechanisms for LyC photons between galaxies with relatively low (fesc ≲ 10%) and higher escape fractions. Conclusions. We conclude that two-zone photoionization models are sufficient and required to explain the observed emission line properties of z ∼ 0.3 − 0.4 LyC emitters. This is in agreement with UV absorption line studies, which also show the co-existence of regions with high hydrogen column density (i.e., no escape of ionizing photons) and density-bounded or very low column density regions responsible for the observed escape of LyC radiation. These simple but consistent models provide a first step towards the use of optical emission lines and their ratios as quantitative diagnostics of LyC escape from galaxies.

Io's SO2 atmosphere from HST Lyman-α images: 1997 to 2018

The atmosphere of Jupiter's volcanic moon Io consists of mainly sulfur dioxide (SO2), and this main constituent has been studied with a variety of observing techniques across many wavelengths over the years. Here we study absorption by SO2 at the hydrogen Ly-α line (1216 Å) in a large set of images taken by the Space Telescope Imaging Spectrograph (STIS) onboard the Hubble Space Telescope (HST) between 1997 and 2018. An advanced statistical analysis using a Monte-Carlo trial method is applied to derive the SO2 column density from the Ly-α intensity, which includes the uncertainties of the used variables such as solar and background flux. Our analysis produces a probability distribution function of the SO2 column density and highlights some short-comings of the observing technique. Most importantly, the HST/STIS images of the surface-reflected Ly-α flux are only sensitive to SO2 column densities between ~1015 cm−2 and ~5×1016 cm−2. Due to strong non-linearity in the relationship between the SO2 abundance and the Ly-α flux at the low and high values of detected flux, SO2 abundance directly retrieved from the STIS images will generally fall within these boundaries. This explains the relatively low equatorial column density of about 1016 cm−2 reported by previous studies using the Ly-α images (e.g., Feldman et al., 2000; Feaga et al., 2009) compared to other studies (e.g., Spencer et al., 2005; Tsang et al., 2013; Jessup and Spencer, 2015; Lellouch et al., 2015), where the obtained column density is often 1017 cm−2. By assuming a log-normal probability distribution function for the column density, a new estimate of the SO2 column density is then fitted, indirectly accounting for abundances beyond the detectability limits. This method suggests slightly higher equatorial SO2 abundances and much larger upper-limits, revealing that the Ly-α observations are in fact consistent with the higher abundances found in other studies. We then investigate the SO2 abundances at three volcanic sites (Loki, Marduk, Thor), where plumes were observed before and where the sensitivity in our images is comparably high. The observations did not reveal transient changes due to local outgassing at any of the three sites. Finally, the heliocentric distance of Io changed from 4.95 AU to 5.45 AU between the observation dates, potentially allowing us to investigate the influence of solar intensity changes on the SO2 column density via surface frost sublimation. However, the derived error bars are significantly larger than the derived variability, preventing any firm conclusion on seasonal changes and local volcanic outgassing.

MeerKAT HI commissioning observations of MHONGOOSE galaxy ESO 302-G014

Aims. We present the results of three commissioning H I observations obtained with the MeerKAT radio telescope. These observations make up part of the preparation for the forthcoming MHONGOOSE nearby galaxy survey, which is a MeerKAT large survey project that will study the accretion of gas in galaxies and the link between gas and star formation. Methods. We used the available H I data sets, along with ancillary data at other wavelengths, to study the morphology of the MHONGOOSE sample galaxy, ESO 302-G014, which is a nearby gas-rich dwarf galaxy. Results. We find that ESO 302-G014 has a lopsided, asymmetric outer disc with a low column density. In addition, we find a tail or filament of H I clouds extending away from the galaxy, as well as an isolated H I cloud some 20 kpc to the south of the galaxy. We suggest that these features indicate a minor interaction with a low-mass galaxy. Optical imaging shows a possible dwarf galaxy near the tail, but based on the current data, we cannot confirm any association with ESO 302-G014. Nonetheless, an interaction scenario with some kind of low-mass companion is still supported by the presence of a significant amount of molecular gas, which is almost equal to the stellar mass, and a number of prominent stellar clusters, which suggest recently triggered star formation. Conclusions. These data show that MeerKAT produces exquisite imaging data. The forthcoming full-depth survey observations of ESO 302-G014 and other sample galaxies will, therefore, offer insights into the fate of neutral gas as it moves from the intergalactic medium onto galaxies.

Excitation and acceleration of molecular outflows in LIRGs: The extended ESO 320-G030 outflow on 200-pc scales

We used high-spatial resolution (70 pc; 0$ {{\overset{\prime\prime}{.}}} $3) CO multi-transition (J = 1–0, 2–1, 4–3, and 6–5) ALMA data to study the physical conditions and kinematics of the cold molecular outflow in the local luminous infrared galaxy (LIRG) ESO 320-G030 (d = 48 Mpc, LIR/L⊙ = 1011.3). ESO 320-G030 is a double-barred isolated spiral, but its compact and obscured nuclear starburst (SFR ∼ 15 M⊙ yr−1; AV ∼ 40 mag) resembles those of ultra-luminous infrared galaxies (LIR/L⊙ > 1012). In the outflow, the CO(1–0)/CO(2–1) ratio is enhanced with respect to the rest of the galaxy and the CO(4–3) transition is undetected. This indicates that the outflowing molecular gas is less excited than the molecular gas in the nuclear starburst (i.e., outflow launching site) and in the galaxy disk. Non-local thermodynamic equilibrium radiative transfer modeling reveals that the properties of the molecular clouds in the outflow differ from those of the nuclear and disk clouds: The kinetic temperature is lower (Tkin ∼ 9 K) in the outflow, and the outflowing clouds have lower column densities. Assuming a 10−4 CO abundance, the large internal velocity gradients, 60−45+250 km s−1 pc−1, imply that the outflowing molecular clouds are not bound by self-gravity. All this suggests that the life-cycle (formation, collapse, dissipation) of the galaxy disk molecular clouds might differ from that of the outflowing molecular clouds which might not be able to form stars. The low kinetic temperature of the molecular outflow remains constant at radial distances between 0.3 and 1.7 kpc. This indicates that the heating by the hotter ionized outflow phase is not efficient and may favor the survival of the molecular gas phase in the outflow. The spatially resolved velocity structure of the outflow shows a 0.8 km s−1 pc−1 velocity gradient between 190 pc and 560 pc and then a constant maximum outflow velocity of about 700–800 km s−1 up to 1.7 kpc. This could be compatible with a pure gravitational evolution of the outflow, which would require coupled variations of the mass outflow rate and the outflow launching velocity distribution. Alternatively, a combination of ram pressure acceleration and cloud evaporation could explain the observed kinematics and the total size of the cold molecular phase of the outflow.

Quantifying Light Dilution in Ultraviolet Spectroscopic Measurements of Volcanic SO2 Using Dual-Band Modeling

High precision and accuracy in volcanic SO2 emission rate quantification is critical for eruption forecasting and, in combination with in-plume gas ratios, quantifying global volcanic emission inventories. Light dilution, where scattering of ultraviolet light dilutes plume SO2 absorbance signals, has been recognized for more than 50 years, but is still not routinely corrected for during gas flux quantification. Here we use modelling and empirical observations to show that light dilution produces: (i) underestimates in SO2 that can reach a factor of 5 and, at low column densities, cause little impact on standard retrieval fit quality, even for heavily diluted spectra; (ii) retrieved SO2 amounts that are capped by a maximum value regardless of the true amount of SO2, with this maximum amount being reduced as light dilution increases. Global volcanic volatile emission rates may therefore be significantly underestimated. An easily implementable dual-waveband analysis provides a means to detect, and in clear sky conditions, correct dilution effects directly from the spectra, opening a path to more accurate SO2 quantifications.

A Suzaku sample of unabsorbed narrow-line and broad-line Seyfert 1 galaxies – I. X-ray spectral properties

ABSTRACT A sample of narrow-line Seyfert 1 (NLS1) and broad-line Seyfert 1 (BLS1) galaxies observed with Suzaku is presented. The final sample consists of 22 NLS1s and 47 BLS1s, for a total of 69 active galactic nuclei (AGNs) that are all at low redshift (z < 0.5) and exhibit low host-galaxy column densities (<1022${\rm cm}^{-2}\,$). The average spectrum for each object is fit with a toy model to characterize important parameters, including the photon index, soft excess, Compton hump (or hard excess), narrow iron line strength, luminosity, and X-ray Eddington ratio (Lx/LEdd). We confirm previous findings that NLS1s have steeper power laws and higher X-ray Eddington ratios, but also find that NLS1 galaxies have stronger soft and hard excesses than their BLS1 counterparts. Studying the correlations between parameters shows that the soft and hard excesses are correlated for NLS1 galaxies, while no such correlation is observed for BLS1s. Performing a principal component analysis (PCA) on the measured X-ray parameters shows that while the X-ray Eddington ratio is the main source of variations within our sample (PC1), variations in the soft and hard excesses form the second principal component (PC2) and it is dominated by the NLS1s. The correlation between the soft and hard excesses in NLS1 galaxies may suggest a common origin for the two components, such as a blurred reflection model. The presented Suzaku sample of Seyfert 1 galaxies is a useful tool for the analysis of the X-ray properties of AGNs, and for the study of the soft and hard excesses observed in AGNs.

Star-forming versus Quiescent Regions in the Galaxy: A Case Study of ISM Properties Based on 18 cm OH and 12CO (1–0) Observations

Abstract We infer the density of hydrogen nuclei and optical depths associated with molecular gas in front of a star-forming region and in a quiescent region of the Galactic interstellar medium (ISM) using 18 cm OH and archival 3 mm 12CO observations, together with a diffuse cloud model from the literature. The results indicate that the volume densities of hydrogen nuclei are greatest near the star-forming region, intermediate in the CO-bright portions of the quiescent region, and lowest in the CO-dark portions of the quiescent region. Low volume density and low column density appear to be correlated with the presence of CO-dark molecular gas, which is primarily located in the vast space outside of star-forming regions. Molecular gas containing CO may exist in similar quantities both outside of and in the vicinity of star-forming regions; however, the CO emission radiates more efficiently in regions where the ISM is locally compressed, leading to the observational phenomenon of CO-dark and CO-bright molecular gas. We suggest that processes occurring as a natural consequence of star formation such as shock waves and stellar winds lead to compression of the gas into smaller pockets of higher volume density, and such “clumpy” structures will naturally emit more efficiently in CO owing to higher rates of collisional excitation. In this picture, the apparent prominence of CO emission in and near star-forming regions is itself a by-product of the star formation process.

The Dust Attenuation Law in Galaxies

Understanding the properties of dust attenuation curves in galaxies and the physical mechanisms that shape them are among the fundamental questions of extragalactic astrophysics, with great practical significance for deriving the physical properties of galaxies. Attenuation curves result from a combination of dust grain properties, dust content, and the spatial arrangement of dust and different populations of stars. In this review, we assess the state of the field, paying particular attention to extinction curves as the building blocks of attenuation laws. We introduce a quantitative framework to characterize extinction and attenuation curves, present a theoretical foundation for interpreting empirical results, overview an array of observational methods, and review observational results at low and high redshifts. Our main conclusions include the following: ▪ Attenuation curves exhibit a wide range of UV-through-optical slopes, from curves with shallow (Milky Way–like) slopes to those exceeding the slope of the Small Magellanic Cloud extinction curve. ▪ The slopes of the curves correlate strongly with the effective optical opacities, in the sense that galaxies with lower dust column density (lower visual attenuation) tend to have steeper slopes, whereas the galaxies with higher dust column density have shallower (grayer) slopes. ▪ Galaxies exhibit a range of 2175-Å UV bump strengths, including no bump, but, on average, are suppressed compared with the average Milky Way extinction curve. ▪ Theoretical studies indicate that both the correlation between the slope and the dust column as well as variations in bump strength may result from geometric and radiative transfer effects.

Magnetized filamentary gas flows feeding the young embedded cluster in Serpens South

Observations indicate that molecular clouds are strongly magnetized, and that magnetic fields influence the formation of stars. A key observation supporting the conclusion that molecular clouds are significantly magnetized is that the orientation of their internal structure is closely related to that of the magnetic field. At low column densities, the structure aligns parallel with the field, whereas at higher column densities, the gas structure is typically oriented perpendicular to magnetic fields, with a transition at visual extinctions AV ≳ 3 mag. Here we use far-infrared polarimetric observations from the HAWC+ polarimeter on SOFIA to report the discovery of a further transition in relative orientation, that is, a return to parallel alignment at AV ≳ 21 mag in parts of the Serpens South cloud. This transition appears to be caused by gas flow and indicates that magnetic supercriticality sets in near AV ≳ 21 mag, allowing gravitational collapse and star cluster formation to occur even in the presence of relatively strong magnetic fields.

In search of infall motion in molecular clumps II: 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 1021 and 1023 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 0/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 0/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 100 km s−1, which is consistent with previous studies.

The VANDELS survey: Discovery of massive overdensities of galaxies at z > 2

Context. The advent of deep, multi-wavelength surveys, together with the availability of extensive numerical simulations, now allow us for the systematic search and study of (proto)clusters and their surrounding environment as a function of redshift. Aims. We aim to define the environment and to identify overdensities in the VANDELS Chandra Deep Field-South (CDFS) and UKIDSS Ultra Deep Survey (UDS) fields. We want to investigate whether we can use Lyα emission to obtain additional information of the environment properties and whether Lyα emitters show different characteristics as a function of their environment. Methods. We estimated local densities using a three-dimensional algorithm which works in the RA-dec-redshift space. We took advantage of the physical parameters of all the sources in the VANDELS fields to study their properties as a function of environment. In particular, we focused on the rest-frame U − V color to evaluate the stage of evolution of the galaxies located in the overdensities and in the field. Then we selected a sample of 131 Lyα-emitting galaxies (EW(Lyα) > 0 Å), unbiased with respect to environmental density, from the first two seasons of the VANDELS survey to study their location with respect to the over- or under-dense environment and infer whether they are useful tracers of overdense regions. Results. We identify 13 (proto)cluster candidates in the CDFS and nine in the UDS at 2 < z < 4, based on photometric and spectroscopic redshifts from VANDELS and from all the available literature. No significant difference is observed in the rest-frame U − V color between field and galaxies located within the identified overdensities, but the star-forming galaxies in overdense regions tend to be more massive and to have low specific SFRs than in the field. We study the distribution of the VANDELS Lyα emitters (LAEVs) and we find that Lyα emitters lie preferentially outside of overdense regions as the majority of the galaxies with spectroscopic redshifts from VANDELS. The LAEVs in overdense regions tend to have low Lyα equivalent widths and low specific SFRs, and they also tend to be more massive than the LAEVs in the field. Their stacked Lyα profile shows a dominant red peak and a hint of a blue peak. There is evidence that their Lyα emission is more extended and offset with respect to the UV continuum. Conclusions. LAEVs are likely to be influenced by the environment. In fact, our results favour a scenario that implies outflows of low expansion velocities and high HI column densities for galaxies in overdense regions. An outflow with low expansion velocity could be related to the way galaxies are forming stars in overdense regions; the high HI column density can be a consequence of the gravitational potential of the overdensity. Therefore, Lyα-emitting galaxies can provide useful insights on the environment in which they reside.

The Turbulent Gas Structure in the Centers of NGC 253 and the Milky Way

Abstract We compare molecular gas properties in the starbursting center of NGC 253 and the Milky Way Galactic center (GC) on scales of ∼1–100 pc using dendrograms and resolution-, area-, and noise-matched data sets in CO (1–0) and CO (3–2). We find that the size–line width relations in NGC 253 and the GC have similar slope, but NGC 253 has larger line widths by factors of ∼2–3. The σ 2/R dependency on column density shows that, in the GC, on scales of 10–100 pc the kinematics of gas over N > 3 × 1021 cm−2 are compatible with gravitationally bound structures. In NGC 253 this is only the case for column densities N > 3 × 1022 cm−2. The increased line widths in NGC 253 originate in the lower column density gas. This high velocity dispersion, not gravitationally self-bound gas, is likely in transient structures created by the combination of high average densities and feedback in the starburst. The high densities turn the gas molecular throughout the volume of the starburst, and the injection of energy and momentum by feedback significantly increases the velocity dispersion at a given spatial scale over what is observed in the GC.