Giant Molecular Cloud Formation at the Interface of Colliding Supershells in the Large Magellanic Cloud

We investigate the H i envelope of the young, massive GMCs in the star-forming regions N48 and N49, which are located within the high column density H i ridge between two kpc-scale supergiant shells, LMC 4 and LMC 5. New long-baseline H i 21 cm line observations with the Australia Telescope Compact Array (ATCA) were combined with archival shorter baseline data and single dish data from the Parkes telescope, for a final synthesized beam size of 24.75″ by 20.48″, which corresponds to a spatial resolution of ∼ 6 pc in the LMC. It is newly revealed that the H i gas is highly filamentary, and that the molecular clumps are distributed along filamentary H i features. In total 39 filamentary features are identified and their typical width is ∼ 21 (8–49) [pc]. We propose a scenario in which the GMCs were formed via gravitational instabilities in atomic gas which was initially accumulated by the two shells and then further compressed by their collision. This suggests that GMC formation involves the filamentary nature of the atomic medium.

A two-step gravitational cascade for the fragmentation of self-gravitating disks

Self-gravitating disks are believed to play an important role in astrophysics in particular regarding the star and planet formation process. In this context, disks subject to an idealized cooling process, characterized by a cooling timescale β expressed in unit of orbital timescale, have been extensively studied. We take advantage of the Riemann solver and the 3D Godunov scheme implemented in the code Ramses to perform high resolution simulations, complementing previous studies that have used Smoothed Particle Hydrodynamics (SPH) or 2D grid codes. We observe that the critical value of β for which the disk fragments is consistent with most previous results, and is not well converged with resolution. By studying the probability density function of the fluctuations of the column density (∑-PDF), we argue that there is no strict separation between the fragmented and the unfragmented regimes but rather a smooth transition with the probability of apparition of fragments steadily diminishing as the cooling becames less effective. We find that the high column density part of the ∑-PDF follows a simple power law whose slope turns out to be proportional to β and we propose an explanation based on the balance between cooling and heating through gravitational stress. Our explanation suggests that a more efficient cooling requires more heating implying a larger fraction of dense material which, in the absence of characteristic scales, results in a shallower scale-free power law. We propose that the gravitational cascade proceeds in two steps, first the formation of a dense filamentary spiral pattern through a sequence of quasi-static equilibrium triggered by the viscous transport of angular momentum, and second the collapse alongside these filaments that eventually results in the formation of bounded fragments.

Observations of plumes from the 2019 Raikoke eruption with the Infrared Atmospheric Sounding Interferometer (IASI)

<p>Raikoke, a remote volcano in the Kuril Islands, erupted on the 21<sup>st</sup> June 2019. The eruption injected significant quantities of SO<sub>2</sub> into the atmosphere along with volcanic ash. These plumes have been studied with tools developed for the Infrared Atmospheric Sounding Interferometer (IASI) by the Earth Observation Data Group (EODG) at the University of Oxford. IASI is a hyperspectral sensor onboard of three meteorological satellites (Metop A, B and C). Each instrument obtains near global coverage twice a day and has a spectral range which includes sensitivity to both SO<sub>2</sub> and ash: making them useful for studying the Raikoke plumes. A fast linear SO<sub>2</sub> retrieval was first applied to flag pixels with elevated amounts of SO<sub>2</sub>. With this tool it was possible to follow the Raikoke plume as it circulated the northern hemisphere above 30 degrees, with parts of the plume still visible around 2 months after the eruption took place. Next an iterative SO<sub>2</sub> retrieval was used to quantify the amount and height of the SO<sub>2</sub> in each pixel. In the first few days after the eruption took place, very high column amounts are recorded, in some cases exceeding 600 DU. Using this retrieval, a preliminary estimate of 1.6 Tg was obtained for the total amount of SO<sub>2 </sub>emitted (measured on the 23<sup>rd</sup> of June). Height information from this technique shows that there were probably multiple injection heights during the eruption and that SO<sub>2</sub> was emitted into both the troposphere and stratosphere. The tropospheric plume remains visible for just a few days after the eruption, while the stratospheric portion of the plume persists for several weeks.</p>

The stratified disc wind of MCG-03-58-007

ABSTRACT Past Suzaku, XMM–Newton, and NuSTAR observations of the nearby (z = 0.03233) bright Seyfert 2 galaxy MCG-03-58-007 revealed the presence of two deep and blue-shifted iron K-shell absorption line profiles. These could be explained with the presence of two phases of a highly ionized, high column density accretion disc wind outflowing with vout1 ∼ −0.1c and vout2 ∼ −0.2c. Here we present two new observations of MCG-03-58-007: one was carried out in 2016 with Chandra and one in 2018 with Swift. Both caught MCG-03-58-007 in a brighter state ($F_{{\mathrm{2}-10\, keV}} \sim 4 \times 10^{-12}$ erg cm−2 s−1) confirming the presence of the fast disc wind. The multi-epoch observations of MCG-03-58-007 covering the period from 2010 to 2018 were then analysed. These data show that the lower velocity component outflowing with vout1 ∼ −0.072 ± 0.002c is persistent and detected in all the observations, although it is variable in column density in the range NH ∼ 3–8 × 1023 cm−2. In the 2016 Swift observation we detected again the second faster component outflowing with vout2 ∼ −0.2c, with a column density ($N_{\mbox{H}}=7.0^{+5.6}_{-4.1}\times 10^{23}$ cm−2), similar to that seen during the Suzaku observation. However during the Chandra observation 2 yr earlier, this zone was not present (NH < 1.5 × 1023 cm−2), suggesting that this faster zone is intermittent. Overall the multi-epochs observations show that the disc wind in MCG-03-58-007 is not only powerful, but also extremely variable, hence placing MCG-03-58-007 among unique disc winds such as the one seen in the famous QSO PDS456. One of the main results of this investigation is the consideration that these winds could be extremely variable, sometime appearing and sometime disappearing; thus to reach solid and firm conclusions about their energetics multiple observations are mandatory.

Broad He i 1.08-µm absorption from the obscurer in the active galaxy NGC 5548

ABSTRACT The nucleus of the active galaxy NGC 5548 was the target of two intensive spectroscopic monitoring campaigns at X-ray, ultraviolet (UV), and optical frequencies in 2013/2014. These campaigns detected the presence of a massive obscuration event. In 2016/2017, Landt et al. conducted a near-IR spectroscopic monitoring campaign on NGC 5548 and discovered He i 1.08-μm absorption. Here, we decompose this absorption into its components and study its time variability. We attribute the narrow He i absorption lines to the warm absorber (WA) and, as for the newly appeared low-ionization WA lines in the UV, their presence is most likely due to a reduction in ionization parameter caused by the obscurer. The observed variability of the narrow He i absorption is consistent with what is expected for the WA. Most importantly, we also detect fast, broad He i absorption, which we attribute to the obscurer. This He i broad absorption, which is indicative of a high column density gas, is unsaturated and variable on time-scales of a few months. The observed variability of the obscurer is mainly due to changes in ionization, although density changes also play a role. We test the physical cycle model of Dehghanian et al. which proposes that helium recombination can account for how the obscurer influences the physics of the WA gas. Our results support their model, but also indicate that the reality might be more complex.

Detecting neutral hydrogen at z ≳ 3 in large spectroscopic surveys of quasars

ABSTRACT We present a pipeline based on a random forest classifier for the identification of high column density clouds of neutral hydrogen (i.e. the Lyman limit systems, LLSs) in absorption within large spectroscopic surveys of z ≳ 3 quasars. We test the performance of this method on mock quasar spectra that reproduce the expected data quality of the Dark Energy Spectroscopic Instrument and the WHT (William Herschel Telescope) Enhanced Area Velocity Explorer surveys, finding ${\gtrsim}90{{\ \rm per\ cent}}$ completeness and purity for $N_{\rm H\,\rm{\small I}} \gtrsim 10^{17.2}~\rm cm^{-2}$ LLSs against quasars of g < 23 mag at z ≈ 3.5–3.7. After training and applying our method on 10 000 quasar spectra at z ≈ 3.5–4.0 from the Sloan Digital Sky Survey (Data Release 16), we identify ≈6600 LLSs with $N_{\rm H\,\rm{\small I}} \gtrsim 10^{17.5}~\rm cm^{-2}$ between z ≈ 3.1 and 4.0 with a completeness and purity of ${\gtrsim}90{{\ \rm per\ cent}}$ for the classification of LLSs. Using this sample, we measure a number of LLSs per unit redshift of ℓ(z) = 2.32 ± 0.08 at z = [3.3, 3.6]. We also present results on the performance of random forest for the measurement of the LLS redshifts and H i column densities, and for the identification of broad absorption line quasars.

High-mass star formation in Orion B triggered by cloud–cloud collision: Merging molecular clouds in NGC 2024

We performed new comprehensive 13CO(J = 2–1) observations toward NGC 2024, the most active star-forming region in Orion B, with an angular resolution of ∼100″ obtained with Nanten2. We found that the associated cloud consists of two independent velocity components. The components are physically connected to the H ii region as evidenced by their close correlation with the dark lanes and the emission nebulosity. The two components show complementary distribution with a displacement of ∼0.6 pc. Such complementary distribution is typical to colliding clouds discovered in regions of high-mass star formation. We hypothesize that a cloud–cloud collision between the two components triggered the formation of the late O-type stars and early B stars localized within 0.3 pc of the cloud peak. The duration time of the collision is estimated to be 0.3 million years from a ratio of the displacement and the relative velocity ∼3 km s−1 corrected for probable projection. The high column density of the colliding cloud ∼1023 cm−2 is similar to those in the other high-mass star clusters in RCW 38, Westerlund 2, NGC 3603, and M 42, which are likely formed under trigger by cloud–cloud collision. The present results provide an additional piece of evidence favorable to high-mass star formation by a major cloud–cloud collision in Orion.

QSO obscuration at high redshift (z ≳ 7): predictions from the bluetides simulation

ABSTRACT High-$z$ AGNs hosted in gas-rich galaxies are expected to grow through significantly obscured accretion phases. This may limit or bias their observability. In this work, we use bluetides, a large volume cosmological simulation of galaxy formation to examine quasar obscuration for the highest redshift ($z$ ≥ 7) supermassive black holes residing in the centre of galaxies. We find that for the bright quasars, most of the high-column density gas ($\rm {\gt} 90 {\rm {per\ cent}}$) resides in the innermost regions of the host galaxy (typically within <10 ckpc), while the gas in the outskirts is a minor contributor to the NH. The brightest quasars can have large angular variations in galactic obscuration, over 2 orders of magnitude (ranging from column density $N_\mathrm{H} \sim 10^{21.5 \!-\! 24}\, \rm {cm}^{-2}$), where the lines of sight with the lowest obscuration are those formed via strong gas outflows driven by AGN feedback. The obscured fraction P(NH > 1023 cm−2) typically ranges from 0.6 to 1.0 for increasing LX (with $L_\mathrm{ X} \gt 10^{43} \, \rm {erg\, s}^{-1}$), with no clear trend of redshift evolution. Due to the angular variation in NH, all relations between NH and LX, MBH, and galaxy host properties (global M*, $M_{\rm H_2}$, and star formation rate) show appreciable scatter. The dust optical depth in the UV band τUV has tight positive correlation with NH. Our dust-extincted UV luminosity function (UVLF) is about 1.5 dex lower than the intrinsic UVLF, implying that more than 99 per cent of the $z$ ∼ 7 AGNs are heavily dust extincted and therefore would be missed by the UV-band observation.

Cloud formation in the atomic and molecular phase: H I self absorption (HISA) towards a giant molecular filament

Molecular clouds form from the atomic phase of the interstellar medium. However, characterizing the transition between the atomic and the molecular interstellar medium (ISM) is a complex observational task. Here we address cloud formation processes by combining H I self absorption (HISA) with molecular line data. Column density probability density functions (N-PDFs) are a common tool for examining molecular clouds. One scenario proposed by numerical simulations is that the N-PDF evolves from a log-normal shape at early times to a power-law-like shape at later times. To date, investigations of N-PDFs have been mostly limited to the molecular component of the cloud. In this paper, we study the cold atomic component of the giant molecular filament GMF38.1-32.4a (GMF38a, distance = 3.4 kpc, length ~ 230 pc), calculate its N-PDFs, and study its kinematics. We identify an extended HISA feature, which is partly correlated with the 13CO emission. The peak velocities of the HISA and 13CO observations agree well on the eastern side of the filament, whereas a velocity offset of approximately 4 km s−1 is found on the western side. The sonic Mach number we derive from the linewidth measurements shows that a large fraction of the HISA, which is ascribed to the cold neutral medium (CNM), is at subsonic and transonic velocities. The column density of the CNM part is on the order of 1020 to 1021 cm−2. The column density of molecular hydrogen, traced by 13CO, is an order of magnitude higher. The N-PDFs from HISA (CNM), H I emission (the warm and cold neutral medium), and 13CO (molecular component) are well described by log-normal functions, which is in agreement with turbulent motions being the main driver of cloud dynamics. The N-PDF of the molecular component also shows a power law in the high column-density region, indicating self-gravity. We suggest that we are witnessing two different evolutionary stages within the filament. The eastern subregion seems to be forming a molecular cloud out of the atomic gas, whereas the western subregion already shows high column density peaks, active star formation, and evidence of related feedback processes.

Incoherent fast variability of X-ray obscurers

Context. Obscuration events caused by outflowing clumps or streams of high column density and low ionised gas, shown to absorb the X-ray continuum heavily, have been witnessed across a number of Seyfert galaxies. Aims. We report on the X-ray spectral-timing analysis of the December 2016 obscuration event in NGC 3783, which was aimed at probing variability of the X-ray obscurer on the shortest possible timescales. The main goals of this study are to obtain independent constraints on the density and, ultimately on the distance of the obscuring gas, as well as to characterise the impact of variable obscuration on the observed X-ray spectral-timing characteristics of Seyfert galaxies. Methods. We carried out a comparative analysis of NGC 3783 during unobscured (using archival 2000–2001 XMM-Newton data) and obscured states (using XMM-Newton and NuSTAR data from the 2016 observational campaign). The analysed timescales range between ten hours and about one hour. This study was then generalised to discuss the signatures of variable obscuration in the X-ray spectral-timing characteristics of Seyfert galaxies as a function of the physical properties of the obscuring gas. Results. The X-ray obscurer in NGC 3783 is found to vary on timescales between about one hour to ten hours. This variability is incoherent with respect to the variations of the X-ray continuum. A fast response (on timescales shorter than about 1.5 ks) of the ionisation state of the obscuring gas to the short timescale variability of the primary X-ray continuum provides a satisfactory interpretation of all the observed X-ray spectral-timing properties. This study enabled us to put independent constraints on the density and location of the obscuring gas. We found the gas to have a density of ne > 7.1 × 107 cm−3, which is consistent with a location in the broad line region.