Hydrogen Column Density Research Articles

Physical conditions and redshift evolution of optically thin C iii absorbers: low-z sample

ABSTRACT We present a detailed analysis of 99 optically thin C iii absorption systems at redshift 0.2 ≤ z ≤ 0.9 associated with neutral hydrogen column densities in the range 15 ≤ log $N({\rm{H\,{\small I}}})$ (cm−2) ≤ 16.2. Using photoionization models, we infer the number density (nH), C-abundance ([C/H]), and line-of-sight thickness (L) of these systems in the ranges −3.4 ≤ log nH (cm−3) ≤ −1.6, −1.6 ≤ [C/H] ≤ 0.4, and 1.3 pc ≤ L ≤ 10 kpc, respectively, with most of the systems having sub-kpc scale thickness. We combine the low-z and previously reported high-z (2.1 ≤ z ≤ 3.3) optically thin C iii systems to study the redshift evolution and various correlations between the derived physical parameters. We see a significant redshift evolution in nH, [C/H], and L. We compare the redshift evolution of metallicity in C iii systems with those of various types of absorption systems. We find that the slope of [C/H] versus z for C iii absorbers is steeper compared to the redshift evolution of cosmic metallicity of the damped Ly α sample (DLA) but consistent with that of sub-DLAs. We find the existence of strong anticorrelation between L and [C/H] for the combined sample with a significance level of 8.39σ. We see evidence of two distinct [C/H] branch C iii populations (low-[C/H] branch, $[\mathrm{ C/H}]\, \le -1.2$, and high-[C/H] branch, $[\mathrm{ C/H}]\, \gt -1.2$) in the combined C iii sample when divided appropriately in the L versus $N({\rm{C\,{\small III}}})$ plane. Further studies of C iii absorbers in the redshift range 1.0 ≤ z ≤ 2.0 are important to map the redshift evolution of these absorbers and gain insights into the time evolution physical conditions of the circumgalactic medium.

Chandra View of the LINER-type Nucleus in the Radio-loud Galaxy CGCG 292–057: Ionized Iron Line and Jet–ISM Interactions

Abstract We present an analysis of the new, deep (94 ksec) Chandra ACIS-S observation of radio-loud active galaxy CGCG 292−057, characterized by a LINER-type nucleus and a complex radio structure that indicates intermittent jet activity. On the scale of the host galaxy bulge, we detected excess X-ray emission with a spectrum best fit by a thermal plasma model with a temperature of ∼0.8 keV. We argue that this excess emission results from compression and heating of the hot diffuse fraction of the interstellar medium displaced by the expanding inner, ∼20 kpc-scale lobes observed in this restarted radio galaxy. The nuclear X-ray spectrum of the target clearly displays an ionized iron line at ∼6.7 keV, and is best fitted with a phenomenological model consisting of a power-law (photon index ≃ 1.8) continuum absorbed by a relatively large amount of cold matter (hydrogen column density ≃0.7 × 1023 cm−2), and partly scattered (fraction ∼3%) by ionized gas, giving rise to a soft excess component and Kα line from iron ions. We demonstrate that the observed X-ray spectrum, particularly the equivalent width of Fe XXV Kα (of order 0.3 keV) can in principle, be explained in a scenario involving a Compton-thin gas located at the scale of the broad-lined region in this source and photoionized by nuclear illumination. We compare the general spectral properties of the CGCG 292−057 nucleus, with those of other nearby LINERs studied in X-rays.

Martian Oxygen and Hydrogen Upper Atmospheres Responding to Solar and Dust Storm Drivers: Hisaki Space Telescope Observations

AbstractWe present variations of oxygen 130.4 nm and hydrogen Ly‐β airglow of the Martian upper atmosphere observed by the Hisaki spacecraft in the Earth orbit. In 5‐year intermittent observations covering various Mars seasons, the 130.4 nm brightness varied from ∼700 to ∼1,200 Rayleigh, correlated with the solar 130.4 nm flux. The Ly‐β brightness, on the other hand, varied from ∼50 to ∼260 Rayleigh, correlated with the column dust optical depth rather than the solar Ly‐β flux. This suggests that the global oxygen column density in the upper atmosphere was relatively stable over the observations while the hydrogen column density was highly variable, controlled by dust storms. Although the observations were made in different Mars Years, we suggest that the hydrogen column density increased by at least 2–5 times from non‐dusty to dusty seasons. The source of the hydrogen atoms is likely water vapor transported from the lower atmosphere which subsequently photodissociate into hydrogen atoms. The amount of oxygen atoms dissociated from water vapor is small compared with those dissociated from the main CO2 atmosphere. Large brightness of both emissions was detected during the comet siding spring (CSS) approach. We suggest that a large solar 130.4 nm flux caused the large 130.4 nm brightness and an effect of the CSS approach was small. For Ly‐β, a regional dust storm during the CSS approach likely caused the large brightness. Although it is possible that the hydrogen atoms are transported from the comet, we leave the effect of CSS on the Ly‐β brightness uncertain.

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.

MUSE view of Arp220: Kpc-scale multi-phase outflow and evidence for positive feedback

Context. Arp220 is the nearest and prototypical ultra-luminous infrared galaxy; it shows evidence of pc-scale molecular outflows in its nuclear regions and strongly perturbed ionised gas kinematics on kpc scales. It is therefore an ideal system for investigating outflow mechanisms and feedback phenomena in detail. Aims. We investigate the feedback effects on the Arp220 interstellar medium (ISM), deriving a detailed picture of the atomic gas in terms of physical and kinematic properties, with a spatial resolution that had never before been obtained (0.56″, i.e. ∼210 pc). Methods. We use optical integral-field spectroscopic observations from VLT/MUSE-AO to obtain spatially resolved stellar and gas kinematics, for both ionised ([N II]λ6583) and neutral (Na IDλλ5891, 96) components; we also derive dust attenuation, electron density, ionisation conditions, and hydrogen column density maps to characterise the ISM properties. Results. Arp220 kinematics reveal the presence of a disturbed kpc-scale disc in the innermost nuclear regions as well as highly perturbed multi-phase (neutral and ionised) gas along the minor axis of the disc, which we interpret as a galactic-scale outflow emerging from the Arp220 eastern nucleus. This outflow involves velocities up to ∼1000 km s−1 at galactocentric distances of ≈5 kpc; it has a mass rate of ∼50 M⊙ yr−1 and kinetic and momentum power of ∼1043 erg s−1 and ∼1035 dyne, respectively. The inferred energetics do not allow us to distinguish the origin of the outflows, namely whether they are active galactic nucleus- or starburst-driven. We also present evidence for enhanced star formation at the edges of – and within – the outflow, with a star-formation rate SFR ∼ 5 M⊙ yr−1 (i.e. ∼2% of the total SFR). Conclusions. Our findings suggest the presence of powerful winds in Arp220: They might be capable of heating or removing large amounts of gas from the host (“negative feedback”) but could also be responsible for triggering star formation (“positive feedback”).

The ALPINE-ALMA [CII] survey

Context. The Lyman-α line in the ultraviolet (UV) and the [CII] line in the far-infrared (FIR) are widely used tools to identify galaxies in the early Universe and to obtain insights into interstellar medium (ISM) properties in high-redshift galaxies. By combining data obtained with ALMA in band 7 at ∼320 GHz as part of the ALMA Large Program to INvestigate [CII] at Early Times (ALPINE) with spectroscopic data from DEIMOS at the Keck Observatory, VIMOS and FORS2 at the Very Large Telescope, we assembled a unique sample of 53 main-sequence star-forming galaxies at 4.4 < z < 6 in which we detect both the Lyman-α line in the UV and the [CII] line in the FIR. Aims. The goal of this paper is to constrain the properties of the Lyα emission in these galaxies in relation to other properties of the ISM. Methods. We used [CII], observed with ALMA, as a tracer of the systemic velocity of the galaxies, and we exploited the available optical spectroscopy to obtain the Lyα-[CII] and ISM-[CII] velocity offsets. Results. We find that 90% of the selected objects have Lyα-[CII] velocity offsets in the range 0 < ΔvLyα − [CII] < 400 km s−1, in line with the few measurements available so far in the early Universe, and significantly smaller than those observed at lower redshifts. At the same time, we observe ISM-[CII] offsets in the range −500 < ΔvISM−[CII] < 0 km s−1, in line with values at all redshifts, which we interpret as evidence for outflows in these galaxies. We find significant anticorrelations between ΔvLyα−[CII] and the Lyα rest-frame equivalent width EW0(Lyα) (or equivalently, the Lyα escape fraction fesc(Lyα)): galaxies that show smaller ΔvLyα−[CII] have larger EW0(Lyα) and fesc(Lyα). Conclusions. We interpret these results in the framework of available models for the radiative transfer of Lyα photons. According to the models, the escape of Lyα photons would be favored in galaxies with high outflow velocities, producing large EW0(Lyα) and small ΔvLyα-[CII], in agreement with our observations. The uniform shell model would also predict that the Lyα escape in galaxies with slow outflows (0 < vout < 300 km s−1) is mainly determined by the neutral hydrogen column density (NHI) along the line of sight, while the alternative model by Steidel et al. (2010, ApJ, 717, 289) would more highly favor a combination of NHI at the systemic velocity and covering fraction as driver of the Lyα escape. We suggest that the increase in Lyα escape that is observed in the literature between z ∼ 2 and z ∼ 6 is not due to a higher incidence of fast outflows at high redshift, but rather to a decrease in average NHI along the line of sight, or alternatively, a decrease in HI covering fraction.

An extreme ultraluminous X-ray source X-1 in NGC 5055

Aims. We analysed multi-epoch X-ray data of the ultraluminous X-ray source NGC 5055 X-1, with luminosity up to 2.32 × 1040 erg s−1, to constrain the physical parameters of the source. Methods. We performed a timing and spectral analysis of Chandra and XMM-Newton observations. We used spectral models that assume the emission is from an accreting black hole system. We fit the data with a multicolour disk combined with a powerlaw or a thermal Comptonization (NTHCOMP) component and compared those fits with a slim disk model. Results. The light curves of the source do not show significant variability. From the hardness ratios (3–10 keV/0.3–3 keV flux), we infer that the source is not spectrally variable. We found that the photon index is tightly, positively correlated with the unabsorbed 0.3–10 keV flux and the hydrogen column density. Furthermore, the temperature emissivity profile indicates a deviation from the standard sub-Eddington thin disk model. The source shows an inverse correlation between luminosity and inner disk temperature in all fitted models. Conclusions. Our analysis favours the source to be in an ultraluminous soft state. The positive correlations between the photon index and the flux as well as between the photon index and the hydrogen column density may suggest the source is accreting at high Eddington ratios and might indicate the presence of a wind. The inverse luminosity relation with the inner disk temperature for all spectral models may indicate that the emission is geometrically beamed by an optically thick outflow.

Detection capabilities of the Athena X-IFU for the warm-hot intergalactic medium using gamma-ray burst X-ray afterglows

At low redshifts, the observed baryonic density falls far short of the total number of baryons predicted. Cosmological simulations suggest that these baryons reside in filamentary gas structures, known as the warm-hot intergalactic medium (WHIM). As a result of the high temperatures of these filaments, the matter is highly ionised such that it absorbs and emits far-UV and soft X-ray photons. Athena, the proposed European Space Agency X-ray observatory, aims to detect the “missing” baryons in the WHIM up to redshifts of z = 1 through absorption in active galactic nuclei and gamma-ray burst (GRB) afterglow spectra, allowing for the study of the evolution of these large-scale structures of the Universe. This work simulates WHIM filaments in the spectra of GRB X-ray afterglows with Athena using the SImulation of X-ray TElescopes framework. We investigate the feasibility of their detection with the X-IFU instrument, through O VII (E = 573 eV) and O VIII (E = 674 eV) absorption features, for a range of equivalent widths imprinted onto GRB afterglow spectra of observed starting fluxes ranging between 10−12 and 10−10 erg cm−2 s−1, in the 0.3−10 keV energy band. The analyses of X-IFU spectra by blind line search show that Athena will be able to detect O VII−O VIII absorption pairs with EWO VII > 0.13 eV and EWO VIII > 0.09 eV for afterglows with F > 2 × 10−11 erg cm−2 s−1. This allows for the detection of ≈ 45−137 O VII−O VIII absorbers during the four-year mission lifetime. The work shows that to obtain an O VII−O VIII detection of high statistical significance, the local hydrogen column density should be limited at NH < 8 × 1020 cm−2.

A Carbon-enhanced Lyman Limit System: Signature of the First Generation of Stars?

Abstract We present the study of a Lyman limit system (LLS) at z abs = 1.5441 toward quasar J134122.50+185213.9 observed with VLT X-shooter. This is a very peculiar system with strong C i absorption seen associated with a neutral hydrogen column density of log N(H i) (cm−2) = 18.10, too small to shield the gas from any external UV flux. The low-ionization absorption lines exhibit a simple kinematic structure consistent with a single component. Using CLOUDY models to correct for ionization, we find that the ionization parameter of the gas is in the range −4.5 < log U < –4.2 and the gas density –1.5 < log n(H) (cm−3) < –1.2. The models suggest that carbon is overabundant relative to iron, [C/Fe] > +2.2 at [Fe/H] ∼–1.6. Such a metal abundance pattern is reminiscent of carbon-enhanced metal-poor stars detected in the Galaxy halo. Metal enrichment by the first generation of supernovae provide a plausible explanation for the inferred abundance pattern in this system.

Dust Masses, Compositions, and Luminosities in the Nuclear Disks and the Diffuse Circumnuclear Medium of Arp 220

Abstract We present an analysis of the 4–2600 μm spectral energy distributions (SEDs) of the west and east nuclei and the diffuse infrared (IR) region of the merger-driven starburst Arp 220. We examine several possible source morphologies and dust temperature distributions using a mixture of silicate and carbonaceous grains. From fits to the SEDs we derive dust masses, temperatures, luminosities, and dust-inferred gas masses. We show that the west and east nuclei are powered by central sources deeply enshrouded behind cm−2 column densities of hydrogen with an exponential density distribution, and that the west and east nuclei are optically thick out to wavelengths of ∼1900 and ∼770 μm, respectively. The nature of the central sources cannot be determined from our analysis. We derive star formation rates (SFRs) or black hole masses needed to power the IR emission, and show that the [C ii] 158 μm line cannot be used as a tracer of the SFR in heavily obscured systems. Gas masses inferred from dust are larger than those inferred from CO observations, suggesting either larger dust-to-H mass ratios or the presence of hidden atomic H that cannot be inferred from CO observations. The luminosities per unit mass in the nuclei are ∼450 in solar units, smaller than the Eddington limit of ∼1000–3000 for an optically thick star-forming region, suggesting that the observed gas outflows are primarily driven by stellar winds and supernova shock waves instead of radiation pressure on the dust.

X-ray absorption and 9.7 μm silicate feature as a probe of AGN torus structure

The dusty torus plays a vital role in unifying active galactic nuclei (AGNs). However, the physical structure of the torus remains largely unclear. Here we present a systematical investigation of the torus mid-infrared (MIR) spectroscopic feature, i.e., the 9.7μm silicate line, of 175 AGNs selected from the Swift/BAT Spectroscopic Survey (BASS). Our sample is constructed to ensure that each of the 175 AGNs has Spizter/IRS MIR, optical, and X-ray spectroscopic coverage. Therefore, we can simultaneously measure the silicate strength, optical emission lines, and X-ray properties (e.g., the column density and the intrinsic X-ray luminosity). We show that, consistent with previous works, the silicate strength is weakly correlated with the hydrogen column density (), albeit with large scatters. For X-ray unobscured AGNs, the silicate-strength-derived V-band extinction and the broad-Hα-inferred one are both small; however, for X-ray obscured AGNs, the former is much larger than the latter. In addition, we find that the optical type 1 AGNs with strong X-ray absorption on average show significant silicate absorption, indicating that their X-ray absorption might not be caused by dust-free gas in the broad-line region. Our results suggest that the distribution and structure of the obscuring dusty torus are likely to be very complex. We test our results against the smooth and clumpy torus models and find evidence in favor of the clumpy torus model.

X-Ray Spectroscopy in the Microcalorimeter Era. I. Effects of Fe xxiv Resonant Auger Destruction on Fe xxv Kα Spectra

Abstract We discuss the importance of Fe23+ in determining the line intensities of the Fe xxv Kα complex in an optically thick cloud, and investigate resonant auger destruction (RAD) with CLOUDY. Although initially motivated by the Perseus cluster, our calculations are extended to the wide range of column densities encountered in astronomy. A Fe xxv line photon can change/lose its identity upon absorption by three-electron iron as a result of “line interlocking.” This may lead to the autoionization of the absorbing ion, ultimately destroying the Fe xxv Kα photon by RAD. Out of the four members in the Fe xxv Kα complex, a significant fraction of the x line photons are absorbed by Fe23+ and destroyed, causing the x line intensity to decrease. For example, at a hydrogen column density of 1025 cm−2, ∼32% of x photons are destroyed due to RAD while w is mostly unaffected. The line intensity of y is slightly (≤2%) reduced. z is not directly affected by RAD, but the contrasting behavior between z and x line intensities points toward the possible conversion of a tiny fraction (∼2%) of x photons into z photons. The change in line intensities due to electron scattering escape off fast thermal electrons is also discussed.

Revisiting the Magnetic Field of the L183 Starless Core

Abstract We present observations of linear polarization from dust thermal emission at 850 μm toward the starless cloud L183. These data were obtained at the James Clerk Maxwell Telescope (JCMT) using the SCUBA-2 camera in conjunction with its polarimeter POL-2. Polarized dust emission traces the plane-of-sky magnetic field structure in the cloud, thus allowing us to investigate the role of magnetic fields in the formation and evolution of its starless core. To interpret these measurements, we first calculate the dust temperature and column density in L183 by fitting the spectral energy distribution obtained by combining data from the JCMT and the Herschel space observatory. We used the Davis–Chandrasekhar–Fermi technique to measure the magnetic field strength in five subregions of the cloud, and we find values ranging from ∼120 ± 18 μG to ∼270 ± 64 μG in agreement with previous studies. Combined with an average hydrogen column density ( ) of ∼1.5 × 1022 cm−2 in the cloud, we also find that all five subregions are magnetically subcritical. These results indicate that the magnetic field in L183 is sufficiently strong to oppose the gravitational collapse of the cloud.

New results concerning the environment of the heliosphere, nearby interstellar clouds, and physical processes in the inter–cloud medium

We present our new results concerning the interface between the outer heliosphere and the local interstellar medium (LISM). The three dimensional shape of the Local Interstellar Cloud (LIC) based on 62 sightlines to nearby stars shows a region of very low neutral hydrogen density in the direction of the star ε CMa, the brightest source of extreme-UV (EUV) radiation. This “hydrogen hole” with very weak neutral hydrogen absorption by the LIC and Blue clouds results from photoionization by the EUV radiation from ε CMa. The LIC likely surrounds the heliosphere, but in the direction of the hydrogen hole its neutral hydrogen column density is too low to be measured. Upper limits to this column density and the direction of the Sun’s motion through space indicate that the Sun will leave the outer edge of the LIC in less than 1, 900 years. The measured difference between the speed and direction of incoming neutral hydrogen atoms (measured by IBEX and Ulysses) and the flow vector of the LIC indicate that the plasma at the edge of the LIC has a different flow vector than the LIC core. The inter-cloud plasma and much of the Local Cavity are inside the Str¨omgren sphere (also called an H II region) surrounding ε CMa. The outer edges of the LIC and other clouds are Str¨omgren shells that are partially ionized by the EUV radiation from ε CMa and white dwarfs. The Local Cavity could be a Str¨omgren sphere plasma photoionized by ε CMa and hot white dwarfs that contains low density ionized gas that is not hot. An interstellar probe should measure magnetic field and plasma properties in the VLISM between the heliopause and 600–700 au from the Sun and then enter what is likely the Str¨omgren shell outer edge of the LIC. When the Sun leaves the LIC, it will either enter the G cloud, a transition region between the LIC and the G cloud, or ionized Str¨omgren sphere plasma.

Lyman continuum leakage in faint star-forming galaxies at redshiftz= 3−3.5 probed by gamma-ray bursts

Context.The identification of the sources that reionized the Universe and their specific contribution to this process are key missing pieces of our knowledge of the early Universe. Faint star-forming galaxies may be the main contributors to the ionizing photon budget during the epoch of reionization, but their escaping photons cannot be detected directly due to inter-galactic medium opacity. Hence, it is essential to characterize the properties of faint galaxies with significant Lyman continuum (LyC) photon leakage up toz∼ 4 to define indirect indicators allowing analogs to be found at the highest redshift.Aims.Long gamma-ray bursts (LGRBs) typically explode in star-forming regions of faint, star-forming galaxies. Through LGRB afterglow spectroscopy it is possible to detect directly LyC photons. Our aim is to use LGRBs as tools to study LyC leakage from faint, star-forming galaxies at high redshift.Methods.Here we present the observations of LyC emission in the afterglow spectra ofGRB 191004Batz = 3.5055, together with those of the other two previously known LyC-leaking LGRB host galaxies (GRB 050908atz = 3.3467, andGRB 060607Aatz = 3.0749), to determine their LyC escape fraction and compare their properties.Results.From the afterglow spectrum of GRB 191004B we determine a neutral hydrogen column density at the LGRB redshift of log(NH I/cm−2) = 17.2 ± 0.15, and negligible extinction (AV = 0.03 ± 0.02 mag). The only metal absorption lines detected are C IVand Si IV. In contrast to GRB 050908 and GRB 060607A, the host galaxy of GRB 191004B displays significant Lyman-alpha (Lyα) emission. From its Lyαemission and the non-detection of Balmer emission lines we constrain its star-formation rate (SFR) to 1 ≤ SFR ≤ 4.7 M⊙yr−1. We fit the Lyαemission with a shell model and find parameters values consistent with the observed ones. The absolute (relative) LyC escape fractions we find for GRB 191004B, GRB 050908 and GRB 060607A are of 0.35−0.11+0.10(0.43−0.13+0.12), 0.08−0.04+0.05(0.08−0.04+0.05) and 0.20−0.05+0.05(0.45−0.15+0.15), respectively. We compare the LyC escape fraction of LGRBs to the values of other LyC emitters found from the literature, showing that LGRB afterglows can be powerful tools to study LyC escape for faint high-redshift star-forming galaxies. Indeed we could push LyC leakage studies to much higher absolute magnitudes. The host galaxies of the three LGRBs presented here have allM1600 > −19.5 mag, with the GRB 060607A host atM1600 > −16 mag. LGRB hosts may therefore be particularly suitable for exploring the ionizing escape fraction in galaxies that are too faint or distant for conventional techniques. Furthermore, the time involved is minimal compared to galaxy studies.

CMZoom: Survey Overview and First Data Release

We present an overview of the CMZoom survey and its first data release. CMZoom is the first blind, high-resolution survey of the Central Molecular Zone (CMZ; the inner 500 pc of the Milky Way) at wavelengths sensitive to the pre-cursors of high-mass stars. CMZoom is a 500-hour Large Program on the Submillimeter Array (SMA) that mapped at 1.3 mm all of the gas and dust in the CMZ above a molecular hydrogen column density of 10^23 cm^-2 at a resolution of ~3" (0.1 pc). In this paper, we focus on the 1.3 mm dust continuum and its data release, but also describe CMZoom spectral line data which will be released in a forthcoming publication. While CMZoom detected many regions with rich and complex substructure, its key result is an overall deficit in compact substructures on 0.1 - 2 pc scales (the compact dense gas fraction: CDGF). In comparison with clouds in the Galactic disk, the CDGF in the CMZ is substantially lower, despite having much higher average column densities. CMZ clouds with high CDGFs are well-known sites of active star formation. The inability of most gas in the CMZ to form compact substructures is likely responsible for the dearth of star formation in the CMZ, surprising considering its high density. The factors responsible for the low CDGF are not yet understood but are plausibly due to the extreme environment of the CMZ, having far-reaching ramifications for our understanding of the star formation process across the cosmos.

LEGO – II. A 3 mm molecular line study covering 100 pc of one of the most actively star-forming portions within the Milky Way disc

ABSTRACT The current generation of (sub)mm-telescopes has allowed molecular line emission to become a major tool for studying the physical, kinematic, and chemical properties of extragalactic systems, yet exploiting these observations requires a detailed understanding of where emission lines originate within the Milky Way. In this paper, we present 60 arcsec (∼3 pc) resolution observations of many 3 mm band molecular lines across a large map of the W49 massive star-forming region (∼100 pc × 100 pc at 11 kpc), which were taken as part of the ‘LEGO’ IRAM-30m large project. We find that the spatial extent or brightness of the molecular line transitions are not well correlated with their critical densities, highlighting abundance and optical depth must be considered when estimating line emission characteristics. We explore how the total emission and emission efficiency (i.e. line brightness per H2 column density) of the line emission vary as a function of molecular hydrogen column density and dust temperature. We find that there is not a single region of this parameter space responsible for the brightest and most efficiently emitting gas for all species. For example, we find that the HCN transition shows high emission efficiency at high column density (1022 cm−2) and moderate temperatures (35 K), whilst e.g. N2H+ emits most efficiently towards lower temperatures (1022 cm−2; <20 K). We determine $X_{\mathrm{CO} (1-0)} \sim 0.3 \times 10^{20} \, \mathrm{cm^{-2}\, (K\, km\, s^{-1})^{-1}}$, and $\alpha _{\mathrm{HCN} (1-0)} \sim 30\, \mathrm{M_\odot \, (K\, km\, s^{-1}\, pc^2)^{-1}}$, which both differ significantly from the commonly adopted values. In all, these results suggest caution should be taken when interpreting molecular line emission.

Herschel 158 μm [C ii] Observations of “CO-dark” Gas in the Perseus Giant Molecular Cloud

Abstract We present observations of velocity-resolved [C ii] 158 μm emission from both a dense and a more diffuse photodissociation region (PDR) in the Perseus giant molecular cloud using the Heterodyne Instrument for the Far-Infrared on board the Herschel Space Telescope. We detect [C ii] emission from 80% of the total positions, with a 95% detection rate from the dense boundary region. The integrated intensity of the [C ii] emission remains relatively constant across each boundary, despite the observed range in optical extinction between 1 and 10 mag. This flat profile indicates a constant heating and cooling rate within both regions observed. The integrated intensity of [C ii] emission is reasonably well correlated with the neutral hydrogen (H i) column density, as well as the total gas column density. This, in addition to the 80′ (7 pc) extent of the [C ii] emission from the cloud center, suggests that the H i envelope plays a dominant role in explaining the [C ii] emission emanating from Perseus. We compare the [C ii] and 12CO integrated intensities with predictions from a 1D, two-sided slab PDR model and show that a simple core + envelope, equilibrium model without an additional “CO-dark” H2 component can reproduce observations well. Additional observations are needed to disentangle how much of the [C ii] emission is associated with the “CO-dark” H2 gas, as well as constrain spatial variations of the dust-to-gas ratio across Perseus.

Properties of five z ∼ 0.3–0.4 confirmed LyC leakers: VLT/XShooter observations

ABSTRACT Using new Very Large Telescope (VLT)/XShooter spectral observations we analyse the physical properties of five z ∼ 0.3–0.4 confirmed Lyman continuum (LyC) leakers. Strong resonant Mg ii λλ2796, 2803 Å emission lines (I(λλ2796, 2803)/I(Hβ) ≃ 10–38 per cent) and non-resonant Fe ii* λλ2612, 2626 Å emission lines are observed in spectra of five and three galaxies, respectively. We find high electron densities Ne ∼ 400 cm−3, significantly higher than in typical low-z, but comparable to those measured in z ∼ 2–3 star-forming galaxies (SFGs). The galaxies have a mean value of log N/O = –1.16, close to the maximum values found for SFGs in the metallicity range of 12 + log O/H ≃ 7.7–8.1. All 11 low-z LyC emitting galaxies found by Izotov et al., including the ones considered in this study, are characterized by high equivalent width (EW) (Hβ) ∼ 200–400 Å, high ionization parameter (log(U) = –2.5 to –1.7), high average ionizing photon production efficiency ξ = 1025.54 Hz erg−1, and hard ionizing radiation. On the Baldwin–Phillips–Terlevich (BPT) diagram we find the same offset of our leakers from low-z main-sequence SFGs as that for local analogues of Lyman-break galaxies (LBGs) and extreme SFGs at z ∼ 2–3. We confirm the effectiveness of the He i emission lines diagnostics proposed by Izotov et al. in searching for LyC leaker candidates and find that their intensity ratios correspond to those in a median with low neutral hydrogen column density N(H i) = 1017–5 × 1017 cm−2 that permit leakage of LyC radiation, likely due to their density-bounded H ii regions.

Continuum-fitting the X-Ray Spectra of Tidal Disruption Events

Abstract We develop a new model for X-ray emission from tidal disruption events (TDEs), applying stationary general relativistic “slim disk” accretion solutions to supermassive black holes (SMBHs) and then ray-tracing the photon trajectories from the image plane to the disk surface, including gravitational redshift, Doppler, and lensing effects self-consistently. We simultaneously and successfully fit the multi-epoch XMM-Newton X-ray spectra for two TDEs: ASASSN-14li and ASASSN-15oi. We test explanations for the observed, unexpectedly slow X-ray brightening of ASASSN-15oi, including delayed disk formation and variable obscuration by a reprocessing layer. We propose a new mechanism that better fits the data: a “slimming disk” scenario in which accretion onto an edge-on disk slows, reducing the disk height and exposing more X-rays from the inner disk to the sightline over time. For ASASSN-15oi, we constrain the SMBH mass to . For ASASSN-14li, the SMBH mass is , and the spin is >0.3. For both TDEs, our fitted masses are consistent with independent estimates; for ASASSN-14li, application of the external mass constraint narrows our spin constraint to >0.85. The mass accretion rate of ASASSN-14li decays slowly, as ∝t −1.1, perhaps due to inefficient debris circularization. Over ≈1100 days, its SMBH has accreted ΔM ≈ 0.17M ⊙, implying a progenitor star mass of >0.34M ⊙, i.e., no “missing energy problem.” For both TDEs, the hydrogen column density declines to the host galaxy plus Milky Way value after a few hundred days, suggesting a characteristic timescale for the depletion or removal of obscuring gas.