arXiv subjects

J. -K. Krogager

Publications and source records attributed to J. -K. Krogager.

At least 19 recordsLinked to original sources

arXiv2026

Connecting the dusty dots: dust depletion and extinction of local interstellar clouds

Investigating the chemical complexity of the interstellar medium (ISM) is key for understanding its physical nature and evolution. In this work, we study parsec-scale interstellar dust clouds in the neutral ISM of the Milky Way using two different probes: dust depletion and dust extinction. We examine their relationship to investigate the distribution of metals and dust in the Solar neighbourhood, and how they are related to the Local Bubble. We use measurements of dust depletion for individual gas clouds along sight lines of sight towards bright O/B stars within 1.1 kpc of the Sun, derived from UV absorption-line spectra. We combine these with parsec-scale 3D dust extinction density maps out to 1.25 kpc. We assume a correlation between dust depletion and dust extinction density, which we use to imply that the absorption components are spatially associated with the peaks in dust extinction density, and to pinpoint the likely locations of the gas clouds in physical space. We identify peaks in the dust extinction curves, and then associate the stronger peaks with the strongest dust depletion components. Independent distance measurements along the line of sight towards one of our targets, theta1 Ori C, validates our results. In our sample, the minimum distance between clouds that have significantly different chemical properties is ~ 100pc, giving an indication on the physical scale on which chemical mixing remains incomplete in the ISM of the Milky Way. For five of the eight targets, we report dust depletion values for gas clouds associated with the Local Bubble. Additionally, we find a velocity gradient that is consistent with the expansion of the Local Bubble, further supporting our methodology. Overall, we show that it is possible to use complementary information from dust depletion and dust extinction to build more detailed maps of ISM metal and dust distributions.

astro-ph.GA↗

arXiv2026

The multiple coherence scales of C IV at cosmic noon

The spatial and kinematic structure of the circumgalactic medium (CGM) remains poorly constrained observationally. In this article we compute the clustering of CIV absorption systems at cosmic noon using quasar pairs. We analyze VLT/UVES and Keck/HIRES high-resolution spectra (R = 45000) of a sample of 8 projected and 4 lensed quasar pairs that probe transverse separations, $Δr$, from sub-kpc to a few Mpc, over the redshift range 1.6 < z < 3.3. We detect and fit Voigt profiles to a total of 141 CIV systems, corresponding to 620 velocity components across all quasar lines-of-sight. We compute the two-point correlation function of CIV, $ξ(Δv, Δr)$, where $Δv$ is the velocity difference between components across all available scales. We find a strong dependence of $ξ(Δr)$ with $Δr$ at all velocities. $ξ(Δr)$ reaches a sharp peak at the smallest scales analyzed here, $Δr\approx 0.1$ kpc, decreases steadily up to $Δr\approx 5$ kpc and remains flat up to $Δr\approx 500$ kpc, where it begins to decrease again. By fitting power-laws to the projected transverse correlation function $Ξ(Δr)$, we infer two coherence lengths: $r_1 = 654^{+100}_{-87}$ kpc, which we interpret as a representative size for the CIV enriched regions at $z\approx 2$, and $r_2 = 4.70^{+1.60}_{-1.19}$ kpc for the individual CIV-bearing "clouds". Projecting instead in $Δr$, we find consistent amplitudes of $ξ(Δv)$ with previous work using quasars and extended background sources. Our results suggest that CIV may be a good tracer of not only the small, internal structure of the circumgalactic medium, but also of the way in which galaxies cluster at cosmic noon.

astro-ph.GA↗

arXiv2025

On the Dusty Proximate Damped Lyman-α System toward Q2310-3358 at z = 2.40

Quasar absorption systems not only affect the way quasars are selected, but also serve as key probes of galaxies, providing insight into their chemical evolution and interstellar medium (ISM). Recently, a method based on Gaia astrometric measurements has aided the selection of quasars reddened by dust hitherto overlooked. We conducted a spectroscopic study using VLT/X-Shooter on one such dust-reddened quasar, Q2310-3358. This quasar, at $z = 2.3909\pm0.0022$, is associated with a damped Lyman-$α$ absorber (DLA) at nearly the same redshift $2.4007\pm0.0003$, with a neutral hydrogen column density of $\log N(HI)=21.214\pm0.003$. The DLA is very metal-rich (close to solar metallicity after correction for depletion on dust grains). Its properties align with the metal-to-dust ratio and the mass-metallicity relation established in previous large samples of DLAs. Surprisingly, given its proximity to the quasar in redshift, the absorber has strong cold gas characteristics, including CI and H$_2$. Based on the derived kinetic temperature of $71^{+28}_{-15}$~K, we infer the presence of a strong UV radiation field, which in turn suggests that the quasar and the DLA are in close proximity, i.e., part of the same galaxy and not just different objects in the same overdensity of galaxies. We used the line ratios of the fine-structure lines to constrain the density of the cold gas, yielding $n_{\rm H} \sim 10^{3}~\mathrm{cm}^{-3}$. Our analysis extends the understanding of $z_{abs} \approx z_{em}$ absorption line systems and provides valuable constraints on the interplay between dust, metals, and neutral gas in the ISM of early galaxies.

astro-ph.GA↗

arXiv2025

Investigating chemical variations between interstellar gas clouds in the Solar neighbourhood

The interstellar medium (ISM) is a fundamental component of the Milky Way. Studying its chemical composition and the level of its chemical diversity gives us insight into the evolution of the Milky Way and the role of gas in the Galactic environment. In this paper, we use a novel simulation technique to model the distribution of total hydrogen between gas components, and therefore derive new constraints on the dust depletion and metallicity. We study individual gas components along the lines of sight towards eight bright O/B stars within 1.1 kpc of the Sun using high-resolution HST/STIS absorption spectra (R sim 114 000). We measure the level of dust depletion for these individual components and find components with higher levels of dust depletion compared to Milky Way sightlines in the literature. We find large ranges in the level of dust depletion among components along lines of sight, up to 1.19 dex. Although it is not possible to directly measure the metallicity of individual components due to the saturated and damped Ly-alpha line, we investigate possible metallicity ranges for individual gas components by exploring many different distributions of the total hydrogen gas between components. We select possible combinations of these gas fractions which produce the minimum metallicity difference between components, and for these cases we determine individual metallicities to accuracies that range between sim 0.1 to 0.4 dex. This work shows that full line-of-sight analyses wash out the level of diversity along lines of sight, and that component-by-component studies give a more in-depth understanding of the chemical intricacies of the interstellar medium.

astro-ph.GA↗

arXiv2025

Exploring quasar evolution with proximate molecular absorbers: Insights from the kinematics of highly ionized nitrogen

We investigate the presence and kinematics of NV absorption proximate to high redshift quasars selected upon the presence of strong $H_{2}$ and HI absorption at the quasar redshift. Our spectroscopic observations with X-shooter at the VLT reveal a 70% detection rate of NV (9 of 13 quasars with 2.5 < z < 3.3), remarkably higher than the 10% detection rate in intervening DLA systems and the 30% rate observed within a few thousand km/s of the source in the general quasar population. While many NV components lie within the velocity range of the neutral gas, the kinematic profiles of high-ionization species appear decoupled from those of low-ionization species, with the former extending over much larger velocity ranges, particularly towards bluer velocities. We also observe significant variations in the NV/SiIV, which we attribute to varying ionization conditions, with a velocity-dependent trend: blueshifted NV components systematically exhibit higher ionization parameters compared to those near the quasar's systemic redshift. Furthermore, the most redshifted systems relative to the quasar show no evidence of NV absorption. The results suggest that proximate $H_{2}$ absorption systems select critical stages of quasar evolution, during which the quasar remains embedded in a rich molecular environment. Redshifted systems trace infalling gas, potentially associated with mergers, preceding the onset of outflows. Such outflows may reach or even carry out neutral and molecular gas.This latter stage would correspond to proximate $H_{2}$ systems located around or blueshifted relative to the quasar's systemic z. Finally, the only case in our sample featuring highly blueshifted neutral gas shows no evidence of an association with the quasar.Our findings highlight the need to account for the ionization state when defining a velocity threshold to distinguish quasar-associated systems from intervening.

astro-ph.GA↗

arXiv2024

Modelling of long gamma-ray burst host galaxies at cosmic noon from damped Lyman-α absorption statistics

We study the properties of long gamma-ray burst (GRB) host galaxies using a statistical modelling framework derived to model damped Lyman-$α$ absorbers (DLAs) in quasar spectra at high redshift. The distribution of NHI for GRB-DLAs is $\sim$10 times higher than what is found for quasar-DLAs at similar impact parameters. We interpret this as a temporal selection effect due to the short-lived GRB progenitor probing its host at the onset of a starburst where the interstellar medium may exhibit multiple overdense regions. Owing to the larger NHI, the dust extinction is larger with 29 per cent of GRB-DLAs exhibiting A(V)>1 mag in agreement with the fraction of 'dark bursts'. Despite the differences in NHI distributions, we find that high-redshift 2 < z < 3 quasar- and GRB-DLAs trace the luminosity function of star-forming host galaxies in the same way. We propose that their differences may arise from the fact that the galaxies are sampled at different times in their star formation histories, and that the absorption sightlines probe the galaxy haloes differently. Quasar-DLAs sample the full H I cross-section, whereas GRB-DLAs sample only regions hosting cold neutral medium. Previous studies have found that GRBs avoid high-metallicity galaxies ($\sim$0.5 $Z_{\odot}$). Since at these redshifts galaxies on average have lower metallicities, our sample is only weakly sensitive to such a threshold. Lastly, we find that the modest detection rate of cold gas (H$_2$ or C I) in GRB spectra can be explained mainly by a low volume filling factor of cold gas clouds and to a lesser degree by destruction from the GRB explosion itself.

astro-ph.GA↗

arXiv2024

The MeerKAT Absorption Line Survey Data Release 2: Wideband continuum catalogues and a measurement of the cosmic radio dipole

We present the second data release of the MeerKAT Absorption Line Survey (MALS), consisting of wideband continuum catalogues of 391 pointings observed at L~band. The full wideband catalogue covers 4344 deg$^2$ of sky, reaches a depth of 10 $μ$Jy beam$^{-1}$, and contains 971,980 sources. With its balance between survey depth and sky coverage, MALS DR2 covers five orders of magnitude of flux density, presenting a robust view of the extragalactic radio source population down to 200 $μ$Jy. Using this catalogue, we perform a measurement of the cosmic radio dipole, an anisotropy in the number counts of radio sources with respect to the cosmic background, analogous to the dipole found in the cosmic microwave background (CMB). For this measurement, we present the characterisation of completeness and noise properties of the catalogue, and show that a declination-dependent systematic affects the number density of faint sources. In the dipole measurement on the MALS catalogue, we recover reasonable dipole measurements once we model the declination systematic with a linear fit between the size of the major axis of the restoring beam and the amount of sources of each pointing. The final results are consistent with the CMB dipole in terms of direction and amplitude, unlike many recent measurements of the cosmic radio dipole made with other centimetre wavelength catalogues, which generally show a significantly larger amplitude. This result demonstrates the value of dipole measurements with deeper and more sparse radio surveys, as the population of faint sources probed may have had a significant impact on the measured dipole.

astro-ph.CO↗

arXiv2024

Absence of radio-bright dominance in a near-infrared selected sample of red quasars

(Abridged). We explore the fraction of radio loud quasars in the eHAQ+GAIA23 sample, which contains quasars from the High A(V) Quasar (HAQ) Survey, the Extended High A(V) Quasar (eHAQ) Survey, and the Gaia quasar survey. All quasars in this sample have been found using a near-infrared color selection of target candidates that have otherwise been missed by the Sloan Digital Sky Survey (SDSS). We implemented a redshift-dependent color cut in g-i to select red quasars in the sample and divided them into redshift bins, while using a nearest-neighbors algorithm to control for luminosity and redshift differences between our red quasar sample and a selected blue sample from the SDSS. Within each bin, we cross-matched the quasars to the Faint Images of the Radio Sky at Twenty centimeters (FIRST) survey and determined the radio-detection fraction. We find similar radio-detection fractions for red and blue quasars within 1 sigma, independent of redshift. This disagrees with what has been found in the literature for red quasars in SDSS. It should be noted that the fraction of broad absorption line (BAL) quasars in red SDSS quasars is about five times lower than in our sample. BAL quasars have been observed to be more frequently radio quiet than other quasars, therefore the difference in BAL fractions could explain the difference in radio-detection fraction. The observed higher proportion of BAL quasars in our dataset relative to the SDSS sample, along with the higher rate of radio detections, indicates an association of the redness of quasars and the inherent BAL fraction within the overall quasar population. This finding highlights the need to explore the underlying factors contributing to both the redness and the frequency of BAL quasars, as they appear to be interconnected phenomena.

astro-ph.GA↗

arXiv2023

The galaxy counterpart and environment of the dusty Damped Lyman-alpha Absorber at z=2.226 towards Q1218+0832

We report on further observations of the field of the quasar Q1218+0832. Geier et al. 2019 presented the discovery of the quasar resulting from a search for quasars reddened and dimmed by dust in foreground damped Lyman-alpha absorbers (DLAs). The DLA is remarkable by having a very large HI column density close to 10^22 cm^-2 . Its dust extinction curve shows the 2175 AA bump known from the Local Group. It also shows absorption from cold gas exemplified by CI and CO molecules. For this paper, we present narrow-band observations of the field of Q1218+0832 and also use an archival Hubble Space Telescope (HST) image to search for the galaxy counterpart of the DLA. No emission from the DLA galaxy is found in either the narrow-band imaging or in the HST image. In the HST image, we could probe down to an impact parameter of 0.3 arcsec and a 3-sigma detection limit of 26.8 mag per arcsec^2. In the narrow-band image, we probed down to a 0 arcsec impact parameter and detected nothing down to a 3-sigma detection limit of about 3x10-17 erg s^-1 cm^-2 . We did detect a bright Lyman-alpha emitter 59 arcsec south of Q1218+0832 with a flux of 3x10^-16 erg s^-1 cm^-2 . We conclude that the DLA galaxy must be located at a very small impact parameter (<0.3 arcsec, 2.5 kpc) or it is optically dark. Also, the DLA galaxy most likely is part of a galaxy group.

astro-ph.GA↗

arXiv2023

The cosmic build-up of dust and metals. Accurate abundances from GRB-selected star-forming galaxies at $1.7 < z < 6.3$

The chemical enrichment of dust and metals in the interstellar medium (ISM) of galaxies throughout cosmic time is one of the key driving processes of galaxy evolution. Here we study the evolution of the gas-phase metallicities, dust-to-gas (DTG), and dust-to-metal (DTM) ratios of 36 star-forming galaxies at $1.7 < z < 6.3$ probed by gamma-ray bursts (GRBs). We compile all GRB-selected galaxies with intermediate (R=7000) to high (R>40,000) resolution spectroscopic data for which at least one refractory (e.g. Fe) and one volatile (e.g. S or Zn) element have been detected at S/N>3. This is to ensure that accurate abundances and dust depletion patterns can be obtained. We first derive the redshift evolution of the dust-corrected, absorption-line based gas-phase metallicity [M/H]$_{\rm tot}$ in these galaxies, for which we determine a linear relation with redshift ${\rm [M/H]_{tot}}(z) = (-0.21\pm 0.04)z -(0.47\pm 0.14)$. We then examine the DTG and DTM ratios as a function of redshift and through three orders of magnitude in metallicity, quantifying the relative dust abundance both through the direct line-of-sight visual extinction $A_V$ and the derived depletion level. We use a novel method to derive the DTG and DTM mass ratios for each GRB sightline, summing up the mass of all the depleted elements in the dust-phase. We find that the DTG and DTM mass ratios are both strongly correlated with the gas-phase metallicity and show a mild evolution with redshift as well. While these results are subject to a variety of caveats related to the physical environments and the narrow pencil-beam sightlines through the ISM probed by the GRBs, they provide strong implications for studies of dust masses to infer the gas and metal content of high-redshift galaxies, and particularly demonstrate the large offset from the average Galactic value in the low-metallicity, high-redshift regime.

astro-ph.GA↗

arXiv2023

Proximate molecular quasar absorbers: Chemical enrichment and kinematics of the neutral gas

Proximate molecular quasar absorbers (PH2) are an intriguing population of absorption systems recently uncovered through strong H2 absorption at small velocity separation from the background quasars. We performed a multi-wavelength spectroscopic follow-up of thirteen such systems with VLT/X-Shooter. Here, we present the observations and study the overall chemical enrichment measured from the HI, H2 and metal lines. We combine this with an investigation of the neutral gas kinematics with respect to the quasar host. We find gas-phase metallicities in the range 2% to 40% of the Solar value, i.e. in the upper-half range of HI-selected proximate damped Lyman-alpha systems, but similar to what is seen in intervening H2-bearing systems. This is likely driven by similar selection effects that play against the detection of most metal and molecular rich systems in absorption. Differences are however seen in the abundance of dust (from [Zn/Fe]) and its depletion pattern, when compared to intervening systems, possibly indicating different dust production or destruction close to the AGN. We also note the almost-ubiquitous presence of a high-ionisation phase traced by NV in proximate systems. In spite of the hard UV field from the quasars, we found no strong overall deficit of neutral argon, at least when compared to intervening DLAs. This likely results from argon being mostly neutral in the H2 phase, which actually accounts for a large fraction of the total amount of metals. We measure the quasar systemic redshifts through emission lines from both ionised gas and CO(3-2) emission, the latter being detected in all 6 cases for which we obtained 3-mm data from complementary NOEMA observations. For the first time, we observe a trend between line-of-sight velocity with respect to systemic redshift and metallicity of the absorbing gas. [truncated]

astro-ph.GA↗

arXiv2023

The MeerKAT Absorption Line Survey: Homogeneous continuum catalogues towards a measurement of the cosmic radio dipole

The number counts of homogeneous samples of radio sources are a tried and true method of probing the large scale structure of the Universe, as most radio sources outside the galactic plane are at cosmological distances. As such they are expected to trace the cosmic radio dipole, an anisotropy analogous to the dipole seen in the cosmic microwave background (CMB). Results have shown that although the cosmic radio dipole matches the direction of the CMB dipole, it has a significantly larger amplitude. This result challenges our assumption of the Universe being isotropic, which can have large repercussions for the current cosmological paradigm. Though significant measurements have been made, sensitivity to the radio dipole is generally hampered by systematic effects that can cause large biases in the measurement. Here we assess these systematics with data from the MeerKAT Absorption Line Survey (MALS). We present the analysis of ten MALS pointings, focusing on systematic effects that could lead to an inhomogeneous catalogue. We describe the calibration and creation of full band continuum images and catalogues, producing a combined catalogue containing 16,313 sources and covering 37.5 square degrees of sky down to a sensitivity of 10 $μ$Jy/beam. We measure the completeness, purity, and flux recovery statistics for these catalogues using simulated data. We investigate different source populations in the catalogues by looking at flux densities and spectral indices, and how they might influence source counts. Using the noise characteristics of the pointings, we find global measures that can be used to correct for the incompleteness of the catalogue, producing corrected number counts down to 100 - 200 $μ$Jy. We show that we can homogenise the catalogues and properly account for systematic effects. We determine that we can measure the dipole to $3σ$ significance with 100 MALS pointings.

astro-ph.CO↗

arXiv2023

Chemical diversity of gas in distant galaxies: The metal and dust enrichment and variations within absorbing galaxies

The chemical composition of gas in galaxies can be measured in detail from absorption spectroscopy. By studying gas in galaxies in this way, it is possible to investigate the small and faint galaxies, which are the most numerous in the universe. In particular, the chemical distribution of gas in absorbing systems gives us insight into cycles of gas in and around galaxies. Here we study chemical enrichment within 64 Damped Lyman-alpha Absorption (DLA) systems between $1.7 < z < 4.2$. We use high-resolution spectra from VLT/UVES to infer dust depletion from relative abundances of several metals. We perform a component-by-component analysis within DLAs, and characterise variations in their chemical enrichment. Unlike hydrogen, the metal columns can be characterised for individual components. We use them to derive the dust depletion ([Zn/Fe]fit), as an indicator for chemical enrichment. We find that some DLAs are chemically diverse within themselves, with [Zn/Fe]fit ranging up to 0.62 dex within a single system. This suggests that absorbing gas within these galaxies is chemically diverse. Although we do not find a clear trend of decreasing dust depletion with redshift, we do see that the most chemically enriched systems are at lower redshifts. We also observe evidence for dust-poor components at all redshifts, which may be due to the accretion of pristine gas onto galaxies. We combine the chemical and kinematic properties of the individual gas components and observe potential signatures of infalling gas, with low depletion at velocities below $\sim$100km/s, and outflows, with high depletion and velocities of $\sim$600km/s. We find over-abundances of alpha-elements (an enhancement of $\sim$0.3dex) and under-abundances of Mn in several components, which is likely a signature of core-collapse SNe nucleosythesis in the ISM. We observe these effects mostly at lower levels of chemical enrichment.

astro-ph.GA↗

arXiv2022

Mapping HI 21-cm in the Klemola 31 group at z = 0.029: emission and absorption towards PKS2020-370

We present MeerKAT Absorption Line Survey (MALS) observations of the HI gas in the Klemola31 galaxy group ($z=0.029$), located along the line of sight to the radio-loud quasar PKS2020-370 ($z=1.048$). Four galaxies of the group are detected in HI emission, and HI absorption is also detected in front of PKS2020-370 in Klemola31A. The emission and absorption are somewhat compensating on the line of sight of the quasar, and the derived column density of the absorption appears under-estimated, with respect to the neighbouring emission. A symmetric tilted-ring model of Klemola31A, assuming the absorbing gas in regular rotation in the plane, yields a rather high spin temperature of 530K. An alternative interpretation is that the absorbing gas is extra-planar, which will also account for its non-circular motion. The NaI/CaII ratio also suggests that the absorbing gas is unrelated to cold HI disk. Two of the galaxies in the Klemola group are interacting with a small companion, and reveal typical tidal tails, and velocity perturbations. Only one of the galaxies, ESO400-13, reveals a strong HI deficiency, and a characteristic ram-pressure stripping, with a total asymmetry in the distribution of its gas. Since a small galaxy group as Klemola31 is not expected to host a dense intra-group gas, this galaxy must be crossing the group at a very high velocity, mostly in the sky plane.

astro-ph.GA↗

arXiv2022

First discoveries and localisations of Fast Radio Bursts with MeerTRAP: a real-time, commensal MeerKAT survey

We report on the discovery and localization of fast radio bursts (FRBs) from the MeerTRAP project, a commensal fast radio transient-detection programme at MeerKAT in South Africa. Our hybrid approach combines a coherent search with an average field-of-view of 0.4 $\rm deg^{2}$ with an incoherent search utilizing a field-of-view of $\sim$1.27 $\rm deg^{2}$ (both at 1284~MHz). Here, we present results on the first three FRBs: FRB 20200413A (DM=1990.05 pc cm$^{-3}$), FRB 20200915A (DM=740.65 pc cm$^{-3}$), and FRB 20201123A (DM=433.55 pc cm$^{-3}$). FRB 20200413A was discovered only in the incoherent beam. FRB 20200915A (also discovered only in the incoherent beam) shows speckled emission in the dynamic spectrum which cannot be explained by interstellar scintillation in our Galaxy or plasma lensing, and might be intrinsic to the source. FRB 20201123A shows a faint post-cursor burst about 200 ms after the main burst and warrants further follow-up to confirm whether it is a repeating FRB. FRB 20201123A also exhibits significant temporal broadening consistent with scattering by a turbulent medium. The broadening exceeds that predicted for medium along the sightline through our Galaxy. We associate this scattering with the turbulent medium in the environment of the FRB in the host galaxy. Within the approximately $1'$ localization region of FRB 20201123A, we identify one luminous galaxy ($r \approx 15.67$; J173438.35$-$504550.4) that dominates the posterior probability for a host association. The galaxy's measured properties are consistent with other FRB hosts with secure associations.

astro-ph.HE↗

arXiv2022

HI gas playing hide-and-seek around a powerful FRI-type quasar at z$\sim$2.1

We present optical spectroscopic and milli-arcsecond scale radio continuum observations of the quasar M1540-1453 ($z_{em}$ = 2.104$\pm$0.002) that shows associated HI 21-cm absorption at $z_{abs}$ = 2.1139. At sub-kpc scales, the powerful radio source with 1.4 GHz luminosity of $5.9\times10^{27}$ WHz$^{-1}$ shows Fanaroff-Riley (FR) class I morphology caused by the interaction with dense gas within 70 pc from the AGN. Interestingly, while there are indications for the presence of absorption from low-ionization species like FeII, SiII and SiIII in the optical spectrum, the expected strong damped Ly$α$ absorption is not detected at the redshift of the HI 21-cm absorber. In comparison to typical high-$z$ quasars, the Ly$α$ emission line is much narrower. The `ghostly' nature of the HI Ly$α$ absorber partially covering the broad line region of extent 0.05 pc and the detection of widespread HI 21-cm absorption covering the diffuse radio source (extent $>$425 pc) imply the presence of a large clumpy HI halo -- which may have been blown by the jet-ISM interaction. Further observations are needed to confirm the `ghostly' nature of the Ly$α$ absorber, and obtain a better understanding of the role played by the jet-ISM interaction in shaping the radio morphology of this powerful AGN. The study showcases how joint radio and optical analysis can shed light on gaseous environment and origin of radio morphology in AGN at high redshifts, when these are still the assembly sites of giant galaxies.

astro-ph.GA↗

arXiv2022

Extremely strong DLAs at high redshift: Gas cooling and H$_2$ formation

We present a spectroscopic investigation with VLT/X-shooter of seven candidate extremely strong damped Lyman-$α$ absorption systems (ESDLAs, $N(\text{HI})\ge 5\times 10^{21}$ cm$^{-2}$) observed along quasar sightlines. We confirm the extremely high column densities, albeit slightly (0.1~dex) lower than the original ESDLA definition for four systems. We measured low-ionisation metal abundances and dust extinction for all systems. For two systems we also found strong associated H$_2$ absorption $\log N(\text{H$_2$)[cm$^{-2}$]}=18.16\pm0.03$ and $19.28\pm0.06$ at $z=3.26$ and $2.25$ towards J2205+1021 and J2359+1354, respectively), while for the remaining five we measured conservative upper limits on the H$_2$ column densities of typically $\log N(\text{H$_2$)[cm$^{-2}$]}<17.3$. The increased H$_2$ detection rate ($10-55$% at 68% confidence level) at high HI column density compared to the overall damped Lyman-$α$ population ($\sim 5-10$%) confirms previous works. We find that these seven ESDLAs have similar observed properties as those previously studied towards quasars and gamma-ray burst afterglows, suggesting they probe inner regions of galaxies. We use the abundance of ionised carbon in excited fine-structure level to calculate the cooling rates through the CII $λ$158$μ$m emission, and compare them with the cooling rates from damped Lyman-$α$ systems in the literature. We find that the cooling rates distribution of ESDLAs also presents the same bimodality as previously observed for the general (mostly lower HI column density) damped Lyman-$α$ population.

astro-ph.GA↗

arXiv2021

GRB host galaxies with strong H$_2$ absorption: CO-dark molecular gas at the peak of cosmic star formation

We present a pilot search of CO emission in three H$_2$-absorbing, long-duration gamma-ray burst (GRB) host galaxies at z~2-3. We used the Atacama Large Millimeter/sub-millimeter Array (ALMA) to target the CO(3-2) emission line and report non-detections for all three hosts. These are used to place limits on the host molecular gas masses, assuming a metallicity-dependent CO-to-H$_2$ conversion factor ($α_{\rm CO}$). We find, $M_{\rm mol} < 3.5\times 10^{10}\,M_{\odot}$ (GRB\,080607), $M_{\rm mol} < 4.7\times 10^{11}\,M_{\odot}$ (GRB\,120815A), and $M_{\rm mol} < 8.9\times 10^{11}\,M_{\odot}$ (GRB\,181020A). The high limits on the molecular gas mass for the latter two cases are a consequence of their low stellar masses $M_\star$ ($M_\star \lesssim 10^{8}\,M_{\odot}$) and low gas-phase metallicities ($Z\sim 0.03\,Z_{\odot}$). The limit on the $M_{\rm mol}/M_\star$ ratio derived for GRB\,080607, however, is consistent with the average population of star-forming galaxies at similar redshifts and stellar masses. We discuss the broader implications for a metallicity-dependent CO-to-H$_2$ conversion factor, and demonstrate that the canonical Galactic $α_{\rm CO}$, will severely underestimate the actual molecular gas mass for all galaxies at $z>1$ with $M_\star < 10^{10}\,M_\odot$. To better quantify this we develop a simple approach to estimate the relevant $α_{\rm CO}$ factor based only on the redshift and stellar mass of individual galaxies. The elevated conversion factors will make these galaxies appear CO-"dark" and difficult to detect in emission, as is the case for the majority of GRB hosts. GRB spectroscopy thus offers a complementary approach to identify low-metallicity, star-forming galaxies with abundant molecular gas reservoirs at high redshifts that are otherwise missed by current ALMA surveys.

astro-ph.GA↗