Gas-rich Galaxies Research Articles

The Smallest Scale of Hierarchy Survey (SSH)

We present new deep, wide-field imaging data from the Large Binocular Telescope (LBT) in g and r bands from the Smallest Scale of Hierarchy Survey (SSH) that reveal previously undetected tidal features and stellar streams in the outskirts of six dwarf irregular galaxies (NGC 5238, UGC 6456, UGC 6541, UGC 7605, UGC 8638, and UGC 8760) with stellar masses in the range 1.2 × 107 M⊙ to 1.4 × 108 M⊙. The six dwarfs are located 1-2 Mpc away from large galaxies, which implies that the observed distortions are unlikely to be due to tidal effects from a nearby, massive companion. At the distances of ~3-4 Mpc at which the dwarfs lie, the identified tidal features are all resolved into individual stars in the LBT images and appear to consist of a population older than 1–2 Gyr. This excludes the possibility that they result from irregular and asymmetric star formation episodes that are common in gas-rich dwarf galaxies. The most plausible explanation is that we witness the hierarchical merging assembly of these dwarfs with their satellite populations. This scenario is also supported by the peculiar morphology and disturbed velocity field of their HI component. From the SSH sample, we estimate that a fraction of ~13% of the late-type dwarfs show signs of merging with satellites. This is in agreement with other recent independent studies and theoretical predictions within the ΛCDM cosmological framework.

A Tale of NGC 3785: The formation of an ultra-diffuse galaxy at the end of the longest tidal tail

We present the discovery of an extended and faint tail observed in the isolated environment associated with galaxy NGC\,3785. This study additionally provides observational evidence supporting the formation of ultra-diffuse galaxies (UDGs) at the end of the tail. We utilized the Gnuastro software to detect and analyze the low surface brightness structures in the optical $g$- and $r$-bands using data from the Dark Energy Camera Legacy Survey. We created a detection map to identify the faint tail and measured its length using cubic spline fitting. Additionally, we found 84 star-forming clumps along the tail and performed photometric analysis on the tail portion after applying a significance threshold on the signal-to-noise ratio . We have measured the projected length of the tail, which is sim 390 kpc. We propose that this tail arises from the interaction of the NGC\,3785 with a gas-rich galaxy, which ends up as a UDG at the end of the tail.

Massive star cluster formation

The mode of star formation that results in the formation of globular clusters and young massive clusters is difficult to constrain through observations. We present models of massive star cluster formation using the TORCH framework, which uses the Astrophysical MUltipurpose Software Environment (AMUSE) to couple distinct multi-physics codes that handle star formation, stellar evolution and dynamics, radiative transfer, and magnetohydrodynamics. We upgraded TORCH by implementing the N-body code PETAR, thereby enabling TORCH to handle massive clusters forming from 106 M⊙ clouds with ≥105 individual stars. We present results from TORCH simulations of star clusters forming from 104, 105, and 106 M⊙ turbulent spherical gas clouds (named M4, M5, M6) of radius R = 11.7 pc. We find that star formation is highly efficient and becomes more so at a higher cloud mass and surface density. For M4, M5, and M6 with initial surface densities 2.325 × 101,2,3 M⊙ pc−2, after a free-fall time of tff = 6.7,2.1,0.67 Myr, we find that ∼30%, 40%, and 60% of the cloud mass has formed into stars, respectively. The end of simulation-integrated star formation efficiencies for M4, M5, and M6 are ϵ⋆ = M⋆/Mcloud = 36%, 65%, and 85%. Observations of nearby clusters similar in mass and size to M4 have instantaneous star formation efficiencies of ϵinst ≤ 30%, which is slightly lower than the integrated star formation efficiency of M4. The M5 and M6 models represent a different regime of cluster formation that is more appropriate for the conditions in starburst galaxies and gas-rich galaxies at high redshift, and that leads to a significantly higher efficiency of star formation. We argue that young massive clusters build up through short efficient bursts of star formation in regions that are sufficiently dense (Σ ≥ 102 M⊙ pc−2) and massive (Mcloud ≥ 105 M⊙). In such environments, stellar feedback from winds and radiation is not strong enough to counteract the gravity from gas and stars until a majority of the gas has formed into stars.

The generation of a multiphase medium in ‘Splash’ bridge systems: towards an understanding of star formation suppression in turbulent galaxy systems

ABSTRACT Cloud–cloud collisions in splash bridges produced in gas-rich disc galaxy collisions offer a brief but interesting environment to study the effects of shocks and turbulence on star formation rates in the diffuse intergalactic medium, far from the significant feedback effects of massive star formation and active galactic nucleus. Expanding on our earlier work, we describe simulated collisions between counter-rotating disc galaxies of relatively similar mass, focusing on the thermal and kinematic effects of relative inclination and disc offset at the closest approach. This includes essential heating and cooling signatures, which go some way towards explaining the luminous power in H$_2$ and [C ii] emission in the Taffy bridge, as well as providing a partial explanation of the turbulent nature of the recently observed compact CO-emitting clouds observed in Taffy by the Atacama Large Millimeter Array (ALMA). The models show counter-rotating disc collisions result in swirling, shearing kinematics for the gas in much of the post-collision bridge. Gas with little specific angular momentum due to collisions between counter-rotating streams accumulates near the centre of mass. The disturbances and mixing in the bridge drive continuing cloud collisions, differential shock heating, and cooling throughout. A wide range of relative gas phases and line-of-sight velocity distributions are found in the bridges, depending sensitively on initial disc orientations, and the resulting variety of cloud collision histories. Most cloud collisions can occur promptly or persist for quite a long duration. Cold and hot phases can largely overlap throughout the bridge or can be separated into different parts of the bridge.

The origin of the H α line profiles in simulated disc galaxies

ABSTRACT Observations of ionized H $\alpha$ gas in high-redshift disc galaxies have ubiquitously found significant line broadening, $\sigma _{\rm H\,\alpha }\sim 10{\!-\!}100\, {\rm km\, s^{-1}}$. To understand whether this broadening reflects gas turbulence within the interstellar medium (ISM) of galactic discs, or arises from out-of-plane emission in mass-loaded outflows, we perform radiation hydrodynamic simulations of isolated Milky Way-mass disc galaxies in a gas-poor (low-redshift) and gas rich (high-redshift) condition and create mock H $\alpha$ emission line profiles. We find that the majority of the total (integrated) ${\rm H\,\alpha }$ emission is confined within the ISM, with extraplanar gas contributing ${\sim} 45~{{\ \rm per\ cent}}$ of the extended profile wings ($v_z\ge 200$${\, \rm {km\, s^{-1}} }$) in the gas-rich galaxy. This substantiates using the ${\rm H\,\alpha }$ emission line as a tracer of mid-plane disc dynamics. We investigate the relative contribution of diffuse and dense ${\rm H\,\alpha }$ emitting gas, corresponding to diffuse ionized gas (DIG; $\rho \lesssim 0.1\, {\rm cm^{-3}}$, $T\sim 8\, 000$ K) and H ii regions ($\rho \gtrsim 10\, {\rm cm^{-3}}$, $T\sim 10\, 000$ K), respectively, and find that DIG contributes $f_{\rm DIG}\lesssim 10~{{\ \rm per\ cent}}$ of the total L$_{\rm H\alpha }$. However, the DIG can reach upwards of $\sigma _{\rm H\,\alpha } \sim 60{\!-\!}80\, {\rm km\, s^{-1}}$ while the H ii regions are much less turbulent $\sigma _{\rm H\,\alpha }\sim 10{\!-\!}40\, {\rm km\, s^{-1}}$. This implies that the $\sigma _{\rm H\,\alpha }$ observed using the full ${\rm H\,\alpha }$ emission line is dependent on the relative ${\rm H\,\alpha }$ contribution from DIG/H ii regions and a larger $f_{\rm DIG}$ would shift $\sigma _{\rm H\,\alpha }$ to higher values. Finally, we show that $\sigma _{\rm H\,\alpha }$ evolves, in both the DIG and H ii regions, with the galaxy gas fraction. Our high-redshift equivalent galaxy is roughly twice as turbulent, except for in the DIG which has a more shallow evolution.

Examining the Nature of the Starless Dark Matter Halo Candidate Cloud-9 with Very Large Array Observations

Observations with the Five-hundred-meter Aperture Spherical Telescope recently detected H i 21 cm emission near M94, revealing an intriguing object, Cloud-9, without an optical counterpart. Subsequent analysis suggests that Cloud-9 is consistent with a gas-rich (M H I ≈ 106 M ⊙), starless, dark matter (DM) halo of mass M 200 ≈ 5 × 109 M ⊙. Using the Karl G. Jansky Very Large Array in D-array configuration, we present interferometric observations of Cloud-9, revealing it as a dynamically cold (W 50 ≈ 12 km s−1), nonrotating, and spatially asymmetric system, exhibiting gas compression on one side and a tail-like structure toward the other—features likely originating from ram pressure. Our observations suggest Cloud-9 is consistent with a starless ΛCDM DM halo if the gas is largely isothermal. If interpreted as a faint dwarf, Cloud-9 is similar to Leo T, a nearby gas-rich galaxy that would fall below current optical detection limits at Cloud-9's distance (d ≈ 5 Mpc). Further observations with the Hubble Space Telescope reaching magnitudes m g ≈ 30 would help identify such a galaxy or dramatically lower the current limits on its stellar mass (M gal ≲ 105 M ⊙). Cloud-9 thus stands as the firmest starless DM halo candidate to date or the faintest galaxy known at its distance.

Merging Gas-rich Galaxies That Harbor Low-luminosity Twin Quasars at z = 6.05: A Promising Progenitor of the Most Luminous Quasars

We present Atacama Large Millimeter/submillimeter Array [C ii] 158 μm line and underlying far-IR continuum emission observations (0.″57 × 0.″46 resolution) toward a quasar–quasar pair system recently discovered at z = 6.05. The quasar nuclei (C1 and C2) are faint (M 1450 ≳ −23 mag), but we detect very bright [C ii] emission bridging the 12 kpc between the two objects and extending beyond them (total luminosity L [C ii] ≃ 6 × 109 L ⊙). The [C ii]-based total star formation rate of the system is ∼550 M ⊙ yr−1 (the IR-based dust-obscured star formation is ∼100 M ⊙ yr−1), with a [C ii]-based total gas mass of ∼1011 M ⊙. The dynamical masses of the two galaxies are large (∼9 × 1010 M ⊙ for C1 and ∼5 × 1010 M ⊙ for C2). There is a smooth velocity gradient in [C ii], indicating that these quasars are a tidally interacting system. We identified a dynamically distinct, fast-[C ii] component around C1: detailed inspection of the line spectrum there reveals the presence of a broad-wing component, which we interpret as the indication of fast outflows with a velocity of ∼600 km s−1. The expected mass-loading factor of the outflows, after accounting for multiphase gas, is ≳2 − 3, which is intermediate between AGN-driven and starburst-driven outflows. Hydrodynamic simulations in the literature predict that this pair will evolve to a luminous (M 1450 ≲ −26 mag), starbursting (≳1000 M ⊙ yr−1) quasar after coalescence, one of the most extreme populations in the early Universe.

Corvus A: A Low-mass, Isolated Galaxy at 3.5 Mpc

We report the discovery of Corvus A, a low-mass, gas-rich galaxy at a distance of approximately 3.5 Mpc, identified in DR10 of the Dark Energy Camera Legacy Imaging Survey during the initial phase of our ongoing SEmi-Automated Machine LEarning Search for Semi-resolved galaxies (SEAMLESS). Jansky Very Large Array observations of Corvus A detect H i line emission at a radial velocity of 523 ± 2 km s−1. Magellan/Megacam imaging reveals an irregular and complex stellar population with both young and old stars. We detect UV emission in Neil Gehrels Swift observations, indicative of recent star formation. However, there are no signs of H ii regions in Hα imaging from Steward Observatory’s Kuiper telescope. Based on the Megacam color–magnitude diagram we measure the distance to Corvus A via the tip of the red giant branch standard candle as 3.48 ± 0.24 Mpc. This makes Corvus A remarkably isolated, with no known galaxy within ∼1 Mpc. Based on this distance, we estimate the H i and stellar mass of Corvus A to be logMHI/M⊙=6.59 and logM*/M⊙=6.0 , respectively. Although there are some signs of rotation, the H i distribution of Corvus A appears to be close to face on, analogous to that of Leo T, and we therefore do not attempt to infer a dynamical mass from its H i line width. Higher-resolution synthesis imaging is required to confirm this morphology and to draw robust conclusions from its gas kinematics.

Subaru Suprime-Cam Wide-field BVI Stellar Photometry of the M33 Galaxy

We have surveyed the complete extent of the disk of M33—a gas-rich low-mass dwarf spiral galaxy in the Local Group. The B-, V-, and I-passband (the Johnson–Cousins system) CCD images (typical seeing ∼0.″8) were obtained with the Subaru Telescope equipped with the Suprime-Cam mosaic camera. The wide-field (∼1.°0 × 1.°5) catalog of 803,095 (15 ≤ V ≤ 25) starlike objects, measured using the point-spread function and aperture photometry techniques, is presented. We determined the distance modulus of M33 using the tip of the red giant branch (I TRGB = 20.64 ± 0.02) as a reference point of (m − M)0 = 24.63 ± 0.02stat ± 0.06syst (843 kpc). We found young (≲100 Myr) stellar populations residing up to the deprojected radius of ∼10 kpc. The scale length of the young main-sequence (MS) star surface-number density in the range of radial distances from 7 to 9 kpc is 0.53 ± 0.03 kpc. The youngest MS stars (≲15 Myr) reside up to the radius of ∼8 kpc. This distribution of stellar populations may suggest an outside-in scenario of recent star formation in the disk of M33.

Globular cluster orbital decay in dwarf galaxies with MOND and CDM: Impact of supernova feedback

Dynamical friction works very differently for Newtonian gravity with dark matter and in modified Newtonian dynamics (MOND). While the absence of dark matter considerably reduces the friction in major galaxy mergers, analytic calculations indicate the opposite for very small perturbations, such as globular clusters (GCs) sinking in dwarf galaxies. Here, we study the decay of GCs in isolated gas-rich dwarf galaxies using simulations with the Phantom of Ramses code, which enables both the Newtonian and the QUMOND MOND gravity. We modeled the GCs as point masses, and we simulated the full hydrodynamics, with star formation and supernovae feedback. We explored whether the fluctuations in gravitational potential caused by the supernovae can prevent GCs from sinking toward the nucleus. For GCs of typical mass or lighter, we find that this indeed works in both Newtonian and MOND simulations. The GC can even make a random walk . However, we find that supernovae cannot prevent massive GCs ($M from sinking in MOND. The resulting object looks similar to a galaxy with an offset core, which embeds the sunk GC. The problem is much milder in the Newtonian simulations. This result thus favors Newtonian over QUMOND gravity, but we note that it relies on the correctness of the difficult modeling of baryonic feedback. We propose that the fluctuations in the gravitational potential could be responsible for the thickness of the stellar disks of dwarf galaxies and that strong supernova winds in modified gravity can transform dwarf galaxies into ultra-diffuse galaxies.

ALMA Reveals Spatially Resolved Properties of Molecular Gas in the Host Galaxy of FRB 20191001A at z = 0.2340

We report the detection of the CO(2–1) emission line with a spatial resolution of 0.″9 (3.5 kpc) from the host galaxy of the fast radio burst (FRB), FRB 20191001A at z = 0.2340, using the Atacama Large Millimeter/submillimeter Array. This is the first detection of spatially resolved CO emission from the host galaxy of an FRB at a cosmological distance. The inferred molecular gas mass of the host galaxy is (2.3 ± 0.4) × 1010 M ⊙, indicating that it is gas-rich, as evidenced by the measured molecular gas fraction μ gas = 0.50 ± 0.22. This molecular gas mass and the star formation rate of the host, SFR = (8.06 ± 2.42) M ⊙ yr−1, differ from those observed in the other FRB host galaxies with the average M gas = 9.6 × 108 M ⊙ and SFR = 0.90M ⊙ yr−1. This lends further credibility to the hypothesis that FRBs may originate from single or multiple progenitors across a diverse range of galaxy environments. Based on the observed velocity field modeling, we find that the molecular gas disk is dominated by an ordered circular rotation, despite the fact that the host galaxy has a gas-rich companion galaxy with a projected separation of ∼25 kpc. The formation of the FRB’s progenitor might not have been triggered by this interaction. We derive the 3σ upper limit of the molecular gas column density at the FRB detection site to be <2.1 × 1021 cm−2 with a 3σ upper limit.

From giant clumps to clouds

The clumpy nature of gas-rich galaxies at cosmic noon raises the question of universality of the scaling relations and average properties of the star-forming structures. Using controlled simulations of disk galaxies and varying only the gas fraction, we show that the influence of the galactic environments (large-scale turbulence, tides, and shear) contributes, together with the different regime of instabilities, to setting a diversity of physical conditions for the formation and evolution of gas clumps from low to high gas fractions. However, the distributions of gas clumps at all gas fractions follow similar scaling relations as Larson’s, suggesting the universality of median properties. Yet, we find that the scatter around these relations significantly increases with the gas fraction, allowing for the presence of massive, large, and highly turbulent clouds in gas-rich disks in addition to a more classical population of clouds. Clumps with an excess of mass for their size are slightly denser, more centrally concentrated, and host more abundant and faster star formation. We find that the star formation activity (rate, efficiency, and depletion time) correlates much more strongly with the excess of mass than with the mass itself. Our results suggest the existence of universal scaling relations for gas clumps but with redshift-dependent scatters, which calls for deeper and more complete census of the populations of star-forming clumps and young stellar clusters at cosmic noon and beyond.

Galaxies with grains: unraveling dust evolution and extinction curves with hydrodynamical simulations

Dust in galaxies is an important tracer of galaxy properties and their evolution over time. The physical origin of the grain size distribution, the dust chemical composition, and, hence, the associated ultraviolet-to-optical extinctions in diverse galaxies remains elusive. To address this issue, we introduce a model for dust evolution in the RAMSES code for simulations of galaxies with a resolved multiphase interstellar medium. Dust is modelled as a fluid transported with the gas component, and is decomposed into two sizes, 5 nm and 0.1 μm, and two chemical compositions for carbonaceous and silicate grains. This dust model includes the growth of dust by accretion of elements from the gas phase and by the release of dust in stellar ejecta, the destruction by thermal sputtering, supernovae, and astration, and the exchange of dust mass between the two main populations of grain sizes by coagulation and shattering. Using a suite of isolated disc simulations with different masses and metallicities, the simulations can explore the role of these processes in shaping the key properties of dust in galaxies. The simulated Milky Way analogue reproduces the dust-to-metal mass ratio, depletion factors, size distribution and extinction curves of the Milky Way. Galaxies with lower metallicities reproduce the observed decrease in the dust-to-metal mass ratio with metallicity at around a few 0.1 Z⊙. This break in the dust-to-metal ratio corresponds to a galactic gas metallicity threshold that marks the transition from an ejecta-dominated to an accretion-dominated grain growth, and that is different for silicate and carbonaceous grains, with ≃0.1 Z⊙ and ≃0.5 Z⊙ respectively. This leads to more Magellanic Cloud-like extinction curves, i.e. with steeper slopes in the ultraviolet and a weaker bump feature at 2175 Å, in galaxies with lower masses and lower metallicities. Steeper slopes in these galaxies are caused by the combination of the higher efficiency of gas accretion by silicate relative to carbonaceous grains and by the low rates of coagulation that preserves the amount of small silicate grains. Weak bumps are due to the overall inefficient accretion growth of carbonaceous dust at low metallicity, whose growth is mostly supported by the release of large grains in SN ejecta. We also show that the formation of CO molecules is a key component to limit the ability of carbonaceous dust to grow, in particular in low-metallicity gas-rich galaxies.

MHONGOOSE discovery of a gas-rich low surface brightness galaxy in the Dorado group

We present the discovery of a low-mass, gas-rich low surface brightness galaxy in the Dorado group, at a distance of 17.7 Mpc. Combining deep MeerKAT 21-cm observations from the MeerKAT Observations of Nearby Galactic Objects: Observing Southern Emitters (MHONGOOSE) survey with deep photometric images from the VST Early-type Galaxy Survey (VEGAS) we find a stellar and neutral atomic hydrogen ( gas mass of $M_ and respectively. This low surface brightness galaxy is the lowest-mass detection found in a group beyond the local Universe ($D 10$ Mpc). The dwarf galaxy has the typical overall properties of gas-rich low surface brightness galaxies in the Local group, but with some striking differences. Namely, the MHONGOOSE observations reveal a very low column density ($ disk with asymmetrical morphology possibly supported by rotation and higher velocity dispersion in the centre. There, deep optical photometry and UV observations suggest a recent enhancement of the star formation. Found at galactocentric distances where in the Local Group dwarf galaxies are depleted of cold gas (at a projected distance of $390$ kpc from the group centre), this galaxy is likely on its first orbit within the Dorado group. We discuss the possible environmental effects that may have caused the formation of the disk and the enhancement of star formation (SF), highlighting the short-lived phase (a few hundreds million years) of the gaseous disk, before either SF or hydrodynamical forces will deplete the gas of the galaxy.

H i galaxy signatures in the SARAO MeerKAT galactic plane survey − III. Unveiling the obscured part of the Vela Supercluster

ABSTRACT We conducted a search for $\textrm {H}\, \scriptstyle \mathrm{I}$ emission of the gas-rich galaxies in the Vela region (260° ≤ ℓ ≤ 290°, −2° ≤ b ≤ 1°) to explore the Vela Supercluster (VSCL) at Vhel ∼ 18 000 $\rm km\, s^{-1}$, largely obscured by Galactic dust. Within the mostly Radio Frequency Interference-free band (250 &amp;lt; Vhel &amp;lt; 25 000 $\rm km\, s^{-1}$) of MeerKAT, the analysis focuses on 157 hexagonally distributed pointings extracted from the South African Radio Astronomy Observatory MeerKAT Galactic Plane Survey located in the Vela region (Vela−SMGPS). These were combined into 10 contiguous mosaics, covering an ∼90 square degrees area. Among the 843 $\textrm {H}\, \scriptstyle \mathrm{I}$ detected sources, 39 were previously discovered in the H i Parkes Zone of Avoidance survey (Vhel &amp;lt; 12 000 $\rm km\, s^{-1}$; rms ∼ 6 $\rm mJy\, beam^{-1}$). With the improved rms level of the Vela−SMGPS, i.e. 0.29–0.56 $\rm mJy\, beam^{-1}$, our study unveils nearly 12 times more detections (471 candidates) in that same velocity range. We furthermore could identify 187 galaxy candidates with an $\textrm {H}\, \scriptstyle \mathrm{I}$ mass limit reaching $\log (M_{\rm HI}/\rm {\rm M}_{\odot }) = 9.44$ in the VSCL velocity range Vhel ∼ 19 500 ± 3500 $\rm km\, s^{-1}$. We find indications of two wall-like overdensities that confirm the original suspicion that these walls intersect at low latitudes around longitudes of ℓ ∼ 272°–278°. We also find a strong signature most likely associated with the Hydra/Antlia extension and evidence of a previously unknown narrow filament at Vhel ∼ 12 000 $\rm km\, s^{-1}$. This paper demonstrates the efficiency of systematic $\textrm {H}\, \scriptstyle \mathrm{I}$ surveys with the Square Kilometre Array (SKA) precursor MeerKAT, even in the most obscured part of the Zone of Avoidance.

The cold molecular gas in z ≳ 6 quasar host galaxies

Context. Probing the molecular gas reservoirs of z ≳ 6 quasar (QSO) host galaxies is fundamental to understanding the coevolution of star formation and black hole growth in these extreme systems. Yet, there is still an inhomogeneous coverage of molecular gas tracers for z ≳ 6 QSO hosts. Aims. To measure the average excitation and mass of the molecular gas reservoirs in the brightest z &gt; 6.5 QSO hosts, we combined new observations of CO(2–1) emission with existing observations of CO(6–5), CO(7–6), [C I] (2–1), [C II] 158 μm, and dust-continuum emission. Methods. We reduced and analysed observations of CO(2–1), taken with the Karl G. Jansky Very Large Array, in three z = 6.5 − 6.9 QSO hosts – the highest redshift observations of CO(2–1) to date. By combining these with the nine z = 5.7 − 6.4 QSO hosts for which CO(2–1) emission has already been observed, we studied the spread in molecular gas masses and CO excitation of z ≳ 6 QSOs. Results. Two of our three QSOs, P036+03 and J0305–3150, were not detected in CO(2–1), implying more highly excited CO than in the well-studied z = 6.4 QSO J1148+5251. However, we detected CO(2–1) emission at 5.1σ for our highest-redshift target, J2348–3054, yielding a molecular gas mass of (1.2 ± 0.2)×1010 M⊙, assuming αCO = 0.8 (K km s−1 pc2)−1 and r2, 1 = 1. This molecular gas mass is equivalent to the lower limit on the dynamical mass measured previously from resolved [C II] 158 μm observations, implying that there is little mass in stars or neutral gas within the [C II]-emitting region and that a low CO-to-H2 conversion factor is applicable. On average, these z ≳ 6 QSO hosts have far higher CO(6–5)-, CO(7–6)-, and [C II] 158 μm versus CO(2–1) line ratios than the local gas-rich and IR-luminous galaxies that host active galactic nuclei, but with a large range of values, implying some variation in their interstellar medium conditions. We derived a mean CO(6–5)-to-CO(1–0) line luminosity ratio of r6, 1 = 0.9 ± 0.2. Conclusions. Our new CO(2–1) observations show that even at 780 Myr after the Big Bang, QSO host galaxies can already have molecular gas masses of 1010 M⊙, consistent with a picture in which these z ≳ 6 QSOs reside in massive starbursts that are coevolving with the accreting supermassive black holes. Their high gas versus dynamical masses and extremely high line excitation imply the presence of extremely dense and warm molecular gas reservoirs illuminated by strong interstellar radiation fields.

Retrograde infall of the intergalactic gas onto S-galaxy and activity of galactic nuclei

Abstract We present the results of numerical simulations focused on the accretion of intergalactic gas onto a gas-rich S-type disc galaxy. Our investigation explores the conditions favouring the emergence of counterrotating stellar and gaseous components within the galaxy, leading to the inflow of gas towards the central kiloparsec of the galaxy. Notably, we find that the most substantial reservoir of gas, serving as fuel for galactic nucleus activity, resides within the central region during the retrograde infall of gas at an incident angle of approximately 2 0 ° 2{0}^{^\circ } relative to the galactic plane. Departures from this angle significantly diminish the gas flow rate towards the galactic centre. Conversely, the prograde infall of intergalactic gas makes a marginal contribution to the gas content in the central region and cannot supply fuel to the active galactic nucleus. An intriguing characteristic of the observed retrograde impact is the emergence of a rotating polar ring at the galaxy’s periphery, primarily originating from intergalactic gas.

Simulations of spin-driven AGN jets in gas-rich galaxy mergers

ABSTRACT In this work, we use hydrodynamical simulations to explore the effects of kinetic active galactic nuclei (AGN) jet feedback on the progression and outcome of the major merger of two isolated, gas-rich galaxies. We present simulations that use the moving-mesh code arepo to follow the progression of the merger through the first passage and up to the final coalescence, modelling the black holes at the centres of both of the merging galaxies using our prescription for black hole accretion via an α-disc and feedback in the form of a spin-driven jet. We find that the jets drive large-scale, multiphase outflows which launch large quantities of cold gas out to distances greater than 100 kpc and with velocities that reach $\sim 2500 \, {\rm km \, s^{-1}}$. Gas in the outflows that decelerates, cools, and falls back on the galaxies can provide a rich source of fuel for the black hole, leading to intense episodes of jet activity in which the jet can become significantly misaligned. The presence of AGN jets affects the growth of the stellar component: star formation is moderately suppressed at all times during the merger and the peak of the star formation rate, attained during the final coalescence of the galaxies, is reduced by a factor of ∼2. Analysis of simulations such as these will play a central role in making precise predictions for multimessenger investigations of dual radio-AGN, which next-generation observational facilities such as LISA, Athena and SKA will make possible.

Chemical evolution of local post-starburst galaxies: implications for the mass–metallicity relation

ABSTRACT We use the stellar fossil record to constrain the stellar metallicity evolution and star-formation histories of the post-starburst (PSB) regions within 45 local PSB galaxies from the MaNGA survey. The direct measurement of the regions’ stellar metallicity evolution is achieved by a new two-step metallicity model that allows for stellar metallicity to change at the peak of the starburst. We also employ a Gaussian process noise model that accounts for correlated errors introduced by the observational data reduction or inaccuracies in the models. We find that a majority of PSB regions (69 per cent at &amp;gt;1σ significance) increased in stellar metallicity during the recent starburst, with an average increase of 0.8 dex and a standard deviation of 0.4 dex. A much smaller fraction of PSBs are found to have remained constant (22 per cent) or declined in metallicity (9 per cent, average decrease 0.4 dex, standard deviation 0.3 dex). The pre-burst metallicities of the PSB galaxies are in good agreement with the mass–metallicity (MZ) relation of local star-forming galaxies. These results are consistent with hydrodynamic simulations, which suggest that mergers between gas-rich galaxies are the primary formation mechanism of local PSBs, and rapid metal recycling during the starburst outweighs the impact of dilution by any gas inflows. The final mass-weighted metallicities of the PSB galaxies are consistent with the MZ relation of local passive galaxies. Our results suggest that rapid quenching following a merger-driven starburst is entirely consistent with the observed gap between the stellar mass–metallicity relations of local star-forming and passive galaxies.

Apparent Effect of Dust Extinction on the Observed Outflow Velocity of Ionized Gas in Galaxy Mergers

In this study, we examine photoionization outflows during the late stages of galaxy mergers, with a specific focus on the relation between the observed velocity of outflowing gas and the apparent effects of dust extinction. We used the N-body/smoothed particle hydrodynamics code ASURA for galaxy merger simulations. These simulations concentrated on identical galaxy mergers featuring supermassive black holes of 108 M ⊙ and gas fractions of 30% and 10%. From the simulation data, we derived velocity and velocity dispersion diagrams for the active galactic nuclei (AGN)-driven ionized outflowing gas. Our findings show that high-velocity outflows with velocity dispersions of 500 km s−1 or greater can be observed in the late stages of galactic mergers. Particularly, in buried AGNs, both the luminosity-weighted outflow velocity and velocity dispersion increase owing to the apparent effects of dust extinction. Owing to these effects, velocity–velocity dispersion diagrams display a noticeable blue-shifted tilt in models with higher gas fractions. Crucially, this tilt is not influenced by the AGN luminosity but emerges from the observational impacts of dust extinction. Our results imply that the observed high-velocity [O iii] λ5007 outflow exceeding 1000 km s−1 in buried AGNs may be linked to the dust extinction that occurs during the late stages of gas-rich galaxy mergers.