Gas-rich Galaxies Research Articles

Testing the Galaxy-collision-induced Formation Scenario for the Trail of Dark-matter-deficient Galaxies with the Susceptibility of Globular Clusters to the Tidal Force

It has been suggested that a trail of diffuse galaxies, including two dark-matter-deficient galaxies (DMDGs), in the vicinity of NGC 1052 formed because of a high-speed collision between two gas-rich dwarf galaxies, one bound to NGC 1052 and the other one on an unbound orbit. The collision compresses the gas reservoirs of the colliding galaxies, which in turn triggers a burst of star formation. In contrast, the dark matter and preexisting stars in the progenitor galaxies pass through it. Since the high pressures in the compressed gas are conducive to the formation of massive globular clusters (GCs), this scenario can explain the formation of DMDGs with large populations of massive GCs, consistent with the observations of NGC 1052-DF2 (DF2) and NGC 1052-DF4. A potential difficulty with this “mini bullet cluster” scenario is that the observed spatial distributions of GCs in DMDGs are extended. GCs experience dynamical friction causing their orbits to decay with time. Consequently, their distribution at formation should have been even more extended than that observed at present. Using a semianalytic model, we show that the observed positions and velocities of the GCs in DF2 imply that they must have formed at a radial distance of 5–10 kpc from the center of DF2. However, as we demonstrate, the scenario is difficult to reconcile with the fact that the strong tidal forces from NGC 1052 strip the extendedly distributed GCs from DF2, requiring 33–59 massive GCs to form at the collision to explain observations.

Forming stars in a dual AGN host: molecular and ionized gas in the nearby, luminous infrared merger, Mrk 266

ABSTRACT We present star formation rates based on cold and ionized gas measurements of Mrk 266 (NGC 5256), a system composed of two colliding gas-rich galaxies, each hosting an active galactic nucleus. Using 12CO (1-0) observations with the Combined Array for Research in Millimeter-Wave Astronomy (CARMA), we find a total H2 mass in the central region of 1.1 ± 0.3 × 1010 M⊙ which leads to a possible future star formation rate of 25 ± 10 M⊙ yr−1. With the Fourier Transform Spectrograph (SITELLE) on the Canada–France–Hawaii Telescope, we measure an integrated H α luminosity and estimate a present-day star formation rate of 15 ± 2 M⊙ yr−1 in the core of the system (avoiding the two active nuclei). These results confirm that Mrk 266 is an intermediate stage merger with a relatively high recent star formation rate and enough molecular gas to sustain it for a few hundred million years. Inflowing gas associated with the merger may have triggered both the starburst episode and two active galactic nuclei but the two galaxy components differ: the region around the south-western nucleus appears to be more active than the north-east nucleus, which seems relatively quiet. We speculate that this difference may originate in the properties of the interstellar medium in the two systems.

Monochromatic Globular Clusters as a Critical Test of Formation Models for the Dark Matter–deficient Galaxies NGC 1052-DF2 and NGC 1052-DF4

It was recently proposed that the dark matter–deficient ultradiffuse galaxies DF2 and DF4 in the NGC 1052 group could be the products of a “bullet dwarf” collision between two gas-rich progenitor galaxies. In this model, DF2 and DF4 formed at the same time in the immediate aftermath of the collision, and a strong prediction is that their globular clusters should have nearly identical stellar populations. Here we test this prediction by measuring accurate V 606 − I 814 colors from deep HST/ACS imaging. We find that the clusters are extremely homogeneous. The mean color difference between the globular clusters in DF2 and DF4 is ΔDF2−DF4 = −0.003 ± 0.005 mag, and the observed scatter for the combined sample of 18 clusters with M 606 < −8.6 in both galaxies is σ obs = 0.015 ± 0.002 mag. After accounting for observational uncertainties and stochastic cluster-to-cluster variation in the number of red giants, the remaining scatter is mag. Both the color difference and the scatter are an order of magnitude smaller than in other dwarf galaxies, and we infer that the bullet scenario passes an important test that could have falsified it. No other formation models have predicted this extreme uniformity of the globular clusters in the two galaxies. We find that the galaxies themselves are slightly redder than the clusters, consistent with a previously measured metallicity difference. Numerical simulations have shown that such differences are expected in the bullet scenario, as the galaxies continued to self-enrich after the formation of the globular clusters.

A Virgo Environmental Survey Tracing Ionised Gas Emission (VESTIGE)

Context. Low-surface-brightness galaxies (LSBs) contribute to a significant fraction of all the galaxies in the Universe. Ultra-diffuse galaxies (UDGs) form a subclass of LSBs that has attracted a lot of attention in recent years (although its definition may vary between studies). Although UDGs are found in large numbers in galaxy clusters, groups, and in the field, their formation and evolution are still very much debated. Aims. Using a comprehensive set of multiwavelength data from the NGVS (optical), VESTIGE (Hα narrowband), and GUViCS (UV) surveys, we studied a sample of 64 diffuse galaxies and UDGs in the Virgo cluster to investigate their formation history. Methods. We analyzed the photometric colors and surface-brightness profiles of these galaxies and then compared them to models of galaxy evolution, including ram-pressure stripping (RPS) events to infer any possible strong interactions with the hot cluster gas in the past. Results. While our sample consists mainly of red LSBs, which is typical in cluster environments, we found evidence of a color variation with the cluster-centric distance. Blue, H I-bearing, star-forming diffuse galaxies are found at larger distances from the cluster center than the rest of the sample. The comparison of our models with multifrequency observations suggests that most of the galaxies of the sample might have undergone a strong RPS event in their lifetime, on average 1.6 Gyr ago (with a large dispersion, and RPS still ongoing for some of them). This process resulted in the transformation of initially gas-rich diffuse blue galaxies into gas-poor and red ones that form the dominant population now, the more extreme UDGs having undergone the process in a more distant past on average. Conclusions. The RPS in dense environments could be one of the major mechanisms for the formation of the large number of quiescent UDGs we observe in galaxy clusters.

Star-forming and gas-rich brightest cluster galaxies at z ∼ 0.4 in the Kilo-Degree Survey

Brightest cluster galaxies (BCGs) are typically massive ellipticals at the centers of clusters. They are believed to experience strong environmental processing, and their mass assembly and star formation history are still debated. We have selected three star-forming BCGs in the equatorial field of the Kilo-Degree Survey (KiDS). They are KiDS 0920 (z = 0.3216), KiDS 1220 (z = 0.3886), and KiDS 1444 (z = 0.4417). We have observed them with the IRAM 30 m telescope in the first three CO transitions. We remarkably detected all BCGs at high signal-to-noise ratio, S/N ≃ (3.8 − 10.2), for a total of seven detected lines out of eight, corresponding to a success rate of 88%. This allows us to double the number of distant BCGs with clear detections in at least two CO lines. We then combined our observations with available stellar, star formation, and dust properties of the BCGs and compared them with a sample of ∼100 distant cluster galaxies with observations in CO. Our analysis yields large molecular gas reservoirs MH2 ≃ (0.5 − 1.4)×1011 M⊙, high excitation ratios r31 = LCO(3→2)′/LCO(1→0)′ ≃ (0.1 − 0.3), long depletion times τdep ≃ (2 − 4) Gyr, and high MH2/Mdust ≃ (170 − 300) for the three targeted BCGs. The excitation ratio r31 of intermediate-z BCGs, including RX1532 and M1932 from previous studies, appears to be well correlated with the star formation rate and efficiency, which suggests that excited gas is found only in highly star-forming and cool-core BCGs. By performing color-magnitude plots and a red-sequence modeling, we find that recent bursts of star formation are needed to explain the fact that the BCGs are measurably bluer than photometrically selected cluster members. To explain the global observed phenomenology, we suggest that a substantial amount of the molecular gas has been accreted by the KiDS BCGs but still not efficiently converted into stars. KiDS 1220 also shows a double-horn emission in CO(3→2), which implies a low gas concentration. The modeling of the spectrum yields an extended molecular gas reservoir of ∼9 kpc, which is reminiscent of the mature extended-disk phase observed in some local BCGs.

Do post-starburst galaxies host compact molecular gas reservoirs?

ABSTRACT We analysed the high-resolution (up to ∼0.2 arcsec) ALMA CO (2–1) and 1.3 mm dust continuum data of eight gas-rich post-starburst galaxies (PSBs) in the local Universe, six of which had been studied by a recent work. In contrast to this study reporting the detections of extraordinarily compact (i.e. unresolved) reservoirs of molecular gas in the six PSBs, our visibility-plane analysis resolves the CO (2–1) emission in all eight PSBs with effective radii (Re, CO) of $0.8_{-0.4}^{+0.9}$ kpc, typically consisting of gaseous components at both circumnuclear and extended disc scales. With this new analysis, we find that the CO sizes of gas-rich PSBs are compact with respect to their stellar sizes (median ratio $=0.43_{-0.21}^{+0.27}$), but comparable to the sizes of the gas discs seen in local luminous infrared galaxies (LIRGs) and early-type galaxies. We also find that the CO-to-stellar size ratio of gas-rich PSBs is potentially correlated with the gas depletion time-scale, placing them as transitional objects between LIRGs and early-type galaxies from an evolutionary perspective. Finally, the star formation efficiency of the observed PSBs appear consistent with those of star-forming galaxies on the Kennicutt–Schmidt relation, showing no sign of suppressed star formation from turbulent heating.

Strong constraints on decay and annihilation of dark matter from heating of gas-rich dwarf galaxies

Gas-rich dwarf galaxies located outside the virial radius of their host are relatively pristine systems and have ultra-low gas cooling rates. This makes them very sensitive to heat injection by annihilation or decay of dark matter (DM). Such dwarfs are particularly sensitive to DM producing e$^\pm$ with energies 1$-$100 MeV or photons with energies 13.6 eV$-1$ keV, because these products can be efficiently thermalized in the neutral hydrogen gas of the dwarfs. Following the methodology of Wadekar and Farrar (2021), we require the rate of heat injection by DM to not exceed the ultra-low radiative cooling rate of gas in the Leo T dwarf galaxy. This gives model-independent bounds on $(i)$ the decay lifetime of DM to $e^\pm$ (photons) which are stronger than all the previous literature for $m_\mathrm{DM}\sim$ 1$-$10 MeV ($m_\mathrm{DM}\sim0.02-1$ keV), $(ii)$ annihilation of DM to $e^{\pm}$ comparable to constraints from CMB and X/$\gamma$-ray surveys. We also translate our bounds for the case of the following DM models: axion-like particles (ALPs), sterile neutrinos, excited DM states, higgs portal scalars and dark baryons. Observations of gas-rich low-mass dwarfs from upcoming 21cm and optical surveys can therefore be powerful probes of a multitude of models of DM.

Central star formation in double-peak, gas-rich radio galaxies

The respective contributions of gas accretion, galaxy interactions, and mergers to the mass assembly of galaxies, as well as the evolution of their molecular gas and star-formation activity are still not fully understood. In a recent work, a large sample of double-peak (DP) emission-line galaxies have been identified from the SDSS. While the two peaks could represent two kinematic components, they may be linked to the large bulges that their host galaxies tend to have. Star-forming DP galaxies display a central star-formation enhancement and have been discussed as compatible with a sequence of recent minor mergers. In order to probe merger-induced star-formation mechanisms, we conducted observations of the molecular-gas content of 35 star-forming DP galaxies in the upper part of the main sequence (MS) of star formation (SF) with the IRAM 30 m telescope. Including similar galaxies 0.3 dex above the MS and with existing molecular-gas observations from the literature, we finally obtained a sample of 52 such galaxies. We succeeded in fitting the same kinematic parameters to the optical ionised- and molecular-gas emission lines for ten (19%) galaxies. We find a central star-formation enhancement resulting most likely from a galaxy merger or galaxy interaction, which is indicated by an excess of gas extinction found in the centre. This SF is traced by radio continuum emissions at 150 MHz, 1.4 GHz, and 3 GHz, all three of which are linearly correlated in log with the CO luminosity with the same slope. The 52 DP galaxies are found to have a significantly larger amount of molecular gas and longer depletion times, and hence a lower star-formation efficiency, than the expected values at their distance of the MS. The large bulges in these galaxies might be stabilising the gas, hence reducing the SF efficiency. This is consistent with a scenario of minor mergers increasing the mass of bulges and driving gas to the centre. We also excluded the inwards-directed gas migration and central star-formation enhancement as the origin of a bar morphology. Hence, these 52 DP galaxies could be the result of recent minor mergers that funnelled molecular gas towards their centre, triggering SF, but with moderate efficiency.

Seeing the forest and the trees: A radio investigation of the ULIRG Mrk 273

Galaxy mergers have been observed to trigger nuclear activity by feeding gas to the central supermassive black hole. One such class of objects are Ultra Luminous InfraRed Galaxies (ULIRGs), which are mostly late stage major mergers of gas-rich galaxies. Recently, large-scale (∼100 kpc) radio continuum emission has been detected in a select number of ULIRGs, all of which also harbour powerful Active Galactic Nuclei (AGN). This hints at the presence of large-scale radio emission being evidence for nuclear activity. Exploring the origin of this radio emission and its link to nuclear activity requires high sensitivity multi-frequency data. We present such an analysis of the ULIRG Mrk 273. Using the International LOFAR telescope (ILT), we detected spectacular large-scale arcs in this system. This detection includes, for the first time, a giant ∼190 kpc arc in the north. We propose these arcs are fuelled by a low power radio AGN triggered by the merger. We also identified a bright ∼45 kpc radio ridge, which is likely related to the ionised gas nebula in that region. We combined this with high sensitivity data from APERture Tile In Focus (Apertif) and archival data from the Very Large Array (VLA) to explore the spectral properties. The ILT simultaneously allowed us to probe the nucleus at a resolution of ∼0.3″, where we detected three components, and, for the first time, diffuse emission around these components. Combining this with archival high frequency VLA images of the nucleus allowed us to detect absorption in one component, and a steep spectrum radio AGN in another. We then extrapolate from this case study to the importance of investigating the presence of radio emission in more ULIRGs and what it can tell us about the link between mergers and the presence of radio activity.

The kinematics and ionization structure of the extended emission-line region of QSO E1821+643

ABSTRACT The most luminous quasars are created by major, gas-rich mergers and E1821+643, an optically luminous quasar situated at the centre of a cool-core cluster, appears to be in the late stages of the post-merger blowout phase. This quasar is also identified as a gravitational recoil candidate, in which the supermassive black hole (SMBH) has received a recoil kick due to anisotropic emission of gravitational waves during the coalescence of a progenitor SMBH binary. We analyse long-slit spectra of the extended, ionized gas surrounding E1821+643 to study its kinematics and ionization. We have identified three kinematically distinct components, which we associate, respectively, with a wide-angle polar wind from the nucleus, kinematically undisturbed gas, and a redshifted arc-like structure of gas, at a distance of 3–4 arcsec (13–18 kpc) from the nucleus. The latter component coincides with the northern and eastern extremities of an arc of [O iii] emission seen in HST images. This feature could trace a tidal tail originating from a merger with a gas-rich galaxy to the south-east of the nucleus, whose presence has been inferred by Aravena et al. from the detection of CO emission. Alternatively, the arc could be the remnant of a shell of gas swept up by a powerful quasar wind. The emission-line ratios of the extended gas are consistent with photoionization by the quasar, but a contribution from radiative shocks cannot be excluded.

Massive Black Hole Binaries from the TNG50-3 Simulation. I. Coalescence and LISA Detection Rates

We evaluate the cosmological coalescence and detection rates for massive black hole (MBH) binaries targeted by the gravitational wave observatory Laser Interferometer Space Antenna (LISA). Our calculation starts with a population of gravitationally unbound MBH pairs, drawn from the TNG50-3 cosmological simulation, and follows their orbital evolution from kiloparsec scales all the way to coalescence using a semi-analytic model developed in our previous work. We find that for the majority of MBH pairs that coalesce within a Hubble time dynamical friction is the most important mechanism that determines their coalescence rate. Our model predicts an MBH coalescence rate ≲0.45 yr−1 and a LISA detection rate ≲0.34 yr−1. Most LISA detections should originate from 106 to 106.8 M ⊙ MBHs in gas-rich galaxies at redshifts 1.6 ≤ z ≤ 2.4 and have a characteristic signal-to-noise ratio S/N ∼100. We however find a dramatic reduction in the coalescence and detection rates, as well as the average S/N, if the effects of radiative feedback from accreting MBHs are taken into account. In this case, the MBH coalescence rate is reduced by 78% (to ≲0.1 yr−1), and the LISA detection rate is reduced by 94% (to 0.02 yr−1), whereas the average S/N is ∼10. We emphasize that our model provides a conservative estimate of the LISA detection rates, due to the limited MBH mass range in TNG50-3, consistent with other works in the literature that draw their MBH pairs from cosmological simulations.

The Black Hole Mass Function across Cosmic Time. II. Heavy Seeds and (Super)Massive Black Holes

This is the second paper in a series aimed at modeling the black hole (BH) mass function from the stellar to the (super)massive regime. In the present work, we focus on (super)massive BHs and provide an ab initio computation of their mass function across cosmic time. We consider two main mechanisms to grow the central BH that are expected to cooperate in the high-redshift star-forming progenitors of local massive galaxies. The first is the gaseous dynamical friction process, which can cause the migration toward the nuclear regions of stellar mass BHs originated during the intense bursts of star formation in the gas-rich host progenitor galaxy and the buildup of a central heavy BH seed, M • ∼ 103−5 M ⊙, within short timescales of ≲some 107 yr. The second mechanism is the standard Eddington-type gas disk accretion onto the heavy BH seed through which the central BH can become (super)massive, M • ∼ 106−10 M ⊙, within the typical star formation duration, ≲1 Gyr, of the host. We validate our semiempirical approach by reproducing the observed redshift-dependent bolometric AGN luminosity functions and Eddington ratio distributions and the relationship between the star formation and the bolometric luminosity of the accreting central BH. We then derive the relic (super)massive BH mass function at different redshifts via a generalized continuity equation approach and compare it with present observational estimates. Finally, we reconstruct the overall BH mass function from the stellar to the (super)massive regime over more than 10 orders of magnitudes in BH mass.

Cold Gas Reservoirs of Low- and High-mass Central Galaxies Differ in Response to Active Galactic Nucleus Feedback

The growth of supermassive black holes, especially the associated state of active galactic nuclei (AGNs), is generally believed to be the key step in regulating star formation in massive galaxies. As the fuel of star formation, the cold gas reservoir is a direct probe of the effect of AGN feedback on their host galaxies. However, in observations, no clear connection has been found between AGN activity and the cold gas mass. In this paper, we find observational signals of the significant depletion of the total neutral hydrogen gas reservoir in optically selected Type 2 AGN-host central galaxies of stellar mass 109–1010 M ⊙. The effect of AGN feedback on the cold gas reservoir is stronger for higher star formation rates and higher AGN luminosity. But it becomes much weaker above this mass range, consistent with previous findings focusing on massive galaxies. Our result suggests that low-mass and gas-rich AGN-host central galaxies would first form dense cores before AGN feedback is triggered, removing their neutral hydrogen gas. More massive central galaxies may undergo a significantly different formation scenario by gradually building up dense cores with less effective and recurrent AGN feedback.

SDSS IV MaNGA: characteristics of edge-on galaxies with a counter-rotating gaseous disc

ABSTRACT Counter-rotating components in galaxies are one of the most direct forms of evidence for past gas accretion or merging. We discovered 10 edge-on disc gaseous counter-rotators in a sample of 523 edge-on galaxies identified in the final MaNGA (Mapping Nearby Galaxies at APO) IFU sample. The counter-rotators tend to located in small groups. The gaseous counter-rotators have intermediate stellar masses and and located in the green valley and red sequence of the colour–magnitude diagram. The average vertical extents of the stellar and ionized gas discs are the same as for the rest of the sample while their radial gas and stellar distributions are more centrally concentrated. This may point at angular momentum loss during the formation process of the counter-rotating discs. The counter-rotators have low gas and dust content, weak emission-line strengths, and low star formation rates. This suggests that the formation of counter-rotators may be an efficient way to quench galaxies. One counter-rotator, SDSS J080016.09+292817.1 (Galaxy F), has a post-starburst region and a possible AGN at the centre. Another counter-rotator, SDSS J131234.03+482159.8 (Galaxy H), is identified as a potential ongoing galaxy interaction with its companion satellite galaxy, a gas-rich spiral galaxy. This may be representative case of a gaseous counter-rotator forming through a merger origin. However, tidal distortions expected in mergers are only found in a few of the galaxies and we cannot rule out direct gas accretion as another formation mechanism.

The VIMOS Ultra Deep Survey: The reversal of the star-formation rate − density relation at 2 &lt;z&lt; 5

Utilizing spectroscopic observations taken for the VIMOS Ultra-Deep Survey (VUDS), new observations from Keck/DEIMOS, and publicly available observations of large samples of star-forming galaxies, we report here on the relationship between the star-formation rate (SFR) and the local environment (δgal) of galaxies in the early universe (2 &lt; z &lt; 5). Unlike what is observed at lower redshifts (z ≲ 2), we observe a definite, nearly monotonic increase in the average SFR with increasing galaxy overdensity over more than an order of magnitude inδgal. The robustness of this trend is quantified by accounting for both uncertainties in our measurements and galaxy populations that are either underrepresented or not present in our sample (e.g., extremely dusty star-forming and quiescent galaxies), and we find that the trend remains significant under all circumstances. This trend appears to be primarily driven by the fractional increase of galaxies in high-density environments that are more massive in their stellar content and are forming stars at a higher rate than their less massive counterparts. We find that, even after stellar mass effects are accounted for, there remains a weak but significant SFR–δgaltrend in our sample implying that additional environmentally related processes are helping to drive this trend. We also find clear evidence that the average SFR of galaxies in the densest environments increases with increasing redshift. These results lend themselves to a picture in which massive gas-rich galaxies coalesce into proto-cluster environments atz ≳ 3, interact with other galaxies or with a forming large-scale medium, subsequently using or losing most of their gas in the process, and begin to seed the nascent red sequence that is present in clusters at slightly lower redshifts.

Detecting PAHs in high-z galaxies in proxy: modelling physical conditions in an extremely strong damped Lyman α absorber towards QSO SDSS J1143+1420 at z = 2.323

ABSTRACT We explore indirect methods to detect polycyclic aromatic hydrocarbons (PAHs) in gas-rich, absorption-selected galaxies at high redshift. We look at the optical X-Shooter observations of an intervening, extremely strong damped Lyman α absorber (or ESDLA, with log [N(H i) ≳ 21.7)] towards QSO SDSS J1143+1420 at redshift, zESDLA = 2.323. Literature studies have shown that this ESDLA contains signatures of dust and diffuse molecular hydrogen and it was specifically chosen for our study due to its close spatial proximity (impact parameter, $\rm \rho = 0.6\, \pm \, 0.3$kpc) with its associated galaxy. There is no direct detection of PAHs emission in the limited observations of infrared spectra along this sightline. Hence, we use cloudy numerical simulation modelling to indirectly probe the presence of PAHs in the ESDLA. We note that PAHs need to be included in the models to reproduce the observed column densities of warm H2 and C i. Thus, we infer the presence of PAHs indirectly in our ESDLA, with an abundance of PAH/H = 10−7.046. We also measure a low 2175 Å bump strength (E$\rm _{bump}\, \sim$0.03–0.19 mag) relative to star-forming galaxies by modelling extinction of quasar QSO spectra by dust at the absorber rest frame. This is consistent with the low PAH abundance obtained indirectly using cloudy modelling. Our study highlights the usage of cloudy modelling to indirectly detect PAH in high-redshift gas-rich absorption-selected galaxies.

Cosmological simulations predict that AGN preferentially live in gas-rich, star-forming galaxies despite effective feedback

ABSTRACT Negative feedback from active galactic nuclei (AGN) is the leading mechanism for the quenching of massive galaxies in the vast majority of modern galaxy evolution models. However, direct observational evidence that AGN feedback causes quenching on a population scale is lacking. Studies have shown that luminous AGN are preferentially located in gas-rich and star-forming galaxies, an observation that has sometimes been suggested to be in tension with a negative AGN feedback picture. We investigate three of the current cosmological simulations (illustrisTNG, EAGLE, and SIMBA) along with post-processed models for molecular hydrogen gas masses and perform similar tests to those used by observers. We find that the simulations predict: (i) no strong negative trends between Lbol and $f_{\mathrm{ H}_2}$ or specific star formation rate (sSFR); (ii) both high-luminosity ($L_{\rm {bol}} \ge 10^{44}\rm {\, erg\, s^{-1}}$) and high Eddington ratio (λEdd $\ge 1{{\ \rm per\ cent}}$) AGN are preferentially located in galaxies with high molecular gas fractions and sSFR; and (iii) that the gas-depleted and quenched fractions of AGN host galaxies are lower than a control sample of non-active galaxies. These three findings are in qualitative agreement with observational samples at z = 0 and z = 2 and show that such results are not in tension with the presence of strong AGN feedback, which all simulations we employ require to produce realistic massive galaxies. However, we also find quantifiable differences between predictions from the simulations, which could allow us to observationally test the different subgrid feedback models.

Looking at the Distant Universe with the MeerKAT Array: Discovery of a Luminous OH Megamaser at z > 0.5

In the local universe, OH megamasers (OHMs) are detected almost exclusively in infrared-luminous galaxies, with a prevalence that increases with IR luminosity, suggesting that they trace gas-rich galaxy mergers. Given the proximity of the rest frequencies of OH and the hyperfine transition of neutral atomic hydrogen (H i), radio surveys to probe the cosmic evolution of H i in galaxies also offer exciting prospects for exploiting OHMs to probe the cosmic history of gas-rich mergers. Using observations for the Looking At the Distant Universe with the MeerKAT Array (LADUMA) deep H i survey, we report the first untargeted detection of an OHM at z > 0.5, LADUMA J033046.20−275518.1 (nicknamed “Nkalakatha”). The host system, WISEA J033046.26−275518.3, is an infrared-luminous radio galaxy whose optical redshift z ≈ 0.52 confirms the MeerKAT emission-line detection as OH at a redshift z OH = 0.5225 ± 0.0001 rather than H i at lower redshift. The detected spectral line has 18.4σ peak significance, a width of 459 ± 59 km s−1, and an integrated luminosity of (6.31 ± 0.18 [statistical] ± 0.31 [systematic]) × 103 L ⊙, placing it among the most luminous OHMs known. The galaxy’s far-infrared luminosity L FIR = (1.576 ±0.013) × 1012 L ⊙ marks it as an ultraluminous infrared galaxy; its ratio of OH and infrared luminosities is similar to those for lower-redshift OHMs. A comparison between optical and OH redshifts offers a slight indication of an OH outflow. This detection represents the first step toward a systematic exploitation of OHMs as a tracer of galaxy growth at high redshifts.

Overestimated inclinations of Milgromian disc galaxies: the case of the ultradiffuse galaxy AGC 114905

ABSTRACT We present two hydrodynamical star-forming simulations in the Milgromian dynamics (MOND) framework of a gas-rich disc galaxy with properties similar to AGC 114905, which has recently been argued to have a rotation curve (RC) that is inconsistent with the MOND prediction. Our first model considers the galaxy in isolation, while our second model includes an external field of $0.05 \, a_{_0}$, the estimated gravitational field from large-scale structure. We show that isophotes in the face-on view can differ from circular at the 50 per cent level. This could mislead observers into overestimating the inclination i between disc and sky planes. Because RCs require a correction factor of 1/sin i, the actual RC could be much higher than that reported by observers. This plausibly reconciles AGC 114905 with MOND expectations.

Stellar and black hole assembly inz&amp;lt; 0.3 infrared-luminous mergers: intermittent starbursts versus super-Eddington accretion

ABSTRACTWe study stellar and black hole mass assembly in a sample of 42 infrared-luminous galaxy mergers at z &amp;lt; 0.3 by combining results from radiative transfer modelling with archival measures of molecular gas and black hole mass. The ratios of stellar mass, molecular gas mass, and black hole mass to each other are consistent with those of massive gas-rich galaxies at z &amp;lt; 0.3. The advanced mergers may show increased black hole mass to stellar mass ratios, consistent with the transition from active galactic nucleus (AGN) to ellipticals and implying substantial black hole mass growth over the course of the merger. Star formation rates are enhanced relative to the local main sequence, by factors of ∼100 in the starburst and ∼1.8 in the host, respectively. The starburst star formation rates appear distinct to star formation in the main sequence at all redshifts up to at least z ∼ 5. Starbursts may prefer late-stage mergers, but are observed at any merger stage. We do not find evidence that the starbursts in these low-redshift systems substantially increase the total stellar mass, with a soft upper limit on the stellar mass increase from starburst activity of about a factor of two. In contrast, 12 objects show evidence for super-Eddington accretion, associated with late-stage mergers, suggesting that many AGN in infrared-luminous mergers go through a super-Eddington phase. The super-Eddington phase may increase black hole mass by up to an order of magnitude at an accretion efficiency of $42\pm 33{{\ \rm per\ cent}}$ over a period of 44 ± 22 Myr. Our results imply that super-Eddington accretion is an important black hole growth channel in infrared-luminous galaxies at all redshifts.