Galaxy Interactions Research Articles

Interacting galaxies in the IllustrisTNG simulations – VII: the connection between the most luminous active galactic nuclei and galaxy interactions

ABSTRACT We investigate the connection between the most luminous active galactic nuclei (AGN), galaxy pairs, and post-mergers in the IllustrisTNG simulation. We select galaxy pairs and post-mergers with a mass ratio between 1:10 < μ < 1:1 and a redshift between 0 < z < 1. We compare the incidence of luminous AGN in pairs with matched non-pair controls, finding that AGN with luminosity Lbol > 1044 erg s−1 occur in ∼26 per cent of paired galaxies with a companion within 20 kpc, compared with ∼14 per cent in controls (a fractional excess of ∼2). The enhancement of AGN in galaxy pairs is luminosity dependent, with the highest excess (a factor of ∼6 ± 2 at the closest separations) for AGN with Lbol > 1045 erg s−1. Additionally, pairs exhibit a modest yet statistically significant excess of luminous AGN up to ∼150 kpc in separation. For pairs which merge between 0 < z < 1, AGN fractions are elevated between 1.5 Gyr before and after coalescence, with the highest excess closest to coalescence. Our results indicate that pre-coalescence interactions drive excesses of luminous AGN, but that luminous AGN in galaxy pairs are not ubiquitous. Finally, we investigate what fraction of AGN can be associated with an interaction (either having a companion within 100 kpc or a merger within the last 500 Myr). For AGN with Lbol > 1045 erg s−1, ∼55 per cent are interacting, compared with a 30 per cent interaction fraction in AGN with 1044 < Lbol < 1044.5 erg s−1. Our results support a picture in which interactions play a dominant role in (but are not the sole cause of) triggering the most luminous AGN.

Stochastic gravitational wave background from the collisions of dark matter halos

We investigate the effect of the dark matter (DM) halos collisions, namely collisions of galaxies and galaxy clusters, through gravitational bremsstrahlung, on the stochastic gravitational wave background. We first calculate the gravitational wave signal of a single collision event, assuming point masses and linear perturbation theory. Then we proceed to the calculation of the energy spectrum of the collective effect of all dark matter collisions in the Universe. Concerning the DM halo collision rate, we show that it is given by the product of the number density of DM halos, which is calculated by the extended Press–Schechter (EPS) theory, with the collision rate of a single DM halo, which is given by simulation results, with a function of the linear growth rate of matter density through cosmological evolution. Hence, integrating over all mass and distance ranges, we finally extract the spectrum of the stochastic gravitational wave background created by DM halos collisions. As we show, the resulting contribution to the stochastic gravitational wave background is of the order of hc≈10-29\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$h_{c} \\approx 10^{-29}$$\\end{document} in the band of f≈10-15Hz\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$f \\approx 10^{-15} Hz$$\\end{document}. However, in very low frequency band, it is larger. With current observational sensitivity it cannot be detected.

Pre- and post-processing of cluster galaxies out to 5 × R200: the extreme case of A2670

ABSTRACT We study galaxy interactions in the large-scale environment around A2670, a massive (M200 = $8.5 \pm 1.2~\times 10^{14} \, \mathrm{{M}_{\odot }}$) and interacting galaxy cluster at z = 0.0763. We first characterize the environment of the cluster out to 5× R200 and find a wealth of substructures, including the main cluster core, a large infalling group, and several other substructures. To study the impact of these substructures (pre-processing) and their accretion into the main cluster (post-processing) on the member galaxies, we visually examined optical images to look for signatures indicative of gravitational or hydrodynamical interactions. We find that ∼21 per cent of the cluster galaxies have clear signs of disturbances, with most of those (∼60 per cent) likely being disturbed by ram pressure. The number of ram-pressure stripping candidates found (101) in A2670 is the largest to date for a single system, and while they are more common in the cluster core, they can be found even at >4 × R200, confirming cluster influence out to large radii. In support of a pre-processing scenario, most of the disturbed galaxies follow the substructures found, with the richest structures having more disturbed galaxies. Post-processing also seems plausible, as many galaxy–galaxy mergers are seen near the cluster core, which is not expected in relaxed clusters. In addition, there is a comparable fraction of disturbed galaxies in and outside substructures. Overall, our results highlight the complex interplay of gas stripping and gravitational interactions in actively assembling clusters up to 5 × R200, motivating wide-area studies in larger cluster samples.

Effect of low-mass galaxy interactions on their star formation

According to the Λ cold dark matter model of galaxy formation, the hierarchical assembly process is scale-free and interactions between galaxies in all mass ranges are expected. The effects of interactions between dwarf galaxies on their evolution are not well understood. In this study, we aim to understand the effect of low-mass galaxy interactions on their star formation rate (SFR). We estimated the SFR of 22 interacting and 36 single gas-rich dwarf galaxies in the Lynx-Cancer void region using their far-ultraviolet (FUV) images from the GALEX mission. We find an enhancement in SFR by a factor of 3.4 ± 1.2 for interacting systems compared to single dwarf galaxies in the stellar mass range of 107–108 M⊙. Our results indicate that dwarf–dwarf galaxy interactions can lead to an enhancement in their SFR. These observations are similar to the predictions based on the simulations of dwarf galaxies at lower redshifts. Future deeper and higher-spatial-resolution UV studies will help us to understand the effect of dwarf galaxy interactions on the spatial distribution of star forming clumps and to identify star formation in tidal tails.

Tidally offset neutral gas in Lyman continuum emitting galaxy Haro 11

ABSTRACT Around 400 Myr after the big bang, the ultraviolet emission from star-forming galaxies reionized the Universe. Ionizing radiation (Lyman continuum, LyC) is absorbed by cold neutral hydrogen gas (H i) within galaxies, hindering the escape of LyC photons. Since the H i reservoir of LyC emitters has never been mapped, major uncertainties remain on how LyC photons escape galaxies and ionize the intergalactic medium. We have directly imaged the neutral gas in the nearby reionization-era analogue galaxy Haro 11 with the 21 cm line to identify the mechanism enabling ionizing radiation escape. We find that merger-driven interactions have caused a bulk offset of the neutral gas by about $6\,$ kpc from the centre of the galaxy, where LyC emission production sites are located. This could facilitate the escape of ionizing radiation into our line of sight. Galaxy interactions can cause both elevated LyC production and large-scale displacement of H i from the regions where these photons are produced. They could contribute to the anisotropic escape of LyC radiation from galaxies and the reionization of the Universe. We argue for a systematic assessment of the effect of environment on LyC production and escape.

Bursty star formation and galaxy–galaxy interactions in low-mass galaxies 1 Gyr after the Big Bang

ABSTRACT We use CANUCS JWST/NIRCam imaging of galaxies behind the gravitationally-lensing cluster MACS J0417.5-1154 to investigate star formation burstiness in low-mass (M⋆ ∼ 108 M⊙) galaxies at z ∼ 4.7–6.5. Our sample of 123 galaxies is selected using the Lyman break selection and photometric emission-line excess methods. Sixty per cent of the 123 galaxies in this sample have Hα-to-UV flux ratios that deviate significantly from the range of Hα-to-UV ratio values consistent with smooth and steady star formation histories. This large fraction indicates that the majority of low-mass galaxies is experiencing bursty star formation histories at high redshift. We also searched for interacting galaxies in our sample and found that they are remarkably common ($\sim 40~{{\ \rm per\ cent}}$ of the sample). Compared to non-interacting galaxies, interacting galaxies are more likely to have very low Hα-to-UV ratios, suggesting that galaxy–galaxy interactions enhance star formation burstiness and enable faster quenching (with time-scales of ≲100 Myr) that follows the rapid rise of star formation activity. Given the high frequency of galaxy–galaxy interactions and the rapid SFR fluctuations they appear to cause, we conclude that galaxy–galaxy interactions could be a leading cause of bursty star formation in low-mass, high-z galaxies. They could thus play a significant role in the evolution of the galaxy population at early cosmological times.

Starbursts driven by central gas compaction

ABSTRACT Starburst (SB) galaxies are a rare population of galaxies with star formation rates (SFRs) greatly exceeding those of the majority of star-forming galaxies with similar stellar mass. It is unclear whether these bursts are the result of either especially large gas reservoirs or enhanced efficiencies in converting gas into stars. Tidal torques resulting from gas-rich galaxy mergers are known to enhance the SFR by funnelling gas towards the centre. However, recent theoretical works show that mergers do not always trigger an SB and not all SB galaxies are interacting systems, raising the question of what drives an SB. We analyse a large sample of SB galaxies and a mass- and redshift-matched sample of control galaxies, drawn from the FIREbox cosmological volume at z = 0–1. We find that SB galaxies have both larger molecular gas fractions and shorter molecular depletion times than control galaxies, but similar total gas masses. Control galaxies evolve towards the SB regime by gas compaction in their central regions, over time-scales of ∼70 Myr, accompanied by an increase in the fraction of ultradense and molecular gas. The driving mechanism behind the SB varies depending on the mass of the galaxy. Massive ($M_\star \gtrsim 10^{10}~\rm {M}_\odot$) galaxies undergoing intense, long-lasting SBs are mostly driven by galaxy interactions. Conversely, SBs in non-interacting galaxies are often triggered by a global gravitational instability, which can result in a ‘breathing’ mode in low-mass galaxies.

Bulge–disc decomposition of the Hydra cluster galaxies in 12 bands

ABSTRACT When a galaxy falls into a cluster, its outermost parts are the most affected by the environment. In this paper, we are interested in studying the influence of a dense environment on different galaxy’s components to better understand how this affects the evolution of galaxies. We use, as laboratory for this study, the Hydra cluster which is close to virialization; yet it still shows evidence of substructures. We present a multiwavelength bulge–disc decomposition performed simultaneously in 12 bands from S-PLUS (Southern Photometric Local Universe Survey) data for 52 galaxies brighter than mr = 16. We model the galaxies with a Sérsic profile for the bulge and an exponential profile for the disc. We find that the smaller, more compact, and bulge-dominated galaxies tend to exhibit a redder colour at a fixed stellar mass. This suggests that the same mechanisms (ram-pressure and tidal stripping) that are causing the compaction in these galaxies are also causing them to stop forming stars. The bulge size is unrelated to the galaxy’s stellar mass, while the disc size increases with greater stellar mass, indicating the dominant role of the disc in the overall galaxy mass–size relation found. Furthermore, our analysis of the environment unveils that quenched galaxies are prevalent in regions likely associated with substructures. However, these areas also harbour a minority of star-forming galaxies, primarily resulting from galaxy interactions. Lastly, we find that ∼37 per cent of the galaxies exhibit bulges that are bluer than their discs, indicative of an outside-in quenching process in this type of dense environments.

The origin of the metallicity distributions of the NE and W stellar shelves in the Andromeda Galaxy

ABSTRACT Tidal streams and stellar shells are naturally formed in galaxy interactions and mergers. The Giant Stellar Stream (GSS), the North-East (NE), and Western (W) stellar shelves observed in Andromeda Galaxy (M31) are examples of these structures and were formed through the merger of M31 and a satellite galaxy. Recent observational papers have provided strong evidence that the shells and GSS originate from a single progenitor. In this paper, we investigate the formation of these two stellar shelves and the detailed nature of their relationship to the GSS. We present numerical simulations of tidal disruption of a satellite galaxy assuming that it is a progenitor of the GSS and the shell system. We represent the progenitor as a dwarf spheroidal galaxy with the stellar mass of 109 M⊙ and evolve its merger with M31 for 3 Gyr to reproduce the chemodynamical properties of the NE and W shelves. We find that an initial metallicity of the progenitor with a negative radial gradient of ΔFeH = −0.3 ± 0.2 successfully reproduces observed metallicities of the NE, W shelves, and the GSS, showing that all these structures can originate from the same merger event.

Merger Interactions Enhance Star Formation Rates in Galaxy Group Housing QSO PKS0405−123 and Gaseous Nebulae

The star formation rate (SFR) of galaxies can change due to interactions between galaxies, stellar feedback ejection of gas into the circumgalactic medium, and energy injection from accretion onto black holes. However, it is not clear which of these processes dominantly alters the formation of stars within galaxies. Johnson et al. reported the discovery of large gaseous nebulae in the intragroup medium of a galaxy group housing QSO PKS0405−123 and hypothesized they were created by galaxy interactions. We identify a sample of 30 group member galaxies at z ∼ 0.57 from the Very Large Telescope/Multi Unit Spectroscopic Explorer observations of the field and calculate their [O ii]λλ3727,3729 SFRs in order to investigate whether the QSO and nebulae have affected the SFRs of the surrounding galaxies. We find that star formation is more prevalent in galaxies within the nebulae, signifying galaxy interactions are fueling higher SFRs.

Galaxy mergers in Subaru HSC-SSP: A deep representation learning approach for identification, and the role of environment on merger incidence

Context.Galaxy mergers and interactions are an important process within the context of galaxy evolution, however, there is still no definitive method which identifies pure and complete merger samples is still not definitive. A method for creating such a merger sample is required so that studies can be conducted to deepen our understanding of the merger process and its impact on galaxy evolution.Aims.In this work, we take a deep-learning-based approach for galaxy merger identification in Subaru HSC-SSP, using deep representation learning and fine-tuning, with the aim of creating a pure and complete merger sample within the HSC-SSP survey. We can use this merger sample to conduct studies on how mergers affect galaxy evolution.Methods.We used Zoobot, a deep learning representation learning model pretrained on citizen science votes on Galaxy Zoo DeCALS images. We fine-tuned Zoobot for the purpose of merger classification of images of SDSS and GAMA galaxies in HSC-SSP public data release 3. Fine-tuning was done using ∼1200 synthetic HSC-SSP images of galaxies from the TNG simulation. We then found merger probabilities on observed HSC images using the fine-tuned model. Using our merger probabilities, we examined the relationship between merger activity and environment.Results.We find that our fine-tuned model returns an accuracy on the synthetic validation data of ∼76%. This number is comparable to those of previous studies in which convolutional neural networks were trained with simulation images, but with our work requiring a far smaller number of training samples. For our synthetic data, our model is able to achieve completeness and precision values of ∼80%. In addition, our model is able to correctly classify both mergers and non-mergers of diverse morphologies and structures, including those at various stages and mass ratios, while distinguishing between projections and merger pairs. For the relation between galaxy mergers and environment, we find two distinct trends. Using stellar mass overdensity estimates for TNG simulations and observations using SDSS and GAMA, we find that galaxies with higher merger scores favor lower density environments on scales of 0.5 to 8h−1Mpc. However, below these scales in the simulations, we find that galaxies with higher merger scores favor higher density environments.Conclusions.We fine-tuned a citizen-science trained deep representation learning model for purpose of merger galaxy classification in HSC-SSP, and make our merger probability catalog available to the public. Using our morphology-based catalog, we find that mergers are more prevalent in lower density environments on scales of 0.5–8h−1Mpc.

Mini-quenching of z = 4–8 galaxies by bursty star formation

ABSTRACT The recent reported discovery of a low-mass z = 5.2 and an intermediate-mass z = 7.3 quenched galaxy with JWST/NIRSpec is the first evidence of halted star formation above z ≈ 5. Here, we show how bursty star formation at z = 4–8 gives rise to temporarily quenched, or mini-quenched galaxies in the mass range M⋆ = 107–109 M⊙ using four models of galaxy formation: the periodic box simulation IllustrisTNG, the zoom-in simulations vela and FirstLight and an empirical halo model. The main causes for mini-quenching are stellar feedback, lack of gas accretion onto galaxies, and galaxy–galaxy interactions. The abundance of (mini-)quenched galaxies agrees across the models: the population first appears below z ≈ 8, after which their proportion increases with cosmic time, from ∼0.5–1.0 per cent at z = 7 to ∼2–4 per cent at z = 4, corresponding to comoving number densities of ∼10−5 and ∼10−3 Mpc−3, respectively. These numbers are consistent with star formation rate duty cycles inferred for vela and FirstLight galaxies. Their star formation histories (SFHs) suggest that mini-quenching at z = 4–8 is short-lived with a duration of ∼20–40 Myr, which is close to the free-fall time-scale of the inner halo. However, mock spectral energy distributions of mini-quenched galaxies in IllustrisTNG and vela do not match JADES-GS-z7-01-QU photometry, unless their SFHs are artificially altered to be more bursty on time-scales of ∼40 Myr. Studying mini-quenched galaxies might aid in calibrating sub-grid models governing galaxy formation, as these may not generate sufficient burstiness at high redshift to explain the SFH inferred for JADES-GS-z7-01-QU.

Galaxy Interactions in Filaments and Sheets: Insights from EAGLE Simulations

We study the color and star formation rates of paired galaxies in filaments and sheets using the EAGLE simulations. We find that the major pairs with pair separation <50 kpc are bluer and more star-forming in filamentary environments compared to those hosted in sheet-like environments. This trend reverses beyond a pair separation of ∼50 kpc. The interacting pairs with larger separations (>50 kpc) in filaments are on average redder and low-star-forming compared to those embedded in sheets. The galaxies in filaments and sheets may have different stellar mass and cold gas mass distributions. Using a KS test, we find that for paired galaxies with pair separation <50 kpc, there are no significant differences in these properties in sheets and filaments. The filaments transport gas toward the cluster of galaxies. Some earlier studies find preferential alignment of galaxy pairs with the filament axis. Such alignment of galaxy pairs may lead to different gas accretion efficiency in galaxies residing in filaments and sheets. We propose that the enhancement of star formation rate at smaller pair separation in filaments is caused by the alignment of galaxy pairs. A recent study with SDSS data reports the same findings. The confirmation of these results by the EAGLE simulations suggests that the hydrodynamical simulations are powerful theoretical tools for studying galaxy formation and evolution in the cosmic web.

Bright submillimeter galaxies do trace galaxy protoclusters

There is controversy in the literature regarding whether distant, massive, and dusty starbursts selected at (sub)millimeter wavelengths can trace galaxy overdensities. We thus performed the first systematic search for distant protoclusters around a homogeneously selected sample of 12 spectroscopically confirmed submillimeter galaxies (SMGs) at z ∼ 1.2 − 5.3, which we selected from the GOODS-N field. We applied the well-established Poisson probability method (PPM) to search for megaparsec-scale overdensities around these SMGs, using three different photometric redshift catalogs. We robustly detect galaxy overdensities for 11 out of the 12 SMGs (i.e., 92%±8%), distributed over eight large-scale protoclusters. We confirm all three previously discovered protoclusters, and we detect five new ones around the SMGs SMM J123634 (z = 1.225), ID.19 (z = 2.047), SMM J123607 (z = 2.487), SMM J123606 (z = 2.505), and GN10 (z = 5.303). A wavelet-based analysis of the protocluster fields shows that the SMGs are located in protocluster cores with a complex morphology (compact, filamentary, or clumpy) and an average size of ∼(0.4 − 1) Mpc. By comparing the PPM results obtained using the three redshift catalogs independently, each of which trace different galaxy populations and redshift ranges, we speculate that we are possibly witnessing a transitioning phase at z ≳ 4 for the galaxy population of protoclusters. While z ≲ 4 protoclusters appear to be populated by dusty galaxies, those at the highest redshifts, z ∼ 5, are detected as overdensities of Lyman α emitters or Lyman break galaxies. Further investigation with larger samples is required to reach a definitive conclusion. We also find a good correlation between the molecular (H2) gas mass of the SMGs and the significance of the associated overdensity. To explain the overall phenomenology, we suggest that galaxy interactions in dense environments likely triggered the starburst and gas-rich phase of the SMGs. Altogether, our findings support the scenario that SMGs are excellent tracers of distant protoclusters. The ones presented in this work are excellent targets for the James Webb Space Telescope. Similarly, future surveys with forthcoming facilities (e.g., Euclid and LSST) can be tuned to detect even larger samples of distant protoclusters.

Galaxy Cruise: Deep Insights into Interacting Galaxies in the Local Universe

Abstract We present the first results from GALAXY CRUISE, a community (or citizen) science project based on data from the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP). The current paradigm of galaxy evolution suggests that galaxies grow hierarchically via mergers, but our observational understanding of the role of mergers is still limited. The data from HSC-SSP are ideally suited to improve our understanding with improved identifications of interacting galaxies thanks to the superb depth and image quality of HSC-SSP. We launched a community science project, GALAXY CRUISE, in 2019 and have collected over two million independent classifications of 20686 galaxies at z &amp;lt; 0.2. We first characterize the accuracy of the participants’ classifications and demonstrate that it surpasses previous studies based on shallower imaging data. We then investigate various aspects of interacting galaxies in detail. We show that there is a clear sign of enhanced activities of super-massive black holes and star formation in interacting galaxies compared to those in isolated galaxies. The enhancement seems particularly strong for galaxies undergoing violent mergers. We also show that the mass growth rate inferred from our results is roughly consistent with the observed evolution of the stellar mass function. The second season of GALAXY CRUISE is currently underway and we conclude with future prospects. We make the morphological classification catalog used in this paper publicly available at the GALAXY CRUISE website, which will be particularly useful for machine-learning applications.

The AMIGA sample of isolated galaxies

Context. In the standard cosmological model of galaxy evolution, mergers and interactions play a fundamental role in shaping galaxies. Galaxies that are currently isolated are thus interesting because they allow us to distinguish between internal and external processes that affect the galactic structure. However, current observational limits may obscure crucial information in the low-mass or low-brightness regime. Aims. We use optical imaging of a subsample of the AMIGA catalogue of isolated galaxies to explore the impact of different factors on the structure of these galaxies. In particular, we study the type of disc break as a function of the degree of isolation and the presence of interaction indicators such as tidal streams or plumes, which are only detectable in the ultra-low surface brightness regime. Methods. We present ultra-deep optical imaging in the r band of a sample of 25 low-redshift (z &lt; 0.035) isolated galaxies. Through careful data processing and analysis techniques, the nominal surface brightness limits achieved are comparable to those to be obtained on the ten-year LSST coadds (μr,lim ≳ 29.5 mag arcsec−2 [3σ; 10″ × 10″]). We place special emphasis on preserving the low surface brightness features throughout the processing. Results. The extreme depth of our imaging allows us to study the interaction signatures of 20 galaxies since Galactic cirrus is a strong limiting factor in the characterisation of interactions for the remaining 5 of them. We detect previously unreported interaction features in 8 (40% ± 14%) galaxies in our sample. We identify 9 galaxies (36% ± 10%) with an exponential disc (Type I), 14 galaxies (56% ± 10%) with a down-bending (Type II) profile, and only 2 galaxies (8% ± 5%) with up-bending (Type III) profiles. Isolated galaxies have considerably more purely exponential discs and fewer up-bending surface brightness profiles than field or cluster galaxies. We find clear minor merger activity in some of the galaxies with single exponential or down-bending profiles, and both of the galaxies with up-bending profiles show signatures of a past interaction. Conclusions. We show the importance of ultra-deep optical imaging in revealing faint external features in galaxies that indicate a probable history of interaction. We confirm that up-bending profiles are likely produced by major mergers, while down-bending profiles are probably formed by a threshold in star formation. Unperturbed galaxies that slowly evolve with a low star formation rate could induce the high rate of Type I discs in isolated galaxies.

The EDGE-CALIFA Survey: Spatially Resolved 13CO(1–0) Observations and Variations in 12CO(1–0)/13CO(1–0) in Nearby Galaxies on Kiloparsec Scales

We present 13CO(J = 1 → 0) observations for the EDGE-CALIFA survey, which is a mapping survey of 126 nearby galaxies at a typical spatial resolution of 1.5 kpc. Using detected 12CO emission as a prior, we detect 13CO in 41 galaxies via integrated line flux over the entire galaxy and in 30 galaxies via integrated line intensity in resolved synthesized beams. Incorporating our CO observations and optical IFU spectroscopy, we perform a systematic comparison between the line ratio and the properties of the stars and ionized gas. Higher values are found in interacting galaxies compared to those in noninteracting galaxies. The global slightly increases with infrared color F 60/F 100 but appears insensitive to other host-galaxy properties such as morphology, stellar mass, or galaxy size. We also present azimuthally averaged profiles for our sample up to a galactocentric radius of 0.4r 25 (∼6 kpc), taking into account the 13CO nondetections by spectral stacking. The radial profiles of are quite flat across our sample. Within galactocentric distances of 0.2r 25, the azimuthally averaged increases with the star formation rate. However, Spearman rank correlation tests show the azimuthally averaged does not strongly correlate with any other gas or stellar properties in general, especially beyond 0.2r 25 from the galaxy centers. Our findings suggest that in the complex environments in galaxy disks, is not a sensitive tracer for ISM properties. Dynamical disturbances, like galaxy interactions or the presence of a bar, also have an overall impact on , which further complicates the interpretations of variations.

ViCTORIA project: MeerKAT H I observations of the ram pressure stripped galaxy NGC 4523

We present the first results of a 21 cm H I line pilot observation carried out with the MeerKAT radio telescope in preparation for the ViCTORIA (Virgo Cluster multi Telescope Observations in Radio of Interacting galaxies and AGN) project, an untargeted survey of the Virgo galaxy cluster. The extraordinary quality of the data in terms of sensitivity and angular resolution (rms ∼ 0.65 mJy beam−1 at ∼27″ × 39″ and 11 km s−1 resolution) allowed us to detect an extended (∼10 kpc projected length) low column density (N(HI)≲2.5 × 1020 cm−2) H I gas tail associated with the dwarf (Mstar = 1.6 × 109 M⊙) irregular galaxy NGC 4523 at the northern edge of the cluster. The morphology of the tail and of the stellar disc suggest that the galaxy is suffering a hydrodynamic interaction with the surrounding hot intracluster medium (ICM; ram pressure stripping). The orientation of the trailing tail, the gradient in the H I gas column density at the interface between the cold interstellar medium (ISM) and the hot ICM, the velocity of the galaxy with respect to that of the cluster, and its position indicate that NGC 4523 is infalling for the first time into Virgo from the north-north-west background of the cluster. Using a grid of hydrodynamic simulations, we derive the impact parameters with the surrounding ICM, and estimate that the galaxy will be at pericentre (D ∼ 500–600 kpc) in ∼1 Gyr, where ram pressure stripping will be able to remove most, if not all, of its gas. The galaxy is located on the star formation main sequence when its star formation rate is derived using Hα narrow-band images obtained during the VESTIGE survey, suggesting that NGC 4523 is only at the beginning of its interaction with the surrounding environment. A few H II regions are detected in the deep Hα narrow-band images within the H I gas tail outside the stellar disc. Their ages, derived by comparing their Hα, far-ultraviolet (ASTROSAT/UVIT), near-ultraviolet (GALEX/GUViCS), and optical (NGVS) colours with the predictions of spectral energy distribution fitting models, are ≲30 Myr, and suggest that these H II regions have formed within the stripped gas.

VICTORIA project: The LOFAR HBA Virgo Cluster Survey

Context. The Virgo cluster is the nearest (d = 16.5 Mpc) massive (M ≥ 1014 M⊙) galaxy cluster and is therefore a prime target for studying astrophysical processes in dense large-scale environments. In the radio band, we can probe the nonthermal components of the interstellar medium, the intracluster medium (ICM), and of active galactic nuclei (AGN). This allows an investigation of the impact of the environment on the evolution of galaxies and the contribution of AGN to ICM heating. With the ViCTORIA (VIrgo Cluster multi-Telescope Observations in Radio of Interacting galaxies and AGN) project, we are carrying out multiple wide-field surveys of the Virgo cluster at different frequencies. Aims. We aim to investigate the impact of the environment on the evolution of galaxies and the contribution of AGN to ICM heating – from the inner cluster regions out to beyond the virial radius. Methods. We performed a survey of the cluster at 120–168 MHz using the LOw-Frequency ARray (LOFAR). We imaged a 132 deg2 region of the cluster, reaching an order-of-magnitude greater sensitivity than existing wide-field radio surveys of this field at three times higher spatial resolution compared to other low-frequency observations. We developed a tailored data processing strategy to subtract the bright central radio galaxy M 87 from the data. This allowed us to correct for systematic effects due to ionospheric variation as a function of time and direction. Results. In the final mosaic, which has a resolution of 9″ × 5″, we reach a median noise level of 140 μJy beam−1 inside the virial radius and 280 μJy beam−1 for the full area. We detect 112 Virgo member galaxies and 114 background galaxies. In at least 18 cases, the radio morphology of the cluster member galaxies shows clear signs of ram-pressure stripping. This includes three previously unreported candidates. In addition, we reveal previously undiscovered tails of 150 kpc in length from a previous epoch of AGN activity for NGC 4472 (M 49). While no cluster-scale diffuse radio sources are discovered, we find the presence of an extended radio signature of the W′ group. This feature is coincident with an X-ray filament detected with SRG/eROSITA in the outskirts of the cluster. We speculate that this emission is synchrotron radiation, which could be related to shocks or turbulence from accretion processes. Conclusions. The data published in this paper serve as a valuable resource for future studies. In the follow-up work of the ViCTORIA project, we will use these data for an analysis of environmental effects on the radio properties of star-forming galaxies in Virgo.

VLBI imaging of high-redshift galaxies and protoclusters at low radio frequencies with the International LOFAR Telescope

It is generally known that luminous, ultra-steep spectrum radio sources are preferentially associated with massive galaxies at high redshifts. In this paper, we describe a pilot project directed at such objects to demonstrate the feasibility and importance of using the LOw-Frequency ARray (LOFAR) data to study the most distant massive galaxies undergoing formation and protoclusters. We successfully imaged four high-redshift (z &gt; 2), high-luminosity radio galaxies with sub-arcsecond resolution, at 144 MHz, using the International LOFAR Telescope (ILT). Our targets were 4C 41.17, which we dubbed “the Anthill” galaxy (z = 3.8), as well as B2 0902+34 (z = 3.4), 4C 34.34 (z = 2.4), and 4C 43.15 (z = 2.5). We mapped their low-frequency morphologies and the spatial distributions of their low-frequency spectral indices and then compared these results with the available optical, infrared, and X-ray images. Both for the Anthill at z = 3.8 and B2 0902+34 at z = 3.4, the location of the steepest radio emission coincides with the Lyα–emitting ionized gas halo. Our pilot project demonstrates that thanks to its outstanding sensitivity and high angular resolution at low frequencies, the ILT is a unique facility for studying the co-evolution and interaction of massive galaxies, galaxy clusters, and supermassive black holes in the early Universe.