Single Galaxy Research Articles

Galaxy deblending using residual dense neural networks

We present a new neural network approach for deblending galaxy images in astronomical data using Residual Dense Neural network (RDN) architecture. We train the network on synthetic galaxy images similar to the typical arrangements of field galaxies with a finite point spread function (PSF) and realistic noise levels. The main novelty of our approach is the usage of two distinct neural networks: i) a deblending network which isolates a single galaxy postage stamp from the composite and, ii) a classifier network which counts the remaining number of galaxies. The deblending proceeds by iteratively peeling one galaxy at a time from the composite until the image contains no further objects as determined by the classifier, or by other stopping criteria. By looking at the consistency in the outputs of the two networks, we can assess the quality of the deblending. We characterize the flux and shape reconstructions in different quality bins and compare our deblender with the industry standard, SExtractor. We also discuss possible future extensions for the project with variable PSFs and noise levels.

A Visually Classified Spectroscopic Object Catalog for SDSS-IV MaNGA

We provide a catalog of visually classified objects in the MaNGA integral field spectroscopic survey. The MaNGA survey is designed to target a single galaxy with each of its integral field units; however, many of these fields will host ancillary objects. We identify these discrete objects by cleaning up SDSS photometric objects in MaNGA’s fields-of-view. We then use the spectra from MaNGA’s data cubes to spectrally classify the identified objects. The catalog contains the positions and classifications of 1385 stars, 11,439 galaxies, and 107 broad-line active galactic nucleus (BLAGN) from the 10,130 unique MaNGA fields. We also provide spectroscopically derived parameters for the galaxies including; stellar masses, gas and stellar kinematics, and emission-line fluxes and equivalent widths. This catalog effectively expands the size of the MaNGA catalog by ∼50%, increasing the utility of the MaNGA project.

Nonlinear Color–Metallicity Relations of Globular Clusters. XI. Nonlinearity Effect Revealed by NGC 5128 (Centaurus A) and NGC 4594 (Sombrero) Galaxies

Metallicity distributions (MDs) of globular clusters (GCs) provide crucial clues for the assembly and star formation history of their host galaxies. GC colors, when GCs are old, have been used as a proxy of GC metallicities. Bimodal GC color distributions (CDs) observed in most early-type galaxies have been interpreted as bimodal MDs for decades, suggesting the presence of merely two GC subpopulations within single galaxies. However, the conventional view has been challenged by a new theory that nonlinear metallicity-to-color conversion can cause bimodal CDs from unimodal MDs. The unimodal MDs seem natural given that MDs involved many thousand protogalaxies. The new theory has been tested and corroborated by various observational and theoretical studies. Here we examine the nonlinear nature of GC color−metallicity relations (CMRs) using photometric and spectroscopic GC data of NGC 5128 (Centaurus A) and NGC 4594 (Sombrero), in comparison with stellar population simulations. We find that, with a slight offset in color, the overall shapes of observed and modeled CMRs agree well for all available colors. Diverse color-depending morphologies of GC CDs of the two galaxies are well reproduced based on their observed spectroscopic MDs via our CMR models. The results corroborate the nonlinear CMR interpretation of the GC color bimodality, shedding further light on theories of galaxy formation.

Unveiling the population of dual and lensed active galactic nuclei at sub-arcsec separations

All cosmological models of structure formation predict the existence of a widespread population of dual supermassive black holes in-spiralling inside their common host galaxy, eventually merging and giving rise to intense gravitational waves. These systems can be identified as dual AGNs at kilo parsec separations, but only very few have been confirmed at z>0.5. The appearance of multiple AGNs at small angular separations can also be due to gravitational lensing of single AGNs, which are themselves very important systems for many astrophysical topics. Here we present a novel technique, dubbed the Gaia Multipeak (GMP) method, to obtain large and reliable samples of dual/lensed AGN candidates with sub-arcsec separations by looking for AGNs showing multiple peaks in the light profiles observed by the Gaia satellite. All the GMP-selected sources with high resolution images (26 from the HST archive and 5 from dedicated adaptive-optics assisted imaging at the Large Binocular Telescope) show multiple components with sub-arcsec separation pointing toward a very high reliability of the method. By sampling separations down to ~2 kpc at z>1, this method allows us to probe the physical processes that drive the inspiralling of a pair of SMBHs inside a single galaxy.

On the possibility of formation of binary cosmic systems from the single cosmic objects

In this paper, the fission of the single stars and galaxies in the mass-asymmetry (mass transfer) coordinate is considered. Using the potential energy derived as a function of mass asymmetry, the possibility of the formation of binary stars (binary galaxies) from the single stars (single galaxies) is studied.

Variations in the ΣSFR − Σmol − Σ⋆ plane across galactic environments in PHANGS galaxies

Aims. There exists some consensus that the stellar mass surface density (Σ⋆) and molecular gas mass surface density (Σmol) are the main quantities responsible for locally setting the star formation rate. This regulation is inferred from locally resolved scaling relations between these two quantities and the star formation rate surface density (ΣSFR), which have been extensively studied in a wide variety of works. However, the universality of these relations is debated. Here, we probe the interplay between these three quantities across different galactic environments at a spatial resolution of 150 pc. Methods. We performed a hierarchical Bayesian linear regression to find the best set of parameters C⋆, Cmol, and Cnorm that describe the star-forming plane conformed by Σ⋆, Σmol, and ΣSFR, such that logΣSFR = C⋆logΣ⋆ + CmollogΣmol + Cnorm. We also explored variations in the determined parameters across galactic environments, focusing our analysis on the C⋆ and Cmol slopes. Results. We find signs of variations in the posterior distributions of C⋆ and Cmol across different galactic environments. The dependence of ΣSFR on Σ⋆ spans a wide range of slopes, with negative and positive values, while the dependence of ΣSFR on Σmol is always positive. Bars show the most negative value of C⋆ (−0.41), which is a sign of longer depletion times, while spiral arms show the highest C⋆ among all environments (0.45). Variations in Cmol also exist, although they are more subtle than those found for C⋆. Conclusions. We conclude that systematic variations in the interplay of Σ⋆, Σmol, and ΣSFR across different galactic environments exist at a spatial resolution of 150 pc, and we interpret these variations to be produced by an additional mechanism regulating the formation of stars that is not captured by either Σ⋆ or Σmol. Studying environmental variations in single galaxies, we find that these variations correlate with changes in the star formation efficiency across environments, which could be linked to the dynamical state of the gas that prevents it from collapsing and forming stars, or to changes in the molecular gas fraction.

A Bayesian Population Model for the Observed Dust Attenuation in Galaxies

Dust plays a pivotal role in determining the observed spectral energy distribution (SED) of galaxies. Yet our understanding of dust attenuation is limited and our observations suffer from the dust–metallicity–age degeneracy in SED fitting (single galaxies), large individual variances (ensemble measurements), and the difficulty in properly dealing with uncertainties (statistical considerations). In this study, we create a population Bayesian model to rigorously account for correlated variables and non-Gaussian error distributions and demonstrate the improvement over a simple Bayesian model. We employ a flexible 5D linear interpolation model for the parameters that control dust attenuation curves as a function of stellar mass, star formation rate (SFR), metallicity, redshift, and inclination. Our setup allows us to determine the complex relationships between dust attenuation and these galaxy properties simultaneously. Using Prospector fits of nearly 30,000 3D–Hubble Space Telescope galaxies, we find that the attenuation slope (n) flattens with increasing optical depth (τ), though less so than in previous studies. τ increases strongly with SFR, though when , τ remains roughly constant over a wide range of stellar masses. Edge-on galaxies tend to have larger τ than face-on galaxies, but only for , reflecting the lack of triaxiality for low-mass galaxies. Redshift evolution of dust attenuation is strongest for low-mass, low-SFR galaxies, with higher optical depths but flatter curves at high redshift. Finally, n has a complex relationship with stellar mass, highlighting the intricacies of the star–dust geometry. We have publicly released software for users to access our population model.

Towards a full general relativistic approach to galaxies

We analyse the dynamics of a single disk galaxy from a general relativistic viewpoint. We investigate dark matter (DM) effects in terms of a known family of stationary axially-symmetric solutions of Einstein equations coupled to a rotating dust. These effects are generated by the non-Newtonian features of such solutions and are ascribed to the essential role of frame dragging. Indeed, in such models, the off-diagonal elements of the metric are, in general, of the same order of magnitude of the diagonal ones. We generalize the results of Balasin and Grumiller (BG) to the physical case of differentially rotating dust. In particular, we find that for differential rotation the amount of energy density required to account for the flat rotation curves of disk galaxies is reduced with respect to the BG rigid rotation case. This stresses the discrepancy between Newtonian gravity and general relativity (GR), even at low velocities and low energy densities.

Supernovae and their cosmological implications

Supernovae (SNe) are luminous optical transients, which, despite being rare events in a single galaxy, nowadays are discovered at a rate of ten per day. They populate a growing zoo of different types linked to a variety of progenitors and explosion mechanisms. Through a biased selection of a century of researches, I will review how SNe have became a leading tool to probe the cosmic expansion. I will mainly focus on SNe of type Ia, originating from the thermonuclear explosion of white dwarfs, that were demonstrated to be excellent distance indicators after proper calibration and standardisation. Two decades ago, SNe Ia provided the surprising result that the cosmic expansion is accelerated by a misterious dark energy. Recently, with the decisive contribution of Cepheids variables as primary distance indicators, SN Ia allowed to measure the value of the Hubble-Lemaître constant, H_0, with an error of only 1%. It turned out that the value of H_0 based on SNe Ia is in tension with the one measured through the fit of the CMB fluctuations. If confirmed, this finding requires a deep revision of the standard cosmological model.

Cosmology with One Galaxy?

Galaxies can be characterized by many internal properties such as stellar mass, gas metallicity, and star formation rate. We quantify the amount of cosmological and astrophysical information that the internal properties of individual galaxies and their host dark matter halos contain. We train neural networks using hundreds of thousands of galaxies from 2000 state-of-the-art hydrodynamic simulations with different cosmologies and astrophysical models of the CAMELS project to perform likelihood-free inference on the value of the cosmological and astrophysical parameters. We find that knowing the internal properties of a single galaxy allows our models to infer the value of Ωm, at fixed Ωb, with a ∼10% precision, while no constraint can be placed on σ 8. Our results hold for any type of galaxy, central or satellite, massive or dwarf, at all considered redshifts, z ≤ 3, and they incorporate uncertainties in astrophysics as modeled in CAMELS. However, our models are not robust to changes in subgrid physics due to the large intrinsic differences the two considered models imprint on galaxy properties. We find that the stellar mass, stellar metallicity, and maximum circular velocity are among the most important galaxy properties to determine the value of Ωm. We believe that our results can be explained by considering that changes in the value of Ωm, or potentially Ωb/Ωm, affect the dark matter content of galaxies, which leaves a signature in galaxy properties distinct from the one induced by galactic processes. Our results suggest that the low-dimensional manifold hosting galaxy properties provides a tight direct link between cosmology and astrophysics.

The discovery of a radio galaxy of at least 5 Mpc

Context. Giant radio galaxies (GRGs, or colloquially ‘giants’) are the Universe’s largest structures generated by individual galaxies. They comprise synchrotron-radiating active galactic nucleus ejecta and attain cosmological (megaparsec-scale) lengths. However, the main mechanisms that drive their exceptional growth remain poorly understood. Aims. To deduce the main mechanisms that drive a phenomenon, it is usually instructive to study extreme examples. If there exist host galaxy characteristics that are an important cause for GRG growth, then the hosts of the largest GRGs are likely to possess them. Similarly, if there exist particular large-scale environments that are highly conducive to GRG growth, then the largest GRGs are likely to reside in them. For these reasons, we aim to perform a case study of the largest GRG available. Methods. We reprocessed the LOFAR Two-Metre Sky Survey DR2 by subtracting compact sources and performing multi-scale CLEAN de-convolutions at 60″ and 90″ resolution. The resulting images constitute the most sensitive survey yet for radio galaxy lobes, whose diffuse nature and steep synchrotron spectra have allowed them to evade previous detection attempts at higher resolution and shorter wavelengths. We visually searched these images for GRGs. Results. We have discovered Alcyoneus, a low-excitation radio galaxy with a projected proper length lp = 4.99 ± 0.04 Mpc. Both its jets and lobes are detected at very high significance, and the SDSS-based identification of the host, at spectroscopic redshift zspec = 0.24674 ± 6 × 10−5, is unambiguous. The total luminosity density at ν = 144 MHz is Lν = 8 ± 1 × 1025 W Hz−1, which is below average, though near median (percentile 45 ± 3%) for GRGs. The host is an elliptical galaxy with a stellar mass M⋆ = 2.4 ± 0.4 × 1011 M⊙ and a super-massive black hole mass M• = 4 ± 2 × 108 M⊙, both of which tend towards the lower end of their respective GRG distributions (percentiles 25 ± 9% and 23 ± 11%). The host resides in a filament of the Cosmic Web. Through a new Bayesian model for radio galaxy lobes in three dimensions, we estimate the pressures in the megaparsec-cubed-scale northern and southern lobes to be Pmin, 1 = 4.8 ± 0.3 × 10−16 Pa and Pmin, 2 = 4.9 ± 0.6 × 10−16 Pa, respectively. The corresponding magnetic field strengths are Bmin, 1 = 46 ± 1 pT and Bmin, 2 = 46 ± 3 pT. Conclusions. We have discovered what is in projection the largest known structure made by a single galaxy – a GRG with a projected proper length lp = 4.99 ± 0.04 Mpc. The true proper length is at least lmin = 5.04 ± 0.05 Mpc. Beyond geometry, Alcyoneus and its host are suspiciously ordinary: the total low-frequency luminosity density, stellar mass, and super-massive black hole mass are all lower than, though similar to, those of the medial GRG. Thus, very massive galaxies or central black holes are not necessary to grow large giants, and, if the observed state is representative of the source over its lifetime, neither is high radio power. A low-density environment remains a possible explanation. The source resides in a filament of the Cosmic Web, with which it might have significant thermodynamic interaction. The pressures in the lobes are the lowest hitherto found, and Alcyoneus therefore represents the most promising radio galaxy yet to probe the warm–hot inter-galactic medium.

The Missing Metal Problem in Galaxy Clusters: Characterizing the Early Enrichment Population

Abstract Rich and poor galaxy clusters have the same measured halo metallicity, 0.35–0.4 Z ⊙, even though they are an order of magnitude apart in stellar fraction, M */M gas. The measured intracluster medium (ICM) metallicity in high-mass clusters cannot be explained by the visible stellar population as stars typically make up 3%–20% of the total baryon mass. The independence of metallicity of M */M gas suggests an external and universal source of metals such as an early enrichment population (EEP). Galaxy cluster RX J1416.4+2315, classified as a fossil system, has a stellar fraction of M */M gas = 0.054 ± 0.018, and here we improve the halo metallicity determination using archival Chandra and XMM-Newton observations. We determine the ICM metallicity of RXJ1416 to be 0.303 ± 0.053 Z ⊙ within 0.3 < R/R 500 < 1, excluding the central galaxy. We combine this measurement with other clusters with a wider range of M */M gas, resulting in the fit of Z tot = (0.36 ± 0.01) + (0.10 ± 0.17)(M */M gas). This fit is largely independent of M */M gas and shows that for a low M */M gas system, the observed stellar population can make only 10%–20% of the total metals. We quantify the Fe contribution of the EEP further by adopting a standard Fe yield for visible stellar populations, and find that Z EEP = (0.36 ± 0.01) − − (0.96 ± 0.17)(M */M gas). To account for the observed Fe mass, a supernova (SN) rate of 10 ± 5 SNe yr−1 (Type Ia) and 40 ± 19 SNe yr−1 (core collapse) is required over the redshift range 3 < z < 10 for a single galaxy cluster with mass ∼3 × 1014 M ⊙ at z = 0. These SNe might be visible in observations of high-redshift clusters and protoclusters with the James Webb Space Telescope.

Practical galaxy morphology tools from deep supervised representation learning

ABSTRACT Astronomers have typically set out to solve supervised machine learning problems by creating their own representations from scratch. We show that deep learning models trained to answer every Galaxy Zoo DECaLS question learn meaningful semantic representations of galaxies that are useful for new tasks on which the models were never trained. We exploit these representations to outperform several recent approaches at practical tasks crucial for investigating large galaxy samples. The first task is identifying galaxies of similar morphology to a query galaxy. Given a single galaxy assigned a free text tag by humans (e.g. ‘#diffuse’), we can find galaxies matching that tag for most tags. The second task is identifying the most interesting anomalies to a particular researcher. Our approach is 100 per cent accurate at identifying the most interesting 100 anomalies (as judged by Galaxy Zoo 2 volunteers). The third task is adapting a model to solve a new task using only a small number of newly labelled galaxies. Models fine-tuned from our representation are better able to identify ring galaxies than models fine-tuned from terrestrial images (ImageNet) or trained from scratch. We solve each task with very few new labels; either one (for the similarity search) or several hundred (for anomaly detection or fine-tuning). This challenges the longstanding view that deep supervised methods require new large labelled data sets for practical use in astronomy. To help the community benefit from our pretrained models, we release our fine-tuning code zoobot. Zoobot is accessible to researchers with no prior experience in deep learning.

PHANGS–MUSE: The H II region luminosity function of local star-forming galaxies

We use an unprecedented sample of about 23 000 H II regions detected at an average physical resolution of 67 pc in the PHANGS–MUSE sample to study the extragalactic H II region Hα luminosity function (LF). Our observations probe the star-forming disk of 19 nearby spiral galaxies with low inclination and located close to the star formation main sequence at z = 0. The mean LF slope, α, in our sample is =1.73 with a σ of 0.15. We find that α decreases with the galaxy’s star formation rate surface density, ΣSFR, and argue that this is driven by an enhanced clustering of young stars at high gas surface densities. Looking at the H II regions within single galaxies, we find that no significant variations occur between the LF of the inner and outer part of the star-forming disk, whereas the LF in the spiral arm areas is shallower than in the inter-arm areas for six out of the 13 galaxies with clearly visible spiral arms. We attribute these variations to the spiral arms increasing the molecular clouds’ arm–inter-arm mass contrast and find suggestive evidence that they are more evident for galaxies with stronger spiral arms. Furthermore, we find systematic variations in α between samples of H II regions with a high and low ionization parameter, q, and argue that they are driven by the aging of H II regions.

Mapping the Diversity of Galaxy Spectra with Deep Unsupervised Machine Learning

Abstract Modern spectroscopic surveys of galaxies such as MaNGA consist of millions of diverse spectra covering different regions of thousands of galaxies. We propose and implement a deep unsupervised machine-learning method to summarize the entire diversity of MaNGA spectra onto a 15 × 15 map (DESOM-1), where neighboring points on the map represent similar spectra. We demonstrate our method as an alternative to conventional full spectral fitting for deriving physical quantities and full probability distributions much more efficiently than traditional resource-intensive Bayesian methods. Since spectra are grouped by similarity, the distribution of spectra onto the map for a single galaxy, i.e., its “fingerprint,” reveals the presence of distinct stellar populations within the galaxy, indicating smoother or episodic star formation histories. We further map the diversity of galaxy fingerprints onto a second map (DESOM-2). Using galaxy images and independent measures of galaxy morphology, we confirm that galaxies with similar fingerprints have similar morphologies and inclination angles. Since morphological information was not used in the mapping algorithm, relating galaxy morphology to the star formation histories encoded in the fingerprints is one example of how the DESOM maps can be used to make scientific inferences.

Gaseous Halos of Simulated Milky Way-like Galaxies

Abstract In order to investigate the role of the host halo in quenching satellite galaxies, we have characterized a single Milky Way-like host galaxy from the FIRE simulations from z = 0–1.76 by quantifying the gas density of the host halo environment with respect to distance from the host and galactocentric latitude. The gas density decreases with increasing distance from the host according to a broken power law. At earlier times (2–10 Gyr ago), the density in the inner regions of the host halo was enhanced relative to z = 0. Thus, earlier infalling satellites experienced more ram-pressure and were more efficiently quenched compared to later infalling satellites. We also find that in the inner halo (<150 kpc) the density is 2–3 times larger close to the plane of the host galaxy disk versus above or below the disk, so satellites that orbit at low galactocentric latitudes may be more efficiently quenched.

Evolution of C iv Absorbers. II. Where Does C iv Live?

Abstract We use the observed cumulative statistics of C iv absorbers and dark matter halos to infer the distribution of C iv-absorbing gas relative to galaxies at redshifts 0 ≤ z ≤ 5. We compare the cosmic incidence dN/dX of C iv absorber populations and galaxy halos, finding that massive L ≥ L ⋆ halos alone cannot account for all the observed W r ≥ 0.05 Å absorbers. However, the dN/dX of lower-mass halos exceeds that of W r ≥ 0.05 Å absorbers. We also estimate the characteristic gas radius of absorbing structures required for the observed C iv dN/dX, assuming each absorber is associated with a single galaxy halo. The W r ≥ 0.3 Å and W r ≥ 0.6 Å C iv gas radii are ∼30%–70% (∼20%–40%) of the virial radius of L ⋆ (0.1L ⋆) galaxies, and the W r ≥ 0.05 Å gas radius is ∼100%–150% (∼60%–100%) of the virial radius of L ⋆ (0.1L ⋆) galaxies. For stronger absorbers, the gas radius relative to the virial radius rises across Cosmic Noon and falls afterwards, while for weaker absorbers, the relative gas radius declines across Cosmic Noon and then dramatically rises at z < 1. A strong luminosity-dependence of the gas radius implies highly extended C iv envelopes around massive galaxies before Cosmic Noon, while a luminosity-independent gas radius implies highly extended envelopes around dwarf galaxies after Cosmic Noon. From available absorber-galaxy and C iv evolution data, we favor a scenario in which low-mass galaxies enrich the volume around massive galaxies at early epochs and propose that the outer halo gas (>0.5 R v ) was produced primarily in ancient satellite dwarf galaxy outflows, while the inner halo gas (<0.5 R v ) originated from the central galaxy and persists as recycled accreting gas.

Finding High-redshift Galaxies with JWST

Abstract One of the primary goals for the upcoming James Webb Space Telescope is to observe the first galaxies. Predictions for planned and proposed surveys have typically focused on average galaxy counts, assuming a random distribution of galaxies across the observed field. The first and most-massive galaxies, however, are expected to be tightly clustered, an effect known as cosmic variance. We show that cosmic variance is likely to be the dominant contribution to uncertainty for high-redshift mass and luminosity functions, and that median high-redshift and high-mass galaxy counts for planned observations lie significantly below average counts. Several different strategies are considered for improving our understanding of the first galaxies, including adding depth, area, and independent pointings. Adding independent pointings is shown to be the most efficient both for discovering the single highest-redshift galaxy and also for constraining mass and luminosity functions.

Period-change rates in Large Magellanic Cloud Cepheids revisited

ABSTRACT The period-change rate (PCR) of pulsating variable stars is a useful probe of changes in their interior structure, and thus of their evolutionary stages. So far, the PCRs of classical Cepheids in the Large Magellanic Cloud (LMC) have been explored in a limited sample of the total population of these variables. Here, we use a template-based method to build observed-minus-computed (O − C) period diagrams, from which we can derive PCRs for these stars by taking advantage of the long time baseline afforded by the Digital Access to a Sky Century @ Harvard light curves, combined with additional data from the Optical Gravitational Lensing Experiment, the MAssive Compact Halo Object project, Gaia’s Data Release 2, and in some cases the All-Sky Automated Survey. From an initial sample of 2315 sources, our method provides an unprecedented sample of 1303 LMC classical Cepheids with accurate PCRs, the largest for any single galaxy, including the Milky Way. The derived PCRs are largely compatible with theoretically expected values, as computed by our team using the Modules for Experiments in Stellar Astrophysics code, as well as with similar previous computations available in the literature. Additionally, five long-period ($P\,\gt\, 50\, \rm {d}$) sources display a cyclic behaviour in their O − C diagrams, which is clearly incompatible with evolutionary changes. Finally, on the basis of their large positive PCR values, two first-crossing Cepheid candidates are identified.

A strong lensing model of the galaxy cluster PSZ1 G311.65-18.48

We present a strong lensing analysis of the galaxy cluster PSZ1 G311.65-18.48 (z = 0.443) using multi-band observations with Hubble Space Telescope complemented with VLT/MUSE spectroscopic data. The MUSE observations provide redshift estimates for the lensed sources and help to reduce misidentification of the multiple images. Spectroscopic data are also used to measure the inner velocity dispersions of 15 cluster galaxies and calibrate the scaling relations to model the subhalo cluster component. The model is based on 62 multiple images grouped in 17 families belonging to four different sources. The majority of them are multiple images of compact stellar knots belonging to a single star-forming galaxy at z = 2.3702. This source is strongly lensed by the cluster to form the Sunburst Arc system. To accurately reproduce all the multiple images, we built a parametric mass model, which includes both cluster-scale and galaxy-scale components. The resulting model has a rms separation between the model-predicted and the observed positions of the multiple images of only 0.14″. We conclude that PSZ1 G311.65-18.48 has a relatively round projected shape and a large Einstein radius (29″ for zs = 2.3702), which could indicate that the cluster is elongated along the line of sight. The Sunburst Arc source is located at the intersection of a complex network of caustics, which explains why parts of the arc are imaged with unprecedented multiplicity (up to 12 times).