TY - JOUR A1 - Sparre, Martin A1 - Whittingham, Joseph A1 - Damle, Mitali A1 - Hani, Maan H. A1 - Richter, Philipp A1 - Ellison, Sara L. A1 - Pfrommer, Christoph A1 - Vogelsberger, Mark T1 - Gas flows in galaxy mergers BT - supersonic turbulence in bridges, accretion from the circumgalactic medium, and metallicity dilution JF - Monthly notices of the Royal Astronomical Society N2 - In major galaxy mergers, the orbits of stars are violently perturbed, and gas is torqued to the centre, diluting the gas metallicity and igniting a starburst. In this paper, we study the gas dynamics in and around merging galaxies using a series of cosmological magnetohydrodynamical zoom-in simulations. We find that the gas bridge connecting the merging galaxies pre-coalescence is dominated by turbulent pressure, with turbulent Mach numbers peaking at values of 1.6-3.3. This implies that bridges are dominated by supersonic turbulence, and are thus ideal candidates for studying the impact of extreme environments on star formation. We also find that gas accreted from the circumgalactic medium (CGM) during the merger significantly contributes (27-51 percent) to the star formation rate (SFR) at the time of coalescence and drives the subsequent reignition of star formation in the merger remnant. Indeed, 19-53 percent of the SFR at z = 0 originates from gas belonging to the CGM prior the merger. Finally, we investigate the origin of the metallicity-diluted gas at the centre of merging galaxies. We show that this gas is rapidly accreted on to the Galactic Centre with a time-scale much shorter than that of normal star-forming galaxies. This explains why coalescing galaxies are not well-captured by the fundamental metallicity relation. KW - MHD KW - methods: numerical KW - galaxies: interactions KW - galaxies: starburst Y1 - 2021 U6 - https://doi.org/10.1093/mnras/stab3171 SN - 1365-2966 VL - 509 IS - 2 SP - 2720 EP - 2735 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Whittingham, Joseph A1 - Sparre, Martin A1 - Pfrommer, Christoph A1 - Pakmor, Rüdiger T1 - The impact of magnetic fields on cosmological galaxy mergers BT - I. Reshaping gas and stellar discs JF - Monthly notices of the Royal Astronomical Society N2 - Mergers play an important role in galaxy evolution. In particular, major mergers are able to have a transformative effect on galaxy morphology. In this paper, we investigate the role of magnetic fields in gas-rich major mergers. To this end, we run a series of high-resolution magnetohydrodynamic (MHD) zoom-in simulations with the moving-mesh code arepo and compare the outcome with hydrodynamic simulations run from the same initial conditions. This is the first time that the effect of magnetic fields in major mergers has been investigated in a cosmologically consistent manner. In contrast to previous non-cosmological simulations, we find that the inclusion of magnetic fields has a substantial impact on the production of the merger remnant. Whilst magnetic fields do not strongly affect global properties, such as the star formation history, they are able to significantly influence structural properties. Indeed, MHD simulations consistently form remnants with extended discs and well-developed spiral structure, whilst hydrodynamic simulations form more compact remnants that display distinctive ring morphology. We support this work with a resolution study and show that whilst global properties are broadly converged across resolution and physics models, morphological differences only develop given sufficient resolution. We argue that this is due to the more efficient excitement of a small-scale dynamo in higher resolution simulations, resulting in a more strongly amplified field that is better able to influence gas dynamics. KW - MHD KW - methods: numerical KW - galaxies: interactions KW - galaxies: magnetic KW - fields Y1 - 2021 U6 - https://doi.org/10.1093/mnras/stab1425 SN - 0035-8711 SN - 1365-2966 VL - 506 IS - 1 SP - 229 EP - 255 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Sparre, Martin A1 - Pfrommer, Christoph A1 - Ehlert, Kristian T1 - Interaction of a cold cloud with a hot wind BT - the regimes of cloud growth and destruction and the impact of magnetic fields JF - Monthly notices of the Royal Astronomical Society N2 - Multiphase galaxy winds, the accretion of cold gas through galaxy haloes, and gas stripping from jellyfish galaxies are examples of interactions between cold and hot gaseous phases. There are two important regimes in such systems. A sufficiently small cold cloud is destroyed by the hot wind as a result of Kelvin-Helmholtz instabilities, which shatter the cloud into small pieces that eventually mix and dissolve in the hot wind. In contrast, stripped cold gas from a large cloud mixes with the hot wind to intermediate temperatures, and then becomes thermally unstable and cools, causing a net accretion of hot gas to the cold tail. Using the magneto-hydrodynamical code AREPO, we perform cloud crushing simulations and test analytical criteria for the transition between the growth and destruction regimes to clarify a current debate in the literature. We find that the hot-wind cooling time sets the transition radius and not the cooling time of the mixed phase. Magnetic fields modify the wind-cloud interaction. Draping of wind magnetic field enhances the field upstream of the cloud, and fluid instabilities are suppressed by a turbulently magnetized wind beyond what is seen for a wind with a uniform magnetic field. We furthermore predict jellyfish galaxies to have ordered magnetic fields aligned with their tails. We finally discuss how the results of idealized simulations can be used to provide input to subgrid models in cosmological (magneto-)hydrodynamical simulations, which cannot resolve the detailed small-scale structure of cold gas clouds in the circumgalactic medium. KW - methods: numerical KW - ISM: jets and outflows KW - galaxies: formation Y1 - 2020 U6 - https://doi.org/10.1093/mnras/staa3177 SN - 0035-8711 SN - 1365-2966 VL - 499 IS - 3 SP - 4261 EP - 4281 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Davidzon, Iary A1 - Ilbert, Olivier A1 - Faisst, Andreas L. A1 - Sparre, Martin A1 - Capak, Peter L. T1 - An Alternate Approach to Measure Specific Star Formation Rates at 2 < z < 7 JF - The astrophysical journal : an international review of spectroscopy and astronomical physics N2 - We trace the specific star formation rate (sSFR) of massive star-forming galaxies (greater than or similar to 10(10)M(circle dot)) from z similar to 2 to 7. Our method is substantially different from previous analyses, as it does not rely on direct estimates of star formation rate, but on the differential evolution of the galaxy stellar mass function (SMF). We show the reliability of this approach by means of semianalytical and hydrodynamical cosmological simulations. We then apply it to real data, using the SMFs derived in the COSMOS and CANDELS fields. We find that the sSFR is proportional to (1 + z)(1.1) (+/-) (0.2) at z > 2, in agreement with other observations but in tension with the steeper evolution predicted by simulations from z similar to 4 to 2. We investigate the impact of several sources of observational bias, which, however, cannot account for this discrepancy. Although the SMF of high-redshift galaxies is still affected by significant errors, we show that future large-area surveys will substantially reduce them, making our method an effective tool to probe the massive end of the main sequence of star-forming galaxies. KW - galaxies: evolution KW - galaxies: high-redshift KW - galaxies: star formation Y1 - 2018 U6 - https://doi.org/10.3847/1538-4357/aaa19e SN - 0004-637X SN - 1538-4357 VL - 852 IS - 2 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Hani, Maan H. A1 - Sparre, Martin A1 - Ellison, Sara L. A1 - Torrey, Paul A1 - Vogelsberger, Mark T1 - Galaxy mergers moulding the circum-galactic medium BT - I. The impact of a major merger JF - Monthly notices of the Royal Astronomical Society N2 - Galaxies are surrounded by sizeable gas reservoirs which host a significant amount of metals: the circum-galactic medium (CGM). The CGM acts as a mediator between the galaxy and the extragalactic medium. However, our understanding of how galaxy mergers, a major evolutionary transformation, impact the CGM remains deficient. We present a theoretical study of the effect of galaxy mergers on the CGM. We use hydrodynamical cosmological zoom-in simulations of a major merger selected from the Illustris project such that the z = 0 descendant has a halo mass and stellar mass comparable to the Milky Way. To study the CGM we then re-simulated this system at a 40 times better mass resolution, and included detailed post-processing ionization modelling. Our work demonstrates the effect the merger has on the characteristic size of the CGM, its metallicity, and the predicted covering fraction of various commonly observed gas-phase species, such as H I, C IV, and O VI. We show that merger-induced outflows can increase the CGM metallicity by 0.2-0.3 dex within 0.5 Gyr post-merger. These effects last up to 6 Gyr post-merger. While the merger increases the total metal covering fractions by factors of 2-3, the covering fractions of commonly observed UV ions decrease due to the hard ionizing radiation from the active galactic nucleus, which we model explicitly. Our study of the single simulated major merger presented in this work demonstrates the significant impact that a galaxy interaction can have on the size, metallicity, and observed column densities of the CGM. KW - methods: numerical KW - galaxies: evolution KW - galaxies: haloes KW - galaxies: interactions Y1 - 2017 U6 - https://doi.org/10.1093/mnras/stx3252 SN - 0035-8711 SN - 1365-2966 VL - 475 IS - 1 SP - 1160 EP - 1176 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Heintz, Kasper Elm A1 - Watson, D. A1 - Jakobsson, P. A1 - Fynbo, J. P. U. A1 - Bolmer, J. A1 - Arabsalmani, M. A1 - Cano, Zach A1 - Covino, S. A1 - Gomboc, A. A1 - Japelj, J. A1 - Kaper, L. A1 - Krogager, J. -K. A1 - Pugliese, G. A1 - Sanchez-Ramirez, R. A1 - Selsing, J. A1 - Sparre, Martin A1 - Tanvir, N. R. A1 - Thone, C. C. A1 - de Ugarte Postigo, A. A1 - Vergani, S. D. T1 - Highly ionized metals as probes of the circumburst gas in the natal regions of gamma-ray bursts JF - Monthly notices of the Royal Astronomical Society N2 - We present here a survey of high-ionization absorption lines in the afterglow spectra of long-duration gamma-ray bursts (GRBs) obtained with the VLT/X-shooter spectrograph. Our main goal is to investigate the circumburst medium in the natal regions of GRBs. Our primary focus is on the N vλλ 1238, 1242 line transitions, but we also discuss other high-ionization lines such as O vi, C iv, and Si iv. We find no correlation between the column density of N v and the neutral gas properties such as metallicity, H i column density, and dust depletion; however, the relative velocity of N v, typically a blueshift with respect to the neutral gas, is found to be correlated with the column density of H i. This may be explained if the N v gas is part of an H ii region hosting the GRB, where the region’s expansion is confined by dense, neutral gas in the GRB’s host galaxy. We find tentative evidence (at 2σ significance) that the X-ray derived column density, NH, X, may be correlated with the column density of N v, which would indicate that both measurements are sensitive to the column density of the gas located in the vicinity of the GRB. We investigate the scenario where N v (and also O vi) is produced by recombination after the corresponding atoms have been stripped entirely of their electrons by the initial prompt emission, in contrast to previous models where highly ionized gas is produced by photoionization from the GRB afterglow. KW - gamma-ray bursts: general KW - ISM: abundances Y1 - 2018 U6 - https://doi.org/10.1093/mnras/sty1447 SN - 0035-8711 SN - 1365-2966 VL - 479 IS - 3 SP - 3456 EP - 3476 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Bustamante, Sebastian A1 - Sparre, Martin A1 - Springel, Volker A1 - Grand, Robert J. J. T1 - Merger-induced metallicity dilution in cosmological galaxy formation simulations JF - Monthly notices of the Royal Astronomical Society N2 - Observational studies have revealed that galaxy pairs tend to have lower gas-phase metallicity than isolated galaxies. This metallicity deficiency can be caused by inflows of low-metallicity gas due to the tidal forces and gravitational torques associated with galaxy mergers, diluting the metal content of the central region. In this work we demonstrate that such metallicity dilution occurs in state-of-the-art cosmological simulations of galaxy formation. We find that the dilution is typically 0.1 dex for major mergers, and is noticeable at projected separations smaller than 40 kpc. For minor mergers the metallicity dilution is still present, even though the amplitude is significantly smaller. Consistent with previous analysis of observed galaxies we find that mergers are outliers from the fundamental metallicity relation, with deviations being larger than expected for a Gaussian distribution of residuals. Our large sample of mergers within full cosmological simulations also makes it possible to estimate how the star formation rate enhancement and gas consumption timescale behave as a function of the merger mass ratio. We confirm that strong starbursts are likely to occur in major mergers, but they can also arise in minor mergers if more than two galaxies are participating in the interaction, a scenario that has largely been ignored in previous work based on idealised isolated merger simulations. KW - methods: numerical KW - galaxies: interactions KW - galaxies: star formation KW - galaxies: evolution Y1 - 2018 U6 - https://doi.org/10.1093/mnras/sty1692 SN - 0035-8711 SN - 1365-2966 VL - 479 IS - 3 SP - 3381 EP - 3392 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Sparre, Martin A1 - Pfrommer, Christoph A1 - Vogelsberger, Mark T1 - The physics of multiphase gas flows BT - fragmentation of a radiatively cooling gas cloud in a hot wind JF - Monthly notices of the Royal Astronomical Society N2 - Galactic winds exhibit a multiphase structure that consists of hot-diffuse and cold-dense phases. Here we present high-resolution idealized simulations of the interaction of a hot supersonic wind with a cold cloud with the moving-mesh code AREPO in setups with and without radiative cooling. We demonstrate that cooling causes clouds with sizes larger than the cooling length to fragment in 2D and 3D simulations. We confirm earlier 2D simulations by McCourt et al. (2018) and highlight differences of the shattering processes of 3D clouds that are exposed to a hot wind. The fragmentation process is quantified with a friends-of-friends analysis of shattered cloudlets and density power spectra. Those show that radiative cooling causes the power spectral index to gradually increase when the initial cloud radius is larger than the cooling length and with increasing time until the cloud is fully dissolved in the hot wind. A resolution of around 1 pc is required to reveal the effect of cooling-induced fragmentation of a 100 pc outflowing cloud. Thus, state-of-the-art cosmological zoom simulations of the circumgalactic medium fall short by orders of magnitudes from resolving this fragmentation process. This physics is, however, necessary to reliably model observed column densities and covering fractions of Lyman alpha haloes, high-velocity clouds, and broad-line regions of active galactic nuclei. KW - methods: numerical KW - ISM: jets and outflows KW - galaxies: formation Y1 - 2018 U6 - https://doi.org/10.1093/mnras/sty3063 SN - 0035-8711 SN - 1365-2966 VL - 482 IS - 4 SP - 5401 EP - 5421 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Selsing, Jonatan A1 - Malesani, D. A1 - Goldoni, P. A1 - Fynbo, Johan A1 - Krühler, T. A1 - Antonelli, L. A. A1 - Arabsalmani, M. A1 - Bolmer, J. A1 - Cano, Z. A1 - Christensen, L. A1 - Covino, S. A1 - De Cia, A. A1 - de Ugarte Postigo, A. A1 - Flores, H. A1 - Fliis, M. A1 - Gomboc, A. A1 - Greiner, J. A1 - Groot, P. A1 - Hammer, F. A1 - Hartoog, O. E. A1 - Heintz, K. E. A1 - Hjorth, J. A1 - Jakobsson, P. A1 - Japelj, J. A1 - Kann, D. A. A1 - Kaper, L. A1 - Ledoux, C. A1 - Leloudas, G. A1 - Levan, A. J. A1 - Maiorano, E. A1 - Melandri, A. A1 - Milvang-Jensen, B. A1 - Palazzi, E. A1 - Palmerio, J. T. A1 - Perley, D. A. A1 - Pian, E. A1 - Piranomonte, S. A1 - Pugliese, G. A1 - Sanchez-Ramirez, R. A1 - Savaglio, S. A1 - Schady, P. A1 - Schulze, S. A1 - Sollerman, J. A1 - Sparre, Martin A1 - Tagliaferri, G. A1 - Tanvir, N. R. A1 - Thone, C. C. A1 - Vergani, S. D. A1 - Vreeswijk, P. A1 - Watson, D. A1 - Wiersema, K. A1 - Wijers, R. A1 - Xu, D. A1 - Zafar, T. T1 - The X-shooter GRB afterglow legacy sample (XS-GRB) JF - Astronomy and astrophysics : an international weekly journal N2 - In this work we present spectra of all gamma-ray burst (GRB) afterglows that have been promptly observed with the X-shooter spectrograph until 31/03/2017. In total, we have obtained spectroscopic observations of 103 individual GRBs observed within 48 hours of the GRB trigger. Redshifts have been measured for 97 per cent of these, covering a redshift range from 0.059 to 7.84. Based on a set of observational selection criteria that minimise biases with regards to intrinsic properties of the GRBs, the follow-up effort has been focused on producing a homogeneously selected sample of 93 afterglow spectra for GRBs discovered by the Swift satellite. We here provide a public release of all the reduced spectra, including continuum estimates and telluric absorption corrections. For completeness, we also provide reductions for the 18 late-time observations of the underlying host galaxies. We provide an assessment of the degree of completeness with respect to the parent GRB population, in terms of the X-ray properties of the bursts in the sample and find that the sample presented here is representative of the full Swift sample. We have constrained the fraction of dark bursts to be <28 per cent and confirm previous results that higher optical darkness is correlated with increased X-ray absorption. For the 42 bursts for which it is possible, we have provided a measurement of the neutral hydrogen column density, increasing the total number of published HI column density measurements by similar to 33 per cent. This dataset provides a unique resource to study the ISM across cosmic time, from the local progenitor surroundings to the intervening Universe. KW - gamma-ray burst: general KW - galaxies: high-redshift KW - ISM: general KW - techniques: spectroscopic KW - catalogs KW - galaxies: star formation Y1 - 2019 U6 - https://doi.org/10.1051/0004-6361/201832835 SN - 1432-0746 SN - 0004-6361 VL - 623 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - Despali, Giulia A1 - Sparre, Martin A1 - Vegetti, Simona A1 - Vogelsberger, Mark A1 - Zavala, Jesús A1 - Marinacci, Federico T1 - The interplay of self-interacting dark matter and baryons in shaping the halo evolution JF - Monthly notices of the Royal Astronomical Society N2 - We use high-resolution hydrodynamical simulation to test the difference of halo properties in cold dark matter (CDM) and a self-interacting dark matter (SIDM) scenario with a constant cross-section of sigma(T)/m(x) = 1 cm(2) g(-1). We find that the interplay between dark matter self-interaction and baryonic physics induces a complex evolution of the halo properties, which depends on the halo mass and morphological type, as well as on the halo mass accretion history. While high-mass haloes, selected as analogues of early-type galaxies, show cored profiles in the SIDM run, systems of intermediate mass and with a significant disc component can develop a profile that is similar or cuspier than in CDM. The final properties of SIDM haloes - measured at z = 0.2 - correlate with the halo concentration and formation time, suggesting that the differences between different systems are due to the fact that we are observing the impact of self-interaction. We also search for signatures of SIDM in the lensing signal of the main haloes and find hints of potential differences in the distribution of Einstein radii, which suggests that future wide-field survey might be able to distinguish between CDM and SIDM models on this basis. Finally, we find that the subhalo abundances are not altered in the adopted SIDM model with respect to CDM. KW - gravitational lensing: strong KW - methods: numerical KW - galaxies: haloes KW - dark matter Y1 - 2019 U6 - https://doi.org/10.1093/mnras/stz273 SN - 0035-8711 SN - 1365-2966 VL - 484 IS - 4 SP - 4563 EP - 4573 PB - Oxford University Press CY - Oxford ER - TY - JOUR A1 - Hani, Maan H. A1 - Ellison, Sara L. A1 - Sparre, Martin A1 - Grand, Robert J. J. A1 - Pakmor, Rüdiger A1 - Gómez, Facundo A. A1 - Springel, Volker T1 - The diversity of the circumgalactic medium around z=0 Milky Way-mass galaxies from the Auriga simulations JF - Monthly notices of the Royal Astronomical Society N2 - Galaxies are surrounded by massive gas reservoirs ( i.e. the circumgalactic medium; CGM) which play a key role in their evolution. The properties of the CGM, which are dependent on a variety of internal and environmental factors, are often inferred from absorption line surveys which rely on a limited number of single lines-of-sight. In this work we present an analysis of 28 galaxy haloes selected from the Auriga project, a cosmological magneto-hydrodynamical zoom-in simulation suite of isolated MilkyWay-mass galaxies, to understand the impact of CGM diversity on observational studies. Although the Auriga haloes are selected to populate a narrow range in halo mass, our work demonstrates that the CGM of L-star galaxies is extremely diverse: column densities of commonly observed species span similar to 3-4 dex and their covering fractions range from similar to 5 to 90 per cent. Despite this diversity, we identify the following correlations: 1) the covering fractions ( CF) of hydrogen and metals of the Auriga haloes positively correlate with stellar mass, 2) the CF of H I, C IV, and Si II anticorrelate with active galactic nucleus luminosity due to ionization effects, and 3) the CF of H I, C IV, and Si II positively correlate with galaxy disc fraction due to outflows populating the CGM with cool and dense gas. The Auriga sample demonstrates striking diversity within the CGM of L-star galaxies, which poses a challenge for observations reconstructing CGM characteristics from limited samples, and also indicates that long-term merger assembly history and recent star formation are not the dominant sculptors of the CGM. KW - methods: numerical KW - galaxies: evolution KW - galaxies: haloes Y1 - 2019 U6 - https://doi.org/10.1093/mnras/stz1708 SN - 0035-8711 SN - 1365-2966 VL - 488 IS - 1 SP - 135 EP - 152 PB - Oxford Univ. Press CY - Oxford ER -