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Papers by Aura Obreja

Monthly Notices of the Royal Astronomical Society

The formation of galaxies can be understood in terms of the assembly patterns of each type of gal... more The formation of galaxies can be understood in terms of the assembly patterns of each type of galactic component. To perform this kind of analysis, it is necessary to define some criteria to separate those components. Decomposition methods based on dynamical properties are more physically motivated than photometry-based ones. We use the unsupervised Gaussian Mixture model of galactic structure finder to extract the components of a sub-sample of galaxies with Milky Way-like masses from the eagle simulations. A clustering in the space of first- and second-order dynamical moments of all identified substructures reveals five types of galaxy components: thin and thick discs, stellar haloes, bulges and spheroids. We analyse the dynamical, morphological and stellar population (SP) properties of these five component types, exploring to what extent these properties correlate with each other, and how much they depend on the total galaxy stellar and dark matter halo masses. All galaxies contai...

Current simulations have enough resolution to study in detail the evolution of galactic angular m... more Current simulations have enough resolution to study in detail the evolution of galactic angular momentum by looking separately at the various stellar dynamical components. The comparison of this kind of simulation results with observations is non trivial given all the caveats for estimating angular momentum in the latter. Therefore, mock observations of high resolution zoom-in simulations are a necessary step for a more meaningful comparison with observations.

A large number of Ultra-Diffuse Galaxies (UDGs) has been detected over the past few years, both i... more A large number of Ultra-Diffuse Galaxies (UDGs) has been detected over the past few years, both in clusters and in isolation. UDGs have stellar masses typical of dwarf galaxies but effective radii of Milky Way-sized objects, and their origin remains puzzling. Using hydrodynamical zoom-in simulations from the NIHAO project we show that UDGs form naturally in dwarf-mass haloes, as a result of episodic gas outflows associated with star formation. The simulated UDGs live in isolated haloes of masses 1010−11M , have stellar masses of 107−8.5M , effective radii larger than 1 kpc and dark matter cores. Remarkably, they have a non-negligible HI gas mass of 107−9M , which correlates with the extent of the galaxy. Gas availability is crucial to the internal processes that form UDGs: feedback driven gas outflows, and subsequent dark matter and stellar expansion, are the key to reproduce faint, yet unusually extended, galaxies. This scenario implies that UDGs represent a dwarf population of low...

Recent Herschel observations of dwarf galaxies have shown a wide diversity in the shapes of their... more Recent Herschel observations of dwarf galaxies have shown a wide diversity in the shapes of their IR-submm spectral energy distributions as compared to more massive galaxies, presenting features that cannot be explained with the current models. In order to understand the physics driving these differences, we have computed the emission of a sample of simulated dwarf galaxies using the radiative transfer code GRASIL-3D. This code separately treats the radiative transfer in dust grains from molecular clouds and cirri. The simulated galaxies have masses ranging from 10 − 10 M and have evolved within a Local Group environment by using CLUES initial conditions. We show that their IR band luminosities are in agreement with observations, with their SEDs reproducing naturally the particular spectral features observed. We conclude that the GRASIL-3D two-component model gives a physical interpretation to the emission of dwarf galaxies, with molecular clouds (cirri) as the warm (cold) dust comp...

arXiv: Astrophysics of Galaxies, 2017

Selecting disk galaxies from the cosmological, hydrodynamical simulation Magneticum Pathfinder we... more Selecting disk galaxies from the cosmological, hydrodynamical simulation Magneticum Pathfinder we show that almost half of our poster child disk galaxies at z=2z=2z=2 show significantly declining rotation curves and low dark matter fractions, very similar to recently reported observations. These galaxies do not show any anomalous behavior, reside in standard dark matter halos and typically grow significantly in mass until z=0z=0z=0, where they span all morphological classes, including disk galaxies matching present day rotation curves and observed dark matter fractions. Our findings demonstrate that declining rotation curves and low dark matter fractions in rotation dominated galaxies at z=2z=2z=2 appear naturally within the Lambda\LambdaLambdaCDM paradigm and reflect the complex baryonic physics, which plays a role at the peak epoch of star-formation. In addition, we find that dispersion dominated galaxies at z=2z=2z=2, which host a significant gas disk, exhibit similar shaped rotation curves as the disk ga...

Astronomy & Astrophysics, 2021

The spin, or normalized angular momentum λ, of dark matter halos in cosmological simulations foll... more The spin, or normalized angular momentum λ, of dark matter halos in cosmological simulations follows a log normal distribution and has little correlation with galaxy observables such as stellar masses or sizes. There is currently no way to infer the λ parameter of individual halos hosting observed galaxies. Here, we present a first attempt to measure λ starting from the dynamically distinct disks and stellar halos identified in high-resolution cosmological simulations with the Galactic Structure Finder (gsf). In a subsample of NIHAO galaxies analyzed with gsf, we find tight correlations between the total angular momentum of the dark matter halos, Jh, and the azimuthal angular momentum, Jz, of the dynamical distinct stellar components of the form: log(Jh) = α + β⋅log(Jz). The stellar halos have the tightest relation with α = 9.50 ± 0.42 and β = 0.46 ± 0.04. The other tight relation is with the disks, for which α = 6.15 ± 0.92 and β = 0.68 ± 0.07. While the angular momentum is difficu...

Monthly Notices of the Royal Astronomical Society, 2021

With the advent of large spectroscopic surveys the amount of high quality chemodynamical data in ... more With the advent of large spectroscopic surveys the amount of high quality chemodynamical data in the Milky Way (MW) increased tremendously. Accurately and correctly capturing and explaining the detailed features in the high-quality observational data is notoriously difficult for state-of-the-art numerical models. In order to keep up with the quantity and quality of observational data sets, improved prescriptions for galactic chemical evolution need to be incorporated into the simulations. Here we present a new, flexible, time-resolved chemical enrichment model for cosmological simulations. Our model allows us to easily change a number of stellar physics parameters such as the shape of the initial mass function (IMF), stellar lifetimes, chemical yields, or SN Ia delay times. We implement our model into the Gasoline2 code and perform a series of cosmological simulations varying a number of key parameters, foremost evaluating different stellar yield sets for massive stars from the lite...

Monthly Notices of the Royal Astronomical Society, 2020

The kinematics of the most metal-poor stars provide a window into the early formation and accreti... more The kinematics of the most metal-poor stars provide a window into the early formation and accretion history of the Milky Way (MW). Here, we use five high-resolution cosmological zoom-in simulations (∼ 5 × 106 star particles) of MW-like galaxies taken from the NIHAO-UHD project, to investigate the origin of low-metallicity stars ([Fe/H] ≤ −2.5). The simulations show a prominent population of low-metallicity stars confined to the disc plane, as recently discovered in the MW. The ubiquity of this finding suggests that the MW is not unique in this respect. Independently of the accretion history, we find that ≳90 per cent of the retrograde stars in this population are brought in during the initial build-up of the galaxies during the first few Gyr after the Big Bang. Our results therefore highlight the great potential of the retrograde population as a tracer of the early build-up of the MW. The prograde planar population, on the other hand, is accreted during the later assembly phase and ...

Monthly Notices of the Royal Astronomical Society, 2020

We use cosmological hydrodynamical galaxy formation simulations from the NIHAO project to investi... more We use cosmological hydrodynamical galaxy formation simulations from the NIHAO project to investigate the response of cold dark matter (CDM) haloes to baryonic processes. Previous work has shown that the halo response is primarily a function of the ratio between galaxy stellar mass and total virial mass, and the density threshold above which gas is eligible to form stars, n[cm−3]. At low n all simulations in the literature agree that dwarf galaxy haloes are cuspy, but at high n ≳ 100 there is no consensus. We trace halo contraction in dwarf galaxies with n ≳ 100 reported in some previous simulations to insufficient spatial resolution. Provided the adopted star formation threshold is appropriate for the resolution of the simulation, we show that the halo response is remarkably stable for n ≳ 5, up to the highest star formation threshold that we test, n = 500. This free parameter can be calibrated using the observed clustering of young stars. Simulations with low thresholds n ≤ 1 pred...

Proceedings of the International Astronomical Union, 2017

High resolution cosmological and hydrodynamical simulations have reached a resolution able to res... more High resolution cosmological and hydrodynamical simulations have reached a resolution able to resolve in a self consistent way the disc of our galaxy, the galaxy center and the satellites orbiting around it. We present first results from the NIHAO-UHD project, a set of very high-resolution baryonic zoom-in simulations of Milky Way mass disc galaxies. These simulations model the full cosmological assembly history of the galaxies and their satellite system using the same, well tested physics as the NIHAO project. We show that these simulations can self-consistently reproduce the observed kinematical and morphological features of the X-shaped bulge observed in our own Milky Way.

Monthly Notices of the Royal Astronomical Society, 2019

Simulating thin and extended galactic disks has long been a challenge in computational astrophysi... more Simulating thin and extended galactic disks has long been a challenge in computational astrophysics. We introduce the NIHAO-UHD suite of cosmological hydrodynamical simulations of Milky Way mass galaxies and study stellar disk properties such as stellar mass, size and rotation velocity which agree well with observations of the Milky Way and local galaxies. In particular, the simulations reproduce the age-velocity dispersion relation and a multi-component stellar disk as observed for the Milky Way. Half of our galaxies show a double exponential vertical profile, while the others are well described by a single exponential model which we link to the disk merger history. In all cases, mono-age populations follow a single exponential whose scale height varies monotonically with stellar age and radius. The scale length decreases with stellar age while the scale height increases. The general structure of the stellar disks is already set at time of birth as a result of the inside-out and up...

Monthly Notices of the Royal Astronomical Society, 2019

We implement an optically thin approximation for the effects of the local radiation field from st... more We implement an optically thin approximation for the effects of the local radiation field from stars and hot gas on the gas heating and cooling in the N-body smoothed particle hydrodynamics code gasoline2. We resimulate three galaxies from the NIHAO project: one dwarf, one Milky Way-like, and one massive spiral, and study what are the local radiation field effects on various galaxy properties. We also study the effects of varying the ultraviolet background (UVB) model, by running the same galaxies with two different UVBs. Galaxy properties at zzz = 0 like stellar mass, stellar effective mass radius, H i mass, and radial extent of the H i disc show significant changes between the models with and without the local radiation field, and smaller differences between the two UVB models. The intrinsic effect of the local radiation field through cosmic time is to increase the equilibrium temperature at the interface between the galaxies and their circumgalactic media (CGM), moving this bound...

Monthly Notices of the Royal Astronomical Society, 2018

Based on the stellar orbit distribution derived from orbit-superposition Schwarzschild models, we... more Based on the stellar orbit distribution derived from orbit-superposition Schwarzschild models, we decompose each of 250 representative present-day galaxies into four orbital components: cold with strong rotation, warm with weak rotation, hot with dominant random motion, and counter-rotating (CR). We rebuild the surface brightness () of each orbital component and we present in figures and tables a quantification of their morphologies using the Sersic index n, concentration C = log (0.1R e / R e) and intrinsic flattening q Re and q Rmax , with R e the half-light radius and R max the CALIFA data coverage. We find that: (1) kinematic hotter components are generally more concentrated and rounder than colder components, and (2) all components become more concentrated and thicker/rounder in more massive galaxies; they change from disc-like in low-mass late-type galaxies to bulge-like in high-mass early type galaxies. Our findings suggest that Sersic n is not a good discriminator between rotating bulges and non-rotating bulges. The luminosity fraction of cold orbits f cold is well correlated with the photometrically decomposed disc fractiondisc f disc as f cold = 0.14 + 0.23f disc. Similarly, the hot orbit fraction f hot is correlated with the bulge fraction f bulge as f hot = 0.19 + 0.31f bulge. The warm orbits mainly contribute to discs in low-mass late-type galaxies, and to bulges in high-mass early-type galaxies. The cold, warm, and hot components generally follow the same morphology (= 1 − q Rmax) versus kinematics (σ 2 z /V 2 tot) relation as the thin disc, thick disc/pseudo-bulge, and classical bulge identified from cosmological simulations.

Monthly Notices of the Royal Astronomical Society, 2018

We present the first results of applying Gaussian Mixture Models in the stellar kinematic space o... more We present the first results of applying Gaussian Mixture Models in the stellar kinematic space of normalized angular momentum and binding energy on NIHAO high-resolution galaxies to separate the stars into multiple components. We exemplify this method, using a simulated Milky Way analogue, whose stellar component hosts thin and thick discs, classical and pseudo bulges, and a stellar halo. The properties of these stellar structures are in good agreement with observational expectations in terms of sizes, shapes, and rotational support. Interestingly, the two kinematic discs show surface mass density profiles more centrally concentrated than exponentials, while the bulges and the stellar halo are purely exponential. We trace back in time the Lagrangian mass of each component separately to study their formation history. Between z ∼ 3 and the end of halo virialization, z ∼ 1.3, all components lose a fraction of their angular momentum. The classical bulge loses the most (∼95 per cent) and the thin disc the least (∼60 per cent). Both bulges formed their stars in situ at high redshift, while the thin disc formed ∼98 per cent in situ, but with a constant SFR ∼ 1.5 M yr −1 over the last ∼11 Gyr. Accreted stars (6 per cent of total stellar mass) are mainly incorporated to the thick disc or the stellar halo, which formed ex situ 8 per cent and 45 per cent of their respective masses. Our analysis pipeline is freely available at https://github.com/aobr/gsf.

Monthly Notices of the Royal Astronomical Society, 2019

We introduce algorithms for black hole physics, i.e. black hole formation, accretion, and feedbac... more We introduce algorithms for black hole physics, i.e. black hole formation, accretion, and feedback, into the Numerical Investigation of a Hundred Astrophysical Objects (NIHAO) project of galaxy simulations. This enables us to study high mass, elliptical galaxies, where feedback from the central black hole is generally thought to have a significant effect on their evolution. We furthermore extend the NIHAO suite by 45 simulations that encompass z = 0 halo masses from 1 × 1012 to 4times1013,mathrmModot4 \times 10^{13}\, \mathrm{M}_{\odot }4times1013,mathrmModot, and resimulate five galaxies from the original NIHAO sample with black hole physics, which have z = 0 halo masses from 8 × 1011 to 3times1012,mathrmModot3 \times 10^{12}\, \mathrm{M}_{\odot }3times1012,mathrmModot. Now NIHAO contains 144 different galaxies and thus has the largest sample of zoom-in simulations of galaxies, spanning z = 0 halo masses from 9 × 108 to 4times1013,mathrmModot4 \times 10^{13}\, \mathrm{M}_{\odot }4times1013,mathrmModot. In this paper we focus on testing the algorithms and calibrating their free parameters against the stell...

Monthly Notices of the Royal Astronomical Society, 2019

We use 25 simulated galaxies from the NIHAO project to define and characterize a variety of kinem... more We use 25 simulated galaxies from the NIHAO project to define and characterize a variety of kinematic stellar structures: thin and thick discs, large-scale single discs, classical and pseudobulges, spheroids, inner discs, and stellar haloes. These structures have masses, spins, shapes, and rotational support in good agreement with theoretical expectations and observational data. Above a dark matter halo mass of 2.5 × 10 11 M , all galaxies have a classical bulge and 70 per cent have a thin and thick disc. The kinematic (thin) discs follow a power-law relation between angular momentum and stellar mass J * = 3.4M 1.26±0.06 * , in very good agreement with the prediction based on the empirical stellar-to-halo-mass relation in the same mass range, and show a strong correlation between maximum 'observed' rotation velocity and dark matter halo circular velocity v c = 6.4v 0.64±0.04 max. Tracing back in time these structures' progenitors, we find all of them to lose a fraction 1 − f j of their maximum angular momentum. Thin discs are significantly better at retaining their high-redshift spins (f j ∼ 0.70) than thick ones (f j ∼ 0.40). Stellar haloes have their progenitor baryons assembled the latest (z 1/2 ∼ 1.1) and over the longest timescales (τ ∼ 6.2 Gyr), and have the smallest fraction of stars born in situ (f in situ = 0.35 ± 0.14). All other structures have 1.5 z 1/2 3, τ = 4 ± 2 Gyr, and f in situ 0.9.

The Astrophysical Journal, 2019

The stellar populations in the inner kiloparsecs of the Milky Way (MW) show complex kinematical a... more The stellar populations in the inner kiloparsecs of the Milky Way (MW) show complex kinematical and chemical structures. The origin and evolution of these structures are still under debate. Here we study the central region of a fully cosmological hydrodynamical simulation of a disk galaxy that reproduces key properties of the inner kiloparsecs of the MW: it has a boxy morphology and shows an overall rotation and dispersion profile in agreement with observations. We use a clustering algorithm on stellar kinematics to identify a number of discrete kinematic components: a high-and low-spin disk, a stellar halo, and two bulge components, one fast-rotating and one slowrotating. We focus on the two bulge components and show that the slow-rotating one is spherically symmetric while the fast-rotating component shows a boxy/peanut morphology. Although the two bulge components are kinematically discrete populations at present day, they are both mostly formed over similar timescales, from disk material. We find that stellar particles with lower initial birth angular momentum (most likely thick-disk stars) end up in the slow-rotating low-spin bulge, while stars with higher birth angular momentum (most likely thin-disk stars) are found in the high-spin bulge. This has the important consequence that a bulge population with a spheroidal morphology does not necessarily indicate a merger origin. In fact, we do find that only ∼2.3% of the stars in the bulge components are ex situ stars brought in by accreted dwarf galaxies early on. We identify these ex situ stars as the oldest and most metal-poor stars on highly radial orbits with large vertical excursions from the disk.

Monthly Notices of the Royal Astronomical Society, 2019

We use cosmological hydrodynamical galaxy formation simulations from the NIHAO project to investi... more We use cosmological hydrodynamical galaxy formation simulations from the NIHAO project to investigate the impact of the threshold for star formation on the response of the dark matter (DM) halo to baryonic processes. The fiducial NIHAO threshold, n = 10 [cm −3 ], results in strong expansion of the DM halo in galaxies with stellar masses in the range 10 7.5 M star 10 9.5 M. We find that lower thresholds such as n = 0.1 (as employed by the EAGLE/APOSTLE and Illustris/AURIGA projects) do not result in significant halo expansion at any mass scale. Halo expansion driven by supernova feedback requires significant fluctuations in the local gas fraction on sub-dynamical times (i.e. 50 Myr at galaxy half-light radii), which are themselves caused by variability in the star formation rate. At one per cent of the virial radius, simulations with n = 10 have gas fractions of 0.2 and variations of 0.1, while n = 0.1 simulations have order of magnitude lower gas fractions and hence do not expand the halo. The observed DM circular velocities of nearby dwarf galaxies are inconsistent with CDM simulations with n = 0.1 and n = 1, but in a reasonable agreement with n = 10. Star formation rates are more variable for higher n, lower galaxy masses, and when star formation is measured on shorter timescales. For example, simulations with n = 10 have up to 0.4 dex higher scatter in specific star formation rates than simulations with n = 0.1. Thus observationally constraining the sub-grid model for star formation, and hence the nature of DM, should be possible in the near future.

Monthly Notices of the Royal Astronomical Society, 2019

In this third paper of the series, we investigate the effects of warm dark matter (WDM) with a pa... more In this third paper of the series, we investigate the effects of warm dark matter (WDM) with a particle mass of m WDM = 3 keV on the smallest galaxies in our Universe. We present a sample of 21 hydrodynamical cosmological simulations of dwarf galaxies and 20 simulations of satellite-host galaxy interaction that we performed both in a cold dark matter (CDM) and WDM scenario. In the WDM simulations, we observe a higher critical mass for the onset of star formation. Structure growth is delayed in WDM, as a result WDM haloes have a stellar population on average 2 Gyr younger than their CDM counterparts. Nevertheless, despite this delayed star formation, CDM and WDM galaxies are both able to reproduce the observed scaling relations for velocity dispersion, stellar mass, size, and metallicity at z = 0. WDM satellite haloes in a Milky Way mass host are more susceptible to tidal stripping due to their lower concentrations, but their galaxies can even survive longer than the CDM counterparts if they live in a dark matter halo with a stee per central slope. In agreement with our previous CDM satellite study we observe a steepening of the WDM satellites' central dark matter density slope due to stripping. The difference in the average stellar age for satellite galaxies, between CDM and WDM, could be used in the future for disentangling these two models.

Monthly Notices of the Royal Astronomical Society

The formation of galaxies can be understood in terms of the assembly patterns of each type of gal... more The formation of galaxies can be understood in terms of the assembly patterns of each type of galactic component. To perform this kind of analysis, it is necessary to define some criteria to separate those components. Decomposition methods based on dynamical properties are more physically motivated than photometry-based ones. We use the unsupervised Gaussian Mixture model of galactic structure finder to extract the components of a sub-sample of galaxies with Milky Way-like masses from the eagle simulations. A clustering in the space of first- and second-order dynamical moments of all identified substructures reveals five types of galaxy components: thin and thick discs, stellar haloes, bulges and spheroids. We analyse the dynamical, morphological and stellar population (SP) properties of these five component types, exploring to what extent these properties correlate with each other, and how much they depend on the total galaxy stellar and dark matter halo masses. All galaxies contai...

Current simulations have enough resolution to study in detail the evolution of galactic angular m... more Current simulations have enough resolution to study in detail the evolution of galactic angular momentum by looking separately at the various stellar dynamical components. The comparison of this kind of simulation results with observations is non trivial given all the caveats for estimating angular momentum in the latter. Therefore, mock observations of high resolution zoom-in simulations are a necessary step for a more meaningful comparison with observations.

A large number of Ultra-Diffuse Galaxies (UDGs) has been detected over the past few years, both i... more A large number of Ultra-Diffuse Galaxies (UDGs) has been detected over the past few years, both in clusters and in isolation. UDGs have stellar masses typical of dwarf galaxies but effective radii of Milky Way-sized objects, and their origin remains puzzling. Using hydrodynamical zoom-in simulations from the NIHAO project we show that UDGs form naturally in dwarf-mass haloes, as a result of episodic gas outflows associated with star formation. The simulated UDGs live in isolated haloes of masses 1010−11M , have stellar masses of 107−8.5M , effective radii larger than 1 kpc and dark matter cores. Remarkably, they have a non-negligible HI gas mass of 107−9M , which correlates with the extent of the galaxy. Gas availability is crucial to the internal processes that form UDGs: feedback driven gas outflows, and subsequent dark matter and stellar expansion, are the key to reproduce faint, yet unusually extended, galaxies. This scenario implies that UDGs represent a dwarf population of low...

Recent Herschel observations of dwarf galaxies have shown a wide diversity in the shapes of their... more Recent Herschel observations of dwarf galaxies have shown a wide diversity in the shapes of their IR-submm spectral energy distributions as compared to more massive galaxies, presenting features that cannot be explained with the current models. In order to understand the physics driving these differences, we have computed the emission of a sample of simulated dwarf galaxies using the radiative transfer code GRASIL-3D. This code separately treats the radiative transfer in dust grains from molecular clouds and cirri. The simulated galaxies have masses ranging from 10 − 10 M and have evolved within a Local Group environment by using CLUES initial conditions. We show that their IR band luminosities are in agreement with observations, with their SEDs reproducing naturally the particular spectral features observed. We conclude that the GRASIL-3D two-component model gives a physical interpretation to the emission of dwarf galaxies, with molecular clouds (cirri) as the warm (cold) dust comp...

arXiv: Astrophysics of Galaxies, 2017

Selecting disk galaxies from the cosmological, hydrodynamical simulation Magneticum Pathfinder we... more Selecting disk galaxies from the cosmological, hydrodynamical simulation Magneticum Pathfinder we show that almost half of our poster child disk galaxies at z=2z=2z=2 show significantly declining rotation curves and low dark matter fractions, very similar to recently reported observations. These galaxies do not show any anomalous behavior, reside in standard dark matter halos and typically grow significantly in mass until z=0z=0z=0, where they span all morphological classes, including disk galaxies matching present day rotation curves and observed dark matter fractions. Our findings demonstrate that declining rotation curves and low dark matter fractions in rotation dominated galaxies at z=2z=2z=2 appear naturally within the Lambda\LambdaLambdaCDM paradigm and reflect the complex baryonic physics, which plays a role at the peak epoch of star-formation. In addition, we find that dispersion dominated galaxies at z=2z=2z=2, which host a significant gas disk, exhibit similar shaped rotation curves as the disk ga...

Astronomy & Astrophysics, 2021

The spin, or normalized angular momentum λ, of dark matter halos in cosmological simulations foll... more The spin, or normalized angular momentum λ, of dark matter halos in cosmological simulations follows a log normal distribution and has little correlation with galaxy observables such as stellar masses or sizes. There is currently no way to infer the λ parameter of individual halos hosting observed galaxies. Here, we present a first attempt to measure λ starting from the dynamically distinct disks and stellar halos identified in high-resolution cosmological simulations with the Galactic Structure Finder (gsf). In a subsample of NIHAO galaxies analyzed with gsf, we find tight correlations between the total angular momentum of the dark matter halos, Jh, and the azimuthal angular momentum, Jz, of the dynamical distinct stellar components of the form: log(Jh) = α + β⋅log(Jz). The stellar halos have the tightest relation with α = 9.50 ± 0.42 and β = 0.46 ± 0.04. The other tight relation is with the disks, for which α = 6.15 ± 0.92 and β = 0.68 ± 0.07. While the angular momentum is difficu...

Monthly Notices of the Royal Astronomical Society, 2021

With the advent of large spectroscopic surveys the amount of high quality chemodynamical data in ... more With the advent of large spectroscopic surveys the amount of high quality chemodynamical data in the Milky Way (MW) increased tremendously. Accurately and correctly capturing and explaining the detailed features in the high-quality observational data is notoriously difficult for state-of-the-art numerical models. In order to keep up with the quantity and quality of observational data sets, improved prescriptions for galactic chemical evolution need to be incorporated into the simulations. Here we present a new, flexible, time-resolved chemical enrichment model for cosmological simulations. Our model allows us to easily change a number of stellar physics parameters such as the shape of the initial mass function (IMF), stellar lifetimes, chemical yields, or SN Ia delay times. We implement our model into the Gasoline2 code and perform a series of cosmological simulations varying a number of key parameters, foremost evaluating different stellar yield sets for massive stars from the lite...

Monthly Notices of the Royal Astronomical Society, 2020

The kinematics of the most metal-poor stars provide a window into the early formation and accreti... more The kinematics of the most metal-poor stars provide a window into the early formation and accretion history of the Milky Way (MW). Here, we use five high-resolution cosmological zoom-in simulations (∼ 5 × 106 star particles) of MW-like galaxies taken from the NIHAO-UHD project, to investigate the origin of low-metallicity stars ([Fe/H] ≤ −2.5). The simulations show a prominent population of low-metallicity stars confined to the disc plane, as recently discovered in the MW. The ubiquity of this finding suggests that the MW is not unique in this respect. Independently of the accretion history, we find that ≳90 per cent of the retrograde stars in this population are brought in during the initial build-up of the galaxies during the first few Gyr after the Big Bang. Our results therefore highlight the great potential of the retrograde population as a tracer of the early build-up of the MW. The prograde planar population, on the other hand, is accreted during the later assembly phase and ...

Monthly Notices of the Royal Astronomical Society, 2020

We use cosmological hydrodynamical galaxy formation simulations from the NIHAO project to investi... more We use cosmological hydrodynamical galaxy formation simulations from the NIHAO project to investigate the response of cold dark matter (CDM) haloes to baryonic processes. Previous work has shown that the halo response is primarily a function of the ratio between galaxy stellar mass and total virial mass, and the density threshold above which gas is eligible to form stars, n[cm−3]. At low n all simulations in the literature agree that dwarf galaxy haloes are cuspy, but at high n ≳ 100 there is no consensus. We trace halo contraction in dwarf galaxies with n ≳ 100 reported in some previous simulations to insufficient spatial resolution. Provided the adopted star formation threshold is appropriate for the resolution of the simulation, we show that the halo response is remarkably stable for n ≳ 5, up to the highest star formation threshold that we test, n = 500. This free parameter can be calibrated using the observed clustering of young stars. Simulations with low thresholds n ≤ 1 pred...

Proceedings of the International Astronomical Union, 2017

High resolution cosmological and hydrodynamical simulations have reached a resolution able to res... more High resolution cosmological and hydrodynamical simulations have reached a resolution able to resolve in a self consistent way the disc of our galaxy, the galaxy center and the satellites orbiting around it. We present first results from the NIHAO-UHD project, a set of very high-resolution baryonic zoom-in simulations of Milky Way mass disc galaxies. These simulations model the full cosmological assembly history of the galaxies and their satellite system using the same, well tested physics as the NIHAO project. We show that these simulations can self-consistently reproduce the observed kinematical and morphological features of the X-shaped bulge observed in our own Milky Way.

Monthly Notices of the Royal Astronomical Society, 2019

Simulating thin and extended galactic disks has long been a challenge in computational astrophysi... more Simulating thin and extended galactic disks has long been a challenge in computational astrophysics. We introduce the NIHAO-UHD suite of cosmological hydrodynamical simulations of Milky Way mass galaxies and study stellar disk properties such as stellar mass, size and rotation velocity which agree well with observations of the Milky Way and local galaxies. In particular, the simulations reproduce the age-velocity dispersion relation and a multi-component stellar disk as observed for the Milky Way. Half of our galaxies show a double exponential vertical profile, while the others are well described by a single exponential model which we link to the disk merger history. In all cases, mono-age populations follow a single exponential whose scale height varies monotonically with stellar age and radius. The scale length decreases with stellar age while the scale height increases. The general structure of the stellar disks is already set at time of birth as a result of the inside-out and up...

Monthly Notices of the Royal Astronomical Society, 2019

We implement an optically thin approximation for the effects of the local radiation field from st... more We implement an optically thin approximation for the effects of the local radiation field from stars and hot gas on the gas heating and cooling in the N-body smoothed particle hydrodynamics code gasoline2. We resimulate three galaxies from the NIHAO project: one dwarf, one Milky Way-like, and one massive spiral, and study what are the local radiation field effects on various galaxy properties. We also study the effects of varying the ultraviolet background (UVB) model, by running the same galaxies with two different UVBs. Galaxy properties at zzz = 0 like stellar mass, stellar effective mass radius, H i mass, and radial extent of the H i disc show significant changes between the models with and without the local radiation field, and smaller differences between the two UVB models. The intrinsic effect of the local radiation field through cosmic time is to increase the equilibrium temperature at the interface between the galaxies and their circumgalactic media (CGM), moving this bound...

Monthly Notices of the Royal Astronomical Society, 2018

Based on the stellar orbit distribution derived from orbit-superposition Schwarzschild models, we... more Based on the stellar orbit distribution derived from orbit-superposition Schwarzschild models, we decompose each of 250 representative present-day galaxies into four orbital components: cold with strong rotation, warm with weak rotation, hot with dominant random motion, and counter-rotating (CR). We rebuild the surface brightness () of each orbital component and we present in figures and tables a quantification of their morphologies using the Sersic index n, concentration C = log (0.1R e / R e) and intrinsic flattening q Re and q Rmax , with R e the half-light radius and R max the CALIFA data coverage. We find that: (1) kinematic hotter components are generally more concentrated and rounder than colder components, and (2) all components become more concentrated and thicker/rounder in more massive galaxies; they change from disc-like in low-mass late-type galaxies to bulge-like in high-mass early type galaxies. Our findings suggest that Sersic n is not a good discriminator between rotating bulges and non-rotating bulges. The luminosity fraction of cold orbits f cold is well correlated with the photometrically decomposed disc fractiondisc f disc as f cold = 0.14 + 0.23f disc. Similarly, the hot orbit fraction f hot is correlated with the bulge fraction f bulge as f hot = 0.19 + 0.31f bulge. The warm orbits mainly contribute to discs in low-mass late-type galaxies, and to bulges in high-mass early-type galaxies. The cold, warm, and hot components generally follow the same morphology (= 1 − q Rmax) versus kinematics (σ 2 z /V 2 tot) relation as the thin disc, thick disc/pseudo-bulge, and classical bulge identified from cosmological simulations.

Monthly Notices of the Royal Astronomical Society, 2018

We present the first results of applying Gaussian Mixture Models in the stellar kinematic space o... more We present the first results of applying Gaussian Mixture Models in the stellar kinematic space of normalized angular momentum and binding energy on NIHAO high-resolution galaxies to separate the stars into multiple components. We exemplify this method, using a simulated Milky Way analogue, whose stellar component hosts thin and thick discs, classical and pseudo bulges, and a stellar halo. The properties of these stellar structures are in good agreement with observational expectations in terms of sizes, shapes, and rotational support. Interestingly, the two kinematic discs show surface mass density profiles more centrally concentrated than exponentials, while the bulges and the stellar halo are purely exponential. We trace back in time the Lagrangian mass of each component separately to study their formation history. Between z ∼ 3 and the end of halo virialization, z ∼ 1.3, all components lose a fraction of their angular momentum. The classical bulge loses the most (∼95 per cent) and the thin disc the least (∼60 per cent). Both bulges formed their stars in situ at high redshift, while the thin disc formed ∼98 per cent in situ, but with a constant SFR ∼ 1.5 M yr −1 over the last ∼11 Gyr. Accreted stars (6 per cent of total stellar mass) are mainly incorporated to the thick disc or the stellar halo, which formed ex situ 8 per cent and 45 per cent of their respective masses. Our analysis pipeline is freely available at https://github.com/aobr/gsf.

Monthly Notices of the Royal Astronomical Society, 2019

We introduce algorithms for black hole physics, i.e. black hole formation, accretion, and feedbac... more We introduce algorithms for black hole physics, i.e. black hole formation, accretion, and feedback, into the Numerical Investigation of a Hundred Astrophysical Objects (NIHAO) project of galaxy simulations. This enables us to study high mass, elliptical galaxies, where feedback from the central black hole is generally thought to have a significant effect on their evolution. We furthermore extend the NIHAO suite by 45 simulations that encompass z = 0 halo masses from 1 × 1012 to 4times1013,mathrmModot4 \times 10^{13}\, \mathrm{M}_{\odot }4times1013,mathrmModot, and resimulate five galaxies from the original NIHAO sample with black hole physics, which have z = 0 halo masses from 8 × 1011 to 3times1012,mathrmModot3 \times 10^{12}\, \mathrm{M}_{\odot }3times1012,mathrmModot. Now NIHAO contains 144 different galaxies and thus has the largest sample of zoom-in simulations of galaxies, spanning z = 0 halo masses from 9 × 108 to 4times1013,mathrmModot4 \times 10^{13}\, \mathrm{M}_{\odot }4times1013,mathrmModot. In this paper we focus on testing the algorithms and calibrating their free parameters against the stell...

Monthly Notices of the Royal Astronomical Society, 2019

We use 25 simulated galaxies from the NIHAO project to define and characterize a variety of kinem... more We use 25 simulated galaxies from the NIHAO project to define and characterize a variety of kinematic stellar structures: thin and thick discs, large-scale single discs, classical and pseudobulges, spheroids, inner discs, and stellar haloes. These structures have masses, spins, shapes, and rotational support in good agreement with theoretical expectations and observational data. Above a dark matter halo mass of 2.5 × 10 11 M , all galaxies have a classical bulge and 70 per cent have a thin and thick disc. The kinematic (thin) discs follow a power-law relation between angular momentum and stellar mass J * = 3.4M 1.26±0.06 * , in very good agreement with the prediction based on the empirical stellar-to-halo-mass relation in the same mass range, and show a strong correlation between maximum 'observed' rotation velocity and dark matter halo circular velocity v c = 6.4v 0.64±0.04 max. Tracing back in time these structures' progenitors, we find all of them to lose a fraction 1 − f j of their maximum angular momentum. Thin discs are significantly better at retaining their high-redshift spins (f j ∼ 0.70) than thick ones (f j ∼ 0.40). Stellar haloes have their progenitor baryons assembled the latest (z 1/2 ∼ 1.1) and over the longest timescales (τ ∼ 6.2 Gyr), and have the smallest fraction of stars born in situ (f in situ = 0.35 ± 0.14). All other structures have 1.5 z 1/2 3, τ = 4 ± 2 Gyr, and f in situ 0.9.

The Astrophysical Journal, 2019

The stellar populations in the inner kiloparsecs of the Milky Way (MW) show complex kinematical a... more The stellar populations in the inner kiloparsecs of the Milky Way (MW) show complex kinematical and chemical structures. The origin and evolution of these structures are still under debate. Here we study the central region of a fully cosmological hydrodynamical simulation of a disk galaxy that reproduces key properties of the inner kiloparsecs of the MW: it has a boxy morphology and shows an overall rotation and dispersion profile in agreement with observations. We use a clustering algorithm on stellar kinematics to identify a number of discrete kinematic components: a high-and low-spin disk, a stellar halo, and two bulge components, one fast-rotating and one slowrotating. We focus on the two bulge components and show that the slow-rotating one is spherically symmetric while the fast-rotating component shows a boxy/peanut morphology. Although the two bulge components are kinematically discrete populations at present day, they are both mostly formed over similar timescales, from disk material. We find that stellar particles with lower initial birth angular momentum (most likely thick-disk stars) end up in the slow-rotating low-spin bulge, while stars with higher birth angular momentum (most likely thin-disk stars) are found in the high-spin bulge. This has the important consequence that a bulge population with a spheroidal morphology does not necessarily indicate a merger origin. In fact, we do find that only ∼2.3% of the stars in the bulge components are ex situ stars brought in by accreted dwarf galaxies early on. We identify these ex situ stars as the oldest and most metal-poor stars on highly radial orbits with large vertical excursions from the disk.

Monthly Notices of the Royal Astronomical Society, 2019

We use cosmological hydrodynamical galaxy formation simulations from the NIHAO project to investi... more We use cosmological hydrodynamical galaxy formation simulations from the NIHAO project to investigate the impact of the threshold for star formation on the response of the dark matter (DM) halo to baryonic processes. The fiducial NIHAO threshold, n = 10 [cm −3 ], results in strong expansion of the DM halo in galaxies with stellar masses in the range 10 7.5 M star 10 9.5 M. We find that lower thresholds such as n = 0.1 (as employed by the EAGLE/APOSTLE and Illustris/AURIGA projects) do not result in significant halo expansion at any mass scale. Halo expansion driven by supernova feedback requires significant fluctuations in the local gas fraction on sub-dynamical times (i.e. 50 Myr at galaxy half-light radii), which are themselves caused by variability in the star formation rate. At one per cent of the virial radius, simulations with n = 10 have gas fractions of 0.2 and variations of 0.1, while n = 0.1 simulations have order of magnitude lower gas fractions and hence do not expand the halo. The observed DM circular velocities of nearby dwarf galaxies are inconsistent with CDM simulations with n = 0.1 and n = 1, but in a reasonable agreement with n = 10. Star formation rates are more variable for higher n, lower galaxy masses, and when star formation is measured on shorter timescales. For example, simulations with n = 10 have up to 0.4 dex higher scatter in specific star formation rates than simulations with n = 0.1. Thus observationally constraining the sub-grid model for star formation, and hence the nature of DM, should be possible in the near future.

Monthly Notices of the Royal Astronomical Society, 2019

In this third paper of the series, we investigate the effects of warm dark matter (WDM) with a pa... more In this third paper of the series, we investigate the effects of warm dark matter (WDM) with a particle mass of m WDM = 3 keV on the smallest galaxies in our Universe. We present a sample of 21 hydrodynamical cosmological simulations of dwarf galaxies and 20 simulations of satellite-host galaxy interaction that we performed both in a cold dark matter (CDM) and WDM scenario. In the WDM simulations, we observe a higher critical mass for the onset of star formation. Structure growth is delayed in WDM, as a result WDM haloes have a stellar population on average 2 Gyr younger than their CDM counterparts. Nevertheless, despite this delayed star formation, CDM and WDM galaxies are both able to reproduce the observed scaling relations for velocity dispersion, stellar mass, size, and metallicity at z = 0. WDM satellite haloes in a Milky Way mass host are more susceptible to tidal stripping due to their lower concentrations, but their galaxies can even survive longer than the CDM counterparts if they live in a dark matter halo with a stee per central slope. In agreement with our previous CDM satellite study we observe a steepening of the WDM satellites' central dark matter density slope due to stripping. The difference in the average stellar age for satellite galaxies, between CDM and WDM, could be used in the future for disentangling these two models.