The dominant physical processes responsible for the formation and longevity of giant gaseous and stellar discs in galaxies remain controversial. Although they are rare (less than 10 confirmed as of now), giant low-surface brightness (gLSB) discy galaxies provide interesting insights given their extreme nature. We describe observations of UGC 1378 including deep spectroscopy with the Russian 6m telescope and multi-band imaging with Binospec at the MMT. Galaxy UGC 1378 has both high surface brightness and an extended low surface brightness discs. Our stellar velocity dispersion data for the high-surface brightness, Milky Way-sized, disc appears inconsistent with a recent major merger, a widely discussed formation scenario for the very extended low surface brightness disc. We estimate the star formation rates (SFR) from archival Wide-Field Infrared Survey Explorer data. The SFR surface density in the LSB disc is low relative to its gas density, consistent with recent gas accretion. We argue that the unusually large size of UGC 1378's disc may be the product of a rich gas reservoir (e.g. a cosmic filament) and an isolated environment that has preserved the giant disc.
We present an analysis of physical and morphological properties of an interacting pair of dwarf galaxies UGC 4703 located at the vicinity of an isolated Milky-Way (MW) type spiral galaxy NGC 2718. Based on the comparison of physical and morphological properties with that of the Large and Small Magellanic Clouds (LMC and SMC), we report that the UGC 4703 pair-NGC 2718 system is probably an LMC-SMC-MW analogue. Located at a sky projected distance of 81 kpc from NGC 2718, we find that UGC 4703 is clearly interacting with its nearby smaller mass companion UGC 4703B forming a bridge of stellar stre…
The Milky Way disk consists of two prominent components - a thick, alpha-rich, low-metallicity component and a thin, metal-rich, low-alpha component. External galaxies have been shown to contain thin and thick disk components, but whether distinct components in the [$α$/Fe]-[Z/H] plane exist in other Milky Way-like galaxies is not yet known. We present VLT-MUSE observations of UGC 10738, a nearby, edge-on Milky Way-like galaxy. We demonstrate through stellar population synthesis model fitting that UGC 10738 contains alpha-rich and alpha-poor stellar populations with similar spatial distributio…
We investigate the impact of f(R) modified gravity on the internal properties of Milky Way sized dark matter halos in a set of cosmological zoom simulations of seven halos from the Aquarius suite, carried out with our code MG-GADGET in the Hu & Sawicki f(R) model. Also, we calculate the fifth forces in ideal NFW-halos as well as in our cosmological simulations and compare them against analytic model predictions for the fifth force inside spherical objects. We find that these theoretical predictions match the forces in the ideal halos very well, whereas their applicability is somewhat limited f…
Recent kinematical constraints on the internal densities of the Milky Way's dwarf satellites have revealed a discrepancy with the subhalo populations of simulated Galaxy-scale halos in the standard CDM model of hierarchical structure formation. This has been dubbed the "too big to fail" problem, with reference to the improbability of large and invisible companions existing in the Galactic environment. In this paper, we argue that both the Milky Way observations and simulated subhalos are consistent with the predictions of the standard model for structure formation. Specifically, we show that t…
Collisionless simulations of the CDM cosmology predict a plethora of dark matter substructures in the halos of Milky Way sized galaxies, yet the number of known luminous satellites galaxies is very much smaller, a discrepancy that has become known as the `missing satellite problem'. The most massive substructures have been shown to be plausibly the hosts of the brightest satellites, but it remains unclear which processes prevent star formation in the many other, purely dark substructures. We use high-resolution hydrodynamic simulations of the formation of Milky Way sized galaxies in order to t…
The concordance $Λ$CDM cosmology predicts tens of satellite galaxies distributed in the inner region ($<40\ \lkpc$) of the Milky Way (MW), yet at most $12$ were discovered at present day, including 3 discovered very recently by Dark Energy Survey (DES) and 5 from other surveys (e.g. PanSTARRS, MagLiteS). We use $5$ ultra-high resolution simulations of MW sized dark matter haloes from the $\aqua$ project, combined with $\galf$ semi-analytical galaxy formation model, to investigate properties of the model satellite galaxy population inside $40\ \lkpc$ of MW sized haloes. On average, in each halo…
We study the orbital properties of dark matter haloes by combining a spectral method and cosmological simulations of Milky Way-sized galaxies. We compare the dynamics and orbits of individual dark matter particles from both hydrodynamic and $N$-body simulations, and find that the fraction of box, tube and resonant orbits of the dark matter halo decreases significantly due to the effects of baryons. In particular, the central region of the dark matter halo in the hydrodynamic simulation is dominated by regular, short-axis tube orbits, in contrast to the chaotic, box and thin orbits dominant in …
Stellar streams originating in disrupted dwarf galaxies and star clusters are observed around the Milky Way and nearby galaxies. Such substructures are the important tracers that record how the host haloes have accreted progenitor galaxies. Based on the cosmological context, we investigate the relationship between structural properties of substructures such as length and thinness at $z=0$, and orbits of their progenitors. We model stellar components of a large sample of substructures around Milky Way-sized haloes by combining semi-analytic models with a high-resolution cosmological $N$-body si…
Now almost 70 years since its introduction, the effective or half-light radius has become a very popular choice for characterising galaxy size. However, the effective radius measures the concentration of light within galaxies and thus does not capture the intuitive definition of size which is related to the edge or boundary of objects. For this reason, we aim to demonstrate the undesirable consequence of using the effective radius to draw conclusions about the nature of faint 'ultra-diffuse galaxies' (UDGs) when compared to dwarfs and Milky Way-like galaxies. Instead of the effective radius, w…
We study the properties of the central spheroids located within 10 kpc of the centre of mass of Milky Way mass-sized galaxies simulated in a cosmological context. The simulated central regions are dominated by stars older than 10 Gyr, mostly formed in situ, with a contribution of ~30 per cent from accreted stars. These stars formed in well-defined starbursts, although accreted stars exhibit sharper and earlier ones. The fraction of accreted stars increases with galactocentric distance, so that at a radius of ~8-10 kpc a fraction of ~40 per cent, on average, are detected. Accreted stars are sli…
Solving Milky Way-sized Systems with Haskap Pie: A Halo finding Algorithm with efficient Sampling, K-means clustering, tree-Assembly, Particle tracking, Python modules, Inter-code applicability, and Energy solving
Using realistic cosmological simulations of Milky Way sized haloes, we study their dynamical state and the accuracy of inferring their mass profiles with steady-state models of dynamical tracers. We use a new method that describes the phase-space distribution of a steady-state tracer population in a spherical potential without any assumption regarding the distribution of their orbits. Applying the method to five haloes from the Aquarius $Λ$CDM N-body simulation, we find that dark matter particles are an accurate tracer that enables the halo mass and concentration parameters to be recovered wit…
We introduce a new set of simulations of Milky Way-sized galaxies using the AMR code ART + hydrodynamics in a $Λ$CDM cosmogony. The simulation series is named GARROTXA and follow the formation of a halo/galaxy from z~$=$~60 to z~$=$~0. The final virial mass of the system is $\sim$7.4$\times$10$^{11}$M$_{\odot}$. Our results are as follows: (a) contrary to many previous studies, the circular velocity curve shows no central peak and overall agrees with recent MW observations. (b) Other quantities, such as M$\_{*}$(6$\times$10$^{10}$M$_\odot$) and R$_d$ (2.56 kpc), fall well inside the observatio…
We introduce the MUSCEL Program (MUltiwavelength observations of the Structure, Chemistry and Evolution of LSB galaxies), a project aimed at determining the star-formation histories of low surface brightness galaxies. MUSCEL utilizes ground-based optical spectra and space-based UV and IR photometry to fully constrain the star-formation histories of our targets with the aim of shedding light on the processes that led low surface brightness galaxies down a different evolutionary path from that followed by high surface brightness galaxies, such as our Milky Way. Here we present the spatially-reso…
The long-standing challenge of creating a Milky Way-like disk galaxy from cosmological simulations has motivated significant developments in both numerical methods and physical models in recent years. We investigate these two fundamental aspects in a new comparison project using a set of cosmological hydrodynamic simulations of the formation and evolution of a Milky Way-size galaxy. In this study, we focus on the comparison of two particle-based hydrodynamics methods: the improved smoothed particle hydrodynamics (SPH) code Gadget, and the Lagrangian Meshless Finite-Mass (MFM) code GIZMO. All t…
We study the global and radial stellar mass assembly of eight zoomed-in MW-sized galaxies produced in Hydrodynamics cosmological simulations. The disk-dominated galaxies (4) show a fast initial stellar mass growth in the innermost parts, driven mostly by in-situ SF, but since $z\sim2-1$ the SF enters in a long-term quenching phase. The outer regions follow this trend but more gentle as more external they are. As the result, the radial stellar mass growth is highly inside-out due to both the inside-out structural growth and inside-out SF quenching. The half-mass radius evolves fast; for instanc…
We employ an improved methodology to insert live stellar disks into high-resolution dark matter simulations of Milky Way sized halos, allowing us to investigate the fate of thin stellar disks in the tumultuous environment of cold dark matter structures. We study a set of eight different halos, drawn from the Aquarius simulation project, in which stellar disks are adiabatically grown with a prescribed structure, and then allowed to self-consistently evolve. The initial velocity distribution is set-up in very good equilibrium with the help of the GALIC code. We find that the residual triaxiality…
We use a high resolution cosmological zoom simulation of a Milky Way-sized halo to study the observable features in velocity and metallicity space associated with the dynamical influence of spiral arms. For the first time, we demonstrate that spiral arms, that form in a disc in a fully cosmological environment with realistic galaxy formation physics, drive large-scale systematic streaming motions. In particular, on the trailing edge of the spiral arms the peculiar galacto-centric radial and azimuthal velocity field is directed radially outward and azimuthally backward, whereas it is radially i…
We introduce a new set of eight Milky Way-sized cosmological simulations performed using the AMR code ART + Hydrodynamics in a LCDM cosmology. The set of zoom-in simulations covers present-day virial masses in the 0.83-1.56 x 10^12 msun range and is carried out with our simple but effective deterministic star formation (SF) and ``explosive' stellar feedback prescriptions. The work is focused on showing the goodness of the simulated set of ``field' Milky Way-sized galaxies. Our results are as follows. (a) The circular velocity curves of our simulated galaxies are nearly flat. (b) Runs ending wi…
Vertically extended, high velocity dispersion stellar distributions appear to be a ubiquitous feature of disc galaxies, and both internal and external mechanisms have been proposed to be the major driver of their formation. However, it is unclear to what extent each mechanism can generate such a distribution, which is likely to depend on the assembly history of the galaxy. To this end, we perform 16 high resolution cosmological-zoom simulations of Milky Way-sized galaxies using the state-of-the-art cosmological magneto-hydrodynamical code \textlcsc{AREPO}, and analyse the evolution of the vert…
We show that in N-body simulations of isolated spiral discs, spiral arms appear to transient, recurring features that co-rotate with the stellar disc stars at all radii. As a consequence, stars around the spiral arm continually feel a tangential force from the spiral and gain/lose angular momentum at all radii where spiral structure exists, without gaining significant amounts of random energy. We demonstrate that the ubiquitous radial migration in these simulations can be seen as outward (inward) systematic streaming motions along the trailing (leading) side of the spiral arms. We characterise…
We investigate the kinematic signatures induced by spiral and bar structure in a set of simulations of Milky Way-sized spiral disc galaxies. The set includes test particle simulations that follow a quasi-stationary density wave-like scenario with rigidly rotating spiral arms, and $N$-body simulations that host a bar and transient, co-rotating spiral arms. From a location similar to that of the Sun, we calculate the radial, tangential and line-of-sight peculiar velocity fields of a patch of the disc and quantify the fluctuations by computing the power spectrum from a two-dimensional Fourier tra…
We report the discovery of 47 low surface brightness objects in deep images of a 3 x 3 degree field centered on the Coma cluster, obtained with the Dragonfly Telephoto Array. The objects have central surface brightness mu(g,0) ranging from 24 - 26 mag/arcsec^2 and effective radii r_e = 3"-10", as measured from archival Canada France Hawaii Telescope images. From their spatial distribution we infer that most or all of the objects are galaxies in the Coma cluster. This relatively large distance is surprising as it implies that the galaxies are very large: with r_e = 1.5 - 4.6 kpc their sizes are…
We use high-resolution cosmological hydrodynamical simulations of Milky Way-sized galaxies with varying supernovae feedback strengths and merger histories to investigate the formation of their gaseous halos and especially their hot ($>10^6$~K) X-ray luminous coronae. Our simulations predict the presence of significant hot gas in the halos as early as $z=3-4$, well before the halos ought to be able to sustain hot mode accretion in the conventional picture. The nascent coronae grow inside-out and initially do so primarily as a result of outflows from the central galaxies powered by merger-induce…
Milky Way Near Twins (MWNeTs). I. A Hierarchical Framework for Identifying Candidate Evolutionary Counterparts of the Milky Way
Main Astronomical Observatory of the NAS of Ukraine（乌克兰国家科学院主天文台）；Institute of Astronomy, V.N. Karazin Kharkiv National University（瓦·尼·卡拉金哈尔科夫国立大学天体物理研究所）；International Centre for Astronomical, Medical, and Ecological Research, National Academy of Sciences of Ukraine（乌克兰国家科学院国际天文、医学和生态研究中心）
We study evolution of single subhaloes with their masses of $\sim10^9 M_\odot$ in a Milky-Way-sized host halo for self-interacting dark matter (SIDM) models. We perform dark-matter-only N-body simulations of dynamical evolution of individual subhaloes orbiting its host by varying self-scattering cross sections (including a velocity-dependent scenario), subhalo orbits, and internal properties of the subhalo. We calibrate a gravothermal fluid model to predict time evolution in spherical mass density profiles of isolated SIDM haloes with the simulations. We find that tidal effects of SIDM subhalo…
Stellar cluster formation in a Milky Way-sized galaxy at z>4 -- II. A hybrid formation scenario for the nuclear star cluster and its connection to the nuclear stellar ring
We present ALMA observations at a spatial resolution of 0.2 arcsec (60 pc) of CO emission from the Taffy galaxies (UGC 12914/5). The observations are compared with narrow-band Pa$α$, mid-IR, radio continuum and X-ray imaging, plus optical spectroscopy. The galaxies have undergone a recent head-on collision, creating a massive gaseous bridge which is known to be highly turbulent. The bridge contains a complex web of narrow molecular filaments and clumps. The majority of the filaments are devoid of star formation, and fall significantly below the Kennicutt-Schmidt relationship for normal galaxie…
We investigate the properties of halo gas using three cosmological `zoom-in' simulations of realistic Milky Way-galaxy analogs with varying sub-grid physics. In all three cases, the mass of hot ($T > 10^6$ K) halo gas is $\sim 1\%$ of the host's virial mass. The X-ray luminosity of two of the runs is consistent with observations of the Milky Way, while the third simulation is X-ray bright and resembles more closely a very massive, star-forming spiral. Hot halos extend to 140 kpc from the galactic center and are surrounded by a bubble of warm-hot ($T = 10^5 - 10^6$K) gas that extends to the vir…
We study the luminosity function and the radial distribution of satellite galaxies within Milky Way sized haloes as predicted in Cold Dark Matter based models of galaxy formation, making use of numerical N-body techniques as well as three different semi-analytic model (SAMs) galaxy formation codes. We extract merger trees from very high-resolution dissipationless simulations of four Galaxy-sized DM haloes, and use these as common input for the semi-analytic models. We present a detailed comparison of our predictions with the observational data recently obtained on the Milky Way satellite lumin…
Thin and thick disks are found in most spiral galaxies, yet their formation scenarios remain uncertain. Whether thick disks form through slow or fast, internal or environmental, processes is unclear. The physical origin of outer truncations in thin and thick disks, observed as a drop in optical and near-infrared (NIR) surface brightness profiles, is also a much debated topic. These truncations have been linked to star formation (SF) thresholds in Milky-Way type galaxies, but no such connection has been made for their low-mass counterparts or in thick disks. Our photometric analysis of the edge…
We present results from \textsc{GigaEris}, a cosmological, $N$-body hydrodynamical "zoom-in" simulation of the formation of a Milky Way-sized galaxy halo with unprecedented resolution, encompassing of order a billion particles within the refined region. The simulation employs a modern implementation of smoothed-particle hydrodynamics, including metal-line cooling and metal and thermal diffusion. We focus on the early assembly of the galaxy, down to redshift $z=4.4$. The simulated galaxy has properties consistent with extrapolations of the main sequence of star-forming galaxies to higher redshi…
We simulate the tidal disruption of a collisionless N-body globular star cluster in a total of 300 different orbits selected to have galactocentric radii between 10 and 30 kpc in four dark matter halos: (a) a spherical halo with no subhalos, (b) a spherical halo with subhalos, (c) a realistic halo with no subhalos, and (d) a realistic halo with subhalos. This allows us to isolate and study how the halo's (lack of) dynamical symmetry and substructures affect the dispersal of tidal debris. The realistic halos are constructed from the snapshot of the Via Lactea II simulation at redshift zero. We …
We present the largest number of Milky Way sized dark matter halos simulated at very high mass ($\sim$$10^4$ M$_\odot$/particle) and temporal resolution ($\sim$5 Myrs/snapshot) done to date, quadrupling what is currently available in the literature. This initial suite consists of the first 24 halos of the $Caterpillar$ $Project$ (www.caterpillarproject.org) whose project goal of 60 - 70 halos will be made public when complete. We resolve $\sim$20,000 gravitationally bound subhalos within the virial radius of each host halo. Over the ranges set by our spatial resolution our convergence is excel…
Evidence for the First Globular Cluster Stellar Stream beyond the Milky Way
University of Copenhagen（哥本哈根大学）；DTU Space, Technical University of Denmark（丹麦技术大学空间中心）；Princeton University（普林斯顿大学）；University of Arizona（亚利桑那大学）；Flatiron Institute（平顿研究所）；Northwestern University（西北大学）
According to the hierarchical model of galaxy formation underlying our current understanding of cosmology, the Milky Way (MW) has continued to accrete smaller-sized dwarf galaxies since its formation. Remnants of this process surround the MW as debris streams and satellite galaxies, and provide information that is complementary to studies of the Galaxy itself. The satellite system thus has the potential to teach us about the formation and evolution of the MW. Can the existence of a narrow, co-rotating plane of satellite galaxies (the Vast Polar Structure, VPOS) put constraints on our Galaxy's …
We use simulations of Milky Way-sized dark matter haloes from the Aquarius Project to investigate the orbits of substructure haloes likely, according to a semi-analytic galaxy formation model, to host luminous satellites. These tend to populate the most massive subhaloes and are on more radial orbits than the majority of subhaloes found within the halo virial radius. One reason for this (mild) kinematic bias is that many low-mass subhaloes have apocentres that exceed the virial radius of the main host; they are thus excluded from subhalo samples identified within the virial boundary, reducing …
The nature of dark matter is still unknown and one of the most fundamental scientific mysteries. Although successfully describing large scales, the standard cold dark matter model (CDM) exhibits possible shortcomings on galactic and sub-galactic scales. It is exactly at these highly non-linear scales where strong astrophysical constraints can be set on the nature of the dark matter particle. While observations of the Lyman-$α$ forest probe the matter power spectrum in the mildly non-linear regime, satellite galaxies of the Milky Way provide an excellent laboratory as a test of the underlying c…
We report on the discovery of a new Milky Way (MW) satellite in Boötes based on data from the on-going Hyper Suprime-Cam (HSC) Subaru Strategic Program (SSP). This satellite, named Boötes IV, is the third ultra-faint dwarf that we have discovered in the HSC-SSP. We have identified a statistically significant (32.3$σ$) overdensity of stars having characteristics of a metal-poor, old stellar population. The distance to this stellar system is $D_{\odot}=209^{+20}_{-18}$ kpc with a $V$-band absolute magnitude of $M_V=-4.53^{+0.23}_{-0.21}$ mag. Boötes IV has a half-light radius of $r_h=462^{+98}_{…
We introduce a dust model for cosmological simulations implemented in the moving-mesh code AREPO and present a suite of cosmological hydrodynamical zoom-in simulations to study dust formation within galactic haloes. Our model accounts for the stellar production of dust, accretion of gas-phase metals onto existing grains, destruction of dust through local supernova activity, and dust driven by winds from star-forming regions. We find that accurate stellar and active galactic nuclei feedback is needed to reproduce the observed dust-metallicity relation and that dust growth largely dominates dust…
The Solo (Solitary Local) Dwarf Galaxy survey is a volume limited, wide-field g- and i- band survey of all known nearby (<3 Mpc) and isolated (>300 kpc from the Milky Way or M31) dwarf galaxies. This set of 44 dwarfs are homogeneously analysed for quantitative comparisons to the satellite dwarf populations of the Milky Way and M31. In this paper, an analysis of the 12 closest Solo dwarf galaxies accessible from the northern hemisphere is presented, including derivation of their distances, spatial distributions, morphology, and extended structures, including their inner integrated light propert…
We investigated the habitability of the Milky Way, making use of recent observational analysis on the prevalence of Earth-sized planets, in order to estimate where and when potentially habitable star systems may have formed over the course of the Galaxy's history. We were then able to estimate the age distribution of potential intelligent life in our Galaxy using our own evolution and the age of the Sun as a proxy. To do this we created a galactic chemical evolution model and applied the following habitability constraints to the Sun-like (G-type) stars formed in our model: an environment free …
We examine the spatial distribution and orbital pole correlations of satellites in a suite of zoom-in high-resolution dissipationless simulations of Miky Way (MW) sized haloes. We use the measured distribution to estimate the incidence of satellite configurations as flattened and as correlated in their orbital pole distribution as satellite system of the Milky Way. We confirm that this incidence is sensitive to the radial distribution of subhaloes and thereby to the processes that affect it, such as artificial disruption due to numerical effects and disruption due to the central disk. Controll…
Recent studies suggest that only three of the twelve brightest satellites of the Milky Way (MW) inhabit dark matter halos with maximum circular velocity, V_max, exceeding 30km/s. This is in apparent contradiction with the LCDM simulations of the Aquarius Project, which suggest that MW-sized halos should have at least 8 subhalos with V_max>30km/s. The absence of luminous satellites in such massive subhalos is thus puzzling and may present a challenge to the LCDM paradigm. We note, however, that the number of massive subhalos depends sensitively on the (poorly-known) virial mass of the Milky Way…
We investigate the star formation-feedback cycle in cosmological galaxy formation simulations, focusing on progenitors of Milky Way (MW)-sized galaxies. We find that in order to reproduce key properties of the MW progenitors, such as semi-empirically derived star formation histories and the shape of rotation curves, our implementation of star formation and stellar feedback requires 1) a combination of local early momentum feedback via radiation pressure and stellar winds and subsequent efficient supernovae feedback, and 2) efficacy of feedback that results in self-regulation of the global star…
It has been suggested that the satellite galaxies of the Milky Way reside in a highly-flattened, kinematically-coherent plane called Disk of Satellites (DoS). The origin of the DoS, however, has been hotly debated, and a number of conflicting claims have been reported in the literature on whether or not the DoS is consistent with predictions from the standard Lambda Cold Dark Matter ($Λ$CDM) cosmological model. Here we investigate this issue by comparing a high-resolution, hydrodynamic $Λ$CDM simulation of a Milky Way sized galaxy with its dark matter only counterpart. We find the following re…
The "Disk of satellites" (DoS) around Milky Way is a highly debated topic with conflicting interpretations of observations and their theoretical models. We perform a comprehensive analysis of all dwarfs detected in the Milky Way and find that the DoS structure depends strongly on the plane identification method and the sample size. In particular, we demonstrate that a smaller sample size produces a higher anisotropy of the spatial distribution and a stronger clustering of the angular momentum of the satellites. Moreover, we calculate the evolution of the 11 classical satellites with proper mot…
We present an analysis of the radial age gradients for the stellar halos of five Milky Way mass-sized systems simulated as part of the Aquarius Project. The halos show a diversity of age trends, reflecting their different assembly histories. Four of the simulated halos possess clear negative age gradients, ranging from approximately -7 to -19 Myr/kpc , shallower than those determined by recent observational studies of the Milky Way's stellar halo. However, when restricting the analysis to the accreted component alone, all of the stellar halos exhibit a steeper negative age gradient with values…
