Milky Way Analogues Research Articles

Linking High-z and Low-z: Are We Observing the Progenitors of the Milky Way with JWST?

The recent JWST observation of the Firefly Sparkle at z = 8.3 offers a unique opportunity to link the high- and the low-z Universe. Indeed, the claim of it being a Milky Way (MW) type of assembly at the cosmic dawn opens the possibility of interpreting the observation with locally calibrated galaxy-formation models. Here, we use the a state-of-the-art MW-evolution model to perform forward modeling of our Galaxy's progenitors at high-z. We build a set of mock spectra for the MW building blocks to make predictions for JWST and to interpret the Firefly Sparkle observation. First, we find that the most massive MW progenitor becomes detectable in a deep survey like JADES from z ≈ 8.2, meaning that we could have already observed MW analogs that still need interpretation. Second, we provide predictions for the number of detectable MW progenitors in lensed surveys like the CAnadian NIRISS Unbiased Cluster Survey, and interpret the Firefly Sparkle as a group of MW building blocks. Both the number of detections and the observed NIRCam photometry are consistent with our predictions. By identifying the MW progenitors whose mock photometry best fits the data, we find bursty and extended star formation histories, lasting >150–300 Myr, and estimate their properties: M h ≈ 108−9 M ⊙, M ⋆ ≈ 106.2−7.5 M ⊙, SFR ≈ 0.04–0.20 M ⊙ yr−1, and Z gas ≈ 0.04–0.24 Z ⊙. Uncovering the properties of MW analogs at cosmic dawn by combining JWST observations and locally constrained models will allow us to understand our Galaxy's formation, linking the high- and low-z perspectives.

Dark Matter Distribution in Milky Way analog Galaxies

Our current understanding of how dark matter (DM) is distributed within the Milky Way (MW) halo, particularly in the solar neighborhood, is based on either careful studies of the local stellar orbits, model assumptions on the global shape of the MW halo, or from direct acceleration measurements. In this work, we undertake a study of external galaxies, with the intent of providing insight into the DM distribution in MW-analog galaxies. For this, we carefully select a sample of galaxies similar to the MW, based on maximum atomic hydrogen (H i) rotational velocity ( vmax,HI=200–280 km s−1) and morphological type (Sab–Sbc) criteria. With a need for deep, highly resolved H i, our resulting sample is composed of five galaxies from the VLA Imaging of Virgo in Atomic Gas (VIVA) survey and The H i Nearby Galaxy Survey (THINGS). To perform our baryonic analysis, we use deep mid-IR images at 3.6 and 4.5 μm from the Spitzer Survey of Stellar Structures in Galaxies (S4G). Based on the dynamical three-dimensional modeling software 3D-Based Analysis of Rotating Objects via Line Observations (3D-BAROLO), we construct rotation curves (RCs) and derive the gas and stellar contributions from the galaxy's gaseous and stellar disks' mass surface density profiles. Through a careful decomposition of their RCs into their baryonic (stars, gas) and DM components, we isolate the DM contribution by using a Markov Chain Monte Carlo-based approach. Based on the Sun's location and the MW’s R 25, we define the corresponding location of the solar neighborhood in these systems. We put forward a window for the DM density (ρ dm = 0.21–0.55 GeV cm−3) at these galactocentric distances in our MW analog sample, consistent with the values found for the MW’s local DM density, based on more traditional approaches found in the literature.

Populating the Milky Way

Context.Stellar populations and their distribution differ widely across the Galaxy, which is likely to affect planet demographics. Our local neighbourhood is dominated by young, metal-rich stars in the galactic thin disc, while the stellar halo and galactic bulge host a large fraction of older, metal-poor stars.Aims.We study the impact of these variations on planet populations in different regions of the Galaxy by combining a high-resolution galaxy formation simulation with state-of-the-art planet population synthesis models.Methods.We constructed a population model to estimate occurrence rates of different planet types, based on the New Generation Planet Population Synthesis (NGPPS). We applied this model to a simulated Milky Way (MW) analogue in the HESTIA galaxy formation simulation. We studied the planet occurrence rate in the metal-rich regions of the inner Galaxy, namely, in the galactic bulge and thin disc. We compared these result with the frequencies in the more distant, metal-poor region such as the thick disc and stellar halo.Results.We find that the planet demographics in the central, metal-rich regions of the MW analogue differ strongly from the planet populations in the more distant, metal-poor regions. The occurrence rate of giant planets (>300M⊕) is 10–20 times larger in the thin disc compared to the thick disc, driven by the low amounts of solid material available for planet formation around metal-poor stars. Similarly, low-mass Earth-like planets around Sun-like stars are most abundant in the thick disc, being 1.5 times more frequent than in the thin disc. Moreover, low-mass planets are expected to be abundant throughout the galaxy, from the central regions to the outer halo, due to their formation processes being less dependent on stellar metallicity. The planet populations differ more strongly around Sun-like stars compared to dwarfs with masses 0.3–0.5M⊙, caused by a weaker correlation between [Fe/H] metallicity and planet mass. However, it is important to note that the occurrence rates of low-mass planets are still uncertain, making our findings strongly model-dependent. Massive planets are more comprehensively understood and our findings are more robust. Nonetheless, other systematic effects have the potential to alter the giant planet population that we have not addressed in this study. We discuss some of these limitations and offer further directions for future research.

A Measurement of the Assembly of Milky Way Analogs at Redshifts 0.5 < z < 2 with Resolved Stellar Mass and Star Formation Rate Profiles

The resolved mass assembly of Milky Way–mass galaxies has been previously studied in simulations, the local Universe, and at higher redshifts using infrared (IR) light profiles. To better characterize the mass assembly of Milky Way analogs (MWAs), as well as their changes in star formation rate (SFR) and color gradients, we construct resolved stellar mass and SFR maps of MWA progenitors selected with abundance matching techniques up to z ∼2 using deep, multiwavelength imaging data from the Hubble Frontier Fields. Our results using stellar mass profiles agree well with previous studies that utilize IR light profiles, showing that the inner 2 kpc of the galaxies and the regions beyond 2 kpc exhibit similar rates of stellar mass growth. This indicates the progenitors of MWAs from z ∼ 2 to the present do not preferentially grow their bulges or their disks. The evolution of the SFR profiles indicates a greater decrease in SFR density in the inner regions versus the outer regions. Sérsic parameters indicate modest growth in the central regions at lower redshifts, perhaps indicating slight bulge growth. However, the Sérsic index does not rise above n ∼ 2 until z < 0.5, meaning these galaxies are still disk-dominated systems. We find that the half-mass radii of the MWA progenitors increase between 1.5 < z < 2, but remain constant at later epochs (z < 1.5). This implies mild bulge growth since z ∼ 2 in MWA progenitors, in line with previous MWA mass assembly studies.

Galaxy assembly revealed by globular clusters

Many observable properties of globular clusters (GCs) provide valuable insights for unveiling the hierarchical assembly of their host galaxy. For the Milky Way (MW) in particular, GCs from different accreted satellite galaxies show distinct chemical, spatial, kinematic, and age distributions. Here we examine such clustering features for model GC populations in simulated galaxies, which are carefully selected to match various observational constraints of the MW assembly. We evaluate several widely used clustering, dimensionality reduction, and supervised classification methods on these model GCs, using 10 properties that are observable in the MW. We can categorize in-situ and ex-situ formed GCs with about 90% accuracy, based solely on their clustering features in these 10 variables. The methods are also effective in distinguishing the last major merger in MW analogs with similar accuracy. Although challenging, we still find it possible to identify one, and only one, additional smaller satellite. We develop a new technique to classify the progenitors of MW GCs by combining several methods and weighting them by the validated accuracy. According to this technique, about 60% of GCs belong to the in-situ group, 20% are associated with the Gaia-Sausage/Enceladus event, and 10% are associated with the Sagittarius dwarf galaxy. The remaining 10% of GCs cannot be reliably associated with any single accretion event.

Formation of Galactic Disks. I. Why Did the Milky Way’s Disk Form Unusually Early?

Recent results from spectroscopic and astrometric surveys of nearby stars suggest that the stellar disk of our Milky Way (MW) was formed quite early, within the first few billion years of its evolution. Chemokinematic signatures of disk formation in cosmological zoom-in simulations appear to be in tension with these data, implying that MW-like disk formation is delayed in simulations. We investigate the formation of galactic disks using a representative sample of MW-like galaxies from the cosmological volume simulation TNG50. We find that on average MW-mass disks indeed form later than the local data suggest. However, their formation time and metallicity exhibit a substantial scatter, such that ∼10% of MW-mass galaxies form disks early, similar to the MW. Thus, although the MW is unusual, it is consistent with the overall population of MW-mass disk galaxies. The direct MW analogs assemble most of their mass early, ≳10 Gyr ago, and are not affected by destructive mergers after that. In addition, these galaxies form their disks during the early enrichment stage when the interstellar medium metallicity increases rapidly, with only ∼25% of early-forming disks being as metal-poor as the MW was at the onset of disk formation, [Fe/H] ≈ −1.0. In contrast, most MW-mass galaxies either form disks from already enriched material or experience late destructive mergers that reset the signatures of galactic disk formation to later times and higher metallicities. Finally, we also show that earlier disk formation leads to more dominant rotationally supported stellar disks at redshift zero.

Systematically Measuring Ultradiffuse Galaxies (SMUDGes). IV. Ultradiffuse Satellites of Milky Way Analogs

To better understand the formation of large, low-surface-brightness galaxies, we measure the correlation function between ultradiffuse galaxy (UDG) candidates and Milky Way analogs (MWAs). We find that: (1) the projected radial distribution of UDG satellites (projected surface density ∝r −0.84±0.06) is consistent with that of normal satellite galaxies; (2) the number of UDG satellites per MWA (S UDG) is ∼0.5 ± 0.1 over projected radii from 20 to 250 kpc and −17 < M r < −13.5; (3) S UDG is consistent with a linear extrapolation of the relationship between the number of UDGs per halo versus halo mass obtained over galaxy group and cluster scales; (4) red UDG satellites dominate the population of UDG satellites (∼80%); (5) over the range of satellite magnitudes studied, UDG satellites comprise ∼10% of the satellite galaxy population of MWAs; and (6) a significant fraction of these (∼13%) have estimated total masses >1010.9 M ⊙ or, equivalently, at least half the halo mass of the LMC, and populate a large fraction (∼18%) of the expected subhalos down to these masses. All of these results suggest a close association between the overall low-mass galaxy population and UDGs, which we interpret as favoring models where UDG formation principally occurs within the general context of low-mass galaxy formation over models invoking more exotic physical processes specifically invoked to form UDGs.

The impact of the Large Magellanic Cloud on dark matter direct detection signals

We study the effect of the Large Magellanic Cloud (LMC) on the dark matter (DM) distribution in the Solar neighborhood, utilizing the Auriga magneto-hydrodynamical simulations of Milky Way (MW) analogues that have an LMC-like system. We extract the local DM velocity distribution at different times during the orbit of the LMC around the MW in the simulations. As found in previous idealized simulations of the MW-LMC system, we find that the DM particles in the Solar neighborhood originating from the LMC analogue dominate the high speed tail of the local DM speed distribution. Furthermore, the native DM particles of the MW in the Solar region are boosted to higher speeds as a result of a response to the LMC's motion.We simulate the signals expected in near future xenon, germanium, and silicon direct detection experiments, considering DM interactions with target nuclei or electrons. We find that the presence of the LMC causes a considerable shift in the expected direct detection exclusion limits towards smaller cross sections and DM masses, with the effect being more prominent for low mass DM. Hence, our study shows, for the first time, that the LMC's influence on the local DM distribution is significant even in fully cosmological MW analogues.

The stellar halo in Local Group Hestia simulations

Theory suggests that mergers play an important role in shaping galactic discs and stellar haloes, which was observationally confirmed in the Milky Way (MW) thanks toGaiadata. In this work, aiming to probe the contribution of mergers to the in situ stellar halo formation, we analyse six M 31 and MW analogues from the HESTIA suite of cosmological hydrodynamical zoom-in simulations of the Local Group. We found that all the HESTIA galaxies experience between one to four mergers with stellar mass ratios between 0.2 and 1 relative to the host at the time of the merger. These significant mergers, with a single exception, happened 7 − 11 Gyr ago. The overall impact of the most massive mergers in HESTIA is clearly seen as a sharp increase in the orbital eccentricity (and a corresponding decrease in the rotational velocityVϕ) of pre-existing disc stars of the main progenitor, thus nicely reproducing the Splash-, Plume-like feature that was discovered in the MW. We do find a correlation between mergers and close pericentric passages of massive satellites and bursts of the star formation in the in situ component. Massive mergers sharply increase the disc velocity dispersion of the in situ stars; however, the latest significant merger often heats up the disc up to the numbers when the contribution of the previous ones is less prominent in the age-velocity dispersion relation. In HESTIA galaxies, the in situ halo is an important component of the inner stellar halo where its fraction is about 30 − 40%, while in the outer parts it typically does not exceed ≈5% beyond 15 kpc from the galactic centre. The simulations suggest that this component of the stellar haloes continues to grow well after mergers conclude; however, the most significant contribution comes from stars that formed recently before the merger. The orbital analysis of the HESTIA galaxies suggests that wedges inRmax − Zmax(apocentre – maximum height from the mid-plane) space are mainly populated by the stars born in between significant mergers.

Are Milky-Way-like galaxies like the Milky Way? A view from SDSS-IV/MaNGA

ABSTRACTIn this paper, we place the Milky Way (MW) in the context of similar-looking galaxies in terms of their star-formation and chemical evolution histories. We select a sample of 138 Milky Way analogues (MWAs) from the SDSS-IV/MaNGA survey based on their masses, Hubble types, and bulge-to-total ratios. To compare their chemical properties to the detailed spatially-resolved information available for the MW, we use a semi-analytic spectral fitting approach, which fits a self-consistent chemical-evolution and star-formation model directly to the MaNGA spectra. We model the galaxies’ inner and outer regions assuming that some of the material lost in stellar winds falls inwards. We also incorporate chemical enrichment from type II and Ia supernovae to follow the alpha-element abundance at different metallicities and locations. We find some MWAs where the stellar properties closely reproduce the distribution of age, metallicity, and alpha enhancement at both small and large radii in the MW. In these systems, the match is driven by the longer time-scale for star formation in the outer parts, and the inflow of enriched material to the central parts. However, other MWAs have very different histories. These divide into two categories: self-similar galaxies where the inner and outer parts evolve identically; and centrally-quenched galaxies where there is very little evidence of late-time central star formation driven by material accreted from the outer regions. We find that, although selected to be comparable, there are subtle morphological differences between galaxies in these different classes, and that the centrally-quenched galaxies formed their stars systematically earlier.

The unusual Milky Way-local sheet system: implications for spin strength and alignment

ABSTRACT The Milky Way and the Local Sheet form a peculiar galaxy system in terms of the unusually low velocity dispersion in our neighbourhood and the seemingly high mass of the Milky Way for such an environment. Using the TNG300 simulation, we searched for Milky Way analogues (MWA) located in the cosmological walls with velocity dispersion in their local Hubble flow similar to the one observed around our galaxy. We find that MWAs in Local-Sheet analogues are rare, with one per (160–200 Mpc)3 volume. We find that a Sheet-like cold environment preserves, amplifies, or simplifies environmental effects on the angular momentum of galaxies. In such sheets, there are particularly strong alignments between the sheet and galaxy spins; also, these galaxies have low spin parameters. These both may relate to a lack of mergers since wall formation. We hope our results will bring awareness of the atypical nature of the Milky Way-Local Sheet system. Wrongly extrapolating local observations without a full consideration of the effect of our cosmic environment can lead to a Copernican bias in understanding the formation and evolution of the Milky Way and the nearby Universe.

Multiplicity Statistics of Stars in the Sagittarius Dwarf Spheroidal Galaxy: Comparison to the Milky Way

We use time-resolved spectra from the Apache Point Observatory Galactic Evolution Experiment (APOGEE) to examine the distribution of radial velocity (RV) variations in 249 stars identified as members of the Sagittarius (Sgr) dwarf spheroidal (dSph) galaxy by Hayes et al. We select Milky Way (MW) stars that have stellar parameters (, T eff, and [Fe/H] ) similar to those of the Sagittarius members by means of a k-d tree of dimension 3. We find that the shape of the distribution of RV shifts in Sgr dSph stars is similar to that measured in their MW analogs, but the total fraction of RV variable stars in the Sgr dSph is larger by a factor of ∼2. After ruling out other explanations for this difference, we conclude that the fraction of close binaries in the Sgr dSph is intrinsically higher than in the MW. We discuss the implications of this result for the physical processes leading to the formation of close binaries in dwarf spheroidal and spiral galaxies.

Dust spectral energy distributions in Milky Way-like galaxies in the IllustrisTNG simulations based on the evolution of grain size distribution

ABSTRACT To understand how the evolution of grain size distribution in galaxies affects observed dust properties, we apply a post-processing dust evolution model to galaxy merger trees from the IllustrisTNG cosmological hydrodynamical simulation. Our dust model includes stellar dust production, sputtering in hot gas, dust growth by accretion and coagulation in the dense interstellar medium (ISM), and shattering in the diffuse ISM. We decompose the grain size distribution into different dust species depending on the elemental abundances and the dense ISM fraction given by the simulation. In our previous work, we focused on Milky Way (MW) analogues and reproduced the observed MW extinction curve. In this study, we compute dust spectral energy distributions (SEDs) for the MW analogues. Our simulated SEDs broadly reproduce the observed MW SED within their dispersion and so does the observational data of nearby galaxies, although they tend to underpredict the MW SED at short wavelengths where emission is dominated by polycyclic aromatic hydrocarbons. We find that metallicity and dense gas fraction are the most critical factors for the SED shape, through their influence on coagulation and shattering. The overall success of our models in reproducing the MW SED further justifies the dust evolution processes included in the model and predicts the dispersion in the SEDs caused by the variety in the assembly history. We also show that the most significant increase in the dust SED occurs between redshifts z ∼ 3 and 2 in the progenitors of the simulated MW-like galaxies.

Dynamics of intermediate-mass black holes wandering in the milky way galaxy using the illustris TNG50 simulation

ABSTRACT The detection of Intermediate-Mass Black Holes (IMBHs) in dwarf galaxies is crucial to closing the gap in the wide mass distribution of black holes ($\sim 3 \, {\rm M_\odot }$ to $\sim 5 \times 10^{10} \, {\rm M_\odot }$). IMBHs originally located at the centre of dwarfs that later collide with the Milky Way (MW) could be wandering, undetected, in our Galaxy. We used TNG50, the highest resolution run of the IllustrisTNG project, to study the kinematics and dynamics of star clusters, in the appropriate mass range, acting as IMBH proxies in an MW analogue galaxy. We showed that $\sim 87{{\ \rm per\ cent}}$ of our studied IMBHs drift inward. The radial velocity of these sinking IMBHs has a median magnitude of $\sim 0.44 \, \mathrm{ckpc \, h^{-1} \, Gyr^{-1}}$ and no dependence on the black hole mass. The central $1 \, \rm ckpc \, h^{-1}$ has the highest number density of IMBHs in the galaxy. A physical toy model with linear drag forces was developed to explain the orbital circularization with time. These findings constrain the spatial distribution of IMBHs, suggesting that future searches should focus on the central regions of the Galaxy. Additionally, we found that the 3D velocity distribution of IMBHs with respect to the galactic centre has a mean of $\sim 180 \, \mathrm{km \, s^{-1}}$ and larger variance with decreasing radius. Remarkably, the velocity distribution relative to the local gas shows significantly lower values, with a mean of $\sim 88 \, \mathrm{km \, s^{-1}}$. These results are instrumental for predicting the accretion and radiation properties of IMBHs, facilitating their detection with future surveys.

L-Galaxies 2020: the formation and chemical evolution of stellar haloes in Milky Way analogues and galaxy clusters

ABSTRACT We present an analysis of the formation and chemical evolution of stellar haloes around (a) Milky Way analogue (MWA) galaxies and (b) galaxy clusters in the L-Galaxies 2020 semi-analytic model of galaxy evolution. Observed stellar halo properties are better reproduced when assuming a gradual stripping model for the removal of cold gas and stars from satellites, compared to an instantaneous stripping model. The slope of the stellar mass–metallicity relation for MWA stellar haloes is in good agreement with that observed in the local Universe . This extends the good agreement between L-Galaxies 2020 and metallicity observations from the gas and stars inside galaxies to those outside. Halo stars contribute on average only ∼0.1 per cent of the total circumgalactic medium enrichment by z = 0 in MWAs, ejecting predominantly carbon produced by asymptotic giant branch stars. Around a quarter of MWAs have a single ‘significant progenitor’ with a mean mass of ∼$2.3\times {}10^{9}\, \rm {M}_{\odot }$, similar to that measured for Gaia Enceladus. For galaxy clusters, L-Galaxies 2020 shows good correspondence with observations of stellar halo mass fractions, but slightly overpredicts stellar masses. Assuming a Navarro–Frenk–White profile for the stellar halo mass distribution provides the best agreement. On average, the intracluster stellar component is responsible for 5.4 per cent of the total intracluster medium iron enrichment, exceeding the contribution from the brightest cluster galaxy by z = 0. We find that considering gradual stripping of satellites and realistic radial profiles is crucial for accurately modelling stellar halo formation on all scales in semi-analytic models.

Satellites around Milky Way Analogs: Tension in the Number and Fraction of Quiescent Satellites Seen in Observations versus Simulations

Abstract We compare the star-forming properties of satellites around Milky Way (MW) analogs from the Stage II release of the Satellites Around Galactic Analogs Survey (SAGA-ii) to those from the APOSTLE and Auriga cosmological zoom-in simulation suites. We use archival GALEX UV imaging as a star formation indicator for the SAGA-ii sample and derive star formation rates (SFRs) to compare with those from APOSTLE and Auriga. We compare our detection rates from the NUV and FUV bands to the SAGA-ii Hα detections and find that they are broadly consistent with over 85% of observed satellites detected in all three tracers. We apply the same spatial selection criteria used around SAGA-ii hosts to select satellites around the MW-like hosts in APOSTLE and Auriga. We find very good overall agreement in the derived SFRs for the star-forming satellites as well as the number of star-forming satellites per host in observed and simulated samples. However, the number and fraction of quenched satellites in the SAGA-ii sample are significantly lower than those in APOSTLE and Auriga below a stellar mass of M * ∼ 108 M ⊙, even when the SAGA-ii incompleteness and interloper corrections are included. This discrepancy is robust with respect to the resolution of the simulations and persists when alternative star formation tracers are employed. We posit that this disagreement is not readily explained by vagaries in the observed or simulated samples considered here, suggesting a genuine discrepancy that may inform the physics of satellite populations around MW analogs.

The SAGA Survey. II. Building a Statistical Sample of Satellite Systems around Milky Way–like Galaxies

Abstract We present the Stage II results from the ongoing Satellites Around Galactic Analogs (SAGA) Survey. Upon completion, the SAGA Survey will spectroscopically identify satellite galaxies brighter than M r,o = −12.3 around 100 Milky Way (MW) analogs at z ∼ 0.01. In Stage II, we have more than quadrupled the sample size of Stage I, delivering results from 127 satellites around 36 MW analogs with an improved target selection strategy and deep photometric imaging catalogs from the Dark Energy Survey and the Legacy Surveys. We have obtained 25,372 galaxy redshifts, peaking around z = 0.2. These data significantly increase spectroscopic coverage for very low redshift objects in 17 &lt; r o &lt; 20.75 around SAGA hosts, creating a unique data set that places the Local Group in a wider context. The number of confirmed satellites per system ranges from zero to nine and correlates with host galaxy and brightest satellite luminosities. We find that the number and luminosities of MW satellites are consistent with being drawn from the same underlying distribution as SAGA systems. The majority of confirmed SAGA satellites are star-forming, and the quenched fraction increases as satellite stellar mass and projected radius from the host galaxy decrease. Overall, the satellite quenched fraction among SAGA systems is lower than that in the Local Group. We compare the luminosity functions and radial distributions of SAGA satellites with theoretical predictions based on cold dark matter simulations and an empirical galaxy–halo connection model and find that the results are broadly in agreement.

Radial Distributions of Dwarf Satellite Systems in the Local Volume

Abstract The radial spatial distribution of low-mass satellites around a Milky Way (MW)-like host is an important benchmark for simulations of small-scale structure. The distribution is sensitive to the disruption of subhalos by the central disk and can indicate whether the disruption observed in simulations of MW analogs is artificial (i.e., numerical) or physical in origin. We consider a sample of 12 well-surveyed satellite systems of MW-like hosts in the Local Volume (D &lt; 12 Mpc) that are complete to M V &lt; −9 and within 150 projected kpc. We investigate the radial distribution of satellites and compare with ΛCDM cosmological simulations, including big-box cosmological simulations and high-resolution zoom-in simulations of MW-sized halos. We find that the observed satellites are significantly more centrally concentrated than the simulated systems. Several of the observed hosts, including the MW, are ∼2σ outliers relative to the simulated hosts in being too concentrated, while none of the observed hosts are less centrally concentrated than the simulations. This result is robust to different ways of measuring the radial concentration. We find that this discrepancy is more significant for bright, M V &lt; −12 satellites, suggestive that this is not the result of observational incompleteness. We argue that the discrepancy is possibly due to artificial disruption in the simulations, but, if so, this has important ramifications for what relation between stellar mass and halo mass is allowed in the low-mass regime by the observed abundance of satellites.

Cosmological insights into the assembly of the radial and compact stellar halo of the Milky Way

ABSTRACT Recent studies using Gaia DR2 have identified a massive merger in the early history of the Milky Way (MW) whose debris is dominated by radial and counterrotating orbits. This event, dubbed the Gaia-Enceladus/Gaia-Sausage (GE/GS), is also hypothesized to have built the majority of the inner stellar halo. We use the cosmological hydrodynamic simulation Illustris to place this merger in the context of galaxy assembly within lambda cold dark matter. From ∼150 MW analogues, $\sim \!80 {{ \rm {per\ cent}}}$ have experienced at least one merger of similar mass and infall time as the GE/GS. Within this sample, 37 have debris as radial as the GE/GS, which we dub the ancient radial mergers (ARMs). Counterrotation is not rare among ARMs, with $43 {{ \rm {per\ cent}}}$ having $\gt 40 {{ \rm {per\ cent}}}$ of their debris in counterrotating orbits. However, the compactness inferred for the GE/GS debris given its large radial orbital anisotropy, β, and its substantial contribution to the stellar halo are difficult to reproduce. The median radius of ARM debris is r*,deb ≃ 45 kpc, while GE/GS is thought to be mostly contained within r ∼ 30 kpc. For most MW analogues, few mergers are required to build the inner stellar halo, and ARM debris only accounts for (median) $\sim \!12 {{ \rm {per\ cent}}}$ of inner accreted stars. Encouragingly, we find one ARM that is both compact and dominates the inner halo of its central, making it our best GE/GS analogue. Interestingly, this merger deposits a significant number of stars (M* ≃ 1.5 × 109 M⊙) in the outer halo, suggesting that an undiscovered section of GE/GS may await detection.

Local Group Analogs in ΛCDM Cosmological Simulations

Abstract We use semi-analytic galaxy catalogs based on two high-resolution cosmological N-body simulations, Millennium-WMAP7 and Millennium-II, to investigate the formation of Local Group (LG) analogs. Unlike previous studies, we use the observed stellar masses to select the LG member (Milky Way (MW) and M31) analogs, and then impose constraints using the observed separation, isolation, and kinematics of the two main member galaxies. By comparing radial and low-ellipticity orbits between the MW and M31, we find higher tangential velocity results in higher total mass, which are 10 and 10 for radial and low-ellipticity orbits. The orbits also influence the individual mass distribution of MW and M31 analogs. For radial orbits, typical host halo masses of the MW and M31 are 10 and 10 for low-ellipticity orbits, the masses are 10 and 10 . The LG is located primarily in filaments with tails extending toward higher densities up to . The dark matter velocity anisotropy parameters β of both the MW and M31 analogs are close to zero in the center, increasing to 0.2–0.3 at 50–80 kpc and decreasing slowly outward. The slope is much flatter than computed from the MW satellites, and the amplitude is smaller than traced by halo stars. Values of β from different tracers agree at ∼120 kpc where β ∼ 0.2. We also find that model predictions agree broadly with observations in the radial distribution and luminosity function of satellites around the MW and M31.