Aims. Several kinematic and chemical substructures have been recently found amongst Milky Way halo stars with retrograde motions. It is currently unclear how these various structures are related to ...each other. This Letter aims to shed light on this issue. Methods. We explore the retrograde halo with an augmented version of the Gaia DR2 RVS sample, extended with data from three large spectroscopic surveys, namely RAVE, APOGEE, and LAMOST. In this dataset, we identify several structures using the HDBSCAN clustering algorithm. We discuss their properties and possible links using all the available chemical and dynamical information. Results. In concordance with previous work, we find that stars with Fe/H < −1 have more retrograde motions than those with Fe/H > −1. The retrograde halo contains a mixture of debris from objects like Gaia-Enceladus, Sequoia, and even the chemically defined thick disc. We find that the Sequoia has a smaller range in orbital energies than previously suggested and is confined to high energy. Sequoia could be a small galaxy in itself, but since it overlaps both in integrals-of-motion space and chemical abundance space with the less bound debris of Gaia-Enceladus, its nature cannot yet be fully settled. In the low-energy part of the halo, we find evidence for at least one more distinct structure: Thamnos. Stars in Thamnos are on low-inclination, mildly eccentric retrograde orbits, moving at vϕ ≈ −150 km s−1, and are chemically distinct from the other structures. Conclusions. Even with the excellent Gaia DR2 data, piecing together all the fragments found in the retrograde halo remains challenging. At this point, we are very much in need of large datasets with high-quality high-resolution spectra and tailored high-resolution hydrodynamical simulations of galaxy mergers.
Abstract
Stellar streams from globular clusters (GCs) offer constraints on the nature of dark matter and have been used to explore the dark matter halo structure and substructure of our Galaxy. ...Detection of GC streams in other galaxies would broaden this endeavor to a cosmological context, yet no such streams have been detected to date. To enable such exploration, we develop the
Hough Stream Spotter
(
HSS
), and apply it to the Pan-Andromeda Archaeological Survey (PAndAS) photometric data of resolved stars in M31's stellar halo. We first demonstrate that our code can re-discover known dwarf streams in M31. We then use the
HSS
to blindly identify 27 linear GC stream-like structures in the PAndAS data. For each
HSS
GC stream candidate, we investigate the morphologies of the streams and the colors and magnitudes of all stars in the candidate streams. We find that the five most significant detections show a stronger signal along the red giant branch in color–magnitude diagrams than spurious non-stream detections. Lastly, we demonstrate that the
HSS
will easily detect globular cluster streams in future Nancy Grace Roman Space Telescope data of nearby galaxies. This has the potential to open up a new discovery space for GC stream studies, GC stream gap searches, and for GC stream-based constraints on the nature of dark matter.
ABSTRACT
We introduce and apply a new approach to probe the response of galactic stellar haloes to the interplay between cosmological merger histories and galaxy formation physics. We perform dark ...matter-only, zoomed simulations of two Milky Way-mass hosts and make targeted, controlled changes to their cosmological histories using the genetic modification technique. Populating each history’s stellar halo with a semi-empirical, particle tagging approach then enables a controlled study, with all instances converging to the same large-scale structure, dynamical and stellar mass at z = 0 as their reference. These related merger scenarios alone generate an extended spread in stellar halo mass fractions (1.5 dex) comparable to the observed population, with the largest scatter achieved by growing late (z ≤ 1) major mergers that spread out existing stars to create massive, in-situ dominated stellar haloes. Increasing a last major merger at z ∼ 2 brings more accreted stars into the inner regions, resulting in smaller scatter in the outskirts which are predominantly built by subsequent minor events. Exploiting the flexibility of our semi-empirical approach, we show that the diversity of stellar halo masses across scenarios is reduced by allowing shallower slopes in the stellar mass–halo mass relation for dwarf galaxies, while it remains conserved when central stars are born with hotter kinematics across cosmic time. The merger-dependent diversity of stellar haloes thus responds distinctly to assumptions in modelling the central and dwarf galaxies respectively, opening exciting prospects to constrain star formation and feedback at different galactic mass-scales with the coming generation of deep, photometric observatories.
ABSTRACT
Understanding the variability of galaxy star formation histories (SFHs) across a range of time-scales provides insight into the underlying physical processes that regulate star formation ...within galaxies. We compile the SFHs of galaxies at z = 0 from an extensive set of models, ranging from cosmological hydrodynamical simulations (Illustris, IllustrisTNG, Mufasa, Simba, EAGLE), zoom simulations (FIRE-2, g14, and Marvel/Justice League), semi-analytic models (Santa Cruz SAM) and empirical models (UniverseMachine), and quantify the variability of these SFHs on different time-scales using the power spectral density (PSD) formalism. We find that the PSDs are well described by broken power laws, and variability on long time-scales (≳1 Gyr) accounts for most of the power in galaxy SFHs. Most hydrodynamical models show increased variability on shorter time-scales (≲300 Myr) with decreasing stellar mass. Quenching can induce ∼0.4−1 dex of additional power on time-scales >1 Gyr. The dark matter accretion histories of galaxies have remarkably self-similar PSDs and are coherent with the in situ star formation on time-scales >3 Gyr. There is considerable diversity among the different models in their (i) power due to star formation rate variability at a given time-scale, (ii) amount of correlation with adjacent time-scales (PSD slope), (iii) evolution of median PSDs with stellar mass, and (iv) presence and locations of breaks in the PSDs. The PSD framework is a useful space to study the SFHs of galaxies since model predictions vary widely. Observational constraints in this space will help constrain the relative strengths of the physical processes responsible for this variability.
The morphology of thin stellar streams can be used to test the nature of dark matter. It is therefore crucial to extend searches for globular cluster (GC) streams to other galaxies than the Milky ...Way. In this paper, we investigate the current and future prospects of detecting GC streams in external galaxies in resolved stars (e.g., with Wide Field InfraRed Survey Telescope (WFIRST)) and using integrated light (e.g., with Hyper Suprime-cam (HSC), the Large Synoptic Survey Telescope (LSST), and Euclid). In particular, we inject mock streams to data from the PAndAS M31 survey and produce simulated M31 backgrounds mimicking what WFIRST will observe in M31. Additionally, we estimate the distance limit to which GC streams will be observable. Our results demonstrate that for a 1 hr (1000 s) exposure, using conservative estimates, WFIRST should detect GC streams in resolved stars in galaxies out to distances of ∼3.5 Mpc (∼2 Mpc). This volume contains 199 (122) galaxies, of which >90% are dwarfs. With integrated light, thin streams can be resolved out to ∼100 Mpc with HSC and LSST and to ∼600 Mpc with WFIRST and Euclid. The low surface brightness of the streams (typically >30 mag arcsec−2), however, will make them difficult to detect, unless the streams originate from very young clusters. We emphasize that if the external galaxies do not host spiral arms or galactic bars, gaps in their stellar streams provide an ideal test case for evidence of interactions with dark matter subhalos. Furthermore, obtaining a large samples of thin stellar streams can help constrain the orbital structure and hence the potentials of external halos.
In spatially resolved galaxy observations, star formation rate (SFR) radial profiles are found to correlate with total specific SFRs. A central depletion in star formation is thought to correlate ...with the globally depressed SFRs of, for example, galaxies within the Green Valley (GV). We present, for the first time, radial specific SFR profiles for a statistical sample of simulated galaxies from the Illustris and EAGLE large cosmological simulations. For galaxies on the star-forming sequence, simulated specific SFR profiles are in reasonable agreement with observations. However, both galaxy samples show centrally concentrated star formation for galaxies in the GV at all galaxy stellar masses, suggesting that quenching occurs from the outside-in, in strong conflict with observations of inside-out quenching. This difference between simulations and observations may be due to resolution issues and/or possible failures in the star formation and feedback implementation in current large-scale cosmological simulations. We conclude that the distribution of star formation within galaxies is a strong additional constraint for simulations and models, in particular, related to the quenching of star formation.
Stars in galaxies form from the cold rotationally supported gaseous disks that settle at the center of dark matter halos. In the simplest models, such angular momentum is acquired early on at the ...time of collapse of the halo and preserved thereafter, implying a well-aligned spin for the stellar and gaseous component. Observations, however, have shown the presence of gaseous disks in counterrotation with the stars. We use the Illustris numerical simulations to study the origin of such counterrotation in low-mass galaxies (M = 2 × 109-5 × 1010 M ), a sample where mergers have not played a significant role. Only ∼1% of our sample shows a counterrotating gaseous disk at z = 0. These counterrotating disks arise in galaxies that have had a significant episode of gas removal followed by the acquisition of new gas with misaligned angular momentum. In our simulations, we identify two main channels responsible for the gas loss: a strong feedback burst and gas stripping during a flyby passage through a more massive group environment. Once settled, counterrotation can be long-lived with several galaxies in our sample displaying misaligned components consistently for more than 2 Gyr. As a result, no major correlation with the present-day environment or structural properties might remain, except for a slight preference for early-type morphologies and a lower than average gas content at a given stellar mass.
ABSTRACT
Understanding the rate at which stars form is central to studies of galaxy formation. Observationally, the star formation rates (SFRs) of galaxies are measured using the luminosity in ...different frequency bands, often under the assumption of a time-steady SFR in the recent past. We use star formation histories (SFHs) extracted from cosmological simulations of star-forming galaxies from the FIRE project to analyse the time-scales to which the H α and far-ultraviolet (FUV) continuum SFR indicators are sensitive. In these simulations, the SFRs are highly time variable for all galaxies at high redshift, and continue to be bursty to z = 0 in dwarf galaxies. When FIRE SFHs are partitioned into their bursty and time-steady phases, the best-fitting FUV time-scale fluctuates from its ∼10 Myr value when the SFR is time-steady to ≳100 Myr immediately following particularly extreme bursts of star formation during the bursty phase. On the other hand, the best-fitting averaging time-scale for H α is generally insensitive to the SFR variability in the FIRE simulations and remains ∼5 Myr at all times. These time-scales are shorter than the 100 and 10 Myr time-scales sometimes assumed in the literature for FUV and H α, respectively, because while the FUV emission persists for stellar populations older than 100 Myr, the time-dependent luminosities are strongly dominated by younger stars. Our results confirm that the ratio of SFRs inferred using H α versus FUV can be used to probe the burstiness of star formation in galaxies.
ABSTRACT
We study the relationship between supermassive black hole (BH) feedback, BH luminosity, and the kinematics of stars and gas for galaxies in IllustrisTNG. We use galaxies with mock MaNGA ...observations to identify kinematic misalignment at z = 0 (difference in rotation of stars and gas), for which we follow the evolutionary history of BH activity and gas properties over the last 8 Gyr. Misaligned low-mass galaxies ($\mathrm{\mathit{ M}_{stel} \lt 10^{10.2}\, \mathrm{M}_{\odot }}$) typically have boosted BH luminosity and BH growth, and have had more energy injected by BHs into the gas over the last 8 Gyr in comparison to low-mass aligned galaxies. These properties likely lead to outflows and gas removal, in agreement with active low mass galaxies in observations. Splitting on BH luminosity at z = 0 produces consistent distributions of kinematic misalignment at z = 0; however, splitting on the maximum BH luminosity over the last 8 Gyr produces statistically significant different distributions. While instantaneous correlation at z = 0 is difficult due to misalignment persisting on longer time-scales, the relationship between BH activity and misalignment is clear. High-mass quenched galaxies ($\mathrm{\mathit{ M}_{stel} \gt 10^{10.2}\, \mathrm{M}_{\odot }}$) with misalignment typically have similar BH luminosities, show lower gas fractions, and have typically lower gas phase metallicity over the last 8 Gyr in comparison to the high mass aligned.
The observed breakBRD ("break bulges in red disks") galaxies are a nearby sample of face-on disk galaxies with particularly centrally concentrated star formation: they have red disks but recent star ...formation in their centers as measured by the Dn4000 spectral index. In this paper, we search for breakBRD analogs in the IllustrisTNG simulation and describe their history and future. We find that a small fraction (∼4% at z = 0; ∼1% at z = 0.5) of galaxies fulfill the breakBRD criteria, in agreement with observations. In comparison with the mass-weighted parent IllustrisTNG sample, these galaxies tend to consist of a higher fraction of satellite and splashback galaxies. However, the central, non-splashback breakBRD galaxies show similar environments, black hole masses, and merger rates, indicating that there is not a single formation trigger for inner star formation and outer quenching. We determine that breakBRD analog galaxies as a whole are in the process of quenching. The breakBRD state, with its highly centrally concentrated star formation, is uncommon in the history of either currently quiescent or star-forming galaxies; however, approximately 10% of 1010 < M*/M < 1011 quiescent galaxies at z = 0 have experienced SFR concentrations comparable to those of the breakBRDs in their past. Additionally, the breakBRD state is short lived, lasting a few hundred Myr up to ∼2 Gyr. The observed breakBRD galaxies may therefore be a unique sample of outside-in quenching galaxies.
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