We explore the structure of the element abundance-age-orbit distribution of the stars in the Milky Way's low- disk, by (re-)deriving precise Fe/H, X/Fe, and ages, along with orbits, for red clump ...stars from the apogee survey. There has been a long-standing theoretical expectation and observational evidence that metallicity (Fe/H) and age are informative about a star's orbit, e.g., about its angular momentum and the corresponding mean Galactocentric distance or its vertical motion. Indeed, our analysis of the apogee data confirms that Fe/H or age alone can predict the stars' orbits far less well than a combination of the two. Remarkably, we find and show explicitly that, for known Fe/H and age, the other abundances X/Fe of Galactic disk stars can be predicted well (on average to 0.02 dex) across a wide range of Galactocentric radii, and therefore provide little additional information, e.g., for predicting their orbit. While the age-abundance space for metal-poor stars and potentially for stars near the Galactic center is rich or complex, for the bulk of the Galaxy's low- disk it is simple: Fe/H and age contain most information, unless X/Fe can be measured to 0.02 or better. Consequently, we do not have the precision with current (and likely near-future) data to assign stars to their individual (coeval) birth clusters, from which the disk is presumably formed. We can, however, place strong constraints on future models of Galactic evolution, chemical enrichment, and mixing.
If the favored hierarchical cosmological model is correct, then the Milky Way system should have accreted 6100-200 luminous satellite galaxies in the past 612 Gyr. We model this process using a ...hybrid semianalytic plus N-body approach that distinguishes explicitly between the evolution of light and dark matter in accreted satellites. This distinction is essential to our ability to produce a realistic stellar halo, with mass and density profile much like that of our own Galaxy, and a surviving satellite population that matches the observed number counts and structural parameter distributions of the satellite galaxies of the Milky Way. Our accreted stellar halos have density profiles that typically drop off with radius faster than the dark matter and follow power laws at r > 30 kpc with r 8 r super(-a), a 3-4. They are well fit by Hernquist profiles over the full radial range. We find that stellar halos are assembled from the inside out, with the majority of mass (680%) coming from the 615 most massive accretion events. The satellites that contribute to the stellar halo have median accretion times of 69 Gyr in the past, while surviving satellite systems have median accretion times of 65 Gyr in the past. This implies that stars associated with the inner halo should be quite different chemically from stars in surviving satellites and also from stars in the outer halo or those liberated in recent disruption events. We briefly discuss the expected spatial structure and phase-space structure for halos formed in this manner. Searches for this type of structure offer a direct test of whether cosmology is indeed hierarchical on small scales.
We use the Bullock & Johnston suite of simulations to study the density profiles of L*-type galaxy stellar halos. Observations of the Milky Way and M31 stellar halos show contrasting results: the ...Milky Way has a "broken" profile, where the density falls off more rapidly beyond ~25 kpc, while M31 has a smooth profile out to 100 kpc with no obvious break. Simulated stellar halos, built solely by the accretion of dwarf galaxies, also exhibit this behavior: some halos have breaks, while others do not. The presence or absence of a break in the stellar halo profile can be related to the accretion history of the galaxy. We find that a break radius is strongly related to the buildup of stars at apocenters. We relate these findings to observations, and find that the "break" in the Milky Way density profile is likely associated with a relatively early (~6-9 Gyr ago) and massive accretion event. In contrast, the absence of a break in the M31 stellar halo profile suggests that its accreted satellites have a wide range of apocenters. Hence, it is likely that M31 has had a much more prolonged accretion history than the Milky Way.
We present 1201 galaxies at 0.05 < z < 0.45 that host tidal features in the first ∼200 deg2 of imaging from the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP). We select these galaxies from a ...sample of 21,208 galaxies with spectroscopic redshifts drawn from the Sloan Digital Sky Survey (SDSS) spectroscopic campaigns. Of these galaxies, we identify 214 shell systems and 987 stream systems. For 575 of these systems, we are additionally able to measure the (g−i) colors of the tidal features. We find evidence for star formation in a subset of the streams, with the exception of streams around massive ellipticals, and find that stream host galaxies span the full range of stellar masses in our sample. Galaxies that host shells are predominantly red and massive: we find that observable shells form more frequently around ellipticals than around disk galaxies of the same stellar mass. Although the majority of the shells in our sample are consistent with being formed by minor mergers, 15% 4.4% of shell host galaxies have (g−i) colors as red as their host galaxy, consistent with being formed by major mergers. These "red shell" galaxies are preferentially aligned with the major axis of the host galaxy, as previously predicted from simulations. We suggest that although the bulk of the observable shell population originates from fairly minor mergers, which preferentially form shells that are not aligned with the major axis of the galaxy, major mergers produce a significant number of observable shells.
ABSTRACT
Interactions between galaxies leave distinguishable imprints in the form of tidal features, which hold important clues about their mass assembly. Unfortunately, these structures are ...difficult to detect because they are low surface brightness features, so deep observations are needed. Upcoming surveys promise several orders of magnitude increase in depth and sky coverage, for which automated methods for tidal feature detection will become mandatory. We test the ability of a convolutional neural network to reproduce human visual classifications for tidal detections. We use as training ∼6000 simulated images classified by professional astronomers. The mock Hyper Suprime Cam Subaru (HSC) images include variations with redshift, projection angle, and surface brightness (μlim = 26–35 mag arcsec−2). We obtain satisfactory results with accuracy, precision, and recall values of Acc = 0.84, P = 0.72, and R = 0.85 for the test sample. While the accuracy and precision values are roughly constant for all surface brightness, the recall (completeness) is significantly affected by image depth. The recovery rate shows strong dependence on the type of tidal features: we recover all the images showing shell features and 87 per cent of the tidal streams; these fractions are below 75 per cent for mergers, tidal tails, and bridges. When applied to real HSC images, the performance of the model worsens significantly. We speculate that this is due to the lack of realism of the simulations, and take it as a warning on applying deep learning models to different data domains without prior testing on the actual data.
Tidal streams are a powerful probe of the Milky Way (MW) potential shape. In this paper, we introduce a simple test-particle method to fit stream data, using a Markov Chain Monte Carlo technique to ...marginalize over uncertainties in the progenitor's orbit and the MW halo shape parameters. Applying it to mock data of thin streams in the MW halo, we show that, even for very cold streams, stream-orbit offsets - not modelled in our simple method - introduce systematic biases in the recovered shape parameters. For the streams that we consider, and our particular choice of potential parametrization, these errors are of the order of ∼20 per cent on the halo flattening parameters. However, larger systematic errors can arise for more general streams and potentials; such offsets need to be correctly modelled in order to obtain an unbiased recovery of the underlying potential. Assessing which of the known MW streams are most constraining, we find that NGC 5466 and Pal 5 are the most promising candidates. These form an interesting pair as their orbital planes are both approximately perpendicular to each other and to the disc, giving optimal constraints on the MW halo shape. We show that - while with current data their constraints on potential parameters are poor - good radial velocity data along the Pal 5 stream will provide constraints on q
z
- the flattening perpendicular to the disc. Furthermore, as discussed in a companion paper, NGC 5466 can provide rather strong constraints on the MW halo shape parameters, if the tentative evidence for a departure from the smooth orbit towards its western edge is confirmed.
M giants recovered from the Two Micron All-Sky Survey have recently been used to map the position and velocity distributions of tidal debris from the Sagittarius (Sgr) dwarf spheroidal galaxy around ...the entire Galaxy. We compare this data set to both test-particle orbits and N-body simulations of satellite destruction run within a variety of rigid Milky Way potentials, and we find that the mass of the Milky Way within 50 kpc of its center should be (3.8-5.6) x 10 super(11) M sub( )in order for any Sgr orbit to simultaneously fit the velocity gradient in the Sgr trailing debris and the apocenter of the Sgr leading debris. Orbital pole precession of young debris and leading debris velocities in regions corresponding to older debris provide contradictory evidence in favor of oblate/prolate Galactic halo potentials, respectively, leading us to conclude that the orbit of Sgr has evolved over the past few gigayears. In light of this discrepancy, we consider constraints from only the younger portions of the debris within three models of the flattening of the Galactic potential q = 0.90 (O), 1.0 (S), and 1.25 (P), i.e., oblate, spherical, and prolate in our further N-body simulations. On the basis of the velocity dispersion and width along the trailing tidal stream, we estimate the current bound mass of Sgr to be M sub(Sgr) = (2-5) x 10 super(8) M sub( )independent of the form of the Galactic potential; this corresponds to a range of mass-to-light ratios (M/L) sub(Sgr) = 14-36(M/L) sub( )for the Sgr core. Models with masses in this range best fit the apocenter of leading Sgr tidal debris when they orbit with a radial period of roughly 0.85 Gyr and have perigalactica and apogalactica of about 15 and 60 kpc, respectively. These distances scale with the assumed distance to the Sgr dwarf and the assumed depth of the Galactic potential. The density distribution of debris along the orbit in these models is consistent with the M giant observations, and debris at all orbital phases in which M giants are obviously present is younger (i.e., was lost more recently from the satellite) than the typical age of a Sgr M giant star.
Differentially methylated or hydroxymethylated regions (DMRs) in mammalian DNA are often associated with tissue-specific gene expression but the functional relationships are still being unraveled. To ...elucidate these relationships, we studied 16 human genes containing myogenic DMRs by analyzing profiles of their epigenetics and transcription and quantitatively assaying 5-hydroxymethylcytosine (5hmC) and 5-methylcytosine (5mC) at specific sites in these genes in skeletal muscle (SkM), myoblasts, heart, brain, and diverse other samples. Although most human promoters have little or no methylation regardless of expression, more than half of the genes that we chose to study-owing to their myogenic DMRs-overlapped tissue-specific alternative or cryptic promoters displaying corresponding tissue-specific differences in histone modifications. The 5mC levels in myoblast DMRs were significantly associated with 5hmC levels in SkM at the same site. Hypermethylated myogenic DMRs within CDH15, a muscle- and cerebellum-specific cell adhesion gene, and PITX3, a homeobox gene, were used for transfection in reporter gene constructs. These intragenic DMRs had bidirectional tissue-specific promoter activity that was silenced by in vivo-like methylation. The CDH15 DMR, which was previously associated with an imprinted maternal germline DMR in mice, had especially strong promoter activity in myogenic host cells. These findings are consistent with the controversial hypothesis that intragenic DNA methylation can facilitate transcription and is not just a passive consequence of it. Our results support varied roles for tissue-specific 5mC- or 5hmC-enrichment in suppressing inappropriate gene expression from cryptic or alternative promoters and in increasing the plasticity of gene expression required for development and rapid responses to tissue stress or damage.
Most spatial conservation planning for wide-ranging or migratory species is constrained by poor knowledge of species’ spatiotemporal dynamics and is only based on static species’ ranges. However, ...species have substantial variation in abundance across their range and migratory species have important spatiotemporal population dynamics. With growing ecological data and advancing analytics, both of these can be estimated and incorporated into spatial conservation planning. However, there is limited information on the degree to which including this information affects conservation planning. We compared the performance of systematic conservation prioritizations for different scenarios based on varying the input species’ distributions by ecological metric (abundance distributions versus range maps) and temporal sampling resolution (weekly, monthly, or quarterly). We used the example of a community of 41 species of migratory shorebirds that breed in North America, and we used eBird data to produce weekly estimates of species’ abundances and ranges. Abundance distributions at a monthly or weekly resolution led to prioritizations that most efficiently protected species throughout the full annual cycle. Conversely, spatial prioritizations based on species’ ranges required more sites and left most species insufficiently protected for at least part of their annual cycle. Prioritizations with only quarterly species ranges were very inefficient as they needed to target 40% of species’ ranges to include 10% of populations. We highlight the high value of abundance information for spatial conservation planning, which leads to more efficient and effective spatial prioritization for conservation. Overall, we provide evidence that spatial conservation planning for wide-ranging migratory species is most robust and efficient when informed by species’ abundance information from the full annual cycle.
M giants selected from the Two Micron All Sky Survey (2MASS) have been used to trace streams of tidal debris apparently associated with the Sagittarius dwarf spheroidal galaxy (Sgr) that entirely ...encircle the Galaxy. While the Sgr M giants are generally aligned with a single great circle on the sky, we measure a difference of 10 degree .4 plus or minus 2 degree .6 between the mean orbital poles of the great circles that best fit debris leading and trailing Sgr, which can be attributed to the precession of Sgr's orbit over the range of phases explored by the data set. Simulations of the destruction of Sgr in potentials containing bulge, disk, and halo components best reproduce this level of precession along the same range of orbital phases if the potential contours of the halo are only slightly flattened, with the ratio of the axis length perpendicular to and in the disk in the range q = 0.90-0.95 (corresponding to isodensity contours with q rho similar to 0.83-0.92). Oblate halos are strongly preferred over prolate (q rho > 1) halos, and flattenings in the potential of q less than or equal to 0.85 (q rho less than or equal to 0.75) and q greater than or equal to 1.05 (q rho greater than or equal to 1.1) are ruled out at the 3 sigma level. More extreme values of q less than or equal to 0.80 (q rho less than or equal to 0.6) and q greater than or equal to 1.25 (q rho greater than or equal to 1.6) are ruled out at the 7 and 5 sigma levels, respectively. These constraints will improve as debris with larger separation in orbital phases is found.
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