Metal-poor Population Research Articles

Unravelling the origin of the counter-rotating core in IC 1459 with KMOS and MUSE

ABSTRACTThe massive early-type galaxy IC 1459 is a slowly rotating galaxy that exhibits a rapidly counter-rotating kinematically decoupled core (KDC, RKDC ≈ 5 arcsec ≈ 0.1Re). To investigate the origin of its KDC, we coupled large data mosaics from the near-infrared and optical integral field unit instruments K-band Multi-Object Spectrograph (KMOS) and Multi-Unit Spectroscopic Explorer (MUSE), respectively. We studied IC 1459’s stellar populations and, for the first time for a KDC, the spatially resolved initial mass function (IMF). We used full-spectral-fitting to fit the stellar populations and IMF simultaneously, and an alternative spectral-fitting method that does not assume a star formation history (SFH; although does not constrain the IMF) for comparison. When no SFH is assumed, we derived a negative metallicity gradient for IC 1459 that could be driven by a distinct metal-poor population in the outer regions of the galaxy, and a radially constant old stellar age. We found a radially constant bottom-heavy IMF out to ${\sim }\frac {1}{3}R_{\rm e}$. The radially flat IMF and age extend beyond the counter-rotating core. We detected high-velocity dispersion along the galaxy’s major axis. Our results potentially add weight to findings from orbital modelling of other KDCs that the core is not a distinct population of stars but in fact two smooth co-spatial counter-rotating populations. No clear picture of formation explains the observational results of IC 1459, but we propose it could have included a gas-rich intense period of star formation at early times, perhaps with counter-rotating accreting cold streams, followed by dry and gas-rich mergers through to the present day.

HERBS II: Detailed chemical compositions of Galactic bulge stars

ABSTRACT This work explores the detailed chemistry of the Milky Way bulge using the HERMES spectrograph on the Anglo-Australian Telescope. Here, we present the abundance ratios of 13 elements for 832 red giant branch and clump stars along the minor bulge axis at latitudes b = −10○, − 7.5○, and −5○. Our results show that none of the abundance ratios vary significantly with latitude. We also observe disc-like [Na/Fe] abundance ratios, which indicate that the bulge does not contain helium-enhanced populations as observed in some globular clusters. Helium enhancement is therefore not the likely explanation for the double red-clump observed in the bulge. We confirm that bulge stars mostly follow abundance trends observed in the disc. However, this similarity is not confirmed across all elements and metallicity regimes. The more metal-poor bulge population at [Fe/H] ≲ − 0.8 is enhanced in the elements associated with core collapse supernovae (SNeII). In addition, the [La/Eu] abundance ratio suggests higher r-process contribution, and likely higher star formation in the bulge compared to the disc. This highlights the complex evolution in the bulge, which should be investigated further, both in terms of modelling; and with additional observations of the inner Galaxy.

Testing massive star evolution, star formation history, and feedback at low metallicity

Stars that start their lives with spectral types O and early B are the progenitors of core-collapse supernovae, long gamma-ray bursts, neutron stars, and black holes. These massive stars are the primary sources of stellar feedback in star-forming galaxies. At low metallicities, the properties of massive stars and their evolution are not yet fully explored. Here we report a spectroscopic study of 320 massive stars of spectral types O (23 stars) and B (297 stars) in the Wing of the Small Magellanic Cloud (SMC). The spectra, which we obtained with the ESO Very Large Telescope, were analyzed using state-of-the-art stellar atmosphere models, and the stellar parameters were determined. We find that the stellar winds of our sample stars are generally much weaker than theoretically expected. The stellar rotation rates show broad, tentatively bimodal distributions. The upper Hertzsprung–Russell diagram (HRD) is well populated by the stars of our sample from a specific field in the SMC Wing. A few very luminous O stars are found close to the main sequence, while all other, slightly evolved stars obey a strict luminosity limit. Considering additional massive stars in evolved stages, with published parameters and located all over the SMC, essentially confirms this picture. The comparison with single-star evolutionary tracks suggests a dichotomy in the fate of massive stars in the SMC. Only stars with an initial mass below ∼30 M⊙ seem to evolve from the main sequence to the cool side of the HRD to become a red supergiant and to explode as type II-P supernova. In contrast, stars with initially more than ∼30 M⊙ appear to stay always hot and might evolve quasi chemically homogeneously, finally collapsing to relatively massive black holes. However, we find no indication that chemical mixing is correlated with rapid rotation. We measured the key parameters of stellar feedback and established the links between the rates of star formation and supernovae. Our study demonstrates that in metal-poor environments stellar feedback is dominated by core-collapse supernovae in combination with winds and ionizing radiation supplied by a few of the most massive stars. We found indications of the stochastic mode of massive star formation, where the resulting stellar population is fully capable of producing large-scale structures such as the supergiant shell SMC-SGS 1 in the Wing. The low level of feedback in metal-poor stellar populations allows star formation episodes to persist over long timescales.

HERBS I: Metallicity and alpha enhancement along the Galactic bulge minor axis

ABSTRACT To better understand the origin and evolution of the Milky Way bulge, we have conducted a survey of bulge red giant branch and clump stars using the High Efficiency and Resolution Multi-Element Spectrograph on the Anglo–Australian Telescope. We targeted ARGOS survey stars with predetermined bulge memberships, covering the full metallicity distribution function. The spectra have signal-to-noise ratios comparable to, and were analysed using the same methods as the GALAH survey. In this work, we present the survey design, stellar parameters, distribution of metallicity, and alpha-element abundances along the minor bulge axis at latitudes b = −10°, − 7.5°, and −5°. Our analysis of ARGOS stars indicates that the centroids of ARGOS metallicity components should be located ≈0.09 dex closer together. The vertical distribution of α-element abundances is consistent with the varying contributions of the different metallicity components. Closer to the plane, alpha abundance ratios are lower as the metal-rich population dominates. At higher latitudes, the alpha abundance ratios increase as the number of metal-poor stars increases. However, we find that the trend of alpha-enrichment with respect to metallicity is independent of latitude. Comparison of our results with those of GALAH DR2 revealed that for [Fe/H] ≈ −0.8, the bulge shares the same abundance trend as the high-α disc population. However, the metal-poor bulge population ([Fe/H] ≲ −0.8) show enhanced alpha abundance ratios compared to the disc/halo. These observations point to fairly rapid chemical evolution in the bulge, and that the metal-poor bulge population does not share the same similarity with the disc as the more metal-rich populations.

Four Metal-poor Stars in the Sagittarius Dwarf Spheroidal Galaxy∗

Abstract We present the metallicities and carbon abundances of four newly discovered metal-poor stars with −2.2 < [Fe/H] < −1.6 in the Sagittarius dwarf spheroidal galaxy. These stars were selected as metal-poor member candidates using a combination of public photometry from the SkyMapper Southern Sky Survey and proper-motion data from the second data release from the Gaia mission. The SkyMapper filters include a metallicity-sensitive narrowband v filter centered on the Ca ii K line, which we use to identify metal-poor candidates. In tandem, we use proper-motion data to remove metal-poor stars that are not velocity members of the Sagittarius dwarf spheroidal galaxy. We find that these two data sets allow for efficient identification of metal-poor members of the Sagittarius dwarf galaxy to follow up with further spectroscopic study. Two of the stars we present have [Fe/H] < −2.0, which adds to the few other such stars currently identified in the Sagittarius dwarf galaxy that are likely not associated with the globular cluster M54, which resides in the nucleus of the system. Our results confirm that there exists a very metal-poor stellar population in the Sagittarius dwarf galaxy. We find that none of our stars can be classified as carbon-enhanced metal-poor stars. Efficiently identifying members of this population will be helpful to further our understanding of the early chemical evolution of the system.

Mapping the inner kpc of the interacting Seyfert galaxy NGC 2992: Stellar populations and gas kinematics

AbstractWe present Gemini Multi-Object Spectrograph Integral Field Unit (GMOS-IFU) observations of the inner 1.1 kpc of the interacting Seyfert galaxy NGC 2992. From full spectral synthesis we found that the stellar population is mainly (up to 80 per cent of the total light) composed by an old (t ≥ 1.4 Gyr) metal-rich (Z ≥ 2.0) populations with a smaller but considerable contribution (up to 30 per cent) from young (t ≤ 100 Myr) metal-poor (Z ≥ 1.0) populations. The gas kinematics presents two main components: one from gas in orbit in the galaxy disk and an outflow with mass outflow rate of ˜2 Mʘ yr–1 and a kinematic power of ˜ 2 × 1040 erg s–1.

Nine tiny star clusters inGaiaDR1, PS1, and DES

We present the results of a systematic Milky Way satellite search performed across an array of publicly available wide-area photometric surveys. Our aim is to complement previous searches by widening the parameter space covered. Specifically, we focus on objects smaller than $1'$ and include old, young, metal poor and metal rich stellar population masks. As a result we find 9 new likely genuine stellar systems in data from GAIA, DES, and Pan-STARRS, which were picked from the candidate list because of conspicuous counterparts in the cut-out images. The presented systems are all very compact ($r_h<1'$) and faint ($M_V\gtrsim-3$), and are associated either with the Galactic disk, or the Magellanic Clouds. While most of the stellar systems look like Open Clusters, their exact classification is, as of today, unclear. With these discoveries, we extend the parameter space occupied by star clusters to sizes and luminosities previously unexplored and demonstrate that rather than two distinct classes of Globular and Open clusters, there appears to be a continuity of objects, unmarked by a clear decision boundary.

The HST Large Programme on NGC 6752. I. Serendipitous discovery of a dwarf Galaxy in background

ABSTRACT As part of a large Hubble Space Telescope investigation aiming at reaching the faintest stars in the Galactic globular cluster NGC 6752, an Advanced Camera for Surveys/Wide Field Channel field was the subject of deep optical observations reaching magnitudes as faint as V ∼ 30. In this field, we report the discovery of Bedin I, a dwarf spheroidal galaxy too faint and too close to the core of NGC 6752 for detection in earlier surveys. As it is of broad interest to complete the census of galaxies in the local Universe, in this letter we provide the position of this new object along with preliminary assessments of its main parameters. Assuming the same reddening as for NGC 6752, we estimate a distance modulus of (m − M)0 = 29.70 ± 0.13 from the observed red giant branch, i.e. 8.7$^{+0.5}_{-0.7}$ Mpc, and size of ∼840 × 340 pc, about one-fifth the size of the Large Magellanic Cloud. A comparison of the observed colour–magnitude diagram with synthetic counterparts, which account for the galaxy distance modulus, reddening, and photometric errors, suggests the presence of an old (∼13 Gyr) and metal-poor ([Fe/H] ∼ −1.3) population. This object is most likely a relatively isolated satellite dwarf spheroidal galaxy of the nearby great spiral NGC 6744, or potentially the most distant isolated dwarf spheroidal known with a secure distance.

Mass and shape of the Milky Way’s dark matter halo with globular clusters from Gaia and Hubble

Aims. We estimate the mass of the inner (&lt; 20 kpc) Milky Way and the axis ratio of its inner dark matter halo using globular clusters as tracers. At the same time, we constrain the distribution in phase-space of the globular cluster system around the Galaxy. Methods. We use the Gaia Data Release 2 catalogue of 75 globular clusters’ proper motions and recent measurements of the proper motions of another 20 distant clusters obtained with the Hubble Space Telescope. We describe the globular cluster system with a distribution function (DF) with two components: a flat, rotating disc-like one and a rounder, more extended halo-like one. While fixing the Milky Way’s disc and bulge, we let the mass and shape of the dark matter halo and we fit these two parameters, together with six others describing the DF, with a Bayesian method. Results. We find the mass of the Galaxy within 20 kpc to be M(&lt;20 kpc) = 1.91−0.17+0.18×1011 M⊙, of which MDM(&lt;20 kpc) = 1.37−0.17+0.18×1011 M⊙ is in dark matter, and the density axis ratio of the dark matter halo to be q = 1.30 ± 0.25. Assuming a concentration-mass relation, this implies a virial mass Mvir = 1.3±0.3×1012 M⊙. Our analysis rules out oblate (q &lt; 0.8) and strongly prolate halos (q &gt; 1.9) with 99% probability. Our preferred model reproduces well the observed phase-space distribution of globular clusters and has a disc component that closely resembles that of the Galactic thick disc. The halo component follows a power-law density profile ρ ∝ r−3.3, has a mean rotational velocity of Vrot ≃ −14km s−1 at 20 kpc, and has a mildly radially biased velocity distribution (β ≃ 0.2 ± 0.07, which varies significantly with radius only within the inner 15 kpc). We also find that our distinction between disc and halo clusters resembles, although not fully, the observed distinction in metal-rich ([Fe/H] &gt; −0.8) and metal-poor ([Fe/H] ≤ −0.8) cluster populations.

From globular clusters to the disc: the dual life of our Galaxy

The halo and disc globular cluster population can be used as a tracer of the primordial epochs of the Milky Way formation. In this work, literature data of globular clusters ages, chemical abundances, and structural parameters are studied, explicitly focussing on the origin of the known split in the age-metallicity relation (AMR) of globular clusters. When the α-element abundances, which are less strongly affected by the internal light-element spread of globular clusters (Si, Ca), are considered, a very low observational scatter among metal-poor clusters is observed. A plateau at [SiCa/Fe],∼ 0.35 dex, with a dispersion of only 0.05 dex (including abundance errors) is observed up to a metallicity of about −0.75 dex. Only a few metal-poor clusters in this metallicity interval present low [SiCa/Fe] abundances. Moreover, metal-rich globular clusters show a knee in the [α/Fe] vs. [Fe/H] plane around [Fe/H] ∼ −0.75 dex. As a consequence, if a substantial fraction of galactic globular clusters has an external origin, they have to be mainly formed either in galaxies that are massive enough to ensure high levels of α-element abundances even at intermediate metallicity, or in lower mass dwarf galaxies accreted by the Milky Way in their early phases of formation. Finally, clusters in the metal-poor branch of the AMR present an anti-correlation of [SiCa/Fe] with the total cluster magnitude, while this is not the case for metal-rich branch clusters. In addition, this lack of faint high-α clusters in the young metal-poor population is in contrast with what is observed for old and more metal-poor clusters, possibly reflecting a higher heterogeneity of formation environments at lower metallicity. Accretion of high-mass satellites, as a major contribution to the current Milky Way globular cluster system both in the metal-poor and the metal-intermediate regime is compatible with the observations.

Understanding the orbital periods of CEMP-s stars

The chemical enrichments detected in carbon- and s-element-enhanced metal-poor (CEMP-s) stars are believed to be the consequence of a past episode of mass transfer from a now extinct asymptotic-giant-branch primary star. This hypothesis is borne out by the evidence that most CEMP-s stars exhibit radial-velocity variations suggesting that they belong to binary systems in which the companion is not directly visible. We used the orbital-period distribution of an unbiased sample of observed CEMP-s stars to investigate the constraints it imposes on our models of binary evolution and on the properties of the metal-poor binary population in the Galactic halo. We generated synthetic populations of metal-poor binary stars using different assumptions about the initial period distribution and about the physics of the mass-transfer process, and we compared the predicted period distributions of our synthetic CEMP-s stars with the observed one. With a set of default assumptions often made in binary population-synthesis studies, the observed period distribution cannot be reproduced. The percentage of observed CEMP-s systems with periods shorter than about 2000 days is underestimated by almost a factor of three, and by about a factor of two between 3000 and 10 000 days. Conversely, about 40% of the simulated systems have periods longer than 104 days, which is approximately the longest measured period among CEMP-s stars. Variations in the assumed stability criterion for Roche-lobe overflow and the efficiency of wind mass transfer do not alter the period distribution enough to overcome this discrepancy. To reconcile the results of the models with the orbital properties of observed CEMP-s stars, one or both of the following conditions are necessary: (i) the specific angular momentum carried away by the material that escapes the binary system is approximately two to five times higher than currently predicted by analytical models and hydrodynamical simulations of wind mass transfer, and (ii) the initial period distribution of very metal-poor binary stars is significantly different from that observed in the solar vicinity and weighted towards periods shorter than about ten thousand days. Our simulations show that some, perhaps all, of the observed CEMP-s stars with apparently constant radial velocity could be undetected binaries with periods longer than 104 days, but the same simulations also predict that twenty to thirty percent of detectable binaries should have periods above this threshold, much more than are currently observed.

Globular clusters formed within dark haloes I: present-day abundance, distribution, and kinematics

We explore a scenario where metal poor globular clusters (GCs) form at the centres of their own dark matter halos in the early universe before reionization. This hypothesis leads to predictions about the abundance, distribution and kinematics of GCs today that we explore using cosmological N-body simulations and analytical modelling. We find that selecting the massive tail of collapsed objects at $z\gtrsim 9$ as GC formation sites leads to four main predictions: i) a highly clustered population of GCs around galaxies today, ii) a natural scaling between number of GCs and halo virial mass that follows roughly the observed trend, iii) a very low number of free floating GCs outside massive halos and iv) GCs should be embedded within massive and extended dark matter (sub)halos. We find that the strongest constraint to the model is given by the combination of (i) and (ii): a mass cut to tagged GCs halos which accounts for the number density of metal poor GCs today predicts a radial distribution that is too extended compared to recent observations. On the other hand, a mass cut sufficient to match the observed half number radius could only explain 60% of the metal poor population. In all cases, observations favour early redshifts for GC formation ($z\geq 15$).

SDSS-IV MaNGA: the formation sequence of S0 galaxies

Gas stripping of spiral galaxies or mergers are thought to be the formation mechanisms of lenticular galaxies. In order to determine the conditions in which each scenario dominates, we derive stellar populations of both the bulge and disk regions of 279 lenticular galaxies in the MaNGA survey. We find a clear bimodality in stellar age and metallicity within the population of S0s and this is strongly correlated with stellar mass. Old and metal-rich bulges and disks belong to massive galaxies, and young and metal-poor bulges and disks are hosted by low-mass galaxies. From this we conclude that the bulges and disks are co-evolving. When the bulge and disk stellar ages are compared, we find that the bulge is almost always older than the disk for massive galaxies ($\textrm{M}_{\star} > 10^{10}~\textrm{M}_{\odot}$). The opposite is true for lower mass galaxies. We conclude that we see two separate populations of lenticular galaxies. The old, massive, and metal-rich population possess bulges that are predominantly older than their disks, which we speculate may have been caused by morphological or inside-out quenching. In contrast, the less massive and more metal-poor population have bulges with more recent star formation than their disks. We postulate they may be undergoing bulge rejuvenation (or disk fading), or compaction. Environment doesn't play a distinct role in the properties of either population. Our findings give weight to the notion that while the faded spiral scenario likely formed low-mass S0s, other processes, such as mergers, may be responsible for high-mass S0s.

Discovery of a thin stellar stream in the SLAMS survey

We report the discovery of a thin stellar stream - which we name the Jet stream - crossing the constellations of Hydra and Pyxis. The discovery was made in data from the SLAMS survey, which comprises deep $g$ and $r$ imaging for a $650$ square degree region above the Galactic disc performed by the CTIO Blanco + DECam. SLAMS photometric catalogues will be made publicly available. The stream is approximately 0.18 degrees wide and 10 degrees long, though it is truncated by the survey footprint. Its colour-magnitude diagram is consistent with an old, metal-poor stellar population at a heliocentric distance of approximately 29 kpc. We corroborate this measurement by identifying a spatially coincident overdensity of likely blue horizontal branch stars at the same distance. There is no obvious candidate for a surviving stream progenitor.

The VVV Survey RR Lyrae Population in the Galactic Center Region*

Abstract Deep near-IR images from the VISTA Variables in the Vía Láctea (VVV) Survey were used to search for RR Lyrae stars within 100 arcmin from the Galactic Center. A large sample of 960 RR Lyrae of type ab (RRab) stars were discovered. A catalog is presented featuring the positions, magnitudes, colors, periods, and amplitudes for the sample, in addition to estimated reddenings, distances, and metallicities, and measured individual relative proper motions. We use the reddening-corrected Wesenheit magnitudes, defined as , in order to isolate bona fide RRL belonging to the Galaxy Center, finding that 30 RRab are foreground/background objects. We measure a range of extinctions from to 1.75 mag for the RRab in this region, finding that large extinction is the main cause of the sample incompleteness. The mean period is P = 0.5446 ± 0.0025 days, yielding a mean metallicity of [Fe/H] = −1.30 ± 0.01 (σ = 0.33) dex for the RRab sample in the Galactic Center region. The median distance for the sample is D = 8.05 ± 0.02 kpc. We measure the RRab surface density using the less reddened region sampled here, finding a density of 1000 RRab/sq deg at a projected Galactocentric distance R G = 1.6 deg. Under simple assumptions, this implies a large total mass (M &gt; 109 M ⊙) for the old and metal-poor population contained inside R G . We also measure accurate relative proper motions, from which we derive tangential velocity dispersions of σV l = 125.0 and σV b = 124.1 km s−1 along the Galactic longitude and latitude coordinates, respectively. The fact that these quantities are similar indicate that the bulk rotation of the RRab population is negligible, and implies that this population is supported by velocity dispersion. In summary, there are two main conclusions of this study. First, the population as a whole is no different from the outer bulge RRab, predominantly a metal-poor component that is shifted with respect to the Oosterhoff type I population defined by the globular clusters in the halo. Second, the RRab sample, as representative of the old and metal-poor stellar population in the region, has high velocity dispersions and zero rotation, suggesting a formation via dissipational collapse.

The disc origin of the Milky Way bulge

There is a long-standing debate over the origin of the metal-poor stellar populations of the Milky Way (MW) bulge, with the two leading scenarios being that these populations are either (i) part of a classical metal-poor spheroid or (ii) the same population as the chemically defined thick disc seen at the solar neighbourhood. Here we test whether the latter scenario can reproduce the observed chemical properties of the MW bulge. To do so we compare an N-body simulation of a composite (thin+thick) stellar disc – which evolves secularly to form a bar and a boxy/peanut (b/p) bulge – to data from APOGEE DR13. This model, in which the thick disc is massive and centrally concentrated, can reproduce the morphology of the metal-rich and metal-poor stellar populations in the bulge, as well as the mean metallicity and [α/Fe] maps as obtained from the APOGEE data. It also reproduces the trends, in both longitude and latitude, of the bulge metallicity distribution function (MDF). Additionally, we show that the model predicts small but measurable azimuthal metallicity variations in the inner disc due to the differential mapping of the thin and thick disc in the bar. We therefore see that the chemo-morphological relations of stellar populations in the MW bulge are naturally reproduced by mapping the thin and thick discs of the inner MW into a b/p.

Full-spectral fitting techniques to characterise the stellar content of ultra diffuse galaxies

AbstractUnderstanding the peculiar properties of Ultra Diffuse Galaxies (UDGs) via spectroscopic analysis is a challenging task that is now becoming feasible. The advent of 10m-class telescopes and high sensitivity instruments is enabling the gathering of high quality spectra even for the faintest systems. In addition, advances in the modelling of stellar populations, stellar libraries, and full-spectral fitting codes are allowing the recovery of the stellar content shaping those spectra with unprecedented reliability. In this contribution we report on the extensive tests we have carried out using the inversion code STECKMAP. The similarities between the Star Formation Histories (SFH) recovered from STECKMAP (applied to high-quality spectra) and deep Colour-Magnitude diagrams fitting (resolved stars) in two Local Group dwarf galaxies (LMC and LeoA) are remarkable, demonstrating the impressive performance of STECKMAP. We exploit the capabilities of STECKMAP and perform one of the most complete and reliable characterisations of the stellar component of UDGs to date using deep spectroscopic data. We measure radial and rotation velocities, SFHs and mean population parameters, such as ages and metallicities, for a sample of five UDG candidates in the Coma cluster. From the radial velocities, we confirm the Coma membership of these galaxies. We find that their rotation properties, if detected at all, are compatible with dwarf-like galaxies. The SFHs of the UDG are dominated by old (∼ 7 Gyr), metal-poor ([M/H] ∼ -1.1) and alpha-enhanced ([Mg/Fe]∼ 0.4) populations followed by a smooth or episodic decline which halted ∼ 2 Gyr ago, possibly a sign of cluster-induced quenching. We find no obvious correlation between individual SFH shapes and any UDG morphological properties. The recovered stellar properties for UDGs are similar to those found for DDO 44, a local UDG analogue resolved into stars. We conclude that the UDGs in our sample are extended dwarfs whose properties are likely the outcome of both internal processes, such as bursty SFHs and/or high-spin haloes, as well as environmental effects within the Coma cluster.

Radial elemental abundance gradients in galaxies from cosmological chemodynamical simulations

AbstractCosmological chemodynamical simulations are nowadays among the best tools to study how chemical elements are produced within galaxies, to reconstruct also the spatial distribution of the chemical elements as a function of time within different galaxy environments. Our simulation code includes the main stellar nucleosynthetic sources in the cosmos (core-collapse and Type Ia supernovae, hypernovae, asymptotic giant branch stars, and stellar winds from stars of all masses and metallicities). We present the predictions of our simulation for the evolution of the radial gradients of O/H, N/O and C/N in the gas-phase of a sample of ten star-forming disc galaxies, all characterised by very different star formation histories at the present time (see Figure 1.). On average, our simulated disc galaxies show a clear inside-out growth of the stellar mass as a function of time, and more negative slopes of the radial gas-phase O/H versus radius at earlier epochs of the galaxy evolution; we predict negative slopes of N/O and positive slopes of C/N at almost all redshifts, because of the main secondary origin of N in stars, even though the high-redshift simulation data are highly scattered because of the more turbulent conditions of the interstellar medium. Finally, we show that similar results are found with zoom-in simulations, where a spiral galaxy is re-simulated with a larger number of resolution elements. With zoom-in simulations, we study how stellar migrations (particularly old and metal-poor stellar populations migrating outwards) and radial gas flows are capable of influencing the galaxy chemical evolution at different galactic radii.

What the Milky Way bulge reveals about the initial metallicity gradients in the disc

AbstractWe examine the metallicity trends in the Milky Way (MW) bulge – using APOGEE DR13 data – and explore their origin by comparing two N-body models of isolated galaxies which develop a bar and a boxy/peanut (b/p) bulge. Both models have been proposed as scenarios for reconciling a disc origin of the MW bulge with a negative vertical metallicity gradient. The first is a superposition of co-spatial disc populations, different scaleheights and metallicities (with flat gradients) where the thick, metal-poor populations contribute significantly to the stellar mass budget in the inner galaxy. The second model is a single disc with an initial steep radial metallicity gradient which gets mapped by the bar into the b/p bulge in such a way that the vertical metallicity gradient of the MW bulge is reproduced – as shown already in previous works in the literature. As we show here, the latter model does not reproduce the positive longitudinal metallicity gradient of the inner disc, nor the metal-poor innermost regions seen in the data. The model with co-spatial thin and thick disc populations reproduces all the aforementioned trends. We therefore see that it is possible to reconcile a (primarily) disc origin for the MW bulge with the observed trends in metallicity by mapping the inner thin and thick discs of the MW into a b/p.

Stellar Streams Discovered in the Dark Energy Survey

Abstract We perform a search for stellar streams around the Milky Way using the first 3 yr of multiband optical imaging data from the Dark Energy Survey (DES). We use DES data covering ∼5000 deg2 to a depth of g &gt; 23.5 with a relative photometric calibration uncertainty of &lt;1%. This data set yields unprecedented sensitivity to the stellar density field in the southern celestial hemisphere, enabling the detection of faint stellar streams to a heliocentric distance of ∼50 kpc. We search for stellar streams using a matched filter in color–magnitude space derived from a synthetic isochrone of an old, metal-poor stellar population. Our detection technique recovers four previously known thin stellar streams: Phoenix, ATLAS, Tucana III, and a possible extension of Molonglo. In addition, we report the discovery of 11 new stellar streams. In general, the new streams detected by DES are fainter, more distant, and lower surface brightness than streams detected by similar techniques in previous photometric surveys. As a by-product of our stellar stream search, we find evidence for extratidal stellar structure associated with four globular clusters: NGC 288, NGC 1261, NGC 1851, and NGC 1904. The ever-growing sample of stellar streams will provide insight into the formation of the Galactic stellar halo, the Milky Way gravitational potential, and the large- and small-scale distribution of dark matter around the Milky Way.