Major Accretion Event Research Articles

Globular Cluster Intrinsic Iron Abundance Spreads. II. Protocluster Metallicities and the Age–Metallicity Relations of Milky Way Progenitors

Abstract Intrinsic iron abundance spreads in globular clusters (GCs), although usually small, are very common, and are signatures of self-enrichment: some stars within the cluster have been enriched by supernova ejecta from other stars within the same cluster. We use the Bailin self-enrichment model to predict the relationship between properties of the protocluster—its mass and the metallicity of the protocluster gas cloud—and the final observable properties today—its current metallicity and the internal iron abundance spread. We apply this model to an updated catalog of Milky Way GCs where the initial mass and/or the iron abundance spread is known to reconstruct their initial metallicities. We find that with the exception of the known anomalous bulge cluster Terzan 5 and three clusters strongly suspected to be nuclear star clusters from stripped dwarf galaxies, the model provides a good lens for understanding their iron spreads and initial metallicities. We then use these initial metallicities to construct age–metallicity relations for kinematically identified major accretion events in the Milky Way’s history. We find that using the initial metallicity instead of the current metallicity does not alter the overall picture of the Milky Way’s history because the difference is usually small but does provide information that can help distinguish which accretion event some individual GCs with ambiguous kinematics should be associated with and points to potential complexity within the accretion events themselves.

The AMBRE Project: Solar neighbourhood chemodynamical constraints on Galactic disc evolution

Context. The abundance of α-elements relative to iron ([α/Fe]) is an important fossil signature in Galactic archaeology for tracing the chemical evolution of disc stellar populations. High-precision chemical abundances, together with accurate stellar ages, distances, and dynamical data, are crucial to infer the Milky Way formation history. Aims. The aim of this paper is to analyse the chemodynamical properties of the Galactic disc using precise magnesium abundance estimates for solar neighbourhood stars with accurate Gaia astrometric measurements. Methods. We estimated ages and dynamical properties for 366 main sequence turn-off stars from the AMBRE Project using PARSEC isochrones together with astrometric and photometric values from Gaia DR2. We use precise global metallicities [M/H] and [Mg/Fe] abundances from a previous study in order to estimate gradients and temporal chemodynamic relations for these stars. Results. We find a radial gradient of −0.099 ± 0.031 dex kpc−1 for [M/H] and +0.023 ± 0.009 dex kpc−1 for the [Mg/Fe] abundance. The steeper [Mg/Fe] gradient than that found in the literature is a result of the improvement of the AMBRE [Mg/Fe] estimates in the metal-rich regime. In addition, we find a significant spread of stellar age at any given [Mg/Fe] value, and observe a clear correlated dispersion of the [Mg/Fe] abundance with metallicity at a given age. While for [M/H] ≤ − 0.2, a clear age–[Mg/Fe] trend is observed, more metal-rich stars display ages from 3 up to 12 Gyr, describing an almost flat trend in the [Mg/Fe]–age relation. Moreover, we report the presence of radially migrated and/or churned stars for a wide range of stellar ages, although we note the large uncertainties of the amplitude of the inferred change in orbital guiding radii. Finally, we observe the appearance of a second chemical sequence in the outer disc, 10–12 Gyr ago, populating the metal-poor, low-[Mg/Fe] tail. These stars are more metal-poor than the coexisting stellar population in the inner parts of the disc, and show lower [Mg/Fe] abundances than prior disc stars of the same metallicity, leading to a chemical discontinuity. Our data favour the rapid formation of an early disc that settled in the inner regions, followed by the accretion of external metal-poor gas –probably related to a major accretion event such as the Gaia-Enceladus/Sausage one– that may have triggered the formation of the thin disc population and steepened the abundance gradient in the early disc.

NGC 1261: An r-process enhanced globular cluster from the Gaia-Enceladus event

Our Milky Way (MW) has witnessed a series of major accretion events in the past. One of the later additions, the Gaia-Enceladus merger, has contributed a considerable mass to the inner Galaxy, but also generously donated to the outer halo. So far, associations with present-day MW globular clusters (GCs) have been chiefly based on their kinematics and ages. In this work, we present a chemical abundance study of the outer halo (RGC ∼ 18 kpc) GC NGC 1261, which has been suggested to be an accreted object based on its younger age. We measured 31 species of 29 elements in two stars from high-resolution Magellan/MIKE spectra and find that the cluster is moderately metal poor, at [Fe/H] = −1.26, with a low scatter of 0.02 dex. NGC 1261 is moderately α-enhanced to the 0.3 dex level. While from the small sample alone it is difficult to assert any abundance correlations, the light elements Na, O, Mg, and Al differ significantly between the two stars in contrast to the majority of other elements with smaller scatter; this argues in favor of multiple generations of stars coexisting in this GC. Intriguingly for its metallicity, NGC 1261 shows heavy element abundances that are consistent with r-process nucleosynthesis and we discuss their origin in various sites. In particular the Eu overabundance quantitatively suggests that one single r-process event, such as a neutron-star neutron-star merger or a rare kind of supernova, can be responsible for the stellar enhancement or even the enrichment of the cluster with the excess r material. Its heavy element pattern makes NGC 1261 resemble the moderately enhanced r-I stars that are commonly found in the halo and have been detected in Gaia-Enceladus as well. Therefore, combining all kinematical, age, and chemical evidence we conclude that NGC 1261 is a chemically intriguing GC that was born in the Gaia-Enceladus galaxy and has been subsequently accreted into the MW halo.

Heavy-elements heritage of the falling sky

Context. A fundamental element of galaxy formation is the accretion of mass through mergers of satellites or gas. Recent dynamical analyses based on Gaia data have revealed major accretion events in the history of the Milky Way. Nevertheless, our understanding of the primordial Galaxy is hindered because the bona fide identification of the most metal-poor and correspondingly oldest accreted stars remains challenging. Aims. Galactic archaeology needs a new accretion diagnostic to understand primordial stellar populations. Contrary to α-elements, neutron-capture elements present unexplained large abundance spreads for low-metallicity stars, which could result from a mixture of formation sites. Methods. We analysed the abundances of yttrium, europium, magnesium, and iron in Milky Way satellite galaxies, field halo stars, and globular clusters. The chemical information was complemented by orbital parameters based on Gaia data. In particular, we considered the average inclination of the orbits. Results. The [Y/Eu] abundance behaviour with respect to the [Mg/Fe] turnovers for satellite galaxies of various masses reveals that higher-luminosity systems, for which the [Mg/Fe] abundance declines at higher metallicities, present enhanced [Y/Eu] abundances, particularly in the [Fe/H] regime between −2.25 dex and −1.25 dex. In addition, the analysis has uncovered a chemo-dynamical correlation for both globular clusters and field stars of the Galactic halo, accounting for about half of the [Y/Eu] abundance spread. In particular, [Y/Eu] under-abundances typical of protracted chemical evolutions are preferentially observed in polar-like orbits, pointing to a possible anisotropy in the accretion processes. Conclusions. Our results strongly suggest that the observed [Y/Eu] abundance spread in the Milky Way halo could result from a mixture of systems with different masses. They also highlight that both nature and nurture are relevant to the formation of the Milky Way since its primordial epochs, thereby opening new pathways for chemical diagnostics of the build-up of our Galaxy.

Elevated r-process Enrichment in Gaia Sausage and Sequoia*

Abstract The Gaia Sausage (GS) and the Sequoia represent the major accretion events that formed the stellar halo of the Milky Way. A detailed chemical study of these main building blocks provides a pristine view of the early steps of the Galaxy’s assembly. We present the results of the analysis of the UVES high-resolution spectroscopic observations at the 8.2 m VLT of nine Sausage/Sequoia members selected kinematically using Gaia DR2. We season this set of measurements with archival data from Nissen & Schuster and GALAH DR3 (2020). Here, we focus on the neutron-capture process by analyzing Sr, Y, Ba, and Eu behavior. We detect clear enhancement in Eu abundance ([Eu/Fe] ∼ 0.6–0.7) indicative of large prevalence of the r-process in the stellar n-capture makeup. We are also able to trace the evolution of the heavy element production across a wide range of metallicity. The barium to europium changes from a tight, flat sequence with [Ba/Eu] = −0.7 reflecting dominant contribution from exploding massive stars, to a clear upturn at higher iron abundances, betraying the onset of contamination from asymptotic giant branch (AGB) ejecta. Additionally, we discover two clear sequences in the [Fe/H]−[Ba/Fe] plane likely caused by distinct levels of s-process pollution and mixing within the GS progenitor.

The H IX galaxy survey

Context. This paper presents the analysis of optical integral field spectra for the H I eXtreme (H IX) galaxy sample. H IX galaxies host at least 2.5 times more atomic gas (H I) than expected from their optical R-band luminosity. Previous examination of their star formation activity and H I kinematics suggested that these galaxies stabilise their large H I discs (radii up to 94 kpc) against star formation due to their higher than average baryonic specific angular momentum. A comparison to semi-analytic models further showed that the elevated baryonic specific angular momentum is inherited from the high spin of the dark matter host. Aims. In this paper we now turn to the gas-phase metallicity as well as stellar and ionised gas kinematics in H IX galaxies to gain insights into recent accretion of metal-poor gas or recent mergers. Methods. We compare the stellar, ionised, and atomic gas kinematics, and examine the variation in the gas-phase metallicity throughout the stellar disc of H IX galaxies. Results. We find no indication for counter-rotation in any of the components, the central metallicities tend to be lower than average, but as low as expected for galaxies of similar H I mass. Metallicity gradients are comparable to other less H I-rich, local star forming galaxies. Conclusions. We conclude that H IX galaxies show no conclusive evidence for recent major accretion or merger events. Their overall lower metallicities are likely due to being hosted by high spin halos, which slows down their evolution and thus the enrichment of their interstellar medium.

Uncovering the birth of the Milky Way through accurate stellar ages with Gaia

Knowledge of ages for stars formed over a galaxy's lifetime is fundamental to understand its formation and evolution. However, stellar ages are difficult to obtain since they cannot be measured from observations, being comparison with stellar models (Soderblom 2010) required. Alternatively, age distributions can be derived applying the robust technique of colour-magnitude diagram fitting (Gallart et al. 2005), till now mainly employed to study nearby galaxies. The new distances to individual Milky Way stars from the Gaia mission (Brown et al. 2018) have allowed us to use this technique to derive ages from a thick disk colour-magnitude diagram, and from the enigmatic, two-sequenced colour-magnitude diagram of the kinematically hot local halo (Babusiaux et al. 2018), which blue-sequence has been linked to a major accretion event (Haywood et al. 2018, Helmi et al. 2018). Because accurate ages were lacking, the time of the merger and its role on our Galaxy's early evolution remained unclear. We show that the stars in both halo sequences share identical age distributions, and are older than the bulk of thick disc stars. The sharp halo age cut 10 Gyr ago can be identified with the accretion of Gaia-Enceladus. Along with state-of-the-art cosmological simulations of galaxy formation (Brook et al. 2012), these robust ages allow us to order the early sequence of events that shaped our Galaxy, identifying the red-sequence as the first stars formed within the Milky Way progenitor which, because of their kinematics, can be described as its long sought in-situ halo.

Evidence for two early accretion events that built the Milky Way stellar halo

AbstractThe Gaia Sausage is the major accretion event that built the stellar halo of the Milky Way galaxy. Here, we provide dynamical and chemical evidence for a second substantial accretion episode, distinct from the Gaia Sausage. The Sequoia Event provided the bulk of the high-energy retrograde stars in the stellar halo, as well as the recently discovered globular cluster FSR 1758. There are up to six further globular clusters, including ω Centauri, as well as many of the retrograde substructures in Myeong et al., associated with the progenitor dwarf galaxy, named the Sequoia. The stellar mass in the Sequoia galaxy is ∼5 × 10 M⊙ , whilst the total mass is ∼1010 M⊙ , as judged from abundance matching or from the total sum of the globular cluster mass. Although clearly less massive than the Sausage, the Sequoia has a distinct chemodynamical signature. The strongly retrograde Sequoia stars have a typical eccentricity of ∼0.6, whereas the Sausage stars have no clear net rotation and move on predominantly radial orbits. On average, the Sequoia stars have lower metallicity by ∼0.3 dex and higher abundance ratios as compared to the Sausage. We conjecture that the Sausage and the Sequoia galaxies may have been associated and accreted at a comparable epoch.

Identifying Sagittarius Stream Stars by Their APOGEE Chemical Abundance Signatures

Abstract The SDSS-IV Apache Point Observatory Galactic Evolution Experiment (APOGEE) survey provides precise chemical abundances of 18 chemical elements for ∼176,000 red giant stars distributed over much of the Milky Way Galaxy (MW), and includes observations of the core of the Sagittarius dwarf spheroidal galaxy (Sgr). The APOGEE chemical abundance patterns of Sgr have revealed that it is chemically distinct from the MW in most chemical elements. We employ a k-means clustering algorithm to six-dimensional chemical space defined by [(C+N)/Fe], [O/Fe], [Mg/Fe], [Al/Fe], [Mn/Fe], and [Ni/Fe] to identify 62 MW stars in the APOGEE sample that have Sgr-like chemical abundances. Of the 62 stars, 35 have Gaia kinematics and positions consistent with those predicted by N-body simulations of the Sgr stream, and are likely stars that have been stripped from Sgr during the last two pericenter passages (<2 Gyr ago). Another 20 of the 62 stars exhibit chemical abundances indistinguishable from the Sgr stream stars, but are on highly eccentric orbits with median r apo ∼ 25 kpc. These stars are likely the “accreted” halo population thought to be the result of a separate merger with the MW 8–11 Gyr ago. We also find one hypervelocity star candidate. We conclude that Sgr was enriched to [Fe/H] ∼ −0.2 before its most recent pericenter passage. If the “accreted halo” population is from one major accretion event, then this progenitor galaxy was enriched to at least [Fe/H] ∼ −0.6, and had a similar star formation history to Sgr before merging.

The origin of accreted stellar halo populations in the Milky Way using APOGEE,Gaia, and the EAGLE simulations

Recent work indicates that the nearby Galactic halo is dominated by the debris from a major accretion event. We confirm that result from an analysis of APOGEE-DR14 element abundances and $\textit{Gaia}$-DR2 kinematics of halo stars. We show that $\sim$2/3 of nearby halo stars have high orbital eccentricities ($e \gtrsim 0.8$), and abundance patterns typical of massive Milky Way dwarf galaxy satellites today, characterised by relatively low [Fe/H], [Mg/Fe], [Al/Fe], and [Ni/Fe]. The trend followed by high $e$ stars in the [Mg/Fe]-[Fe/H] plane shows a change of slope at [Fe/H]$\sim-1.3$, which is also typical of stellar populations from relatively massive dwarf galaxies. Low $e$ stars exhibit no such change of slope within the observed [Fe/H] range and show slightly higher abundances of Mg, Al and Ni. Unlike their low $e$ counterparts, high $e$ stars show slightly retrograde motion, make higher vertical excursions and reach larger apocentre radii. By comparing the position in [Mg/Fe]-[Fe/H] space of high $e$ stars with those of accreted galaxies from the EAGLE suite of cosmological simulations we constrain the mass of the accreted satellite to be in the range $10^{8.5}\lesssim M_*\lesssim 10^{9}\mathrm{M_\odot}$. We show that the median orbital eccentricities of debris are largely unchanged since merger time, implying that this accretion event likely happened at $z\lesssim1.5$. The exact nature of the low $e$ population is unclear, but we hypothesise that it is a combination of $\textit{in situ}$ star formation, high $|z|$ disc stars, lower mass accretion events, and contamination by the low $e$ tail of the high $e$ population. Finally, our results imply that the accretion history of the Milky Way was quite unusual.

Project AMIGA: Distance and Metallicity Gradients along Andromeda’s Giant Southern Stream from the Red Clump∗

Abstract The Giant Southern Stream (GSS) of M31, a keystone signature of a major accretion event, yields crucial constraints on M31 formation and evolution models. Currently, our understanding of the GSS, in terms of both its geometry and its chemistry, results from either wide-field imaging probing only a few magnitudes below the red giant branch tip, or deep imaging or spectroscopy of isolated regions. Here, we take an alternative approach, using Hubble Space Telescope (HST) imaging to characterize the horizontal branch red clump (RC) using unbinned maximum likelihood fits to luminosity functions from observed color–magnitude diagrams. Comparing the RC mean magnitude across three fields at projected distances of 21, 52, and 80 kpc from M31, we find a line-of-sight distance gradient identical to recent literature measurements in fields along the core. We also find tentative evidence that the line-of-sight distance dispersion increases with projected distance from M31. Meanwhile, the metallicity in the 52 kpc field westward of the GSS core is at least as high as that in the 21 kpc GSS core field, and the peak colors of the RC in these two fields imply identical metallicities to within 0.2 dex. We discuss implications for distance and metallicity gradients both along and perpendicular to the GSS in the context of recent ground-based photometric and spectroscopic results, including evidence for a dropoff in metallicity moving westward from the GSS, as well as prospects for further constraining stellar populations in the vicinity of the GSS.

The influence of Sagittarius and the Large Magellanic Cloud on the stellar disc of the Milky Way Galaxy

We present N-body simulations of a Sagittarius-like dwarf spheroidal galaxy (Sgr) that follow its orbit about the Milky Way (MW) since its first crossing of the Galaxy's virial radius to the present day. As Sgr orbits around the MW, it excites vertical oscillations, corrugating and flaring the Galactic stellar disc. These responses can be understood by a two-phase picture in which the interaction is first dominated by torques from the wake excited by Sgr in the MW dark halo before transitioning to tides from Sgr's direct impact on the disc at late times. We show for the first time that a massive Sgr model simultaneously reproduces the locations and motions of arc-like over densities, such as the Monoceros Ring and the Triangulum Andromeda stellar clouds, that have been observed at the extremities of the disc, while also satisfying the solar neighbourhood constraints on the vertical structure and streaming motions of the disc. In additional simulations, we include the Large Magellanic Cloud (LMC) self consistently with Sgr. The LMC introduces coupling through constructive and destructive interference, but no new corrugations. In our models, the excitation of the current structure of the outer disk can be traced to interactions as far back as 6-7 Gyr ago (corresponding to $z\leq1$). Given the apparently quiescent accretion history of the MW over this timescale, this places Sgr as the main culprit behind the vertical oscillations of the disc and the last major accretion event for the Galaxy with the capacity to modulate its chemodynamical structure.

Co-formation of the disc and the stellar halo★

Using a large sample of Main Sequence stars with 7-D measurements supplied by Gaia and SDSS, we study the kinematic properties of the local (within ~10 kpc from the Sun) stellar halo. We demonstrate that the halo's velocity ellipsoid evolves strongly with metallicity. At the low [Fe/H] end, the orbital anisotropy (the amount of motion in the radial direction compared to the tangential one) is mildly radial with 0.2<beta<0.4. However, for stars with [Fe/H]>-1.7 we measure extreme values of beta~0.9. Across the metallicity range considered, i.e. -3<[Fe/H]<-1, the stellar halo's spin is minimal, at the level of 20<v_theta (km/s) <30. Using a suite of cosmological zoom-in simulations of halo formation, we deduce that the observed acute anisotropy is inconsistent with the continuous accretion of dwarf satellites. Instead, we argue, the stellar debris in the inner halo were deposited in a major accretion event by a satellite with Mvir>10^10 Msun around the epoch of the Galactic disc formation, i.e. between 8 and 11 Gyr ago. The radical halo anisotropy is the result of the dramatic radialisation of the massive progenitor's orbit, amplified by the action of the growing disc.

3C 57 as an atypical radio-loud quasar: implications for the radio-loud/radio-quiet dichotomy

Lobe-dominated radio-loud (LD RL) quasars occupy a restricted domain in the 4D Eigenvector 1 (4DE1) parameter space which implies restricted geometry/physics/kinematics for this subclass compared to the radio-quiet (RQ) majority of quasars. We discuss how this restricted domain for the LD RL parent population supports the notion for a RQ-RL dichotomy among Type 1 sources. 3C 57 is an atypical RL quasar that shows both uncertain radio morphology and falls in a region of 4DE1 space where RL quasars are rare. We present new radio flux and optical spectroscopic measures designed to verify its atypical optical/UV spectroscopic behaviour and clarify its radio structure. The former data confirms that 3C 57 falls off the 4DE1 quasar "main sequence" with both extreme optical FeII emission (R_{FeII} ~ 1) and a large CIV 1549 profile blueshift (~ -1500 km/s). These parameter values are typical of extreme Population A sources which are almost always RQ. New radio measures show no evidence for flux change over a 50+ year timescale consistent with compact steep-spectrum (CSS or young LD) over core-dominated morphology. In the 4DE1 context where LD RL are usually low L/L_{Edd} quasars we suggest that 3C 57 is an evolved RL quasar (i.e. large Black Hole mass) undergoing a major accretion event leading to a rejuvenation reflected by strong FeII emission, perhaps indicating significant heavy metal enrichment, high bolometric luminosity for a low redshift source and resultant unusually high Eddington ratio giving rise to the atypical CIV 1549.

THE METAL-POOR KNEE IN THE FORNAX DWARF SPHEROIDAL GALAXY

We present alpha-element abundances of Mg, Si, and Ti for a large sample of field stars in two outer fields of the Fornax dwarf spheroidal galaxy (dSph), obtained with VLT/GIRAFFE (R~16,000). Due to the large fraction of metal-poor stars in our sample, we are able to follow the alpha-element evolution from [Fe/H]=-2.5 continuously to [Fe/H]=-0.7 dex. For the first time we are able to resolve the turnover from the Type II supernovae (SNe) dominated, alpha-enhanced plateau down to subsolar [alpha/Fe] values due to the onset of SNe Ia, and thus to trace the chemical enrichment efficiency of the galaxy. Our data support the general concept of an alpha-enhanced plateau at early epochs, followed by a well-defined "knee", caused by the onset of SNe Ia, and finally a second plateau with sub-solar [alpha/Fe] values. We find the position of this knee to be at [Fe/H]=-1.9 and therefore significantly more metal-poor than expected from comparison with other dSphs and standard evolutionary models. Surprisingly, this value is rather comparable to the knee in Sculptor, a dSph about 10 times less luminous than Fornax. Using chemical evolution models, we find that both the position of the knee as well as the subsequent plateau at sub-solar level can hardly be explained unless the galaxy experienced several discrete star formation events with a drastic variation in star formation efficiency, while a uniform star formation can be ruled out. One possible evolutionary scenario is that Fornax experienced one or several major accretion events from gas-rich systems in the past, so that its current stellar mass is not indicative of the chemical evolution environment at ancient times. If Fornax is the product of several smaller building blocks, this may also have implications of the understanding on the formation process of dSphs in general.

Hoag’s Object: evidence for cold accretion on to an elliptical galaxy

(Abridged) We present new photometric and spectroscopic observations of the famous Hoag's Object, a peculiar ring galaxy with a central roundish core. The nature of Hoag's Object is still under controversial discussion. Previous studies demonstrated that a major accretion event that took place at least 2-3 Gyr ago can account for the observational evidence. However, the role of internal nonlinear mechanisms in forming the outer ring was not yet completely ruled out. These new data, together with HI and optical information from the literature, are used to demonstrate that Hoag's Object is a relatively isolated system surrounded by a luminous quasi-spiral pattern and a massive, low-density HI disc. The main stellar body is an old, mildly triaxial elliptical galaxy with very high angular momentum. We review previous formation scenarios of Hoag's Object in light of the new data and conclude that the peculiar morphology could not represent a late phase in barred early-type galaxies evolution. In addition, no observational evidence supports late merging events in the evolution of the galaxy, although further tests are required before safely dismissing this idea. We propose a new scenario where the elliptical core formed in the early Universe with the HI disc forming shortly after the core by prolonged "cold" accretion of primordial gas from the intergalactic medium. The low gas density does not allow intense star formation to occur everywhere in the disc, but only along a tightly wound spiral pattern of enhanced density induced by the triaxial gravitational potential.

V1647 Orionis: The X-Ray Evolution of a Pre-Main-Sequence Accretion Burst

We present Chandra X-ray Observatory monitoring observations of the recent accretion outburst displayed by the pre-main sequence (pre-MS) star V1647 Ori. The X-ray observations were obtained over a period beginning prior to outburst onset in late 2003 and continuing through its apparent cessation in late 2005, and demonstrate that the mean flux of the spatially coincident X-ray source closely tracked the near-infrared luminosity of V1647 Ori throughout its eruption. We find negligible likelihood that the correspondence between X-ray and infrared light curves over this period was the result of multiple X-ray flares unrelated to the accretion burst. The recent Chandra data confirm that the X-ray spectrum of V1647 Ori hardened during outburst, relative both to its pre-outburst state and to the X-ray spectra of nearby pre-MS stars in the L1630 cloud. We conclude that the observed changes in the X-ray emission from V1647 Ori over the course of its 2003-2005 eruption were generated by a sudden increase and subsequent decline in its accretion rate. These results for V1647 Ori indicate that the flux of hard X-ray emission from erupting low-mass, pre-MS stars, and the duration and intensity of such eruptions, reflect the degree to which star-disk magnetic fields are reorganized before and during major accretion events.

Fundamental parameters of six neglected old open clusters

In this paper, we present the first BV I CCD photometry of six overlooked old open clusters (Berkeley 44, NGC 6827, Berkeley 52, Berkeley 56, Skiff 1 and Berkeley 5) and derive estimates of their fundamental parameters by using isochrones from the Padova library. We found that all the clusters are older than the Hyades, with ages ranging from 0.8 Gyr (NGC 6827 and Berkeley 5) to 4.0 Gyr (Berkeley 56). The latter is one of the old open clusters with the largest heliocentric distance. In the field of Skiff 1, we recognize a faint blue main sequence identical to the one found in the background of open clusters in the second and third Galactic quadrant, and routinely attributed to the Canis Major accretion event. We use the synthetic colour–magnitude diagram method and a Galactic model to show that this population can be easily interpreted as a thick disc and halo population towards Skiff 1. We finally revise the age distribution of the old open clusters, showing that the previously suggested peak at 5 Gyr looses importance as additional old clusters are discovered.

Optical Imaging and Spectroscopy of the Edge-on Sbc Galaxy UGC 10043: Evidence for a Galactic Wind and a Peculiar Triaxial Bulge

We present new optical imaging and spectroscopy of the peculiar, edge-on Sbc galaxy UGC 10043. Using the WIYN telescope, we have obtained B, R, and Hα + [N ] images, together with DensePak integral field spectroscopic measurements of the stellar Ca II infrared triplet and the Hα and [N II] lines from the ionized gas. The imaging observations show that the inner bulge of UGC 10043 (a ≤ 75) is elongated perpendicular to the galaxy major axis. At larger r the bulge isophotes twist to become oblate and nearly circular, suggesting the bulge is triaxial. The bulge shows no clear evidence for rotation about either its major or minor axis. The inner, southwestern quadrant of the bulge is girdled by a narrow dust lane parallel to the minor axis; unsharp masking reveals that this minor-axis dust lane may be part of an inner polar ring, although we find no unambiguous kinematic evidence of orthogonally rotating material. The stellar disk of UGC 10043 has a rather low optical surface brightness [μ(0)R,i ~ 23.2 mag arcsec-2], a small scale height (hz = 395 pc for D = 33.4 Mpc), and a mild integral sign warp. A dusty, inner disk component that appears tilted relative to the outlying disk is also seen. The Hα and [N II] emission lines in UGC 10043 resolve into multiple velocity components, indicating the presence of a large-scale galactic wind with an outflow velocity of Vout 104 km s-1. Hα + [N ] imaging reaffirms this picture by revealing ionized gas extended to |z| ~ 3.5 kpc in the form of a roughly biconical structure. The [N II]/Hα line intensity ratio increases with increasing distance from the plane, reaching values as high as 1.7. Unlike most galaxies with large-scale winds, UGC 10043 has only a modest global star formation rate (1 M⊙ yr-1), implying the wind is powered by a rather feeble central starburst. We discuss evolutionary scenarios that could account for both the structural complexities of UGC 10043 and its large-scale wind. The most plausible scenarios require a major accretion or merger event at least a few gigayears ago.

Mapping the Inner Halo of the Galaxy with 2MASS-Selected Horizontal-Branch Candidates

We use 2MASS photometry to select blue horizontal branch (BHB) candidates covering the sky |b|>15 deg. A 12.5<J<15.5 sample of BHB stars traces the thick disk and inner halo to d<9 kpc, with a density comparable to that of M giant stars. We base our sample selection strategy on the Century Survey Galactic Halo Project, a survey that provides a complete, spectroscopically-identified sample of blue stars to a similar depth as the 2MASS catalog. We show that a -0.20<(J-H)_0<0.10, -0.10<(H-K)_0<0.10 color-selected sample of stars is 65% complete for BHB stars, and is composed of 47% BHB stars. We apply this photometric selection to the full 2MASS catalog, and see no spatial overdensities of BHB candidates at high Galactic latitude |b|>50 deg. We insert simulated star streams into the data and conclude that the high Galactic latitude BHB candidates are consistent with having no ~5 deg wide star stream with density greater than 0.33 objects deg^-2 at the 95% confidence level. The absence of structure suggests there have been no major accretion events in the inner halo in the last few Gyr. However, at low Galactic latitudes a two-point angular correlation analysis reveals structure on angular scales <1 deg. This structure is apparently associated with stars in the thick disk, and has a physical scale of 10-100 pc. Interestingly, such structures are expected by cosmological simulations that predict the majority of the thick disk may arise from accretion and disruption of satellite mergers.