arXiv subjects

Nelson Caldwell

Publications and source records attributed to Nelson Caldwell.

At least 55 records · Page 3Linked to original sources

arXiv2020

Orbital Clustering Identifies the Origins of Galactic Stellar Streams

The origins of most stellar streams in the Milky Way are unknown. With improved proper motions provided by Gaia EDR3, we show that the orbits of 23 Galactic stellar streams are highly clustered in orbital phase space. Based on their energies and angular momenta, most streams in our sample can plausibly be associated with a specific (disrupted) dwarf galaxy host that brought them into the Milky Way. For eight streams we also identify likely globular cluster progenitors (four of these associations are reported here for the first time). Some of these stream progenitors are surprisingly far apart, displaced from their tidal debris by a few to tens of degrees. We identify stellar streams that appear spatially distinct, but whose similar orbits indicate they likely originate from the same progenitor. If confirmed as physical discontinuities, they will provide strong constraints on the mass-loss from the progenitor. The nearly universal ex-situ origin of existing stellar streams makes them valuable tracers of galaxy mergers and dynamical friction within the Galactic halo. Their phase-space clustering can be leveraged to construct a precise global map of dark matter in the Milky Way, while their internal structure may hold clues to the small-scale structure of dark matter in their original host galaxies.

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arXiv2020

Ancient Very Metal-Poor Stars Associated With the Galactic Disk in the H3 Survey

Ancient, very metal-poor stars offer a window into the earliest epochs of galaxy formation and assembly. We combine data from the H3 Spectroscopic Survey and Gaia to measure metallicities, abundances of $α$ elements, stellar ages, and orbital properties of a sample of 482 very metal-poor (VMP; [Fe/H]$<-2$) stars in order to constrain their origins. This sample is confined to $1\lesssim |Z| \lesssim3$ kpc from the Galactic plane. We find that >70% of VMP stars near the disk are on prograde orbits and this fraction increases toward lower metallicities. This result unexpected if metal-poor stars are predominantly accreted from many small systems with no preferred orientation, as such a scenario would imply a mostly isotropic distribution. Furthermore, we find there is some evidence for higher fractions of prograde orbits amongst stars with lower [$α$/Fe]. Isochrone-based ages for main sequence turn-off stars reveal that these VMP stars are uniformly old ($\approx12$ Gyr) irrespective of the $α$ abundance and metallicity, suggesting that the metal-poor population was not born from the same well-mixed gas disk. We speculate that the VMP population has a heterogeneous origin, including both in-situ formation in the ancient disk and accretion from a satellite with the same direction of rotation as the ancient disk at early times. Our precisely measured ages for these VMP stars on prograde orbits show that the Galaxy has had a relatively quiescent merging history over most of cosmic time, and implies the angular momentum alignment of the Galaxy has been in place for at least 12 Gyr.

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arXiv2020

Discovery of Magellanic Stellar Debris in the H3 Survey

We report the discovery of 15 stars in the H3 survey that lie, in projection, near the tip of the trailing gaseous Magellanic Stream (MS). The stars have Galactocentric velocities $< -155$ km s$^{-1}$, Galactocentric distances of $\approx 40$ to 80 kpc (increasing along the MS), and [Fe/H] consistent with that of stars in the Small Magellanic Cloud. These 15 stars comprise 94% (15 of 16) of the H3 observed stars to date that have $R_{GAL} > 37.5$ kpc, $-$350 km s$^{-1} < V_{GSR} < -155$ km s$^{-1}$, and are not associated with the Sagittarius Stream. They represent a unique portion of the Milky Way's outer halo phase space distribution function and confirm that unrelaxed structure is detectable even at radii where H3 includes only a few hundred stars. Due to their statistical excess, their close association with the MS and H I compact clouds in the same region, both in position and velocity space, and their plausible correspondence with tidal debris in a published simulation, we identify these stars as debris of past Magellanic Cloud encounters. These stars are evidence for a stellar component of the tidal debris field far from the Clouds themselves and provide unique constraints on the interaction.

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arXiv2020

PHAT XX. AGB stars and other cool giants in M31 star clusters

The presence of AGB stars in clusters provides key constraints for stellar models, as has been demonstrated with historical data from the Magellanic Clouds. In this work, we look for candidate AGB stars in M31 star clusters from the Panchromatic Hubble Andromeda Treasury (PHAT) survey. Our photometric criteria selects stars brighter than the tip of the red giant branch, which includes the bulk of the thermally-pulsing AGB stars as well as early-AGB stars and other luminous cool giants expected in young stellar populations (e.g. massive red supergiants, and intermediate-mass red helium-burning stars). The AGB stars can be differentiated, a posteriori, using the ages already estimated for our cluster sample. 937 candidates are found within the cluster aperture radii, half (450) of which are very likely cluster members. Cross-matching with additional databases reveals two carbon stars and ten secure variables among them. The field-corrected age distribution reveals the presence of young supergiants peaking at ages smaller than 100 Myr, followed by a long tail of AGB stars extending up to the oldest possible ages. This long tail reveals the general decrease in the numbers of AGB stars from initial values of 50e-6/Msun at 100 Myr down to 5e-6/Msun at 10 Gyr. Theoretical models of near-solar metallicity reproduce this general trend, although with localized discrepancies over some age intervals, whose origin is not yet identified. The entire catalogue is released together with finding charts to facilitate follow-up studies.

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arXiv2020

Evidence from the H3 Survey that the Stellar Halo is Entirely Comprised of Substructure

In the $Λ$CDM paradigm the Galactic stellar halo is predicted to harbor the accreted debris of smaller systems. To identify these systems, the H3 Spectroscopic Survey, combined with $Gaia$, is gathering 6D phase-space and chemical information in the distant Galaxy. Here we present a comprehensive inventory of structure within 50 kpc from the Galactic center using a sample of 5684 giants at $|b|>40^{\circ}$ and $|Z|>2$ kpc. We identify known structures including the high-$α$ disk, the in-situ halo (disk stars heated to eccentric orbits), Sagittarius (Sgr), $Gaia$-Sausage-Enceladus (GSE), the Helmi Streams, Sequoia, and Thamnos. Additionally, we identify the following new structures: (i) Aleph ([Fe/H]$=-0.5$), a low eccentricity structure that rises a surprising 10 kpc off the plane, (ii, iii) Arjuna ([Fe/H]$=-1.2$) and I'itoi ([Fe/H]$<-2$), which comprise the high-energy retrograde halo along with Sequoia, and (iv) Wukong ([Fe/H]$=-1.6$), a prograde phase-space overdensity chemically distinct from GSE. For each structure we provide [Fe/H], [$α$/Fe], and orbital parameters. Stars born within the Galaxy are a major component at $|Z|\sim$2 kpc ($\approx$60$\%$), but their relative fraction declines sharply to $\lesssim$5$\%$ past 15 kpc. Beyond 15 kpc, $>$80$\%$ of the halo is built by two massive ($M_{\star}\sim10^{8}-10^{9}M_{\odot}$) accreted dwarfs: GSE ([Fe/H]$=-1.2$) within 25 kpc, and Sgr ([Fe/H]$=-1.0$) beyond 25 kpc. This explains the relatively high overall metallicity of the halo ([Fe/H]$\approx-1.2$). We attribute $\gtrsim$95$\%$ of the sample to one of the listed structures, pointing to a halo built entirely from accreted dwarfs and heating of the disk.

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arXiv2020

Timing the Early Assembly of the Milky Way with the H3 Survey

The archaeological record of stars in the Milky Way opens a uniquely detailed window into the early formation and assembly of galaxies. Here we use 11,000 main-sequence turn-off stars with well-measured ages, [Fe/H], [$α$/Fe], and orbits from the H3 Survey and Gaia to time the major events in the early Galaxy. Located beyond the Galactic plane, $1\lesssim |Z|/\rm kpc \lesssim4$, this sample contains three chemically distinct groups: a low metallicity population, and low-$α$ and high-$α$ groups at higher metallicity. The age and orbit distributions of these populations show that: 1) the high-$α$ group, which includes both disk stars and the in-situ halo, has a star-formation history independent of eccentricity that abruptly truncated $8.3\pm0.1$ Gyr ago ($z\simeq1$); 2) the low metallicity population, which we identify as the accreted stellar halo, is on eccentric orbits and its star formation truncated $10.2.^{+0.2}_{-0.1}$ Gyr ago ($z\simeq2$); 3) the low-$α$ population is primarily on low eccentricity orbits and the bulk of its stars formed less than 8 Gyr ago. These results suggest a scenario in which the Milky Way accreted a satellite galaxy at $z\approx2$ that merged with the early disk by $z\approx1$. This merger truncated star formation in the early high-$α$ disk and perturbed a fraction of that disk onto halo-like orbits. The merger enabled the formation of a chemically distinct, low-$α$ disk at $z\lesssim1$. The lack of any stars on halo-like orbits at younger ages indicates that this event was the last significant disturbance to the Milky Way disk.

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arXiv2020

High-resolution spectroscopy of the GD-1 stellar stream localizes the perturber near the orbital plane of Sagittarius

The $100^\circ$-long thin stellar stream in the Milky Way halo, GD-1, has an ensemble of features that may be due to dynamical interactions. Using high-resolution MMT/Hectochelle spectroscopy we show that a spur of GD-1-like stars outside of the main stream are kinematically and chemically consistent with the main stream. In the spur, as in the main stream, GD-1 has a low intrinsic radial velocity dispersion, $σ_{V_r}\lesssim1\,\rm km\,s^{-1}$, is metal-poor, $\rm [Fe/H]\approx-2.3$, with little $\rm [Fe/H]$ spread and some variation in $\rm [α/Fe]$ abundances, which point to a common globular cluster progenitor. At a fixed location along the stream, the median radial velocity offset between the spur and the main stream is smaller than $0.5\,\rm km\,s^{-1}$, comparable to the measurement uncertainty. A flyby of a massive, compact object can change orbits of stars in a stellar stream and produce features like the spur observed in GD-1. In this scenario, the radial velocity of the GD-1 spur relative to the stream constrains the orbit of the perturber and its current on-sky position to $\approx5,000\,\rm deg^2$. The family of acceptable perturber orbits overlaps the stellar and dark-matter debris of the Sagittarius dwarf galaxy in present-day position and velocity. This suggests that GD-1 may have been perturbed by a globular cluster or an extremely compact dark-matter subhalo formerly associated with Sagittarius.

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arXiv2020

A Gaia-based catalog of candidate stripped nuclei and luminous globular clusters in the halo of Centaurus A

Tidally stripped galaxy nuclei and luminous globular clusters (GCs) are important tracers of the halos and assembly histories of nearby galaxies, but are difficult to reliably identify with typical ground-based imaging data. In this paper we present a new method to find these massive star clusters using Gaia DR2, focusing on the massive elliptical galaxy Centaurus A (Cen A). We show that stripped nuclei and globular clusters are partially resolved by Gaia at the distance of Cen A, showing characteristic astrometric and photometric signatures. We use this selection method to produce a list of 632 new candidate luminous clusters in the halo of Cen A out to a projected radius of 150 kpc. Adding in broadband photometry and visual examination improves the accuracy of our classification. In a spectroscopic pilot program we have confirmed 5 new luminous clusters, which includes the 7th and 10th most luminous GC in Cen\,A. Three of the newly discovered GCs are further away from Cen A in than all previously known GCs. Several of these are compelling candidates for stripped nuclei. We show that our novel Gaia selection method retains at least partial utility out to distances of 25 Mpc and hence is a powerful tool for finding and studying star clusters in the sparse outskirts of galaxies in the local universe.

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arXiv2019

The survey of Planetary Nebulae in Andromeda (M31) II. Age-velocity dispersion relation in the disc from Planetary Nebulae

The age-velocity dispersion relation is an important tool to understand the evolution of the disc of the Andromeda galaxy (M31) in comparison with the Milky Way. We use Planetary Nebulae (PNe) to obtain the age-velocity dispersion relation in different radial bins of the M31 disc. We separate the observed PNe sample based on their extinction values into two distinct age populations. The observed velocities of our high- and low-extinction PNe, which correspond to higher and lower mass progenitors respectively, are fitted in de-projected elliptical bins to obtain their rotational velocities, $V_ϕ$, and corresponding dispersions, $\rmσ_ϕ$. We assign ages to the two PNe populations by comparing central-star properties of an archival sub-sample of PNe, having models fitted to their observed spectral features, to stellar evolution tracks. For the high- and low-extinction PNe, we find ages of $\sim2.5$ Gyr and $\sim4.5$ Gyr respectively, with distinct kinematics beyond a deprojected radius R$\rm_{GC}= 14$ kpc. At R$\rm_{GC}$=17--20 kpc, which is the equivalent distance in disc scale lengths of the Sun in the Milky Way disc, we obtain $\rmσ_{ϕ,~2.5~Gyr}= 61\pm 14$ km s$^{-1}$ and $\rmσ_{ϕ,~4.5~Gyr}= 101\pm 13$ km s$^{-1}$. The age-velocity dispersion relation for the M31 disc is obtained in two radial bins, R$\rm_{GC}$=14--17 and 17--20 kpc. The high- and low-extinction PNe are associated with the young thin and old thicker disc of M31 respectively, whose velocity dispersion values increase with age. These values are almost twice and thrice that of the Milky Way disc stellar population of corresponding ages. From comparison with simulations of merging galaxies, we find that the age-velocity dispersion relation in the M31 disc measured using PNe is indicative of a single major merger that occurred 2.5 -- 4.5 Gyr ago with an estimated merger mass ratio $\approx$ 1:5.

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arXiv2019

Resolving the Metallicity Distribution of the Stellar Halo with the H3 Survey

The Galactic stellar halo is predicted to have formed at least partially from the tidal disruption of accreted dwarf galaxies. This assembly history should be detectable in the orbital and chemical properties of stars. The H3 Survey is obtaining spectra for 200,000 stars, and, when combined with Gaia data, is providing detailed orbital and chemical properties of Galactic halo stars. Unlike previous surveys of the halo, the H3 target selection is based solely on magnitude and Gaia parallax; the survey therefore provides a nearly unbiased view of the entire stellar halo at high latitudes. In this paper we present the distribution of stellar metallicities as a function of Galactocentric distance and orbital properties for a sample of 4232 kinematically-selected halo giants to 100 kpc. The stellar halo is relatively metal-rich, [Fe/H]=-1.2, and there is no discernable metallicity gradient over the range $6 30$ kpc, respectively. Metal-poor stars with [Fe/H]$<-2$ are a small population of the halo at all distances and orbital categories. We associate the "in-situ" stellar halo with stars displaying thick-disk chemistry on halo-like orbits; such stars are confined to $|z|<10$ kpc. The majority of the stellar halo is resolved into discrete features in orbital-chemical space, suggesting that the bulk of the stellar halo formed from the accretion and tidal disruption of dwarf galaxies. (ABRIDGED)

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arXiv2019

A Lower Limit on the Mass of Our Galaxy from the H3 Survey

The timing argument provides a lower limit on the mass of the Milky Way. We find, using a sample of 32 stars at $R > 60$ kpc drawn from the H3 Spectroscopic Survey and mock catalogs created from published numerical simulations, that M$_{200} > 0.91\times 10^{12}$ M$_\odot$ with 90% confidence. We recommend using this limit to refine the allowed prior mass range in more complex and sophisticated statistical treatments of Milky Way dynamics. The use of such a prior would have significantly reduced many previously published uncertainty ranges. Our analysis suggests that the most likely value of M$_{200}$ is $\sim 1.4 \times 10^{12}$ M$_\odot$, but establishing this as the Milky Way mass requires a larger sample of outer halo stars and a more complete analysis of the inner halo stars in H3. The imminent growth in the sample of outer halo stars due to ongoing and planned surveys will make this possible.

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arXiv2019

Mapping the Stellar Halo with the H3 Spectroscopic Survey

Modern theories of galaxy formation predict that the Galactic stellar halo was hierarchically assembled from the accretion and disruption of smaller systems. This hierarchical assembly is expected to produce a high degree of structure in the combined phase and chemistry space; this structure should provide a relatively direct probe of the accretion history of our Galaxy. Revealing this structure requires precise 3D positions (including distances), 3D velocities, and chemistry for large samples of stars. The Gaia satellite is delivering proper motions and parallaxes for >1 billion stars to G~20. However, radial velocities and metallicities will only be available to G~15, which is insufficient to probe the outer stellar halo (>10 kpc). Moreover, parallaxes will not be precise enough to deliver high-quality distances for stars beyond ~10 kpc. Identifying accreted systems throughout the stellar halo therefore requires a large ground-based spectroscopic survey to complement Gaia. Here we provide an overview of the H3 Stellar Spectroscopic Survey, which will deliver precise stellar parameters and spectrophotometric distances for 200,000 stars to r=18. Spectra are obtained with the Hectochelle instrument at the MMT, which is configured for the H3 Survey to deliver resolution R~23,000 spectra covering the wavelength range 5150A-5300A. The survey is optimized for stellar halo science and therefore focuses on high Galactic latitude fields (|b|>30 deg.), sparsely sampling 15,000 sq. degrees. Targets are selected on the basis of Gaia parallaxes, enabling very efficient selection of bone fide halo stars. The survey began in the Fall of 2017 and has collected 88,000 spectra to-date. All of the data, including the derived stellar parameters, will eventually be made publicly available via the survey website: h3survey.rc.fas.harvard.edu.

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arXiv2019

Signatures of Tidal Disruption in Ultra-Faint Dwarf Galaxies: A Combined HST, Gaia, and MMT/Hectochelle Study of Leo V

The ultra-faint dwarf galaxy Leo V has shown both photometric overdensities and kinematic members at large radii, along with a tentative kinematic gradient, suggesting that it may have undergone a close encounter with the Milky Way. We investigate these signs of disruption through a combination of i) high-precision photometry obtained with the Hubble Space Telescope (HST), ii) two epochs of stellar spectra obtained with the Hectochelle Spectrograph on the MMT, and iii) measurements from the Gaia mission. Using the HST data, we examine one of the reported stream-like overdensities at large radii, and conclude that it is not a true stellar stream, but instead a clump of foreground stars and background galaxies. Our spectroscopic analysis shows that one known member star is likely a binary, and challenges the membership status of three others, including two distant candidates that had formerly provided evidence for overall stellar mass loss. We also find evidence that the proposed kinematic gradient across Leo V might be due to small number statistics. We update the systemic proper motion of Leo V, finding $(μ_α\cosδ, μ_δ)= (0.009\pm0.560$, $-0.777\pm0.314)$ mas yr$^{-1}$, which is consistent with its reported orbit that did not put Leo V at risk of being disturbed by the Milky Way. These findings remove most of the observational clues that suggested Leo V was disrupting, however, we also find new plausible member stars, two of which are located >5 half-light radii from the main body. These stars require further investigation. Therefore, the nature of Leo V still remains an open question.

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arXiv2019

The survey of Planetary Nebulae in Andromeda (M31): I. Imaging the disk and halo with MegaCam@CFHT

The Andromeda (M31) galaxy subtends nearly 100 sq. deg. on the sky, with severe contamination from the Milky Way halo stars whose surface density displays a steep gradient across the entire M31 field-of-view. Planetary Nebulae (PNe) are a population of stars firmly associated with M31, that are excellent tracers of light, chemistry and motion in galaxies. We present a 16 sq. deg. survey of the disk and inner halo of M31 with MegaCam@CFHT to identify PNe, characterize their luminosity-specific PN number and luminosity function (PNLF). PNe were identified based on their bright OIII 5007 $\unicode{x212B}$ emission and absence of a continuum. Subsamples of the faint PNe were independently confirmed by matching with resolved Hubble Space Telescope sources from the PHAT survey and spectroscopic follow-up observations with HectoSpec@MMT. The current survey reaches 2 mag fainter than the previous most-sensitive survey. We identify 4289 PNe, of which only 1099 were previously known. By comparing the PN number density with the surface brightness profile of M31 out to ~30 kpc along the minor-axis, we find that the stellar population in the inner halo has a 7 times larger luminosity-specific PN number value than that of the disk. It indicates that the stellar population at deprojected minor-axis radii larger than ~10 kpc is different from that in the M31 disk. We measure the PNLF and find a bright cut-off and a slope consistent with the previous determination by Ciardullo et al. (1989). Interestingly, it shows a significant rise at the faint end, present in all radial bins covered by the survey, much steeper than that observed for the Magellanic clouds and Milky Way bulge. M31 shows two major episodes of star formation and the rise in the faint end of the PNLF is possibly associated with the older stellar population. It may also be a result of varying opacity of the PNe.

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arXiv2019

Light Element Discontinuities Suggest an Early Termination of Star Formation in the Globular Cluster NGC 6402 (M14)

NGC 6402 is among the most massive globular clusters in the Galaxy, but little is known about its detailed chemical composition. Therefore, we obtained radial velocities and/or chemical abundances of 11 elements for 41 red giant branch cluster members using high resolution spectra obtained with the Magellan-M2FS instrument. We find NGC 6402 to be only moderately metal-poor with $\langle$[Fe/H]$\rangle$ = $-$1.13 dex ($σ$ = 0.05 dex) and to have a mean heliocentric radial velocity of $-$61.1 km s$^{\rm -1}$ ($σ$ = 8.5 km s$^{\rm -1}$). In general, NGC 6402 exhibits mean composition properties that are similar to other inner Galaxy clusters, such as [$α$/Fe] $\sim$ $+$0.3 dex, [Cr,Ni/Fe] $\sim$ 0.0 dex, and $\langle$[La/Eu]$\rangle$ = $-$0.08 dex. Similarly, we find large star-to-star abundance variations for O, Na, Mg, Al, and Si that are indicative of gas that experienced high temperature proton-capture burning. Interestingly, we detect three distinct populations, but also find large gaps in the [O/Fe], [Na/Fe], and [Al/Fe] distributions that may provide the first direct evidence of delayed formation for intermediate composition stars. A qualitative enrichment model is discussed where clusters form stars through an early ($<$ 5$-$10 Myr) phase, which results in first generation and "extreme" composition stars, and a delayed phase ($>$ 40 Myr), which results in the dilution of processed and pristine gas and the formation of intermediate composition stars. For NGC 6402, the missing intermediate composition stars suggest the delayed phase terminated prematurely, and as a result the cluster may uniquely preserve details of the chemical enrichment process.

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arXiv2018

A Deeper Look at the New Milky Way Satellites: Sagittarius II, Reticulum II, Phoenix II, and Tucana III

We present deep Magellan/Megacam stellar photometry of four recently discovered faint Milky Way satellites: Sagittarius II (Sgr II), Reticulum II (Ret II), Phoenix II (Phe II), and Tucana III (Tuc III). Our photometry reaches ~2-3 magnitudes deeper than the discovery data, allowing us to revisit the properties of these new objects (e.g., distance, structural properties, luminosity measurements, and signs of tidal disturbance). The satellite color-magnitude diagrams show that they are all old (~13.5 Gyr) and metal-poor ([Fe/H]$\lesssim-2.2$). Sgr II is particularly interesting as it sits in an intermediate position between the loci of dwarf galaxies and globular clusters in the size-luminosity plane. The ensemble of its structural parameters is more consistent with a globular cluster classification, indicating that Sgr II is the most extended globular cluster in its luminosity range. The other three satellites land directly on the locus defined by Milky Way ultra-faint dwarf galaxies of similar luminosity. Ret II is the most elongated nearby dwarf galaxy currently known for its luminosity range. Our structural parameters for Phe II and Tuc III suggest that they are both dwarf galaxies. Tuc III is known to be associated with a stellar stream, which is clearly visible in our matched-filter stellar density map. The other satellites do not show any clear evidence of tidal stripping in the form of extensions or distortions. Finally, we also use archival HI data to place limits on the gas content of each object.

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arXiv2018

Nearby Early-Type Galactic Nuclei at High Resolution: Dynamical Black Hole and Nuclear Star Cluster Mass Measurements

We present a detailed study of the nuclear star clusters (NSCs) and massive black holes (BHs) of four of the nearest low-mass early-type galaxies: M32, NGC205, NGC5012, and NGC5206. We measure dynamical masses of both the BHs and NSCs in these galaxies using Gemini/NIFS or VLT/SINFONI stellar kinematics, Hubble Space Telescope (HST) imaging, and Jeans Anisotropic Models. We detect massive BHs in M32, NGC5102, and NGC5206, while in NGC205, we find only an upper limit. These BH mass estimates are consistent with previous measurements in M32 and NGC205, while those in NGC5102 and NGC5206 are estimated for the first time, and both found to be $<$$10^6~M_{\odot}$. This adds to just a handful of galaxies with dynamically measured sub-million $M_{\odot}$ central BHs. Combining these BH detections with our recent work on NGC404's BH, we find that 80\% (4/5) of nearby, low-mass ($10^9-10^{10}~M_{\odot}$; $σ_{\star}\sim20-70$ km/s) early-type galaxies host BHs. Such a high occupation fraction suggests the BH seeds formed in the early epoch of cosmic assembly likely resulted in abundant seeds, favoring a low-mass seed mechanism of the remnants, most likely from the first generation of massive stars. We find dynamical masses of the NSCs ranging from $2-80\times10^6~M_{\odot}$ and compare these masses to scaling relations for NSCs based primarily on photometric mass estimates. Color gradients suggest younger stellar populations lie at the centers of the NSCs in three of the four galaxies (NGC205, NGC5102, and NGC5206), while the morphology of two are complex and are best-fit with multiple morphological components (NGC5102 and NGC5206). The NSC kinematics show they are rotating, especially in M32 and NGC5102 ($V/σ_{\star}\sim0.7$).

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arXiv2018

Exploring the Chemical Composition and Double Horizontal Branch of the Bulge Globular Cluster NGC 6569

Photometric and spectroscopic analyses have shown that the Galactic bulge cluster Terzan 5 hosts several populations with different metallicities and ages that manifest as a double red horizontal branch (HB). A recent investigation of the massive bulge cluster NGC 6569 revealed a similar, though less extended, HB luminosity split, but little is known about the cluster's detailed chemical composition. Therefore, we have used high-resolution spectra from the Magellan-M2FS and VLT-FLAMES spectrographs to investigate the chemical compositions and radial velocity distributions of red giant branch and HB stars in NGC 6569. We found the cluster to have a mean heliocentric radial velocity of -48.8 km/s (sigma = 5.3 km/s; 148 stars) and a mean [Fe/H] =-0.87 dex (19 stars), but the cluster's 0.05 dex [Fe/H] dispersion precludes a significant metallicity spread. NGC 6569 exhibits light- and heavy-element distributions that are common among old bulge/inner Galaxy globular clusters, including clear (anti)correlations between [O/Fe], [Na/Fe], and [Al/Fe]. The light-element data suggest that NGC 6569 may be composed of at least two distinct populations, and the cluster's low mean [La/Eu] = -0.11 dex indicates significant pollution with r-process material. We confirm that both HBs contain cluster members, but metallicity and light-element variations are largely ruled out as sources for the luminosity difference. However, He mass fraction differences as small as delta Y ~ 0.02 cannot be ruled out and may be sufficient to reproduce the double HB.

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