Characterization and history of the Helmi streams with Gaia DR2

Context. The halo of the Milky Way has long been hypothesized to harbour significant amounts of merger debris. For more than a decade this view has been supported by wide-field photometric surveys which have revealed the outer halo to be lumpy. Aims. The recent release of Gaia DR2 is allowing us to establish that mergers also have been important and possibly built up the majority of the inner halo. In this work we focus on the Helmi streams, a group of streams crossing the solar vicinity and known for almost two decades. We characterize their properties and relevance for the build-up of the Milky Way’s halo. Methods. We identify new members of the Helmi streams in an unprecedented dataset with full phase-space information combining Gaia DR2, and the APOGEE DR2, RAVE DR5, and LAMOST DR4 spectroscopic surveys. Based on the orbital properties of the stars, we find new stream members up to a distance of 5 kpc from the Sun, which we characterized using photometry and metallicity information. We also perform N-body experiments to constrain the time of accretion and properties of the progenitor of the streams. Results. We find nearly 600 new members of the Helmi streams. Their HR diagram reveals a broad age range, from ≈11 to 13 Gyr, while their metallicity distribution goes from −2.3 to −1.0, and peaks at [Fe/H] ∼ −1.5. These findings confirm that the streams originate in a dwarf galaxy. Furthermore, we find seven globular clusters to be likely associated, and which follow a well-defined age-metallicity sequence whose properties suggest a relatively massive progenitor object. Our N-body simulations favour a system with a stellar mass of ∼108 M⊙ accreted 5 − 8 Gyr ago. Conclusions. The debris from the Helmi streams is an important donor to the Milky Way halo, contributing ≈15% of its mass in field stars and 10% of its globular clusters.

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Globular clusters in the stellar stream surrounding the Milky Way analogue NGC 5907

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/stz3382 ◽

2019 ◽

Vol 491 (4) ◽

pp. 5693-5701 ◽

Cited By ~ 4

Author(s):

Adebusola B Alabi ◽

Duncan A Forbes ◽

Aaron J Romanowsky ◽

Jean P Brodie

Keyword(s):

Spiral Galaxy ◽

Globular Clusters ◽

Milky Way ◽

Dwarf Galaxy ◽

Young Star ◽

Star Cluster ◽

Wide Field ◽

Stellar Disc ◽

The Galaxy ◽

Specific Frequency

ABSTRACT We study the globular clusters (GCs) in the spiral galaxy NGC 5907 well-known for its spectacular stellar stream – to better understand its origin. Using wide-field Subaru/Suprime-Cam gri images and deep Keck/DEIMOS multi-object spectroscopy, we identify and obtain the kinematics of several GCs superimposed on the stellar stream and the galaxy disc. We estimate the total number of GCs in NGC 5907 to be 154 ± 44, with a specific frequency of 0.73 ± 0.21. Our analysis also reveals a significant, new population of young star cluster candidates found mostly along the outskirts of the stellar disc. Using the properties of the stream GCs, we estimate that the disrupted galaxy has a stellar mass similar to the Sagittarius dwarf galaxy accreted by the Milky Way, i.e. $\sim 10^8~\rm M_\odot$.

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Search for globular clusters associated with the Milky Way dwarf galaxies using Gaia DR2

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/staa3297 ◽

2020 ◽

Vol 500 (1) ◽

pp. 986-997

Author(s):

Kuan-Wei Huang ◽

Sergey E Koposov

Keyword(s):

Globular Clusters ◽

Milky Way ◽

Dwarf Galaxies ◽

Dwarf Galaxy ◽

Galactic Centre ◽

Imaging Data ◽

Credible Intervals ◽

Specific Frequency ◽

Number Counts ◽

Gaia Dr2

ABSTRACT We report the result of searching for globular clusters (GCs) around 55 Milky Way (MW) satellite dwarf galaxies within the distance of 450 kpc from the Galactic Centre except for the Large and Small Magellanic Clouds and the Sagittarius dwarf. For each dwarf, we analyse the stellar distribution of sources in Gaia DR2, selected by magnitude, proper motion, and source morphology. Using the kernel density estimation of stellar number counts, we identify 11 possible GC candidates. Cross-matched with existing imaging data, all 11 objects are known either GCs or galaxies and only Fornax GC 1–6 among them are associated with the targeted dwarf galaxy. Using simulated GCs, we calculate the GC detection limit $M_{\rm V}^{\rm lim}$ that spans the range from $M_{\rm V}^{\rm lim}\sim -7$ for distant dwarfs to $M_{\rm V}^{\rm lim}\sim 0$ for nearby systems. Assuming a Gaussian GC luminosity function, we compute that the completeness of the GC search is above 90 per cent for most dwarf galaxies. We construct the 90 per cent credible intervals/upper limits on the GC specific frequency SN of the MW dwarf galaxies: 12 < SN < 47 for Fornax, SN < 20 for the dwarfs with −12 < MV < −10, SN < 30 for the dwarfs with −10 < MV < −7, and SN < 90 for the dwarfs with MV > −7. Based on SN, we derive the probability of galaxies hosting GCs given their luminosity, finding that the probability of galaxies fainter than MV = −9 to host GCs is lower than 0.1.

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The accreted nuclear clusters of the Milky Way

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/staa3407 ◽

2020 ◽

Vol 500 (2) ◽

pp. 2514-2524

Author(s):

Joel Pfeffer ◽

Carmela Lardo ◽

Nate Bastian ◽

Sara Saracino ◽

Sebastian Kamann

Keyword(s):

Globular Clusters ◽

Milky Way ◽

Dwarf Galaxy ◽

Star Clusters ◽

Host Galaxy ◽

The Galaxy ◽

Nuclear Clusters ◽

The Common ◽

Massive Clusters ◽

Peculiar Case

ABSTRACT A number of the massive clusters in the halo, bulge, and disc of the Galaxy are not genuine globular clusters (GCs) but instead are different beasts altogether. They are the remnant nuclear star clusters (NSCs) of ancient galaxies since accreted by the Milky Way. While some clusters are readily identifiable as NSCs and can be readily traced back to their host galaxy (e.g. M54 and the Sagittarius Dwarf galaxy), others have proven more elusive. Here, we combine a number of independent constraints, focusing on their internal abundances and overall kinematics, to find NSCs accreted by the Galaxy and trace them to their accretion event. We find that the true NSCs accreted by the Galaxy are: M54 from the Sagittarius Dwarf, ω Centari from Gaia-Enceladus/Sausage, NGC 6273 from Kraken, and (potentially) NGC 6934 from the Helmi Streams. These NSCs are prime candidates for searches of intermediate-mass black holes (BHs) within star clusters, given the common occurrence of galaxies hosting both NSCs and central massive BHs. No NSC appears to be associated with Sequoia or other minor accretion events. Other claimed NSCs are shown not to be such. We also discuss the peculiar case of Terzan 5, which may represent a unique case of a cluster–cluster merger.

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Chemical Abundances of Seven Outer Halo M31 Globular Clusters from the Pan-Andromeda Archaeological Survey

Proceedings of the International Astronomical Union ◽

10.1017/s1743921316011200 ◽

2016 ◽

Vol 11 (S321) ◽

pp. 10-12

Author(s):

Charli M. Sakari

Keyword(s):

Globular Clusters ◽

Milky Way ◽

Dwarf Galaxy ◽

Large Magellanic Cloud ◽

Archaeological Survey ◽

Chemical Abundances ◽

Light Spectra ◽

Sigma Level ◽

Stellar Streams ◽

Detailed Chemical

AbstractObservations of stellar streams in M31’s outer halo suggest that M31 is actively accreting several dwarf galaxies and their globular clusters (GCs). Detailed abundances can chemically link clusters to their birth environments, establishing whether or not a GC has been accreted from a satellite dwarf galaxy. This talk presents the detailed chemical abundances of seven M31 outer halo GCs (with projected distances from M31 greater than 30 kpc), as derived from high-resolution integrated-light spectra taken with the Hobby Eberly Telescope. Five of these clusters were recently discovered in the Pan-Andromeda Archaeological Survey (PAndAS)—this talk presents the first determinations of integrated Fe, Na, Mg, Ca, Ti, Ni, Ba, and Eu abundances for these clusters. Four of the target clusters (PA06, PA53, PA54, and PA56) are metal-poor ([Fe/H] < -1.5), α-enhanced (though they are possibly less alpha-enhanced than Milky Way stars at the 1 sigma level), and show signs of star-to-star Na and Mg variations. The other three GCs (H10, H23, and PA17) are more metal-rich, with metallicities ranging from [Fe/H] = -1.4 to -0.9. While H23 is chemically similar to Milky Way field stars, Milky Way GCs, and other M31 clusters, H10 and PA17 have moderately-low [Ca/Fe], compared to Milky Way field stars and clusters. Additionally, PA17’s high [Mg/Ca] and [Ba/Eu] ratios are distinct from Milky Way stars, and are in better agreement with the stars and clusters in the Large Magellanic Cloud (LMC). None of the clusters studied here can be conclusively linked to any of the identified streams from PAndAS; however, based on their locations, kinematics, metallicities, and detailed abundances, the most metal-rich PAndAS clusters H23 and PA17 may be associated with the progenitor of the Giant Stellar Stream, H10 may be associated with the SW Cloud, and PA53 and PA56 may be associated with the Eastern Cloud.

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A Gaia DR2 view of the open cluster population in the Milky Way

Astronomy and Astrophysics ◽

10.1051/0004-6361/201833476 ◽

2018 ◽

Vol 618 ◽

pp. A93 ◽

Cited By ~ 180

Author(s):

T. Cantat-Gaudin ◽

C. Jordi ◽

A. Vallenari ◽

A. Bragaglia ◽

L. Balaguer-Núñez ◽

...

Keyword(s):

Stellar Evolution ◽

Milky Way ◽

Galactic Plane ◽

Galactic Disk ◽

Open Cluster ◽

Open Clusters ◽

Number Of Clusters ◽

History Of ◽

Gaia Dr2

Context. Open clusters are convenient probes of the structure and history of the Galactic disk. They are also fundamental to stellar evolution studies. The second Gaia data release contains precise astrometry at the submilliarcsecond level and homogeneous photometry at the mmag level, that can be used to characterise a large number of clusters over the entire sky. Aims. In this study we aim to establish a list of members and derive mean parameters, in particular distances, for as many clusters as possible, making use of Gaia data alone. Methods. We compiled a list of thousands of known or putative clusters from the literature. We then applied an unsupervised membership assignment code, UPMASK, to the Gaia DR2 data contained within the fields of those clusters. Results. We obtained a list of members and cluster parameters for 1229 clusters. As expected, the youngest clusters are seen to be tightly distributed near the Galactic plane and to trace the spiral arms of the Milky Way, while older objects are more uniformly distributed, deviate further from the plane, and tend to be located at larger Galactocentric distances. Thanks to the quality of Gaia DR2 astrometry, the fully homogeneous parameters derived in this study are the most precise to date. Furthermore, we report on the serendipitous discovery of 60 new open clusters in the fields analysed during this study.

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Did the Bulge Form All at Once?

Symposium - International Astronomical Union ◽

10.1017/s0074180900229999 ◽

1996 ◽

Vol 169 ◽

pp. 403-410

Author(s):

R.M. Rich

Keyword(s):

Galaxy Evolution ◽

Globular Clusters ◽

Milky Way ◽

Galactic Center ◽

Field Population ◽

Dynamical Instability ◽

Galactic Bulge ◽

Recent Developments ◽

Order Of Magnitude ◽

History Of

It is reasonable to say that if Jan Oort were alive today, he would no doubt find recent developments in the study of the Galactic bulge to be fascinating. Oort considered the Galactic bulge in two contexts. First, he was interested in the use of the RR Lyrae stars as probes to determine the distance to the Galactic Center. No doubt, Oort would have been excited about the growing evidence of the bulge's triaxiality, as well as by the debate over the age of the bulge. His second interest was in the nature of activity at the center, an issue that I will not discuss in this review. The latter also remains an unsolved problem of the Milky Way, and (based on his work) one that might have been nearer to his heart than this one. Yet the question of when the bulge formed is ultimately a question about the formation history of the Galaxy. The oldest stars (those whose ages we are certain of) are found in Galactic globular clusters, the sum total of which are ≈ 5 × 107M⊙. The field population of the bulge is ≈ 2–3 × 1010M⊙, an order of magnitude more massive than the field population of the metal poor spheroid. So if the bulge formed all at once, and early, then the Milky Way had a luminous, even cataclysmic youth. But if the bulge formed later in the history of our galaxy, as a starburst or dynamical instability of the central disk, then the young Milky Way may have been inconspicuous and primeval galaxies may be hard to find indeed. If our bulge formed very early, its stellar population might have much in common with the giant ellipticals, while a late bulge might teach us much about processes that affect galaxy evolution.

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A deep view into the nucleus of the Sagittarius dwarf spheroidal galaxy: M54

Proceedings of the International Astronomical Union ◽

10.1017/s1743921319007907 ◽

2019 ◽

Vol 14 (S351) ◽

pp. 47-50

Author(s):

M. Alfaro-Cuello ◽

N. Kacharov ◽

N. Neumayer ◽

A. Mastrobuono-Battisti ◽

N. Lützgendorf ◽

...

Keyword(s):

Metal Complex ◽

Detailed Analysis ◽

Globular Clusters ◽

Milky Way ◽

Dwarf Galaxies ◽

Dwarf Galaxy ◽

Star Clusters ◽

High Mass ◽

Data Set

AbstractNuclear star clusters hosted by dwarf galaxies exhibit similar characteristics to high-mass, metal complex globular clusters. This type of globular clusters could, therefore, be former nuclei from accreted galaxies. M54 resides in the photometric center of the Sagittarius dwarf galaxy, at a distance where resolving stars is possible. M54 offers the opportunity to study a nucleus before the stripping of their host by the tidal field effects of the Milky Way. We use a MUSE data set to perform a detailed analysis of over 6600 stars. We characterize the stars by metallicity, age, and kinematics, identifying the presence of three stellar populations: a young metal-rich (YMR), an intermediate-age metal-rich (IMR), and an old metal-poor (OMP). The evidence suggests that the OMP population is the result of accretion of globular clusters in the center of the host, while the YMR population was born in-situ in the center of the OMP population.

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Using ground based data as a precursor for Gaia in getting proper motions of satellites

Proceedings of the International Astronomical Union ◽

10.1017/s1743921317005750 ◽

2017 ◽

Vol 12 (S330) ◽

pp. 210-213

Author(s):

Tobias K. Fritz ◽

Sean T. Linden ◽

Paul Zivick ◽

Nitya Kallivayalil ◽

Jo Bovy

Keyword(s):

Adaptive Optics ◽

Globular Cluster ◽

Proper Motion ◽

Milky Way ◽

Dwarf Galaxy ◽

Optical Data ◽

Line Of Sight ◽

Wide Field ◽

Proper Motions ◽

Tidal Stream

AbstractWe present our effort to measure the proper motions of satellites in the halo of the Milky Way with mainly ground based telescopes as a precursor on what is possible with Gaia. For our first study, we used wide field optical data from the LBT combined with a first epoch of SDSS observations, on the globular cluster Palomar 5 (Pal 5). Since Pal 5 is associated with a tidal stream it is very useful to constrain the shape of the potential of the Milky Way. The motion and other properties of the Pal 5 system constrain the inner halo of the Milky Way to be rather spherical. Further, we combined adaptive optics and HST to get an absolute proper motion of the globular cluster Pyxis. Using the proper motion and the line-of-sight velocity we find that the orbit of Pyxis is rather eccentric with its apocenter at more than 100 kpc and its pericenter at about 30 kpc. The dynamics excludes an association with the ATLAS stream, the Magellanic clouds, and all satellites of the Milky Way at least down to the mass of Leo II. However, the properties of Pyxis, like metallicity and age, point to an origin from a dwarf of at least the mass of Leo II. We therefore propose that Pyxis originated from an unknown relatively massive dwarf galaxy, which is likely today fully disrupted. Assuming that Pyxis is bound to the Milky Way we derive a 68% lower limit on the mass of the Milky Way of 9.5 × 1011 M⊙.

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Variability Survey of ω Centauri in the Near-IR: Period-Luminosity Relations

Proceedings of the International Astronomical Union ◽

10.1017/s1743921315009059 ◽

2015 ◽

Vol 12 (S316) ◽

pp. 351-352

Author(s):

C. Navarrete ◽

M. Catelan ◽

R. Contreras Ramos ◽

F. Gran ◽

J. Alonso-García ◽

...

Keyword(s):

Time Series ◽

Milky Way ◽

Dwarf Galaxy ◽

Variable Stars ◽

Star Cluster ◽

Wide Field ◽

Time Series Study ◽

Near Ir ◽

Series Study

Abstractω Centauri is by far the most massive globular star cluster in the Milky Way, and possibly the remnant of a dwarf galaxy. As such, it contains a large number of variable stars of different classes. Here we report on an extensive, wide-field time-series study of ω Cen in the J and KS bands, which has allowed us to study the near-IR period-luminosity relations for different variability classes, including the first such relations for the SX Phoenicis stars.

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IC 4499 revised: Spectro-photometric evidence of small light-element variations

Astronomy and Astrophysics ◽

10.1051/0004-6361/201833650 ◽

2018 ◽

Vol 618 ◽

pp. A131 ◽

Cited By ~ 12

Author(s):

E. Dalessandro ◽

C. Lardo ◽

M. Cadelano ◽

S. Saracino ◽

N. Bastian ◽

...

Keyword(s):

Globular Clusters ◽

Milky Way ◽

Dwarf Galaxy ◽

Light Element ◽

High Resolution Spectroscopy ◽

Multiple Populations ◽

Number Ratio ◽

Red Giant ◽

Good Agreement

It has been suggested that IC 4499 is one of the very few old globulars to not host multiple populations with light-element variations. To follow-up on this very interesting result, here we have made use of accurate HST photometry and FLAMES at VLT high-resolution spectroscopy to investigate in more detail the stellar population properties of this system. We find that the red giant branch of the cluster is clearly bimodal in near-UV-optical colour-magnitude diagrams, thus suggesting that IC 4499 is actually composed by two sub-populations of stars with different nitrogen abundances. This represents the first detection of multiple populations in IC 4499. Consistently, we also find that one star out of six is Na-rich to some extent, while we do not detect any evidence of intrinsic spread in both Mg and O. The number ratio between stars with normal and enriched nitrogen is in good agreement with the number ratio – mass trend observed in Galactic globular clusters. Also, as typically found in other systems, nitrogen rich stars are more centrally concentrated than normal stars, although this result cannot be considered conclusive because of the limited field of view covered by our observations (∼1rh). On the contrary, we observe that both the RGB UV colour spread, which is a proxy of N variations, and Na abundance variations, are significantly smaller than those observed in Milky Way globular clusters with mass and metallicity comparable to IC 4499. The modest N and Na spreads observed in this system can be tentatively connected to the fact that IC 4499 likely formed in a disrupted dwarf galaxy orbiting the Milky Way, as previously proposed based on its orbit.

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