Metal-poor nuclear star clusters in two dwarf galaxies near Centaurus A suggesting formation from the in-spiraling of globular clusters

Studies of nucleated dwarf galaxies can constrain the scenarios for the formation and evolution of nuclear star clusters (NSC) in low-mass galaxies and give us insights on the origin of ultra compact dwarf galaxies (UCDs). We report the discovery of a NSC in the dwarf galaxy KKs58 and investigate its properties together with those of another NSC in KK197. Both NSCs are hosted by dwarf elliptical galaxies of the Centaurus group. Combining ESO VLT MUSE data with photometry from VLT FORS2, CTIO Blanco DECam, and HST ACS, as well as high-resolution spectroscopy from VLT UVES, we analyse the photometric, kinematic and stellar population properties of the NSCs and their host galaxies. We confirm membership of the NSCs based on their radial velocities and location close to the galaxy centres. We also confirm the membership of two globular clusters (GCs) and detect oblate rotation in the main body of KK197. Based on high signal-to-noise spectra taken with MUSE of the NSCs of both KKs58 and KK197 we measure low metallicities, [Fe/H] = −1.75 ± 0.06 dex and [Fe/H] = −1.84 ± 0.05 dex, and stellar masses of 7.3 × 105 M⊙ and 1.0 × 106 M⊙, respectively. Both NSCs are more metal-poor than their hosts that have metallicities of −1.35 ± 0.23 dex (KKs58) and −0.84 ± 0.12 dex (KK197). This can be interpreted as NSC formation via the in-spiral of GCs. The masses, sizes and metallicities of the two NSCs place them among other NSCs, but also among the known UCDs of the Centaurus group. This indicates that NSCs might constitute the progenitors of a part of the low-mass UCDs, although their properties are almost indistinguishable from typical GCs.

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Abundance of dwarf galaxies around low-mass spiral galaxies in the Local Volume

Astronomy and Astrophysics ◽

10.1051/0004-6361/202038862 ◽

2020 ◽

Vol 644 ◽

pp. A91

Author(s):

Oliver Müller ◽

Helmut Jerjen

Keyword(s):

Surface Brightness ◽

Spiral Galaxies ◽

Dwarf Galaxies ◽

Dwarf Galaxy ◽

Current Model ◽

Host Galaxies ◽

Local Volume ◽

Satellite Problem ◽

Low Mass ◽

Crucial Test

The abundance of satellite dwarf galaxies has long been considered a crucial test for the current model of cosmology leading to the well-known missing satellite problem. Recent advances in simulations and observations have allowed the study of dwarf galaxies around host galaxies in more detail. Using the Dark Energy Camera we surveyed a 72 deg2 area of the nearby Sculptor group, also encompassing the two low-mass Local Volume galaxies NGC 24 and NGC 45 residing behind the group, to search for as yet undetected dwarf galaxies. Apart from the previously known dwarf galaxies we found only two new candidates down to a 3σ surface brightness detection limit of 27.4 r mag arcsec−2. Both systems are in projection close to NGC 24. However, one of these candidates could be an ultra-diffuse galaxy associated with a background galaxy. We compared the number of known dwarf galaxy candidates around NGC 24, NGC 45, and five other well-studied low-mass spiral galaxies (NGC 1156, NGC 2403, NGC 5023, M 33, and the LMC) with predictions from cosmological simulations, and found that for the stellar-to-halo mass models considered, the observed satellite numbers tend to be on the lower end of the expected range. This could mean either that there is an overprediction of luminous subhalos in ΛCDM or that we are missing some of the satellite members due to observational biases.

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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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The nature and nurture of star clusters

Proceedings of the International Astronomical Union ◽

10.1017/s1743921309990809 ◽

2009 ◽

Vol 5 (S266) ◽

pp. 3-13 ◽

Cited By ~ 2

Author(s):

Bruce G. Elmegreen

Keyword(s):

Globular Clusters ◽

Dwarf Galaxy ◽

Cluster Formation ◽

Star Clusters ◽

Mass Function ◽

Radial Gradient ◽

Cluster Mass ◽

Low Mass ◽

Nature And Nurture ◽

Gas Loss

AbstractStar clusters have hierarchical patterns in space and time, suggesting formation processes in the densest regions of a turbulent interstellar medium. Clusters also have hierarchical substructure when they are young, which makes them all look like the inner mixed parts of a pervasive stellar hierarchy. Young field stars share this distribution, presumably because some of them came from dissolved clusters and others formed in a dispersed fashion in the same gas. The fraction of star formation that ends up in clusters is apparently not constant, but may increase with interstellar pressure. Hierarchical structure explains why stars form in clusters and why many of these clusters are self-bound. It also explains the cluster mass function. Halo globular clusters share many properties of disk clusters, including what appears to be an upper cluster cutoff mass. However, halo globulars are self-enriched and often connected with dwarf galaxy streams. The mass function of halo globulars could have initially been like the power-law mass function of disk clusters, but the halo globulars have lost their low-mass members. The reasons for this loss are not understood. It could have happened slowly over time as a result of cluster evaporation, or it could have happened early after cluster formation as a result of gas loss. The latter model explains best the observation that the globular cluster mass function has no radial gradient in galaxies.

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The Faint Globular Cluster in the Dwarf Galaxy Andromeda I

Publications of the Astronomical Society of Australia ◽

10.1017/pasa.2017.35 ◽

2017 ◽

Vol 34 ◽

Cited By ~ 8

Author(s):

Nelson Caldwell ◽

Jay Strader ◽

David J. Sand ◽

Beth Willman ◽

Anil C. Seth

Keyword(s):

Dark Matter ◽

Globular Cluster ◽

Globular Clusters ◽

Local Group ◽

Dwarf Galaxies ◽

Dwarf Galaxy ◽

Dark Matter Halos ◽

History Of ◽

Low Mass ◽

Growing Population

AbstractObservations of globular clusters in dwarf galaxies can be used to study a variety of topics, including the structure of dark matter halos and the history of vigorous star formation in low-mass galaxies. We report on the properties of the faint globular cluster (MV ~ −3.4) in the M31 dwarf galaxy Andromeda I. This object adds to the growing population of low-luminosity Local Group galaxies that host single globular clusters.

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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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An accreting < 105M⊙ black hole at the center of dwarf galaxy IC750

Proceedings of the International Astronomical Union ◽

10.1017/s1743921320003488 ◽

2019 ◽

Vol 15 (S356) ◽

pp. 376-376

Author(s):

Ingyin Zaw

Keyword(s):

Black Holes ◽

Black Hole ◽

Dwarf Galaxies ◽

Dwarf Galaxy ◽

Host Galaxy ◽

Scaling Relations ◽

Host Galaxies ◽

Seed Formation ◽

Maser Emission ◽

Water Masers

AbstractNuclear black holes in dwarf galaxies are important for understanding the low end of the supermassive black hole mass distribution and the black hole-host galaxy scaling relations. IC 750 is a rare system which hosts an AGN, found in ˜0.5% of dwarf galaxies, with circumnuclear 22 GHz water maser emission, found in ˜3–5% of Type 2 AGNs. Water masers, the only known tracer of warm, dense gas in the center parsec of AGNs resolvable in position and velocity, provide the most precise and accurate mass measurements of SMBHs outside the local group. We have mapped the maser emission in IC 750 and find that it traces a nearly edge-on warped disk, 0.2 pc in diameter. The central black hole has an upper limit mass of ˜1 × 105 M⊙ and a best fit mass of ˜8 × 104 M⊙, one to two orders of magnitude below what is expected from black hole-galaxy scaling relations. This has implications for models of black hole seed formation in the early universe, the growth of black holes, and their co-evolution with their host galaxies.

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Formation of massive globular clusters with dark matter and its implication on dark matter annihilation

Monthly Notices of the Royal Astronomical Society Letters ◽

10.1093/mnrasl/slaa089 ◽

2020 ◽

Vol 496 (1) ◽

pp. L70-L74

Author(s):

Henriette Wirth ◽

Kenji Bekki ◽

Kohei Hayashi

Keyword(s):

Dark Matter ◽

Observational Studies ◽

Globular Clusters ◽

Galactic Halo ◽

Dwarf Galaxies ◽

Large Range ◽

Dark Matter Annihilation ◽

Central Regions ◽

Γ Ray ◽

The Masses

ABSTRACT Recent observational studies of γ-ray emission from massive globular clusters (GCs) have revealed possible evidence of dark matter (DM) annihilation within GCs. It is, however, still controversial whether the emission comes from DM or from millisecond pulsars. We here present the new results of numerical simulations, which demonstrate that GCs with DM can originate from nucleated dwarfs orbiting the ancient Milky Way. The simulated stripped nuclei (i.e. GCs) have the central DM densities ranging from 0.1 to several M⊙ pc−3, depending on the orbits and the masses of the host dwarf galaxies. However, GCs born outside the central regions of their hosts can have no/little DM after their hosts are destroyed and the GCs become the Galactic halo GCs. These results suggest that only GCs originating from stellar nuclei of dwarfs can possibly have DM. We further calculate the expected γ-ray emission from these simulated GCs and compare them to observations of ω Cen. Given the large range of DM densities in the simulated GCs, we suggest that the recent possible detection of DM annihilation from GCs should be more carefully interpreted.

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The Structure of the Stellar Hosts in Blue Compact Dwarf Galaxies

Proceedings of the International Astronomical Union ◽

10.1017/s174392130600665x ◽

2006 ◽

Vol 2 (S235) ◽

pp. 300-300

Author(s):

R.O. Amorín ◽

J.A.L. Aguerri ◽

L.M. Cairós ◽

N. Caon ◽

C. Muñoz-Tuñón

Keyword(s):

Galaxy Formation ◽

Dwarf Galaxies ◽

Dwarf Galaxy ◽

Structural Parameters ◽

Host Galaxy ◽

Host Galaxies ◽

Galaxy Formation And Evolution ◽

Radial Profiles ◽

Low Surface Brightness ◽

Formation And Evolution

AbstractBlue compact dwarf (BCD) galaxies are gas-rich, low-luminosity (Mb≳-18 mag) and compact systems, currently undergoing violent star-formation burst (Sargent & Searle 1970). While it was initially hypothesized that they were very young galaxies (e.g. Sargent & Searle 1970, et al. 1988), the subsecuent detection of an extended, redder stellar host galaxy showed that the vast majority of them are old systems (e.g. Gil de Paz et al. 2003,2005). BCDs play an important role for understanding the process of galaxy formation and evolution.The structural properties of the low surface brightness stellar host in BCDs are often studied by fitting r1/n models to the outer regions of their radial profiles. The limitations imposed by the presence of a large starburst emission overlapping the underlying component makes this kind of analysis a difficult task.We propose a two-dimensional fitting methodology in order to improve the extraction of the structural parameters of the LSB host Amorín et al. 2006, submitted). A set of ideal simulations are presented in order to test the reliability of the method and to determine its robustness and flexibility. We present the different steps of the method discussing its advantages and weaknesses. We compare the results for a sample of eight objects with those already obtained using a one-dimensional technique (Caon et al. 2005).We fit a PSF convolved Sérsic model to the BVR images with the GALFIT publicly software (Peng et al. 2002). We restrict the fit to the stellar host by masking out the starburst region and take special care to minimize the sky-subtraction uncertainties. Consistency checks are performed to assess the reliability and accuracy of the derived structural parameters.We obtain robust fits for all the sample galaxies, all of which, except one, show low Sérsic indices n—very close to 1—with good agreement in the three bands. These findings suggest that the stellar hosts in BCDs have near-exponential profiles. Since the Sérsic index n of host galaxies is important in the context of the possible structural and evolutionary connections among the different types of dwarf galaxies, we are currently extending the study to a larger sample of objects. This kind of studies will help us to understand the mechanisms that form and shape BCD galaxies, and how they relate to the other dwarf galaxy classes.

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X-ray sources in Galactic globular clusters and old open clusters

Proceedings of the International Astronomical Union ◽

10.1017/s1743921319007981 ◽

2019 ◽

Vol 14 (S351) ◽

pp. 367-376

Author(s):

Maureen van den Berg

Keyword(s):

Globular Clusters ◽

Cataclysmic Variables ◽

Star Clusters ◽

Open Clusters ◽

Encounter Rate ◽

X Ray ◽

Low Mass ◽

New Perspective ◽

X Ray Binaries ◽

Cluster Age

AbstractThe features and make up of the population of X-ray sources in Galactic star clusters reflect the properties of the underlying stellar environment. Cluster age, mass, stellar encounter rate, binary frequency, metallicity, and maybe other properties as well, determine to what extent we can expect a contribution to the cluster X-ray emission from low-mass X-ray binaries, millisecond pulsars, cataclysmic variables, and magnetically active binaries. Sensitive X-ray observations withXMM-Newton and certainlyChandra have yielded new insights into the nature of individual sources and the effects of dynamical encounters. They have also provided a new perspective on the collective X-ray properties of clusters, in which the X-ray emissivities of globular clusters and old open clusters can be compared to each other and to those of other environments. I will review our current understanding of cluster X-ray sources, focusing on star clusters older than about 1 Gyr, illustrated with recent results.

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Embedding globular clusters in dark matter minihaloes solves the cusp–core and timing problems in the Fornax dwarf galaxy

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/staa011 ◽

2020 ◽

Vol 492 (3) ◽

pp. 3169-3178 ◽

Cited By ~ 1

Author(s):

Pierre Boldrini ◽

Roya Mohayaee ◽

Joseph Silk

Keyword(s):

Dark Matter ◽

Globular Clusters ◽

Dwarf Galaxy ◽

Large Fraction ◽

Star Clusters ◽

Circumstantial Evidence ◽

Core Size ◽

Core Problem ◽

Good Agreement

ABSTRACT We use a fully GPU N-body code to demonstrate that dark matter (DM) minihaloes, as a new component of globular clusters (GCs), resolve both the timing and cusp–core problems in Fornax if the (five or six) GCs were recently accreted (≤3 Gyr ago) by Fornax. Under these assumptions, infall of these GCs does not occur and no star clusters form in the centre of Fornax in accordance with observations. We find that crossings of GCss that have DM minihaloes near the Fornax centre induce a cusp-to-core transition of the DM halo and hence resolve the cusp–core problem in this dwarf galaxy. The DM core size depends on the frequency of GC crossings. Our simulations clearly demonstrate also that between the passages, the DM halo can regenerate its cusp. Moreover, our models are in good agreement with constraints on the DM masses of GCs as our clusters lose a large fraction of their initial DM minihaloes. These results provide circumstantial evidence for the universal existence of DM haloes in GCs.

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