arXiv subjects

M. Hilker

Publications and source records attributed to M. Hilker.

At least 55 records · Page 3Linked to original sources

arXiv2018

The Fornax Deep Survey with VST. V. Exploring the faintest regions of the bright early-type galaxies inside the virial radius

This paper is based on the multi-band (ugri) Fornax Deep Survey (FDS) with the VLT Survey Telescope (VST). We study bright early-type galaxies (m_B<15 mag) in the 9 square degrees around the core of the Fornax cluster, which covers the virial radius. The main goal of the present work is to provide the analysis of the light distribution for all galaxies out to unprecedented limits (in radius and surface brightness) and to release the main products resulting from this analysis in all FDS bands. From the isophote fit, we derive the azimuthally averaged surface brightness profiles, the position angle and ellipticity profiles as a function of the semi-major axis. In each band, we derive the total magnitudes, effective radii, integrated colors and stellar mass-to-light ratios. The long integration times, the arcsec-level angular resolution of OmegaCam@VST and the large covered area of FDS allow us to map the light and color distributions out to large galactocentric distances (up to about 10-15 R_e) and surface brightness levels beyond mu_r = 27 mag/arcsec^2 (mu_B > 28 mag/arcsec^2). Therefore, the new FDS data allow us to explore in great detail the morphology and structure of cluster galaxies out to the region of the stellar halo. The observations suggest that the Fornax cluster is not completely relaxed inside the virial radius. The bulk of the gravitational interactions between galaxies happens in the W-NW core region of the cluster, where most of the bright early-type galaxies are located and where the intra-cluster baryons (diffuse light and GCs) are found. We suggest that the W-NW sub-clump of galaxies results from an infalling group onto the cluster, which has modified the structure of the galaxy outskirts (making asymmetric stellar halos) and has produced the intra-cluster baryons (ICL and GCs), concentrated in this region of the cluster.

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arXiv2018

Mean proper motions, space orbits and velocity dispersion profiles of Galactic globular clusters derived from Gaia DR2 data

We have derived the mean proper motions and space velocities of 154 Galactic globular clusters and the velocity dispersion profiles of 141 globular clusters based on a combination of Gaia DR2 proper motions with ground-based line-of-sight velocities. Combining the velocity dispersion profiles derived here with new measurements of the internal mass functions allows us to model the internal kinematics of 144 clusters, more than 90% of the currently known Galactic globular cluster population. We also derive the initial cluster masses by calculating the cluster orbits backwards in time applying suitable recipes to account for mass loss and dynamical friction. We find a correlation between the stellar mass function of a globular cluster and the amount of mass lost from the cluster, pointing to dynamical evolution as one of the mechanisms shaping the mass function of stars in clusters. The mass functions also show strong evidence that globular clusters started with a bottom-light initial mass function. Our simulations show that the currently surviving globular cluster population has lost about 80% of its mass since the time of formation. If globular clusters started from a log-normal mass function, we estimate that the Milky Way contained about 500 globular clusters initially, with a combined mass of about $2.5 \cdot 10^8$ $M_\odot$. For a power-law initial mass function, the initial mass in globular clusters could have been a factor of three higher.

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arXiv2018

The curious case of the companion: evidence for cold accretion onto a dwarf satellite near the isolated elliptical NGC 7796

The isolated elliptical (IE) NGC 7796 is accompanied by an interesting early-type dwarf galaxy, named NGC7796-DW1. It exhibits a tidal tail, very boxy isophotes, and multiple nuclei or regions (A, B, and C) that are bluer than the bulk population of the galaxy, indicating a younger age. These properties are suggestive of a dwarf-dwarf merger remnant. We use the Multi-Unit Spectroscopic Explorer (MUSE) at the VLT to investigate NGC 7796-DW1. We extract characteristic spectra to which we apply the STARLIGHT population synthesis software to obtain ages and metallicities of the various population components of the galaxy. The galaxy's main body is old and metal-poor. A surprising result is the extended line emission in the galaxy, forming a ring-like structure with a projected diameter of 2.2 kpc. The line ratios fall into the regime of HII-regions, although OB-stellar populations cannot be identified by spectral signatures. Nucleus A is a relatively old (7 Gyr or older) and metal-poor super star cluster, most probably the nucleus of the dwarf, now displaced. The star-forming regions B and C show younger and distinctly more metal-rich components. The emission line ratios of regions B and C indicate an almost solar oxygen abundance, if compared with radiation models of HII regions. NGC7796-DW1 occupies a particular role in the group of transition-type galaxies with respect to its origin and current evolutionary state, being the companion of an IE. The dwarf-dwarf merger scenario is excluded because of the missing metal-rich merger component. A viable alternative is gas accretion from a reservoir of cold, metal-rich gas. NGC7796 has to provide this gas within its X-ray bright halo. As illustrated by NGC7796-DW1, cold accretion may be a general solution to the problem of extended star formation histories in transition dwarf galaxies. (abridged)

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arXiv2018

The Hydra I cluster core - II. Kinematic complexity in a rising velocity dispersion profile around the cD galaxy NGC 3311

NGC 3311, the central galaxy of the Hydra I cluster, shows signatures of recent infall of satellite galaxies from the cluster environment. Previous work has shown that the line-of-sight velocity dispersion of the stars and globular clusters in the extended halo of NGC 3311 rises up to the value of the cluster velocity dispersion. We performed multi-object spectroscopic observations of the diffuse stellar halo of NGC 3311 using VLT/FORS2 in MXU mode to mimic a coarse `IFU'. We use pPXF to extract the kinematic information. We find a homogeneous velocity and velocity dispersion field within r<10 kpc. Beyond this radius, both the velocities and dispersions start to depend on azimuth angle and show a significant intrinsic scatter. The inner spheroid of NGC 3311 can be described as a slow rotator. Outside 10 kpc the cumulative angular momentum is rising. If the radial dependence alone is considered, the velocity dispersion does not simply rise but fills an increasingly large range of values with two well defined envelopes. The lower envelope is about constant at 200 km/s. The upper envelope rises smoothly, joining the velocity dispersion of the outer cluster galaxies. We interpret this behaviour as the superposition of tracer populations with increasingly shallower radial distributions between the extremes of the inner stellar populations and the cluster galaxies. Simple Jeans models illustrate that a range of of mass profiles with different anisotropies can account for all observed velocity dispersions, including radial MOND models. Jeans models using one tracer population with a unique density profile are not able to explain the large range of the observed kinematics. Previous claims about the cored dark halo of NGC 3311 are therefore probably not valid. This may in general apply to central cluster galaxies with rising velocity dispersion profiles, where infall processes are important.

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arXiv2018

A catalogue of masses, structural parameters and velocity dispersion profiles of 112 Milky Way globular clusters

We have determined masses, stellar mass functions and structural parameters of 112 Milky Way globular clusters by fitting a large set of N-body simulations to their velocity dispersion and surface density profiles. The velocity dispersion profiles were calculated based on a combination of more than 15,000 high-precision radial velocities which we derived from archival ESO/VLT and Keck spectra together with ~20,000 published radial velocities from the literature. Our fits also include the stellar mass functions of the globular clusters, which are available for 47 clusters in our sample, allowing us to self-consistently take the effects of mass segregation and ongoing cluster dissolution into account. We confirm the strong correlation between the global mass functions of globular clusters and their relaxation times recently found by Sollima & Baumgardt (2017). We also find a correlation of the escape velocity from the centre of a globular cluster and the fraction of first generation stars (FG) in the cluster recently derived for 57 globular clusters by Milone et al. (2017), but no correlation between the FG star fraction and the global mass function of a globular cluster. This could indicate that the ability of a globular cluster to keep the wind ejecta from the polluting star(s) is the crucial parameter determining the presence and fraction of second generation stars and not its later dynamical mass loss.

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arXiv2018

The Next Generation Fornax Survey (NGFS): IV. Mass and Age Bimodality of Nuclear Clusters in the Fornax Core Region

We present the analysis of 61 nucleated dwarf galaxies in the central regions ($<R_{\rm vir}/4$) of the Fornax galaxy cluster. The galaxies and their nuclei are studied as part of the Next Generation Fornax Survey (NGFS) using optical imaging obtained with the Dark Energy Camera (DECam) mounted at Blanco/CTIO and near-infrared data obtained with VIRCam at VISTA/ESO. We decompose the nucleated dwarfs in nucleus and spheroid, after subtracting the surface brightness profile of the spheroid component and studying the nucleus using PSF photometry.In general, nuclei are consistent with colors of confirmed metal-poor globular clusters, but with significantly smaller dispersion than other confirmed compact stellar systems in Fornax. We find a bimodal nucleus mass distribution with peaks located at $\log({\cal M_*}/M_\odot)\!\simeq\!5.4$ and $\sim\,6.3$. These two nucleus sub-populations have different stellar population properties, the more massive nuclei are older than $\sim\!2$ Gyr and have metal-poor stellar populations ($Z\leq0.02\,Z_\odot$), while the less massive nuclei are younger than $\sim\!2$ Gyr with metallicities in the range $0.02\!<\!Z/Z_\odot\!\leq\!1$. We find that the nucleus mass (${\cal M}_{\rm nuc}$) vs. galaxy mass (${\cal M}_{\rm gal}$) relation becomes shallower for less massive galaxies starting around $10^8\,M_\odot$ and the mass ratio $η_n\!=\!{\cal M}_{\rm nuc}/{\cal M}_{\rm gal}$ shows a clear anti-correlation with ${\cal M}_{\rm gal}$ for the lowest masses, reaching $10\%$. We test current theoretical models of nuclear cluster formation and find that they cannot fully reproduce the observed trends. A likely mixture of in-situ star formation and star-cluster mergers seem to be acting during nucleus growth over cosmic time.

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arXiv2018

The search for multiple populations in Magellanic Cloud Clusters IV: Coeval multiple stellar populations in the young star cluster NGC 1978

We have recently shown that the $\sim2$ Gyr old Large Magellanic Cloud star cluster NGC 1978 hosts multiple populations in terms of star-to-star abundance variations in [N/Fe]. These can be seen as a splitting or spread in the sub-giant and red giant branches (SGB and RGB) when certain photometric filter combinations are used. Due to its relative youth, NGC 1978 can be used to place stringent limits on whether multiple bursts of star-formation have taken place within the cluster, as predicted by some models for the origin of multiple populations. We carry out two distinct analyses to test whether multiple star-formation epochs have occurred within NGC 1978. First, we use UV CMDs to select stars from the first and second population along the SGB, and then compare their positions in optical CMDs, where the morphology is dominantly controlled by age as opposed to multiple population effects. We find that the two populations are indistinguishable, with age differences of $1\pm20$ Myr between them. This is in tension with predictions from the AGB scenario for the origin of multiple populations. Second, we estimate the broadness of the main sequence turnoff (MSTO) of NGC 1978 and we report that it is consistent with the observational errors. We find an upper limit of $\sim$65 Myr on the age spread in the MSTO of NGC 1978. This finding is in conflict with the age spread scenario as origin of the extended MSTO in intermediate age clusters, while it fully supports predictions from the stellar rotation model.

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arXiv2018

The Next Generation Fornax Survey (NGFS): III. Revealing the Spatial Substructure of the Dwarf Galaxy Population inside half of Fornax's Virial Radius

We report the discovery of 271 previously undetected dwarf galaxies in the outer Fornax cluster regions at radii $r_{\rm vir}/4<\!r\!<r_{\rm vir}/2$ using data from the Next Generation Fornax Survey (NGFS) with deep coadded $u'$, $g'$ and $i'$ images obtained with Blanco/DECam at Cerro Tololo Interamerican Observatory. From the 271 dwarf candidates we find 39 to be nucleated. Together with our previous study of the central Fornax region, the new dwarfs detected with NGFS data are 392, of which 56 are nucleated. The total Fornax dwarf galaxy population from NGFS and other catalogs rises, therefore, to a total of 643 with 181 being nucleated, yielding an overall nucleation fraction of $28\%$. The absolute $i'$-band magnitudes for the outer NGFS dwarfs are in the range $-18.80\le\,M_{i'}\le\,-8.78$ with effective radii $r_{{\rm eff},i'}\,=\,0.18-2.22$ kpc and an average Sersic index $\langle n\rangle_{i'}\,=\,0.81$. Non-nucleated dwarfs are found to be fainter and smaller by $Δ\langle M_{i'}\rangle\!=\!2.25$ mag and $Δ\langle r_{{\rm eff},i'}\rangle\!=\!0.4$ kpc than the nucleated dwarfs. We demonstrate a significant clustering of dwarf galaxies on scales $\lesssim\!100$ kpc, and projected surface number density profile estimates, $Σ_N(r)$, show a concentration of dwarfs in the Fornax core region within $r\!\sim\!350$ kpc. $Σ_N(r)$ has a flat distribution up to $\sim\!350$ kpc, beyond which it declines for the non-nucleated dwarfs. The nucleated dwarfs have a steeper $Σ_N(r)$ distribution, being more concentrated towards NGC 1399 and decreasing rapidly outwards. This is the first time the transition from cluster to field environment has been established for the very faint dwarf galaxy population with robust sample statistics.

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arXiv2018

The Fornax Cluster VLT Spectroscopic Survey. I - VIMOS spectroscopy of compact stellar systems in the Fornax core region

We present the results of a wide spectroscopic survey aimed at detecting extragalactic globular clusters (GCs) in the core of the Fornax cluster. About 4500 low resolution spectra (from 4800 to 10000 Å) were observed in 25 VLT/VIMOS masks covering the central 1 deg$^{2}$ around the dominant galaxy NGC 1399 corresponding to $\sim$175 kpc galactocentric radius. We describe the methodology used for data reduction and data analysis. We found a total of 387 unique physical objects (372 GCs and 15 ultra compact dwarfs) in the field covered by our observations. Most of these objects are associated with NGC 1399, with only 10% likely belonging to other giant galaxies. The new VIMOS dataset is complementary to the many GC catalogues already present in the literature and it brings the total number of tracer particles around NGC 1399 to more than 1130 objects. With this comprehensive radial velocity sample we have found that the velocity dispersion of the GC population (equally for red and blue GC populations) shows a relatively sharp increase from low velocity dispersion ($\sim250$-$350$ kms$^{-1}$) to high velocity dispersion ($\sim300$-$400$ kms$^{-1}$) at projected radius of $\approx10$ arcmin ($\sim60$ kpc) from the galaxy centre. This suggests that at a projected radius of $\approx60$ kpc both blue and red GC populations begin to be governed by the dominating Fornax cluster potential, rather than by the central NGC 1399 galaxy potential. This kinematic evidence corroborates similar results found using surface brightness analysis and planetary nebulae kinematics.

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arXiv2018

Kron 3: a fourth intermediate age cluster in the SMC with evidence of multiple populations

We present the results of a spectroscopic study of the intermediate age (approximately 6.5 Gyr) massive cluster Kron 3 in the Small Magellanic Cloud. We measure CN and CH band strengths (at 3839 and 4300 Angstroms respectively) using VLT FORS2 spectra of 16 cluster members and find a sub-population of 5 stars enriched in nitrogen. We conclude that this is evidence for multiple populations in Kron 3, the fourth intermediate age cluster, after Lindsay 1, NGC 416 and NGC 339 (ages 6-8 Gyr), to display this phenomenon originally thought to be a unique characteristic of old globular clusters. At 6.5 Gyr this is one of the youngest clusters with multiple populations, indicating that the mechanism responsible for their onset must operate until a redshift of at least 0.75, much later than the peak of globular cluster formation at redshift ~3.

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arXiv2017

Intra-cluster patches of baryons in the core of the Fornax cluster

In the core of the Fornax cluster, on the West side of NGC1399, we have detected a previously unknown region of intra-cluster light (ICL). It is made up by several faint ($μ_r \simeq 28 - 29$~mag/arcsec$^2$) {\it patches} of diffuse light. The bulk of the ICL is located in between the three bright galaxies in the core, NGC1387, NGC1379 and NGC1381, at $10\leq R \leq40$~arcmin ($\sim 58 - 230$~kpc) from the central galaxy NGC~1399. We show that the ICL is the counterpart in the diffuse light of the known over-density in the population of blue globular clusters (GCs). The total g-band luminosity of the ICL is $L_g\simeq 8.3 \times 10^{9}$ $L_{\odot}$, which is $\sim5\%$ of the total luminosity of NGC1399. This is consistent with the fraction of the blue GCs in the same region of the cluster. The ICL has $g-r \sim 0.7$~mag, which is similar to the colors in the halo of the bright galaxies in the cluster core. The new findings were compared with theoretical predictions for the ICL formation and they support a scenario in which the intra-cluster population detected in the core of the Fornax cluster is build up by the tidal stripping of material (stars and GCs) from galaxy outskirts in a close passage with the cD. Moreover, the diffuse form of the ICL and its location close to the core of the cluster is expected in a dynamically evolved cluster as Fornax.

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arXiv2017

The Fornax Deep Survey (FDS) with the VST: III. Low Surface Brightness (LSB) dwarfs and Ultra Diffuse Galaxies (UDGs) in the center of the Fornax cluster

Studies of low surface brightness (LSB) galaxies in nearby clusters have revealed a sub-population of extremely diffuse galaxies with central surface brightness $μ_{0,g'}$ > 24 mag arcsec$^{-2}$ and effective radius between 1.5 kpc < R$_{e}$ < 10 kpc. The origin of these Ultra Diffuse Galaxies (UDGs) is still unclear, although several theories have been suggested. We exploit the deep g', r' and i'-band images of the Fornax Deep Survey (FDS), in order to identify LSB galaxies in the center of the Fornax cluster. We identified visually all extended structures having r'-band central surface brightness of $μ_{0,r'}$ > 23 mag arcsec$^{-2}$. We classified the objects based on their appearance and performed 2D Sérsic model fitting with GALFIT. We analyzed their distribution and orientations in the cluster, and studied their colors and compared the LSB galaxies in Fornax with those in other environments. Our sample consists of 205 galaxies of which 196 are LSB dwarfs (with R$_e$ < 1.5kpc) and nine are UDGs (R$_e$ > 1.5 kpc). We show that the UDGs have g'-r' colors similar to those of LSB dwarfs. The largest UDGs in our sample appear different from the other LSB galaxies, in that they are significantly more elongated and extended, whereas the smaller UDGs differ from the LSB dwarfs only by their effective radii. We do not find clear differences between the structural parameters of the UDGs in our sample and those of UDGs in other galaxy environments. We find that the dwarf LSB galaxies in our sample are less concentrated in the cluster center than the galaxies with higher surface brightness, and that their number density drops in the core of the cluster. Our findings are consistent with the small UDGs forming the tail of a continuous distribution of less extended LSB galaxies. However, the elongated and distorted shapes of the large UDGs could imply that they are tidally disturbed galaxies.

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arXiv2017

Age as a Major Factor in the Onset of Multiple Populations in Stellar Clusters

It is now well established that globular clusters (GCs) exhibit star-to-star light-element abundance variations (known as multiple stellar populations, MPs). Such chemical anomalies have been found in (nearly) all the ancient GCs (more than 10 Gyr old) of our Galaxy and its close companions, but so far no model for the origin of MPs is able to reproduce all the relevant observations. To gain new insights into this phenomenon, we have undertaken a photometric Hubble Space Telescope survey to study clusters with masses comparable to that of old GCs, where MPs have been identified, but with significantly younger ages. Nine clusters in the Magellanic Clouds with ages between $\sim$ 1.5-11 Gyr have been targeted in this survey. We confirm the presence of multiple populations in all clusters older than 6 Gyr and we add NGC 1978 to the group of clusters for which MPs have been identified. With an age of $\sim$ 2 Gyr, NGC 1978 is the youngest cluster known to host chemical abundance spreads found to date. We do not detect evident star-to-star variations for slightly younger massive clusters ($\sim$ 1.7 Gyr), thus pointing towards an unexpected age dependence for the onset of multiple populations. This discovery suggests that the formation of MPs is not restricted to the early Universe and that GCs and young massive clusters share common formation and evolutionary processes.

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arXiv2017

The Fornax Deep Survey with VST. II. Fornax A: a two-phase assembly caught on act

As part of the Fornax Deep Survey with the ESO VLT Survey Telescope, we present new $g$ and $r$ bands mosaics of the SW group of the Fornax cluster. It covers an area of $3 \times 2$ square degrees around the central galaxy NGC1316. The deep photometry, the high spatial resolution of OmegaCam and the large covered area allow us to study the galaxy structure, to trace stellar halo formation and look at the galaxy environment. We map the surface brightness profile out to 33arcmin ($\sim 200$kpc $\sim15R_e$) from the galaxy centre, down to $μ_g \sim 31$ mag arcsec$^{-2}$ and $μ_r \sim 29$ mag arcsec$^{-2}$. This allow us to estimate the scales of the main components dominating the light distribution, which are the central spheroid, inside 5.5 arcmin ($\sim33$ kpc), and the outer stellar envelope. Data analysis suggests that we are catching in act the second phase of the mass assembly in this galaxy, since the accretion of smaller satellites is going on in both components. The outer envelope of NGC1316 still hosts the remnants of the accreted satellite galaxies that are forming the stellar halo. We discuss the possible formation scenarios for NGC1316, by comparing the observed properties (morphology, colors, gas content, kinematics and dynamics) with predictions from cosmological simulations of galaxy formation. We find that {\it i)} the central spheroid could result from at least one merging event, it could be a pre-existing early-type disk galaxy with a lower mass companion, and {\it ii)} the stellar envelope comes from the gradual accretion of small satellites.

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arXiv2017

The Search for Multiple Populations in Magellanic Cloud Clusters III: No evidence for Multiple Populations in the SMC cluster NGC 419

We present the third paper about our ongoing HST survey for the search for multiple stellar populations (MPs) within Magellanic Cloud clusters. We report here the analysis of NGC 419, a $\sim 1.5$ Gyr old, massive ($\gtrsim 2 \times 10^5 \, {\rm M_{\odot}}$) star cluster in the Small Magellanic Cloud (SMC). By comparing our photometric data with stellar isochrones, we set a limit on [N/Fe] enhancement of $\lesssim$+0.5 dex and hence we find that no MPs are detected in this cluster. This is surprising because, in the first two papers of this series, we found evidence for MPs in 4 other SMC clusters (NGC 121; Lindsay 1, NGC 339, NGC 416), aged from 6 Gyr up to $\sim 10-11$ Gyr. This finding raises the question whether age could play a major role in the MPs phenomenon. Additionally, our results appear to exclude mass or environment as the only key factors regulating the existence of a chemical enrichment, since all clusters studied so far in this survey are equally massive ($\sim 1-2 \times 10^5 \, {\rm M_{\odot}}$) and no particular patterns are found when looking at their spatial distribution in the SMC.

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arXiv2017

The globular cluster system of NGC 1316 IV. Nature of the star cluster complex SH2

The light of the merger remnant NGC 1316 is dominated by old and intermediate-age stars. The only sign of current star formation in this big galaxy is the HII region SH2, an isolated star cluster complex with a ring-like morphology and an estimated age of 0.1 Gyr at a galactocentric distance of about 35 kpc. A nearby intermediate-age globular cluster, surrounded by weak line emission and a few more young star clusters, is kinematically associated. The origin of this complex is enigmatic. The nebular emission lines permit a metallicity determination which can discriminate between a dwarf galaxy or other possible precursors. We used the Integrated Field Unit of the VIMOS instrument at the Very Large Telescope of the European Southern Observatory to study the morphology, kinematics, and metallicity employing line maps, velocity maps, and line diagnostics of a few characteristic spectra. The line ratios of different spectra vary, indicating highly structured HII regions, but define a locus of uniform metallicity. The strong-line diagnostic diagrams and empirical calibrations point to a nearly solar or even super-solar oxygen abundance. The velocity dispersion of the gas is highest in the region offset from the bright clusters. Star formation may be active on a low level. There is evidence for a large-scale disk-like structure in the region of SH2, which would make the similar radial velocity of the nearby globular cluster easier to understand. The high metallicity does not fit to a dwarf galaxy as progenitor. We favour the scenario of a free-floating gaseous complex having its origin in the merger 2 Gyr ago. Over a long period the densities increased secularly until finally the threshold for star formation was reached. SH2 illustrates how massive star clusters can form outside starbursts and without a considerable field population.

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arXiv2016

The Search for Multiple Populations in Magellanic Cloud Clusters II: The Detection of Multiple Populations in Three Intermediate-Age SMC Clusters

This is the second paper in our series about the search for multiple populations in Magellanic Cloud star clusters using the Hubble Space Telescope. Here we report the detection of multiple stellar populations in the colour-magnitude diagrams of the intermediate-age clusters Lindsay 1, NGC 416 and NGC 339. With ages between 6.0 and 7.5 Gyr, these clusters are the youngest ones in which chemical abundance spreads have been detected so far. This confirms that the appearance of multiple populations is not restricted to only ancient globular clusters, but may also be a common feature in clusters as young as 6 Gyr. Our results are in agreement with a recent spectroscopic study of Lindsay 1. We found that the fraction of enriched stars in NGC 416 is ~45% whereas it is ~25% in NGC 339 and ~36% in Lindsay 1. Similar to NGC 121, these fractions are lower than the average value for globular clusters in the Milky Way.

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arXiv2016

The Search for Multiple Populations in Magellanic Cloud Clusters I: Two Stellar Populations in the Small Magellanic Cloud Globular Cluster NGC 121

We started a photometric survey using the WFC3/UVIS instrument onboard the Hubble Space Telescope to search for multiple populations within Magellanic Cloud star clusters at various ages. In this paper, we introduce this survey. As first results of this programme, we also present multi-band photometric observations of NGC 121 in different filters taken with the WFC3/UVIS and ACS/WFC instruments. We analyze the colour-magnitude diagram (CMD) of NGC 121, which is the only "classical" globular cluster within the Small Magellanic Cloud. Thereby, we use the pseudo-colour C_(F336W,F438W,F343N)=(F336W-F438W)-(F438W-F343N) to separate populations with different C and N abundances. We show that the red giant branch splits up in two distinct populations when using this colour combination. NGC 121 thus appears to be similar to Galactic globular clusters in hosting multiple populations. The fraction of enriched stars (N rich, C poor) in NGC 121 is about 32% +/- 3%, which is lower than the median fraction found in Milky Way globular clusters. The enriched population seems to be more centrally concentrated compared to the primordial one. These results are consistent with the recent results by Dalessandro et al. (2016). The morphology of the Horizontal Branch in a CMD using the optical filters F555W and F814W is best produced by a population with a spread in Helium of Delta(Y) =0.025+/-0.005.

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