arXiv subjects

Paul Goudfrooij

Publications and source records attributed to Paul Goudfrooij.

At least 37 records · Page 2Linked to original sources

arXiv2020

The Strikingly Metal-Rich Halo of the Sombrero Galaxy

The nature of the Sombrero galaxy (M 104 = NGC 4594) has remained elusive despite many observational studies at a variety of wavelengths. Here we present Hubble Space Telescope imaging of two fields at $\sim$16 and 33 kpc along the minor axis to examine stellar metallicity gradients in the extended spheroid. We use this imaging, extending more than 2 mag below the tip of the red giant branch (TRGB), in combination with artificial star tests to forward model observed color-magnitude diagrams (CMDs), measuring metallicity distribution functions (MDFs) at different radii along the minor axis. An important and unexpected result is that the halo of the Sombrero is strikingly metal-rich: even the outer field, located at $\sim$17 effective radii of the bulge, has a median metallicity [Z/H]$\sim$-0.15 and the fraction of stars with [Z/H]<-1.0 is negligible. This is unprecedented among massive galaxy halos studied to date, even among giant ellipticals. We find significant radial metallicity gradients, characterized by an increase in the fraction of metal-poor stars with radius and a gradient in median metallicity of $\sim$-0.01 dex/kpc. The density profile is well fit by power laws with slopes that exhibit a dependence on metallicity, with flatter slopes for more metal-poor stars. We discuss our results in the context of recent stellar MDF studies of other nearby galaxies and potential formation scenarios for the Sombrero galaxy.

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arXiv2019

Combined Effects of Rotation and Age Spreads on Extended Main Sequence Turn Offs

The extended main sequence turn offs (eMSTOs) of several young to intermediate age clusters are examined in the Magellanic Clouds and the Milky Way. We explore the effects of extended star formation (eSF) and a range of stellar rotation rates on the behavior of the color-magnitude diagram (CMD), paying particular attention to the MSTO. We create synthetic stellar populations based on MESA stellar models to simulate observed Hubble Space Telescope and Gaia star cluster data. We model the effect of rotation as a non-parametric distribution, allowing for maximum flexibility. In our models the slow rotators comprise the blueward, and fast rotators the redward portion of the eMSTO. We simulate data under three scenarios: non-rotating eSF, a range of rotation rates with a single age, and a combination of age and rotation effects. We find that two of the five clusters (the youngest and oldest) favor an age spread, but these also achieve the overall worst fits of all clusters. The other three clusters show comparable statistical evidence between rotation and an age spread. In all five cases, a rotation rate distribution alone is capable of qualitatively matching the observed eMSTO structure. In future work, we aim to compare our predicted Vsin(i) with observations in order to better constrain the physics related to stellar rotation.

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arXiv2019

Multiple stellar populations in NGC 1866. New clues from Cepheids and Colour-Magnitude Diagram

We perform a comprehensive study of the stellar populations in the young Large Magellanic Cloud cluster NGC 1866, combining the analysis of its best-studied Cepheids with that of a very accurate colour-magnitude diagram (CMD) obtained from the most recent Hubble Space Telescope photometry. We use a Bayesian method based on new PARSEC stellar evolutionary tracks with overshooting and rotation to obtain ages and initial rotation velocities of five well-studied Cepheids of the cluster. We find that four of the five Cepheids belong to an initially slowly rotating young population (of $ 176 \pm 5$ Myr), while the fifth one is significantly older, either $ 288 \pm 20$ Myr for models with high initial rotational velocity ($ω_\mathrm{i} \sim 0.9$), or $ 202 \pm 5$ Myr for slowly rotating models. The complementary analysis of the CMD rules out the latter solution while strongly supporting the presence of two distinct populations of $\sim$176 Myr and $\sim$288 Myr, respectively. Moreover, the observed multiple main sequences and the turn-offs indicate that the younger population is mainly made of slowly rotating stars, as is the case of the four younger Cepheids, while the older population is made mainly of initially fast rotating stars, as is the case of the fifth Cepheid. Our study not only reinforces the notion that some young clusters like NGC 1866 harbor multiple populations, but gives also hints that the first population, the older, may inherit the angular momentum from the parent cloud while stars of the second one, the younger, do not.

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arXiv2019

On the photometric signature of fast rotators

Rapidly rotating stars have been recently recognized as having a major role in the interpretation of colour-magnitude diagrams of young and intermediate-age star clusters in the Magellanic Clouds and in the Milky Way. In this work, we evaluate the distinctive spectra and distributions in colour-colour space that follow from the presence of a substantial range in effective temperatures across the surface of fast rotators. The calculations are inserted in a formalism similar to the one usually adopted for non-rotating stars, which allows us to derive tables of bolometric corrections as a function not only of a reference effective temperature, surface gravity and metallicity, but also of the rotational speed with respect to the break-up value, $ω$, and the inclination angle, $i$. We find that only very fast rotators ($ω>0.95$) observed nearly equator-on ($i>45^\circ$) present sizable deviations from the colour-colour relations of non-rotating stars. In light of these results, we discuss the photometry of the $\sim$ 200-Myr-old cluster NGC 1866 and its split main sequence, which has been attributed to the simultaneous presence of slow and fast rotators. The small dispersion of its stars in colour-colour diagrams allow us to conclude that fast rotators in this cluster either have rotational velocities $ω<0.95$, or are all observed nearly pole-on. Such geometric colour-colour effects, although small, might be potentially detectable in the huge, high-quality photometric samples in the post-Gaia era, in addition to the evolutionary effects caused by rotation-induced mixing.

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arXiv2019

Mixing by overshooting and rotation in intermediate mass stars

Double-line eclipsing binaries (DLEBs) have been recently used to constrain the amount of central mixing as a function of stellar mass, with contrasting results. In this work, we reanalyze the DLEB sample by Claret & Torres, using a Bayesian method and new PARSEC tracks that account for both convective core overshooting and rotational mixing. Using overshooting alone we obtain that, for masses larger than about 1.9 M$_{\odot}$, the distribution of the overshooting parameter, $λ_\mathrm{ov}$, has a wide dispersion between 0.3 and 0.8, with essentially no values below $λ_\mathrm{ov}=$ 0.3 - 0.4. While the lower limit supports a mild convective overshooting efficiency, the large dispersion derived is difficult to explain in the framework of current models of that process, which leave little room for large randomness. We suggest that a simple interpretation of our results can be rotational mixing: different initial rotational velocities, in addition to a fixed amount of overshooting, could reproduce the high dispersion derived for intermediate-mass stars. After a reanalysis of the data, we find good agreement with models computed with fixed overshooting parameter, $λ_\mathrm{ov}=0.4$, and initial rotational rates, $ω$, uniformly distributed in a wide range between $0$ and $0.8$ times the break-up value, at varying initial mass. We also find that our best-fitting models for the components of $α$ Aurigae and TZ Fornacis, agree with their observed rotational velocities, thus providing independent support to our hypothesis. We conclude that a constant efficiency of overshooting in concurrence with a star-to-star variation in the rotational mixing, might be crucial in the interpretation of such data.

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arXiv2018

The Minimum Mass of Rotating Main Sequence Stars and its Impact on the Nature of Extended Main Sequence Turnoffs in Intermediate-Age Star Clusters in the Magellanic Clouds

Extended main sequence turn-offs (eMSTOs) are a common feature in color-magnitude diagrams (CMDs) of young and intermediate-age star clusters in the Magellanic Clouds. The nature of eMSTOs is still debated. The most popular scenarios are extended star formation and ranges of stellar rotation rates. Here we study implications of a kink feature in the main sequence (MS) of young star clusters in the Large Magellanic Cloud (LMC). This kink shows up very clearly in new \emph{Hubble Space Telescope} observations of the 700-Myr-old cluster NGC 1831, and is located below the region in the CMD where multiple or wide MSes, which are known to occur in young clusters and thought to be due to varying rotation rates, merge together into a single MS. The kink occurs at an initial stellar mass of $1.45 \pm 0.02\;M_{\odot}$; we posit that it represents a lower limit to the mass below which the effects of rotation on the energy output of stars are rendered negligible at the metallicity of these clusters. Evaluating the positions of stars with this initial mass in CMDs of massive LMC star clusters with ages of $\sim\,$1.7 Gyr that feature wide eMSTOs, we find that such stars are located in a region where the eMSTO is already significantly wider than the MS below it. This strongly suggests that stellar rotation \emph{cannot} fully explain the wide extent of eMSTOs in massive intermediate-age clusters in the Magellanic Clouds. A distribution of stellar ages still seems necessary to explain the eMSTO phenomenon.

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arXiv2018

The GeMS/GSAOI Galactic Globular Cluster Survey (G4CS) I: A Pilot Study of the stellar populations in NGC 2298 and NGC 3201

We present the first results from the GeMS/GSAOI Galactic Globular Cluster Survey (G4CS) of the Milky-Way globular clusters (GCs) NGC 3201 and NGC 2298. Using the Gemini South Adaptive Optics Imager (GSAOI), in tandem with the Gemini Multi-conjugate adaptive optics System (GeMS) on the 8.1-meter Gemini-South telescope, we collected deep near-IR observations of both clusters, resolving their constituent stellar populations down to $K_s\simeq21$ Vega mag. Point spread function (PSF) photometry was performed on the data using spatially-variable PSFs to generate $JHK_{s}$ photometric catalogues for both clusters. These catalogues were combined with Hubble Space Telescope (HST) data to augment the photometric wavelength coverage, yielding catalogues that span the near-ultraviolet (UV) to near-infrared (near-IR). We then applied 0.14 mas/year accurate proper-motion cleaning, differential-reddening corrections and chose to anchor our isochrones using the lower main-sequence knee (MSK) and the main-sequence turn-off (MSTO) prior to age determination. As a result of the data quality, we found that the $K_{s}$ vs. F606W$-K_{s}$ and F336W vs. F336W$-K_{s}$ color-magnitude diagrams (CMDs) were the most diagnostically powerful. We used these two color combinations to derive the stellar-population ages, distances and reddening values for both clusters. Following isochrone-fitting using three different isochrone sets, we derived best-fit absolute ages of $12.2\pm0.5$ Gyr and $13.2\pm0.4$ Gyr for NGC 3201 and NGC 2298, respectively. This was done using a weighted average over the two aforementioned color combinations, following a pseudo-$χ^2$ determination of the best-fit isochrone set. Our derived parameters are in good agreement with recent age determinations of the two clusters, with our constraints on the ages being or ranking among the most statistically robust.

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arXiv2018

Dissolved Massive Metal-Rich Globular Clusters Can Cause the Range of UV Upturn Strengths Found among Early-type Galaxies

I discuss a scenario in which the ultraviolet (UV) upturn of giant early-type galaxies (ETGs) is primarily due to helium-rich stellar populations that formed in massive metal-rich globular clusters (GCs) which subsequently dissolved in the strong tidal field in the central regions of the massive host galaxy. These massive GCs are assumed to show UV upturns similar to those observed recently in M87, the central giant elliptical galaxy in the Virgo cluster of galaxies. Data taken from the literature reveals a strong correlation between the strength of the UV upturn and the specific frequency of metal-rich GCs in ETGs. Adopting a Schechter function parametrization of GC mass functions, simulations of long-term dynamical evolution of GC systems show that the observed correlation between UV upturn strength and GC specific frequency can be explained by variations in the characteristic truncation mass ${\cal{M}}_{\rm c}$ such that ${\cal{M}}_{\rm c}$ increases with ETG luminosity in a way that is consistent with observed GC luminosity functions in ETGs. These findings suggest that the nature of the UV upturn in ETGs and the variation of its strength among ETGs are causally related to that of helium-rich populations in massive GCs, rather than intrinsic properties of field stars in massive galactic spheroids. With this in mind, I predict that future studies will find that [N/Fe] decreases with increasing galactocentric radius in massive ETGs, and that such gradients have the largest amplitudes in ETGs with the strongest UV upturns.

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arXiv2017

Extended Main Sequence Turnoffs in Intermediate-Age Star Clusters: Stellar Rotation diminishes, but does not eliminate, Age Spreads

Extended main sequence turn-off (eMSTO) regions are a common feature in color-magnitude diagrams of young and intermediate-age star clusters in the Magellanic Clouds. The nature of eMSTOs remains debated in the literature. The currently most popular scenarios are extended star formation activity and ranges of stellar rotation rates. Here we study details of differences in MSTO morphology expected from spreads in age versus spreads in rotation rates, using Monte Carlo simulations with the Geneva SYCLIST isochrone models that include the effects of stellar rotation. We confirm a recent finding of Niederhofer et al. that a distribution of stellar rotation velocities yields an MSTO extent that is proportional to the cluster age, as observed. However, we find that stellar rotation yields MSTO crosscut widths that are generally smaller than observed ones at a given age. We compare the simulations with high-quality Hubble Space Telescope data of NGC 1987 and NGC 2249, which are the two only relatively massive star clusters with an age of ~1 Gyr for which such data is available. We find that the distribution of stars across the eMSTOs of these clusters cannot be explained solely by a distribution of stellar rotation velocities, unless the orientations of rapidly rotating stars are heavily biased towards an equator-on configuration. Under the assumption of random viewing angles, stellar rotation can account for ~60% and ~40% of the observed FWHM widths of the eMSTOs of NGC 1987 and NGC 2249, respectively. In contrast, a combination of distributions of stellar rotation velocities and stellar ages fits the observed eMSTO morphologies very well.

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arXiv2016

Dissecting the Extended Main Sequence Turn-off of the Young Star Cluster NGC1850

We use the Wide Field Camera 3 onboard the Hubble Space Telescope to obtain deep, high-resolution photometry of the young (~ 100 Myr) star cluster NGC1850 in the Large Magellanic Cloud. We analyze the cluster colour-magnitude diagram (CMD) and find that it hosts an extended main sequence turn-off (MSTO) and a double MS. We demonstrate that these features cannot be due to photometric errors, field star contamination, or differential reddening. From a comparison with theoretical models and Monte Carlo simulations, we show that a coeval stellar population featuring a distribution of stellar rotation rates can reproduce the MS split quite well. However, it cannot reproduce the observed MSTO region, which is significantly wider than the simulated ones. Exploiting narrow-band Halpha imaging, we find that the MSTO hosts a population of Halpha-emitting stars which are interpreted as rapidly rotating Be-type stars. We explore the possibility that the discrepancy between the observed MSTO morphology and that of the simulated simple stellar population (SSP) is caused by the fraction of these objects that are highly reddened, but we rule out this hypothesis. We demonstrate that the global CMD morphology is well-reproduced by a combination of SSPs that cover an age range of ~ 35 Myr as well as a wide variety of rotation rates. We derive the cluster mass and escape velocity and use dynamical evolution models to predict their evolution starting at an age of 10 Myr. We discuss these results and their implications in the context of the extended MSTO phenomenon.

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arXiv2016

Evolution of the Mass and Luminosity Functions of Globular Star Clusters

We reexamine the dynamical evolution of the mass and luminosity functions of globular star clusters (GCMF and GCLF). Fall & Zhang (2001, hereafter FZ01) showed that a power-law MF, as commonly seen among young cluster systems, would evolve by dynamical processes over a Hubble time into a peaked MF with a shape very similar to the observed GCMF in the Milky Way and other galaxies. To simplify the calculations, the semi-analytical FZ01 model adopted the "classical" theory of stellar escape from clusters, and neglected variations in the $M/L$ ratios of clusters. Kruijssen & Portegies Zwart (2009, hereafter KPZ09) modified the FZ01 model to include "retarded" and mass-dependent stellar escape, the latter causing significant $M/L$ variations. KPZ09 asserted that their model was compatible with observations whereas the FZ01 model was not. We show here that this claim is not correct; the FZ01 and KPZ09 models fit the observed Galactic GCLF equally well. We also show that there is no detectable correlation between $M/L$ and $L$ for GCs in the Milky Way and Andromeda galaxies, in contradiction with the KPZ09 model. Our comparisons of the FZ01 and KPZ09 models with observations can be explained most simply if stars escape at rates approaching the classical limit for high-mass clusters, as expected on theoretical grounds.

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arXiv2016

Faint Dwarf Galaxies in Hickson Compact Group 90

We report the discovery of a very diverse set of five low-surface brightness (LSB) dwarf galaxy candidates in Hickson Compact Group 90 (HCG 90) detected in deep U- and I-band images obtained with VLT/VIMOS. These are the first LSB dwarf galaxy candidates found in a compact group of galaxies. We measure spheroid half-light radii in the range $0.7\!\lesssim\! r_{\rm eff}/{\rm kpc}\! \lesssim\! 1.5$ with luminosities of $-11.65\!\lesssim\! M_U\! \lesssim\! -9.42$ and $-12.79\!\lesssim\! M_I\! \lesssim\! -10.58$ mag, corresponding to a color range of $(U\!-\!I)_0\!\simeq\!1.1\!-\!2.2$ mag and surface brightness levels of $μ_U\!\simeq\!28.1\,{\rm mag/arcsec^2}$ and $μ_I\!\simeq\!27.4\,{\rm mag/arcsec^2}$. Their colours and luminosities are consistent with a diverse set of stellar population properties. Assuming solar and 0.02 Z$_\odot$ metallicities we obtain stellar masses in the range $M_*|_{Z_\odot} \simeq 10^{5.7-6.3} M_{\odot}$ and $M_*|_{0.02\,Z_\odot}\!\simeq\!10^{6.3-8}\,M_{\odot}$. Three dwarfs are older than 1 Gyr, while the other two significantly bluer dwarfs are younger than $\sim 2$ Gyr at any mass/metallicity combination. Altogether, the new LSB dwarf galaxy candidates share properties with dwarf galaxies found throughout the Local Volume and in nearby galaxy clusters such as Fornax. We find a pair of candidates with $\sim\!2$ kpc projected separation, which may represent one of the closest dwarf galaxy pairs found. We also find a nucleated dwarf candidate, with a nucleus size of $r_{\rm eff}\!\simeq\!46\!-\!63$ pc and magnitude M$_{U,0}=-7.42$ mag and $(U\!-\!I)_0\!=\!1.51$ mag, which is consistent with a nuclear stellar disc with a stellar mass in the range $10^{4.9-6.5}\,M_\odot$.

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arXiv2015

Compact stellar systems in the polar ring galaxies NGC 4650A and NGC 3808B: Clues to polar disk formation

Polar ring galaxies (PRGs) are composed of two kinematically distinct and nearly orthogonal components, a host galaxy (HG) and a polar ring/disk (PR). The HG usually contains an older stellar population than the PR. The suggested formation channel of PRGs is still poorly constrained. Suggested options are merger, gas accretion, tidal interaction, or a combination of both. To constrain the formation scenario of PRGs, we study the compact stellar systems (CSSs) in two PRGs at different evolutionary stages: NGC 4650A with well-defined PR, and NGC 3808B, which is in the process of PR formation. We use archival HST/WFPC2 imaging. PSF-fitting techniques, and color selection criteria are used to select cluster candidates. Photometric analysis of the CSSs was performed to determine their ages and masses using stellar population models at a fixed metallicity. Both PRGs contain young CSSs ($< 1$ Gyr) with masses of up to 5$\times$10$^6$M$_\odot$, mostly located in the PR and along the tidal debris. The most massive CSSs may be progenitors of metal-rich globular clusters or ultra compact dwarf (UCD) galaxies. We identify one such young UCD candidate, NGC 3808 B-8, and measure its size of $r_{\rm eff}=25.23^{+1.43}_{-2.01}$ pc. We reconstruct the star formation history of the two PRGs and find strong peaks in the star formation rate (SFR $\simeq$ 200M$_\odot$/yr) in NGC 3808B, while NGC 4650A shows milder (declining) star formation (SFR $<$ 10M$_\odot$/yr). This difference may support different evolutionary paths between these PRGs. The spatial distribution, masses, and peak star formation epoch of the clusters in NGC 3808 suggest for a tidally triggered star formation. Incompleteness at old ages prevents us from probing the SFR at earlier epochs of NGC 4650A, where we observe the fading tail of CSS formation. This also impedes us from testing the formation scenarios of this PRG.

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arXiv2015

New Constraints on the Star Formation History of the Star Cluster NGC 1856

We use the Wide Field Camera 3 onboard the Hubble Space Telescope to obtain deep, high-resolution photometry of the young (age ~ 300 Myr) star cluster NGC1856 in the Large Magellanic Cloud. We compare the observed colour-magnitude diagram (CMD), after having applied a correction for differential reddening, with Monte Carlo simulations of simple stellar populations (SSPs) of various ages. We find that the main sequence turn-off (MSTO) region is wider than that derived from the simulation of a single SSP. Using constraints based on the distribution of stars in the MSTO region and the red clump, we find that the CMD is best reproduced using a combination of two different SSPs with ages separated by 80 Myr (0.30 and 0.38 Gyr, respectively). However, we can not formally exclude that the width of the MSTO could be due to a range of stellar rotation velocities if the efficiency of rotational mixing is higher than typically assumed. Using a King-model fit to the surface number density profile in conjunction with dynamical evolution models, we determine the evolution of cluster mass and escape velocity from an age of 10 Myr to the present age, taking into account the possible effects of primordial mass segregation. We find that the cluster has an escape velocity Vesc ~ 17 km/s at an age of 10 Myr, and it remains high enough during a period of ~ 100 Myr to retain material ejected by slow winds of first-generation stars. Our results are consistent with the presence of an age spread in NGC1856, in contradiction to the results of Bastian & Silva-Villa (2013).

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arXiv2015

On the Interpretation of Sub-Giant Branch Morphologies of Intermediate-Age Star Clusters with Extended Main Sequence Turnoffs

Recent high-quality photometry of many star clusters in the Magellanic Clouds with ages of 1$\,-\,$2 Gyr revealed main sequence turnoffs (MSTOs) that are significantly wider than can be accounted for by a simple stellar population (SSP). Such extended MSTOs (eMSTOs) are often interpreted in terms of an age spread of several $10^8$ yr, challenging the traditional view of star clusters as being formed in a single star formation episode. Li et al. and Bastian & Niederhofer recently investigated the sub-giant branches (SGBs) of NGC 1651, NGC 1806, and NGC 1846, three star clusters in the Large Magellanic Cloud (LMC) that exhibit an eMSTO. They argued that the SGB of these star clusters can be explained only by a SSP. We study these and two other similar star clusters in the LMC, using extensive simulations of SSPs including unresolved binaries. We find that the shapes of the cross-SGB profiles of all star clusters in our sample are in fact consistent with their cross-MSTO profiles when the latter are interpreted as age distributions. Conversely, SGB morphologies of star clusters with eMSTOs are found to be inconsistent with those of simulated SSPs. Finally, we create PARSEC isochrones from tracks featuring a grid of convective overshoot levels and a very fine grid of stellar masses. A comparison of the observed photometry with these isochrones shows that the morphology of the red clump (RC) of such star clusters is also consistent with that implied by their MSTO in the age spread scenario. We conclude that the SGB and RC morphologies of star clusters featuring eMSTOs are consistent with the scenario in which the eMSTOs are caused by a distribution of stellar ages.

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arXiv2014

Extended Main Sequence Turnoffs in Intermediate-Age Star Clusters: A Correlation Between Turnoff Width and Early Escape Velocity

We present color-magnitude diagram analysis of deep Hubble Space Telescope imaging of a mass-limited sample of 18 intermediate-age (1 - 2 Gyr old) star clusters in the Magellanic Clouds, including 8 clusters for which new data was obtained. We find that ${\it all}$ star clusters in our sample feature extended main sequence turnoff (eMSTO) regions that are wider than can be accounted for by a simple stellar population (including unresolved binary stars). FWHM widths of the MSTOs indicate age spreads of 200-550 Myr. We evaluate dynamical evolution of clusters with and without initial mass segregation. Our main results are: (1) the fraction of red clump (RC) stars in secondary RCs in eMSTO clusters scales with the fraction of MSTO stars having pseudo-ages $\leq 1.35$ Gyr; (2) the width of the pseudo-age distributions of eMSTO clusters is correlated with their central escape velocity $v_{\rm esc}$, both currently and at an age of 10 Myr. We find that these two results are unlikely to be reproduced by the effects of interactive binary stars or a range of stellar rotation velocities. We therefore argue that the eMSTO phenomenon is mainly caused by extended star formation within the clusters; (3) we find that $v_{\rm esc} \geq 15$ km/s out to ages of at least 100 Myr for ${\it all}$ clusters featuring eMSTOs, while $v_{\rm esc} \leq 12$ km/s at all ages for two lower-mass clusters in the same age range that do ${\it not}$ show eMSTOs. We argue that eMSTOs only occur for clusters whose early escape velocities are higher than the wind velocities of stars that provide material from which second-generation stars can form. The threshold of 12-15 km/s is consistent with wind velocities of intermediate-mass AGB stars in the literature.

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arXiv2014

Observations of Transiting Exoplanets with the James Webb Space Telescope (JWST), Publications of the Astronomical Society of the Pacific (PASP), December 2014

This article summarizes a workshop held on March, 2014, on the potential of the James Webb Space Telescope (JWST) to revolutionize our knowledge of the physical properties of exoplanets through transit observations. JWST's unique combination of high sensitivity and broad wavelength coverage will enable the accurate measurement of transits with high signal-to-noise. Most importantly, JWST spectroscopy will investigate planetary atmospheres to determine atomic and molecular compositions, to probe vertical and horizontal structure, and to follow dynamical evolution, i.e. exoplanet weather. JWST will sample a diverse population of planets of varying masses and densities in a wide variety of environments characterized by a range of host star masses and metallicities, orbital semi-major axes and eccentricities. A broad program of exoplanet science could use a substantial fraction of the overall JWST mission.

astro-ph.IM↗

arXiv2014

The HST/ACS Coma Cluster Survey. X. Nuclear star clusters in low-mass early-type galaxies: scaling relations

We present scaling relations between structural properties of nuclear star clusters and their host galaxies for a sample of early-type dwarf galaxies observed as part of the Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS) Coma Cluster Survey. We have analysed the light profiles of 200 early-type dwarf galaxies in the magnitude range $16.0 < m_{F814W} < 22.6 $ mag, corresponding to $-19.0 < M_{F814W} < -12.4 $ mag. Nuclear star clusters are detected in 80% of the galaxies, thus doubling the sample of HST-observed early-type dwarf galaxies with nuclear star clusters. \changed{We confirm that the} nuclear star cluster detection fraction decreases strongly toward faint magnitudes. The luminosities of nuclear star clusters do not scale linearly with host galaxy luminosity. A linear fit yields L$_{nuc} \sim $L$_{gal}^{0.57\pm0.05}$. The nuclear star cluster-host galaxy luminosity scaling relation for low-mass early-type dwarf galaxies is consistent with formation by globular cluster accretion. We find that at similar luminosities, galaxies with higher Sérsic indices have slightly more luminous nuclear star clusters. Rounder galaxies have on average more luminous clusters. Some of the nuclear star clusters are resolved, despite the distance of Coma. We argue that the relation between nuclear star cluster mass and size is consistent with both formation by globular cluster accretion and in situ formation. Our data are consistent with GC inspiraling being the dominant mechanism at low masses, although the observed trend with Sérsic index suggests that in situ star formation is an important second order effect.

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