arXiv subjects

Richard de Grijs

Publications and source records attributed to Richard de Grijs.

At least 217 records · Page 12Linked to original sources

arXiv2007

Star Clusters in the Nearby Late-Type Galaxy NGC 1311

Ultraviolet, optical and near infrared images of the nearby (D ~ 5.5 Mpc) SBm galaxy NGC 1311, obtained with the Hubble Space Telescope, reveal a small population of 13 candidate star clusters. We identify candidate star clusters based on a combination of their luminosity, extent and spectral energy distribution. The masses of the cluster candidates range from ~1000 up to ~100000 Solar masses, and show a strong positive trend of larger mass with increasing with cluster age. Such a trend follows from the fading and dissolution of old, low-mass clusters, and the lack of any young super star clusters of the sort often formed in strong starbursts. The cluster age distribution is consistent with a bursting mode of cluster formation, with active episodes of age ~10 Myr, ~100 Myr and ~1 Gyr. The ranges of age and mass we probe are consistent with those of the star clusters found in quiescent Local Group dwarf galaxies.

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arXiv2007

The poorly constrained cluster disruption time-scale in the Large Magellanic Cloud

We use Monte-Carlo simulations, combined with homogeneously determined age and mass distributions based on multi-wavelength photometry, to constrain the cluster formation history and the rate of bound cluster disruption in the Large Magellanic Cloud (LMC) cluster system. We evolve synthetic star cluster systems formed with a power-law initial cluster mass function (ICMF) of spectral index $α=-2$ assuming different cluster disruption time-scales. For each of these disruption time-scales we derive the corresponding cluster formation rate (CFR) required to reproduce the observed cluster age distribution. We then compare, in a Poissonian $χ^2$ sense, model mass distributions and model two-dimensional distributions in log(mass) vs. log(age) space of the detected surviving clusters to the observations. Because of the bright detection limit ($M_V^{\rm lim} \simeq -4.7$ mag) above which the observed cluster sample is complete, one cannot constrain the characteristic disruption time-scale for a $10^4$ M$_\odot$ cluster, $t_4^{\rm dis}$ (where the disruption time-scale depends on cluster mass as $t_{\rm dis} = t_4^{\rm dis} (M_{\rm cl} / 10^4 {\rm M}_\odot)^0.62$), to better than $t_4^{\rm dis} \ge 1$ Gyr. We conclude that the CFR has increased from 0.3 clusters Myr$^{-1}$ 5 Gyr ago, to a present rate of $(20-30)$ clusters Myr$^{-1}$. For older ages the derived CFR depends sensitively on our assumption of the underlying CMF shape. If we assume a universal Gaussian ICMF, then the CFR has increased steadily over a Hubble time from $\sim 1$ cluster Gyr$^{-1}$ 15 Gyr ago to its present value. If the ICMF has always been a power law with a slope close to $α=-2$, the CFR exhibits a minimum some 5 Gyr ago, which we tentatively identify with the well-known age gap in the LMC's cluster age distribution.

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arXiv2007

Star cluster "infant mortality" in the Small Magellanic Cloud (Redivivus)

The early evolution of star clusters in the Small Magellanic Cloud (SMC) has been the subject of significant recent controversy, particularly regarding the importance and length of the earliest, largely mass-independent disruption phase (referred to as "infant mortality"). Here, we take a fresh approach to the problem, using an independent, homogeneous data set of UBVR imaging observations, from which we obtain the SMC's cluster age and mass distributions in a self-consistent manner. We conclude that the (optically selected) SMC star cluster population has undergone at most ~30 per cent (1 sigma) infant mortality between the age range from about (3-10) Myr, to that of approximately (40-160) Myr. We rule out a 90 per cent cluster mortality rate per decade of age (for the full age range up to 10^9 yr) at a > 6 sigma level. We independently affirm this scenario based on the age distribution of the SMC cluster sample.

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arXiv2007

Young massive star clusters in the era of the Hubble Space Telescope

The Hubble Space Telescope (HST) has been instrumental in the discovery of large numbers of extragalactic young massive star clusters (YMCs), often assumed to be proto-globular clusters (GCs). As a consequence, the field of YMC formation and evolution is thriving, generating major breakthroughs as well as controversies on annual (or shorter) time-scales. Here, I review the long-term survival chances of YMCs, hallmarks of intense starburst episodes often associated with violent galaxy interactions. In the absence of significant external perturbations, the key factor determining a cluster's long-term survival chances is the shape of its stellar initial mass function (IMF). It is, however, not straightforward to assess the IMF shape in unresolved extragalactic YMCs. I also discuss the latest progress in worldwide efforts to better understand the evolution of entire cluster populations, predominantly based on HST observations, and conclude that there is an increasing body of evidence that GC formation appears to be continuing until today; their long-term evolution crucially depends on their environmental conditions, however.

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arXiv2007

The long-term survival chances of young massive star clusters

We review the long-term survival chances of young massive star clusters (YMCs), hallmarks of intense starburst episodes often associated with violent galaxy interactions. We address the key question as to whether at least some of these YMCs can be considered proto-globular clusters (GCs), in which case these would be expected to evolve into counterparts of the ubiquitous old GCs believed to be among the oldest galactic building blocks. In the absence of significant external perturbations, the key factor determining a cluster's long-term survival chances is the shape of its stellar initial mass function (IMF). It is, however, not straightforward to assess the IMF shape in unresolved extragalactic YMCs. We discuss in detail the promise of using high-resolution spectroscopy to make progress towards this goal, as well as the numerous pitfalls associated with this approach. We also discuss the latest progress in worldwide efforts to better understand the evolution of entire cluster systems, the disruption processes they are affected by, and whether we can use recently gained insights to determine the nature of at least some of the YMCs observed in extragalactic starbursts as proto-GCs. We conclude that there is an increasing body of evidence that GC formation appears to be continuing until today; their long-term evolution crucially depends on their environmental conditions, however.

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arXiv2006

Arp 116: Interacting System or Chance Alignment?

Using high spatial resolution Hubble Space Telescope/Advanced Camera for Surveys archival imaging observations of Arp 116, centred on the elliptical galaxy NGC 4649, we explore the novel technique of pixel-by-pixel analysis of the galaxy's colour-magnitude diagramme to search for any evidence of recent enhanced star formation due to the apparent tidal interaction with its spiral companion, NGC 4647. From a detailed analysis of the system's geometry, and based on additional circumstantial evidence from extant multi-wavelength observations, we conclude that, while there may be grounds for the tidal-interaction assumption for this system, any interaction has thus far been of insufficient strength to trigger an enhanced level of recent star formation in the elliptical component, although close inspection of our colour images shows a faint excess of bluer pixels (a ~0.20 mag bluer "loop") in the elliptical galaxy on the side of the spiral companion. Given that there appears to be a moderate reservoir of available gas for ongoing star formation (although at low column density), this suggests that we are currently witnessing the onset of the tidal interaction between NGC 4647 and NGC 4649. In addition, the triggering of new star formation in NGC 4649 may be significantly impeded due to the much lower mass of the spiral component.

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arXiv2006

From nuclear clusters to halo globulars: Star clusters as basic galactic building blocks

I assess the similarities and differences between the star-formation modes in quiescent spiral galaxies versus those in violent starburst regions, including galactic nuclei. As opposed to the quiescent star-formation mode, current empirical evidence on the star-formation processes in the extreme, high-pressure environments induced by galaxy encounters strongly suggests that star cluster formation is an important and perhaps even the dominant mode of star formation in such starburst events. This implies that by using star clusters as unique diagnostic probes, we can trace a galaxy's most violent star formation history very well, at least for the past few Gyr. The sizes, luminosities, and mass estimates of the young massive star clusters are entirely consistent with what is expected for young Milky Way-type globular clusters (GCs). Recent evidence lends support to the scenario that GCs, which were once thought to be the oldest building blocks of galaxies, are still forming today.

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arXiv2006

Diffuse, Non-Thermal X-ray Emission from the Galactic Star Cluster Westerlund 1

We present the diffuse X-ray emission identified in Chandra observations of the young, massive Galactic star cluster Westerlund 1. After removing point-like X-ray sources down to a completeness limit of 2e31 erg/s, we identify 3e34 erg/s (2--8 keV) of diffuse emission. The spatial distribution of the emission can be described as a slightly-elliptical Lorentzian core with a half-width half-maximum along the major axis of 25+/-1", similar to the distribution of point sources in the cluster, plus a 5' halo of extended emission. The spectrum of the diffuse emission is dominated by a hard continuum component that can be described as a kT>3 keV thermal plasma that has a low iron abundance (<0.3 solar), or as non-thermal emission that could be stellar light that is inverse-Compton scattered by MeV electrons. Only 5% of the flux is produced by a kT=0.7 keV plasma. The low luminosity of the thermal emission and the lack of a 6.7 keV iron line suggests that <40,000 unresolved stars with masses between 0.3 and 2 Msun are present in the cluster. Moreover, the flux in the diffuse emission is a factor of two lower than would be expected from a supersonically-expanding cluster wind, and there is no evidence for thermal remnants produced by supernovae. Less than 1e-5 of the mechanical luminosity of the cluster is dissipated as 2--8 keV X-rays, leaving a large amount of energy that either is radiated at other wavelengths, is dissipated beyond the bounds of our image, or escapes into the intergalactic medium.

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arXiv2006

Star Cluster Evolution: From young massive star clusters to old globulars

Young, massive star clusters are the most notable and significant end products of violent star-forming episodes triggered by galaxy collisions, mergers, and close encounters. The question remains, however, whether or not at least a fraction of the compact YMCs seen in abundance in extragalactic starbursts, are potentially the progenitors of globular cluster (GC)-type objects. However, because of the lack of a statistically significant sample of similar nearby objects we need to resort to either statistical arguments or to the painstaking approach of case by case studies of individual objects in more distant galaxies. Despite the difficulties inherent to addressing this issue conclusively, an ever increasing body of observational evidence lends support to the scenario that GCs, which were once thought to be the oldest building blocks of galaxies, are still forming today.

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arXiv2006

The photometric evolution of dissolving star clusters I: First predictions

We calculated the broad-band photometric evolution of unresolved star clusters, including the preferential loss of low-mass stars due to mass segregation. The stellar mass function of a cluster evolves due to three effects: (a) the evolution of massive stars; (b) early tidal effects reduce the mass function independently of the stellar mass; (c) after mass segregation has completed, tidal effects preferentially remove the lowest-mass stars from the cluster. Results: (1) During the first ~40% of the lifetime of a cluster the cluster simply gets fainter due to the loss of stars by tidal effects. (2) Between ~40 and ~80% of its lifetime the cluster gets bluer due to the loss of low-mass stars. This will result in an underestimate of the age of clusters if standard cluster evolution models are used (0.15 -- 0.5 dex). (3) After ~80% of the total lifetime of a cluster it will rapidly get redder. This is because stars at the low-mass end of the main sequence, which are preferentially lost, are bluer than the AGB stars that dominate the light at long wavelengths, resulting in an age overestimate. (4) Clusters with mass segregation and the preferential loss of low-mass stars evolve along almost the same tracks in colour-colour diagrams as clusters without mass segregation. Therefore it will be difficult to distinguish this effect from that due to the cluster age for unresolved clusters, unless the total lifetime of the clusters can be estimated. (5) The changes in the colour evolution of unresolved clusters due to the preferential loss of low-mass stars will affect the determination of the SFHs. (6) The preferential loss of low-mass stars might explain the presence of old (~13 Gyr) clusters in NGC 4365 which are photometrically disguised as intermediate-age clusters (2 - 5 Gyr). [Abridged]

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arXiv2005

How well do we know the age and mass distributions of the star cluster system in the Large Magellanic Cloud?

[ABRIDGED] The LMC star cluster system offers the unique opportunity to independently check the accuracy of age and mass determinations based on a number of complementary techniques, including isochrone analysis. Using our sophisticated tool for star cluster analysis based on broad-band spectral energy distributions (SEDs), we reanalyse the Hunter et al. (2003) LMC cluster photometry. Our main aim is to set the tightest limits yet on the accuracy of ABSOLUTE age determinations based on broad-band SEDs, and therefore on the usefulness of such an approach. Our broad-band SED fits yield reliable ages, with statistical absolute uncertainties within Delta[log(Age/yr)] = 0.4 overall. The systematic differences we find with respect to previous age determinations are caused by conversions of the observational photometry to a different filter system. The LMC's cluster formation rate (CFR) has been roughly constant outside of the well-known age gap between ~3 and 13 Gyr, when the CFR was a factor of ~5 lower. We derive the characteristic cluster disruption time-scale, log(t_4^dis/yr) = 9.9 +- 0.1, where t_dis = t_4^dis (M_cl/10^4 Msun)^0.62. This long characteristic disruption time-scale implies that we are observing the INITIAL cluster mass function (CMF). We conclude that the youngest mass and luminosity-limited LMC cluster subsets show shallower slopes than the slope of alpha = -2 expected (at least below masses of a few x 10^3 Msun), which is contrary to dynamical expectations. This may imply that the initial CMF slope of the LMC cluster system as a whole is NOT well represented by a power-law, although we cannot disentangle the unbound from the bound clusters at the youngest ages.

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arXiv2005

Modes of star formation along the Hubble Sequence and beyond

I assess the similarities and differences between the star-formation modes in quiescent spiral galaxies versus those in violent starburst regions. As opposed to the quiescent star-formation mode in spiral galaxies, current empirical evidence on the star-formation processes in the extreme, high-pressure environments induced by galaxy encounters strongly suggests that star CLUSTER formation is an important and perhaps even the dominant mode of star formation in such starburst events. The sizes, luminosities, and mass estimates of the young massive star clusters (YMCs) are entirely consistent with what is expected for young Milky Way-type globular clusters (GCs). Recent evidence lends support to the scenario that GCs, which were once thought to be the oldest building blocks of galaxies, are still forming today. One of the key unanswered questions in this field relates to their possible survival chances for a Hubble time, and thus to the potential evolutionary connection between YMCs and GCs.

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arXiv2005

Near-infrared integral-field spectroscopy of violent starburst environments

Near-infrared (NIR) integral-field spectroscopy (IFS) of violent starburst environments at high spatial (and spectral) resolution has the potential to revolutionise our ideas regarding the local interactions between the newly-formed massive stars and the interstellar medium (ISM) of their host galaxies. To illustrate this point, I present NIR IFS analysis of the central starburst region of NGC 1140, obtained with CIRPASS on Gemini-South. While strong [FeII] emission is found throughout the galaxy, higher-order Brackett emission is predominantly associated with the northern starburst region. Based on the spatial distributions of the [FeII] versus Brackett line emission, I conclude that a galaxy-wide starburst was induced several x 10^7 yr ago, with more recent starburst activity concentrated around the northern starburst region. I look forward and discuss the exciting prospects that IFS at higher spatial (and spectral) resolution will allow us trace (i) the massive outflows ("superwinds") expected to originate in the dense, young massive star clusters commonly found in intense starburst environments, and (ii) their impact on the galaxy's ISM.

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arXiv2004

Star and cluster formation in extreme environments

Current empirical evidence on the star-formation processes in the extreme, high-pressure environments induced by galaxy encounters (mostly based on high-resolution Hubble Space Telescope observations) strongly suggests that star CLUSTER formation is an important and perhaps even the dominant mode of star formation in such starburst events. The sizes, luminosities, and mass estimates of the young massive star clusters (YMCs) are entirely consistent with what is expected for young Milky Way-type globular clusters (GCs). Recent evidence lends support to the scenario that GCs, which were once thought to be the oldest building blocks of galaxies, are still forming today. Here, I present a novel empirical approach to assess the shape of the initial-to-current YMC mass functions, and hence their possible survival chances for a Hubble time.

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arXiv2004

Young Massive Clusters in Merging and Starburst Galaxies

The currently available empirical evidence on the star formation processes in the extreme, high-pressure environments induced by galaxy encounters, mostly based on high-resolution Hubble Space Telescope imaging observations, strongly suggests that star CLUSTER formation is an important and perhaps even the dominant mode of star formation in the starburst events associated with galaxy interactions. The production of young massive star clusters (YSCs) seems to be a hallmark of intense star formation, particularly in interacting and starburst galaxies. Their sizes, luminosities, and mass estimates are entirely consistent with what is expected for young Milky Way-type globular clusters (GCs). YSCs are important because of what they can tell us about GC formation and evolution (e.g., initial characteristics and early survival rates). They are also of prime importance as probes of the formation and (chemical) evolution of their host galaxies, and of the initial mass function in the extreme environments required for cluster formation. Recent evidence lends support to the scenario that Milky Way-type GCs (although more metal rich), which were once thought to be the oldest building blocks of galaxies, are still forming today.

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arXiv2004

Extragalactic Integral Field Spectroscopy on the Gemini Telescopes

We have been undertaking a programme on the Gemini 8-m telescopes to demonstrate the power of integral field spectroscopy, using the optical GMOS spectrograph, and the new CIRPASS instrument in the near-infrared. Here we present some preliminary results from 3D spectroscopy of extra-galactic objects, mapping the emission lines in a 3CR radio galaxy and in a gravitationally lensed arc, exploring dark matter sub-structure through observations of an Einstein Cross gravitational lens, and the star formation time-scales of young massive clusters in the starburst galaxy NGC 1140.

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arXiv2003

From Young to Old: Spectral Models for Star Cluster Systems

Evolutionary synthesis models for star clusters of various metallicities including gaseous emission during the lifetime of the ionising stars are used to model star cluster systems comprising two populations: an old metal-poor globular clusters (GC) population similar to that of the Milky Way halo and a second GC population of arbitrary metallicity. We investigate the time evolution of color distributions and luminosity functions for the two GC populations in comparison with observations on E/S0 galaxies. We show that multi-passband data for GC populations give clues to the relative ages and metallicities of blue and red subpopulations and help constrain formation scenarios for their parent galaxies.

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arXiv2003

Starbirth in Galaxy Collisions

Young, massive star clusters are the most notable and significant end products of violent star-forming episodes triggered by galaxy collisions, mergers, and close encounters. Their contribution to the total luminosity induced by such extreme conditions dominates, by far, the overall energy output due to the gravitationally-induced star formation. The general characteristics of these newly-formed clusters (such as their masses, luminosities, and sizes) suggest that at least a fraction may eventually evolve into equal, or perhaps slightly more massive, counterparts of the abundant old globular cluster systems in the local Universe. Establishing whether or not such an evolutionary connection exists requires our detailed knowledge of not only the physics underlying the evolution of "simple" stellar populations, but also that of cluster disruption in the time-dependent gravitational potentials of interacting galaxies. Initial results seem to indicate that proto-globular clusters do indeed continue to form today, which would support hierarchical galaxy formation scenarios.

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