arXiv subjects

R. de Grijs

Publications and source records attributed to R. de Grijs.

At least 91 records · Page 5Linked to original sources

arXiv2004

Optical Imaging and Spectroscopy of the Edge-On Sbc Galaxy UGC10043: Evidence for a Galactic Wind and a Peculiar Triaxial Bulge

We present new optical imaging and spectroscopy of the peculiar, edge-on Sbc galaxy UGC10043. B & R imaging reveals that the inner bulge of UGC10043 is elongated perpendicular to the major axis. At larger r, the bulge isophotes twist to become oblate and nearly circular, suggesting the bulge is triaxial. Based on stellar and ionized gas kinematics, the bulge shows no clear evidence for rotation about either its major or minor axis. The southwestern quadrant of the bulge is girdled by a narrow dust lane parallel to the minor axis that may be part of an inner polar ring. The stellar disk of UGC10043 has a low optical surface brightness, a small scale height, a mild integral sign warp, and a dusty, inner region that appears tilted relative to the outlying disk. The HA and [NII] emission lines in UGC10043 resolve into multiple velocity components, indicating the presence of a large-scale galactic wind with an outflow velocity of ~104 km/s. HA+[NII] imaging reaffirms this picture by revealing ionized gas extended to |z|~3.5kpc in a biconical structure. The [NII]/HA line intensity ratio increases with increasing distance from the plane, reaching values as high as 1.7. Unlike most galaxies with large-scale winds, UGC10043 has only a modest global star formation rate (~1M_sun/yr), implying the wind is powered by a rather feeble central starburst. We discuss evolutionary scenarios that could account for both the structural complexities of UGC10043 and its large-scale wind. [Abridged]

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arXiv2003

Nucler star formation in NGC 6240

We have made use of archival HST BVIJH photometry to constrain the nature of the three discrete sources, A1, A2 and B1, identified in the double nucleus of NGC 6240. STARBURST99 models have been fitted to the observed colours, under the assumption, first, that these sources can be treated as star clusters (i.e. single, instantaneous episodes of star formation), and subsequently as star-forming regions (i.e. characterised by continuous star formation). For both scenarios, we estimate ages as young as 4 million years, integrated masses ranging between 7x10^6 Msun (B1) and 10^9 Msun (A1) and a rate of 1 supernova per year, which, together with the stellar winds, sustains a galactic wind of 44 Msun/yr. In the case of continuous star formation, a star-formation rate has been derived for A1 as high as 270 Msun/yr, similar to what is observed for warm Ultraluminous Infrared Galaxies (ULIRGs) with a double nucleus. The A1 source is characterised by a mass density of about 1200 Msun/pc^3 which resembles the CO molecular mass density measured in cold ULIRGs and the stellar density determined in ``elliptical core'' galaxies. This, together with the recent discovery of a supermassive binary black hole in the double nucleus of NGC 6240, might indicate that the ongoing merger could shape the galaxy into a core elliptical.

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arXiv2003

Analysing observed star cluster SEDs with evolutionary synthesis models: Systematic uncertainties

We discuss the systematic uncertainties inherent to analyses of observed (broad-band) Spectral Energy Distributions (SEDs) of star clusters with evolutionary synthesis models. We investigate the effects caused by restricting oneself to a limited number of available passbands, choices of various passband combinations, finite observational errors, non-continuous model input parameter values, and restrictions in parameter space allowed during analysis. Starting from a complete set of UBVRIJH passbands (respectively their Hubble Space Telescope/WFPC2 equivalents) we investigate to which extent clusters with different combinations of age, metallicity, internal extinction and mass can or cannot be disentangled in the various evolutionary stages throughout their lifetimes and what are the most useful passbands required to resolve the ambiguities. We find the U and B bands to be of the highest significance, while the V band and near-infrared data provide additional constraints. A code is presented that makes use of luminosities of a star cluster system in all of the possibly available passbands, and tries to find ranges of allowed age-metallicity-extinction-mass combinations for individual members of star cluster systems. Numerous tests and examples are presented. We show the importance of good photometric accuracies and of determining the cluster parameters independently without any prior assumptions.

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arXiv2003

Multi-color observations of Young Star Clusters

We present a new set of evolutionary synthesis models of our GALEV code, specifically developed to include the gaseous emission of presently forming star clusters, in combination with an advanced tool to compare large model grids with multi-color broad-band observations of YSC systems. Tests and first applications are presented.

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arXiv2003

Young star clusters: Progenitors of globular clusters!?

Star cluster formation is a major mode of star formation in the extreme conditions of interacting galaxies and violent starbursts. Young clusters are observed to form in a variety of such galaxies, a substantial number resembling the progenitors of globular clusters in mass and size, but with significantly enhanced metallicity. From studies of the metal-poor and metal-rich star cluster populations of galaxies, we can therefore learn about the violent star formation history of these galaxies, and eventually about galaxy formation and evolution. We present a new set of evolutionary synthesis models of our GALEV code, with special emphasis on the gaseous emission of presently forming star clusters, and a new tool to compare extensive model grids with multi-color broad-band observations to determine individual cluster masses, metallicities, ages and extinction values independently. First results for young star clusters in the dwarf starburst galaxy NGC 1569 are presented. The mass distributions determined for the young clusters give valuable input to dynamical star cluster system evolution models, regarding survival and destruction of clusters. We plan to investigate an age sequence of galaxy mergers to see dynamical destruction effects in process.

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arXiv2003

Young star clusters: Metallicity tracers in external galaxies

Star cluster formation is a major mode of star formation in the extreme conditions of interacting galaxies and violent starbursts. These newly-formed clusters are built from recycled gas, pre-enriched to various levels within the interacting galaxies. Hence, star clusters of different ages represent a fossil record of the chemical enrichment history of their host galaxy, as well as of the host galaxy's violent star formation history. We present a new set of evolutionary synthesis models of our GALEV code, specifically developed to include the gaseous emission of presently forming star clusters, and a new tool to analyse multi-color observations with our models. First results for newly-born clusters in the dwarf starburst galaxy NGC 1569 are presented.

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arXiv2003

Young star clusters: Clues to galaxy formation and evolution

Young clusters are observed to form in a variety of interacting galaxies and violent starbursts, a substantial number resembling the progenitors of the well-studied globular clusters in mass and size. By studying young clusters in merger remnants and peculiar galaxies, we can therefore learn about the violent star formation history of these galaxies. We present a new set of evolutionary synthesis models of our GALEV code specifically developed to include the gaseous emission of presently forming star clusters, and a new tool that allows to determine individual cluster metallicities, ages, extinction values and masses from a comparison of a large grid of model Spectral Energy Distributions (SEDs) with multi-color observations. First results for the newly-born clusters in NGC 1569 are presented.

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arXiv2003

Star Cluster Formation and Evolution in the Dwarf Starburst Galaxy NGC 1569

We analyse multi-wavelength Hubble Space Telescope (HST) observations of a large number of star clusters in the nearby (post-) starburst dwarf galaxy NGC 1569. Their spectral energy distributions (SEDs) cover at least the wavelength range from U to I in equivalent HST filters, in most cases supplemented by near-infrared data. Using our most up-to-date evolutionary synthesis models of the Goettingen GALEV code we determine ages, metallicities, extinction values and masses for each individual cluster robustly and independently. We confirm the youth of most of these objects. The majority were formed in a very intense starburst starting around 25 Myr ago. While there are two prominent ``super star clusters'' present in this galaxy, with masses of (5-16) * 10^5 Msun, almost all remaining clusters are significantly less massive than an average Milky Way globular cluster, and are generally consistent with open cluster-type objects. We determine the cluster mass function from individual cluster masses derived by scaling the model SEDs of known mass to the observed cluster SEDs for each individual cluster. We find signs of a change in the cluster mass function as the burst proceeds, which we attribute to the special conditions of star cluster formation in this starburst dwarf galaxy environment.

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arXiv2003

On the formation of star clusters in the merger NGC 6240

We identified star clusters in archived HST/WFPC2 images of the merger and ultra-luminous infrared galaxy NGC 6240, with the aim of investigating whether star cluster properties (luminosity, age and mass) in such an extreme environment differ from those of clusters in less luminous starburst galaxies. We found 54 star clusters in all of the F450W, F547M and F814W exposures, of which 41 are located in the main body of NGC 6240 and 13 in the galactic tails. Given that only two colours are available to derive two independent variables (cluster reddening and age), we adopted an ad hoc procedure to statistically derive cluster parameters under the assumption that the cluster metallicity is LMC-like. The colours of each cluster are fitted to STARBURST99 models of fixed mass and variable ages and reddenings. All cluster reddening and age solutions with Chi squared < 1 are considered to be consistent with the data. Masses are derived by scaling the luminosity of the models with best-fit Chi squared < 1 by the observed V luminosity, after correction for reddening and distance. We infer that the most probable age of the observed clusters is (5 - 13) Myr and their most probable mass is about (1 - 2)e05 solar masses. Comparison with star clusters in starburst galaxies seems to point to more massive clusters being formed in more massive galaxies and gas-rich mergers, while the overall cluster mass distribution might be relatively independent of the details of the associated starburst where dense, massive clusters preferentially form.

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arXiv2003

Star Cluster Formation and Evolution in Nearby Starburst Galaxies: II. Initial Conditions

We use the ages, masses and metallicities of the rich young star cluster systems in the nearby starburst galaxies NGC 3310 and NGC 6745 to derive their cluster formation histories and subsequent evolution. We further expand our analysis of the systematic uncertainties involved in the use of broad-band observations to derive these parameters by examining the effects of a priori assumptions on the individual cluster metallicities. The age (and metallicity) distributions of both the clusters in the circumnuclear ring in NGC 3310 and of those outside the ring are statistically indistinguishable, but there is a clear and significant excess of higher-mass clusters IN the ring compared to the non-ring cluster sample; it is likely that the physical conditions in the starburst ring may be conducive for the formation of higher-mass star clusters, on average, than in the relatively more quiescent environment of the main galactic disc. For the NGC 6745 cluster system we derive a median age of ~10 Myr. NGC 6745 contains a significant population of high-mass "super star clusters", with masses in the range 6.5 <= log(M_cl/M_sun) <= 8.0. This detection supports the scenario that such objects form preferentially in the extreme environments of interacting galaxies. The age of the cluster populations in both NGC 3310 and NGC 6745 is significantly lower than their respective characteristic cluster disruption time-scales. This allows us to obtain an independent estimate of the INITIAL cluster mass function slope, alpha = 2.04(+- 0.23)(+0.13)(-0.43) for NGC 3310, and 1.96(+- 0.15)(+- 0.19) for NGC 6745, respectively, for masses M_cl >= 10^5 M_sun and M_cl >= 4 x 10^5 M_sun. These mass function slopes are consistent with those of other young star cluster systems in interacting and starburst galaxies.

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arXiv2003

Chemical evolution of the M82 B fossil starburst

M82 B is an old starburst site located in the eastern part of the M82 disc. We derive the distributions of age and metallicity of the star clusters located in this region of M82 by using theoretical evolutionary population synthesis models. Our analysis is based on the comparison of the $BVIJ$ photometry obtained by de Grijs et al. (2001) with the colours of single-generation stellar populations. We show that M82 B went through a chemical enrichment phase up to super-solar metallicities around the time of the last close encounter between M82 and its large neighbour galaxy M81. We date and confirm the event triggering the enhanced cluster formation at about 1 Gyr ago. At almost the same time an additional, distinct subpopulation of metal-poor clusters formed in the part of M82 B nearest to the galactic centre. The formation of these peculiar clusters may be related to infall of circumgalactic gas onto M82 B.

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arXiv2003

Star Cluster Formation and Evolution in Nearby Starburst Galaxies: I. Systematic Uncertainties

(ABBREVIATED:) The large majority of extragalactic star cluster studies done to date have essentially used two or three-passband aperture photometry, combined with theoretical stellar population synthesis models, to obtain age, mass and extinction estimates, and sometimes also metallicities. The accuracy to which this can be done depends on the choice of passband combination and, crucially, also on the wavelength range covered. Understanding the systematic uncertainties is of the utmost importance for a well-balanced interpretation of the properties of extragalactic star cluster systems. We simultaneously obtain ages, metallicities and extinction values for ~300 clusters in the nearby starburst galaxy NGC 3310, based on archival Hubble Space Telescope observations from the ultraviolet (UV) to the near-infrared (NIR). We show that, for ages 6 <= log(age/yr) <= 9, and if one can only obtain partial coverage of the spectral energy distribution (SED), an optical passband combination of at least four filters including BOTH blue AND red passbands results in the most representative age distribution. We find that while blue-selected passband combinations lead to age distributions that are slightly biased towards younger ages due to the age-metallicity degeneracy, red-dominated passband combinations should be avoided. NGC 3310 underwent a global burst of cluster formation ~3x10^7 yr ago. This coincides closely with the last tidal interaction or merger with a low-metallicity galaxy that likely induced the formation of the large fraction of clusters with (significantly) subsolar metallicities. The observed cluster system has a median mass of = 5.25 +- 0.1, obtained from scaling the appropriate model SEDs for known masses to the observed cluster SEDs.

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arXiv2002

Mass segregation in rich clusters in the Large Magellanic Cloud - III. Implications for the initial mass function

The distribution of core radii of rich clusters in the Large Magellanic Cloud (LMC) systematically increases in both upper limit and spread with increasing cluster age. Cluster-to-cluster variations in the stellar initial mass function (IMF) have been suggested as an explanation. We discuss the implications of the observed degree of mass segregation in our sample clusters for the shape of the initial mass function. Our results are based on Hubble Space Telescope/WFPC2 observations of six rich star clusters in the LMC, selected to include three pairs of clusters of similar age, metallicity, and distance from the LMC centre, and exhibiting a large spread in core radii between the clusters in each pair. All clusters show clear evidence of mass segregation: (i) their luminosity function slopes steepen with increasing cluster radius, and (ii) the brighter stars are characterized by smaller core radii. For all sample clusters, both the slope of the luminosity function in the cluster centres and the degree of mass segregation are similar to each other, within observational errors of a few tenths of power-law slope fits to the data. This implies that their INITIAL mass functions must have been very similar, down to ~0.8 - 1.0 M(sun). We therefore rule out variations in the IMF of the individual sample clusters as the main driver of the increasing spread of cluster core radii with cluster age.

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arXiv2002

On the nature of a secondary main sequence turn-off in the rich LMC cluster NGC 1868

Evidence for a second main-sequence turn-off in a deep colour-magnitude diagram of NGC 1868 is presented. The data were obtained with HST/WFPC2 and reach down to m_{555} ~ 25. Besides the usual t ~ 0.8 Gyr turn-off found in previous analyses, another possible turn-off is seen at m_{555} ~ 21 (M_V ~ 2.5), which is consistent with an age of t ~ 3 Gyrs. This CMD feature stands out clearly especially when contaminating field LMC stars are statistically removed. The background subtracted CMD also visibly displays a red subgiant branch extending about 1.5 mag below the younger turn-off and the clump of red giants. The significance of the secondary turn-off in NGC 1868 was confirmed with Monte-Carlo simulations and bootstrapping techniques. Star-counts in selected regions in the cluster CMD indicate a mass ratio of old population/young population in the range 5% <= M_{old} / M_{young} <= 12%, depending on the mass function slope. The existence of such a subpopulation in NGC 1868 is significant even in the presence of uncertainties in background subtraction. The possibility that the secondary turn-off is associated with the field star population was examined by searching for similar features in CMDs of field stars. Statistically significant excesses of stars redwards of the main-sequence were found in all such fields in the range 20 <= m_{555} <= 22. These however are much broader features that do not resemble the main-sequence termination of a single population. We also discuss other alternative explanations for the feature at m_{555} ~ 21, such as unresolved binarism, peculiar stars or CMD discontinuities associated with the Bohm-Vitense gap.

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arXiv2002

An HST Survey of the mid-UV Morphology of Nearby Galaxies

(Abbreviated) We present an imaging survey of 37 nearby galaxies observed with HST/WFPC2 in the mid-UV F300W filter and in F814W. 11 galaxies were also imaged in F255W. These galaxies were selected to be detectable with WFPC2 in one orbit, and cover a wide range of Hubble types and inclinations. The mid-UV spans the gap between our groundbased optical/NIR images and far-UV images available from the Astro/UIT missions. Our first qualitative results are: (1) Early-type galaxies show a significant decrease in surface brightness going from the red to the mid-UV, and in some cases the presence of dust lanes. Some galaxies would be classified different when viewed in the mid-UV, some become dominated by a blue nuclear feature or point source. (2) Half of the mid-type spiral and star-forming galaxies appear as a later morphological type in the mid-UV, as Astro/UIT also found in the far-UV. Some- times these differences are dramatic. The mid-UV images show a considerable range in the scale and surface brightness of individual star-forming regions. Almost all mid-type spirals have their small bulges bi-sected by a dust-lane. (3) Most of the heterogeneous subset of late-type, irregular, peculiar, and merging galaxies display F300W morphologies that are similar to those seen in F814W, but with differences due to recognizable dust features absorbing the bluer light, and due to UV-bright hot stars, star-clusters, and star-forming ridges. In the rest-frame mid-UV, early- to mid-type galaxies are more likely to be misclassified as later types than vice versa. This morphological K-correction explains only part of the excess faint blue galaxies seen in deep HST fields.

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arXiv2001

Mass segregation in young compact star clusters in the Large Magellanic Cloud: I. Data and Luminosity Functions

We have undertaken a detailed analysis of HST/WFPC2 and STIS imaging observations, and of supplementary wide-field ground-based observations obtained with the NTT of two young ~10-25 Myr) compact star clusters in the LMC, NGC 1805 and NGC 1818. The ultimate goal of our work is to improve our understanding of the degree of primordial mass segregation in star clusters. This is crucial for the interpretation of observational luminosity functions (LFs) in terms of the initial mass function (IMF), and for constraining the universality of the IMF. We present evidence for strong luminosity segregation in both clusters. The LF slopes steepen with cluster radius; in both NGC 1805 and NGC 1818 the LF slopes reach a stable level well beyond the clusters' core or half-light radii. In addition, the brightest cluster stars are strongly concentrated within the inner ~4 R_hl. The global cluster LF, although strongly nonlinear, is fairly well approximated by the core or half-light LF; the (annular) LFs at these radii are dominated by the segregated high-luminosity stars, however. We present tentative evidence for the presence of an excess number of bright stars surrounding NGC 1818, for which we argue that they are most likely massive stars that have been collisionally ejected from the cluster core. We therefore suggest that the cores of massive young stars clusters undergo significant dynamical evolution, even on time-scales as short as ~25 Myr.

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arXiv2001

Mass segregation in young compact star clusters in the Large Magellanic Cloud: II. Mass Functions

We review the complications involved in the conversion of stellar luminosities into masses and apply a range of mass-to-luminosity relations to our Hubble Space Telescope observations of the young LMC star clusters NGC 1805 and NGC 1818. Both the radial dependence of the mass function (MF) and the dependence of the cluster core radii on mass indicate clear mass segregation in both clusters at radii r <= 20-30'', for masses in excess of ~1.6-2.5 Msun. This result does not depend on the mass range used to fit the slopes or the metallicity assumed. It is clear that the cluster MFs, at any radius, are not simple power laws. The global and the annular MFs near the core radii appear to be characterised by similar slopes in the mass range (-0.15 <= log m/Msun <= 0.85), the MFs beyond r >= 30'' have significantly steeper slopes. We estimate that while the NGC 1818 cluster core is between ~5 and ~30 crossing times old, the core of NGC 1805 is likely $\lesssim 3-4$ crossing times old. However, since strong mass segregation is observed out to ~6 Rcore and ~3 Rcore in NGC 1805 and NGC 1818, respectively, it is most likely that significant primordial mass segregation was present in both clusters, particularly in NGC 1805.

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arXiv2001

A search for radio supernovae and supernova remnants in the region of NGC1569's super star clusters

We have used MERLIN, at 1.4 and 5 GHz, to search for radio supernovae (RSNe) and supernova remnants (SNRs) in the unobscured irregular dwarf galaxy NGC1569, and in particular in the region of its super star clusters (SSCs) A and B. Throughout NGC1569 we find some 5 RSNe and SNRs but the SSCs and their immediate surroundings are largely devoid of non-thermal radio sources. Even though many massive stars in the SSCs are expected to have exploded already, when compared with M82 and its many SSCs the absence of RSNe and SNRs in and near A and B may seem plausible on statistical arguments. The absence of RSNe and SNRs in and near A and B may, however, also be due to a violent and turbulent outflow of stellar winds and supernova ejected material, which does not provide a quiescent environment for the development of SNRs within and near the SSCs.

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