arXiv subjects

R. de Grijs

Publications and source records attributed to R. de Grijs.

At least 73 records · Page 4Linked to original sources

arXiv2007

HST/STIS spectroscopy of the environment in the starburst core of M82

[Abridged] We present high-resolution optical HST/STIS observations made with two slits crossing four of the brightest starburst clumps in the vicinity of the nucleus of M82. These provide H_alpha kinematics, extinction, electron density and emission measures. From the radial velocity curves we confirm the presence of a stellar bar, and find that the super star cluster M82-A1 has a position and radial velocity consistent with it being at the end of one of the unique x2 bar orbits formed by an inner Lindblad resonance. We derive a new model for the orientation of the bar and disc with respect to the main starburst clumps, and propose that clump A has formed within the bar region as a result of gas interactions between the bar orbits, whereas region C lies at the edge of the bar and regions D and E are located further out from the nucleus but heavily obscured. We derive extremely high interstellar densities, corresponding to ISM pressures of P/k ~ 0.5-1.0 x 10^7 cm^-3 K, and discuss the implications of the measured gas properties surrounding the nuclear star clusters on the production and evolution of the galactic wind. Despite varying pressures, the ionization parameter is uniform down to parsec-scales, and we discuss why this might be so. Where the S/N of our spectra are high enough, we identify multiple emission-line components. Through detailed Gaussian line-fitting, we identify a ubiquitous broad (200-300 km/s) underlying component to the bright H_alpha line, and discuss the physical mechanism(s) that could be responsible for such widths. We conclude that the evaporation and/or ablation of material from interstellar gas clouds caused by the impact of the high-energy photons and fast-flowing cluster winds produces a highly turbulent layer on the surface of the clouds from which the emission arises.

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arXiv2007

The Young Cluster Population of M82 Region B

We present observations obtained with the Advanced Camera for Surveys on board the Hubble Space Telescope of the "fossil" starburst region B in the nearby starburst galaxy M82. By comparing UBVI photometry with models, we derive ages and extinctions for 35 U-band selected star clusters. We find that the peak epoch of cluster formation occurred ~ 150 Myr ago, in contrast to earlier work that found a peak formation age of 1.1 Gyr. The difference is most likely due to our inclusion of U-band data, which are essential for accurate age determinations of young cluster populations. We further show that the previously reported turnover in the cluster luminosity function is probably due to the neglect of the effect of extended sources on the detection limit. The much younger cluster ages we derive clarifies the evolution of the M82 starburst. The M82-B age distribution now overlaps with the ages of: the nuclear starburst; clusters formed on the opposite side of the disk; and the last encounter with M81, some 220 Myr ago.

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arXiv2007

Mass segregation in very young open clusters -- A case study of NGC 2244 and NGC 6530

We derive the proper motions, membership probabilities, and velocity dispersions of stars in the regions of the young (about 2-4 Myr-old) open clusters NGC 2244 (the central cluster in the Monoceros R2 association) and NGC 6530 (the dominant cluster in the Sgr OB1 association) from photographic plate material obtained at Shanghai Astronomical Observatory, with time baselines of 34 and 87 years, respectively. Both clusters show clear evidence of mass segregation, but they do not exhibit any significant velocity-mass (or, equivalently, a velocity-luminosity) dependence. This provides strong support for the suggestion that the observed mass segregation is -- at least partially -- due to the way in which star formation has proceeded in these complex star-forming regions (``primordial'' mass segregation). Based on arguments related to the clusters' published initial mass functions, in conjunction with our new measurements of their internal velocity dispersions (35 and 8 km/s for NGC 2244 and NGC 6530, respectively), we provide strong arguments in favor of the dissolution of NGC 2244 on very short time-scales, while we speculate that NGC 6530 may be more stable against the effects of internal two-body relaxation. However, this latter object may well be destroyed by the strong tidal field prevalent at its location in the Galactic plane in the direction of the Galactic Center.

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arXiv2007

Exciting the Magnetosphere of the Magnetar CXOU J164710.2-455216 in Westerlund 1

We describe XMM-Newton observations taken 4.3 days prior to and 1.5 days subsequent to two remarkable events that were detected with Swift on 2006 September 21 from the candidate magnetar CXOU J164710.2-455216: (1) a 20 ms burst with an energy of 1e37 erg (15-150 keV), and (2) a rapid spin-down (glitch) with a fractionap period change of 1e-4. We find that the luminosity of the pulsar increased by a factor of 100 in the interval between observations, from 1e33 to 1e35 erg/s (0.5-8.0 keV), and that its spectrum hardened. The pulsed count rate increased by a factor of 10 (0.5-8.0 keV), but the fractional rms amplitude of the pulses decreased from 65 to 11 per cent, and their profile changed from being single-peaked to exhibiting three peaks. Similar changes have been observed from other magnetars in response to outbursts, such as that of 1E 2259+586 in 2002 June. We suggest that a plastic deformation of the neutron star's crust induced a very slight twist in the external magnetic field, which in turn generated currents in the magnetosphere that were the direct cause of the X-ray outburst.

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arXiv2007

Modelling the Near-IR Spectra of Red Supergiant-dominated Populations

We report on recent progress in the modelling of the near-IR spectra of young stellar populations, i.e. populations in which red supergiants (RSGs) are dominant. First, we discuss the determination of fundamental parameters of RSGs using fits to their near-IR spectra with new PHOENIX model spectra; RSG-specific surface abundances are accounted for and effects of the microturbulence parameter are explored. New population synthesis predictions are then described and, as an example, it is shown that the spectra of young star clusters in M82 can be reproduced very well from 0.5 to 2.4 micrometers. We warn of remaining uncertainties in cluster ages.

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arXiv2007

The Young Star Cluster System in the Antennae: Evidence for a Turnover in the luminosity function

The luminosity functions (LFs) of star cluster systems (i.e. the number of clusters per luminosity interval) are vital diagnostics to probe the conditions of star cluster formation. Early studies have revealed a clear dichotomy between old globular clusters and young clusters, with the former characterised by Gaussian-shaped LFs, and the latter following a power law. Recently, this view was challenged by studies of galaxy merger remnants and post-starburst galaxies. In this paper we re-evaluate the young ($\lta$ few hundreds of Myrs, with the majority $\lta$ few tens of Myrs) star cluster system in the ongoing spiral-spiral major merger system NGC 4038/39, the "Antennae" galaxies. The Antennae galaxies represent a very active and complex star-forming environment, which hampers cluster selection and photometry as well as the determination of observational completeness fractions. A main issue of concern is the large number of bright young stars contained in most earlier studies, which we carefully exclude from our cluster sample by accurately determining the source sizes. The resulting LFs are fitted both with Gaussian and with power-law distributions, taking into account both the observational completeness fractions and photometric errors, and compared using a likelihood ratio test. The likelihood ratio results are rigidly evaluated using Monte Carlo simulations. We perform a number of additional tests, e.g. with subsets of the total sample, all confirming our main result: that a Gaussian distribution fits the observed LFs of clusters in this preferentially very young cluster system significantly better than a power-law distribution, at a (statistical) error probability of less than 0.5 per cent.

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arXiv2007

Structural Parameters of Mayall II = G1 in M31

Mayall II = G1 is one of the most luminous globular clusters (GCs) known in M31. New deep, high-resolution observations with the Advanced Camera for Surveys on the {\sl Hubble Space Telescope} are used to provide accurate photometric data to the smallest radii yet. In particular, we present the precise variation of ellipticity and position angle, and of surface brightness for the core of the object. Based on these accurate photometric data, we redetermine the structural parameters of G1 by fitting a single-mass isotropic King model. We derive a core radius, $r_c=0.21\pm0.01\arcsec (=0.78\pm0.04 \rm{pc})$, a tidal radius, $r_t=21.8\pm1.1\arcsec (=80.7\pm3.9 \rm{pc})$, and a concentration index $c=\log (r_t/r_c)=2.01\pm0.02$. The central surface brightness is 13.510 mag arcsec$^{-2}$. We also calculate the half-light radius, at $r_h=1.73\pm0.07\arcsec(=6.5\pm0.3 \rm{pc})$. The results show that, within 10 core radii, a King model fits the surface brightness distribution well. We find that this object falls in the same region of the $M_V$ vs. $\log R_h$ diagram as $ω$ Centauri, M54 and NGC 2419 in the Milky Way. All three of these objects have been claimed to be the stripped cores of now defunct dwarf galaxies. We discuss in detail whether GCs, stripped cores of dwarf spheroidals and normal dwarf galaxies form a continuous distribution in the $M_V$ versus $\log R_h$ plane, or if GCs and dwarf spheroidals constitute distinct classes of objects; we present arguments in favour of this latter view.

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arXiv2006

New age estimates of M31 globular clusters from multi-colour photometry

The large majority of extragalactic star cluster studies performed to date essentially use multi-colour photometry, combined with theoretical stellar synthesis models, to derive ages, masses, extinction estimates, and metallicities. M31 offers a unique laboratory for studies of globular cluster (GC) systems. In this paper, we obtain new age estimates for 91 M31 globular clusters, based on improved photometric data, updated theoretical stellar synthesis models and sophisticated new fitting methods. In particular, we used photometric measurements from the Two Micron All Sky Survey (2MASS), which, in combination with optical photometry, can partially break the well-known age-metallicity degeneracy operating at ages in excess of a few Gyr. We show robustly that previous age determinations based on photometric data were affected significantly by this age-metallicity degeneracy. Except for one cluster, the ages of our other sample GCs are all older than 1 Gyr. Their age distribution shows populations of young and intermediate-age GCs, peaking at $\sim3$ and 8 Gyr respectively, as well as the "usual" complement of well-known old GCs, i.e., GCs of similar age as the majority of the Galactic GCs. Our results also show that although there is significant scatter in metallicity at any age, there is a noticeable lack of young metal-poor and old metal-rich GCs, which might be indicative of an underlying age-metallicity relationship among the M31 GC population.

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arXiv2006

HST/STIS Spectroscopy of Five Super Star Clusters in the Starburst Galaxy M82

We present optical spectroscopy obtained with the Space Telescope Imaging Spectrograph (STIS) of five young massive star clusters in the starburst galaxy M82. A detailed analysis is performed for one cluster `M82-A1' and its immediate environment in the starburst core. From HST archive images, we find that it is elliptical with an effective radius of 3.0+/-0.5 pc and is surrounded by a compact (r=4.5+/-0.5 pc) H II region. We determine the age and reddening of M82-A1 using synthetic spectra from population synthesis models by fitting both the continuum energy distribution and the depth of the Balmer jump. We find an age of 6.4+/-0.5 Myr and a photometric mass estimate of M=7-13 x 10^5 solar masses. We associate its formation with the most recent starburst event 4-6 Myr ago. We find that the oxygen abundance of the H II region surrounding M82-A1 is solar or slightly higher. The H II region has a high pressure P/k = 1-2 x 10^7 cm^-3 K. The diffuse gas in region A has a slightly lower pressure, which together with the broad H alpha emission line width, suggests that both the thermal and turbulent pressures in the M82 starburst core are unusually high. We discuss how this environment has affected the evolution of the cluster wind for M82-A1. We find that the high pressure may have caused the pressure-driven bubble to stall. We also obtain spectroscopic ages for clusters B1-2 and B2-1 in the `fossil' starburst region and for the intermediate age clusters F and L. These are consistent with earlier studies and demonstrate that star formation activity, sufficiently intense to produce super star clusters, has been going on in M82 during the past Gyr, perhaps in discrete and localized episodes.

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arXiv2006

A "super" star cluster grown old: the most massive star cluster in the Local Group

We independently redetermine the reddening and age of the globular cluster 037-B327 in M31 by comparing independently obtained multicolour photometry with theoretical stellar population synthesis models. 037-B327 has long been known to have a very large reddening value, which we confirm to be E(B-V)=1.360+-0.013, in good agreement with the previous results. We redetermine its most likely age at 12.4+- 3.2 Gyr. 037-B327 is a prime example of an unusually bright early counterpart to the ubiquitous ``super'' star clusters presently observed in most high-intensity star-forming regions in the local Universe. In order to have survived for a Hubble time, we conclude that its stellar IMF cannot have been top-heavy. Using this constraint, and a variety of SSP models, we determine a photometric mass of M_{GC}=(3.0+-0.5)*10^7, somewhat depending on the SSP models used, the metallicity and age adopted and the IMF representation. This mass, and its relatively small uncertainties, make this object the most massive star cluster of any age in the Local Group. Assuming that the photometric mass estimate thus derived is fairly close to its dynamical mass, we predict that this GC has a (one-dimensional) velocity dispersion of order (72+-13)km/s. As a surviving "super" star cluster, this object is of prime importance for theories aimed at describing massive star cluster evolution.

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arXiv2006

Accurate photometry of extended spherically symmetric sources

We present a new method to derive reliable photometry of extended spherically symmetric sources from {\it HST} images (WFPC2, ACS/WFC and NICMOS/NIC2 cameras), extending existing studies of point sources and marginally resolved sources. We develop a new approach to accurately determine intrinsic sizes of extended spherically symmetric sources, such as star clusters in galaxies beyond the Local Group (at distances <~ 20 Mpc), and provide a detailed cookbook to perform aperture photometry on such sources, by determining size-dependent aperture corrections (ACs) and taking sky oversubtraction as a function of source size into account. In an extensive Appendix, we provide the parameters of polynomial relations between the FWHM of various input profiles and those obtained by fitting a Gaussian profile (which we have used for reasons of computational robustness, although the exact model profile used is irrelevant), and between the intrinsic and measured FWHM of the cluster and the derived AC. Both relations are given for a number of physically relevant cluster light profiles, intrinsic and observational parameters. AC relations are provided for a wide range of apertures. Depending on the size of the source and the annuli used for the photometry, the absolute magnitude of such extended objects can be underestimated by up to 3 mag, corresponding to an error in mass of a factor of 15. We carefully compare our results to those from the more widely used DeltaMag method, and find an improvement of a factor of 3--40 in both the size determination and the AC.

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arXiv2006

A Neutron Star with a Massive Progenitor in Westerlund 1

We report the discovery of an X-ray pulsar in the young, massive Galactic star cluster Westerlund 1. We detected a coherent signal from the brightest X-ray source in the cluster, CXO J164710.2-455216, during two Chandra observations on 2005 May 22 and June 18. The period of the pulsar is 10.6107(1) s. We place an upper limit to the period derivative of Pdot<2e-10 s/s, which implies that the spin-down luminosity is Edot<3e33 erg/s. The X-ray luminosity of the pulsar is L_X = 3(+10,-2)e33 (D/5 kpc)^2 erg/s, and the spectrum can be described by a kT = 0.61+/-0.02 keV blackbody with a radius of R_bb = 0.27+/-0.03 (D/5 kpc}) km. Deep infrared observations reveal no counterpart with K<18.5, which rules out binary companions with M>1 Msun. Taken together, the properties of the pulsar indicate that it is a magnetar. The rarity of slow X-ray pulsars and the position of CXO J164710.2-455216 only 1.6' from the core of Westerlund 1 indicates that it is a member of the cluster with >99.97% confidence. Westerlund 1 contains 07V stars with initial masses M_i=35 Msun and >50 post-main-sequence stars that indicate the cluster is 4+/-1 Myr old. Therefore, the progenitor to this pulsar had an initial mass M_i>40 Msun. This is the most secure result among a handful of observational limits to the masses of the progenitors to neutron stars.

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arXiv2005

The initial mass distribution of the M82 star cluster system

We explore whether we can constrain the shape of the INITIAL mass distribution of the star cluster population in M82's ~1 Gyr-old post-starburst region "B", in which the present-day cluster mass function (CMF) is closely approximated by a log-normal distribution. We conclude that the M82 B initial CMF must have had a mean mass very close to that of the "equilibrium" CMF of Vesperini (1998). Consequently, if the presently observed M82 B CMF has remained approximately constant since its formation, as predicted, then the INITIAL CMF must have been characterized by a mean mass that was only slightly larger than the present mean mass. From our detailed analysis of the expected evolution of CMFs, we conclude that our observations of the M82 B CMF are inconsistent with a scenario in which the 1 Gyr-old cluster population originated from an initial power-law mass distribution. Our conclusion is supported by arguments related to the initial density in M82 B, which would have been unphysically high if the present cluster population were the remains of an initial power-law distribution.

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arXiv2005

Nearby star cluster formation: Probes of cosmological galaxy evolution

(abridged) The old Star Cluster (SC) systems surrounding any sofar investigated galaxy represent a powerful tool for the understanding of the cosmological evolution of their host galaxies. Phases of enhanced cluster formation can be identified by reliably age dating the clusters. These are symptoms of violent star formation, results of intense starbursts as commonly seen in merging nearby galaxies and high-z forming galaxies. However, the young SC systems of nearby merging galaxies and the old SC systems of nearby passive galaxies (most likely resulting from high-z starburst events of these galaxies) appear to be significantly different in terms of their mass functions. Whether this difference originates in differences in the formation physics/environment or is only due to dynamical evolution of the cluster systems is currently a hot issue. A main diagnostic for the survival probability of a newly formed cluster is most likely its compactness, since less compact, fluffy clusters are more vulnerable to destructive interactions with the galactic environment of the cluster. For these reasons, we developed 1) a tool to significantly improve photometry of nearby SCs (which appear resolved on high-resolution images, provided e.g. by HST), by accurately measuring and taking the cluster size into account, and 2) a tool to effectively determine ages and masses (plus metallicities and extinctions) of SCs from multi-wavelength photometry, ranging from the UV to the NIR.

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arXiv2005

From young massive star cluster to old globular: the L_V-sigma_0 relationship as a diagnostic tool

We present a new analysis of the properties of the young massive star clusters, which demonstrates that these objects are plausible progenitors of old globular clusters (GCs). The method is based on the tight relationship for old GCs between their V-band luminosities and (central) velocity dispersions, sigma_0. We improve the significance of the relationship and find that its functional form, L_V/Lsun \propto sigma_0^(1.57 +- 0.10) (km/s), is fully consistent with previous determinations for smaller Galactic and M31 GC samples. The tightness of the relationship for a GC sample drawn from environments as diverse as those found in the Local Group implies that its origin must be sought in intrinsic properties of the GC formation process itself. We evolve the luminosities of the young massive star clusters (YMCs) in the local Universe to an age of 12 Gyr and find that most YMCs will evolve to loci close to, or to slightly fainter luminosities than the improved GC relationship. In the absence of significant external disturbances, this implies that these objects may potentially survive to become old GC-type objects over a Hubble time. The main advantage of our new method is its simplicity; the only observables required here are the system's velocity dispersion and luminosity. The most important factor affecting the robustness of our conclusions is the adopted form of the initial mass function. We use N-body simulations to confirm that dynamical evolution of the clusters does not significantly alter our conclusions. [ABRIDGED]

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arXiv2005

Systematic uncertainties in the analysis of star cluster parameters based on broad-band imaging observations

(ABRIDGED) We assess the systematic uncertainties in (young) cluster age, mass, and - to a lesser extent - extinction and metallicity determinations, based on broad-band imaging observations with the Hubble Space Telescope. Our aim here is to intercompare the results obtained using a variety of commonly used modelling techniques, specifically with respect to our own extensively tested multi-dimensional approach. Any significant differences among the resulting parameters are due to the details of the various, independently developed modelling techniques used, rather than to the stellar population models themselves. Despite the model uncertainties and the selection effects inherent to most methods used, we find that the PEAKS in the relative age and mass distributions of a given young (<= 10^9 yr) cluster system can be derived relatively robustly and consistently, to accuracies of sigma_t = Delta <= 0.35 and sigma_M = Delta <= 0.14, respectively, assuming Gaussian distributions in cluster ages and masses for reasons of simplicity. The peaks in the relative mass distributions can be obtained with a higher degree of confidence than those in the relative age distributions, as exemplified by the smaller spread among the peak values of the mass distributions derived. This implies that mass determinations are mostly insensitive to the approach adopted. We reiterate that as extensive a wavelength coverage as possible is required to obtain robust and internally consistent age and mass estimates for the individual objects, with reasonable uncertainties. Finally, we conclude that the ACTUAL filter systems used for the observations should be used for constructing model colours, instead of using conversion equations, to achieve more accurate derivations of ages and masses.

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arXiv2004

The Star Cluster Population of M51: II. Age distribution and relations among the derived parameters

We use archival {\it Hubble Space Telescope} observations of broad-band images from the ultraviolet (F255W-filter) through the near infrared (NICMOS F160W-filter) to study the star cluster population of the interacting spiral galaxy M51. We obtain age, mass, extinction, and effective radius estimates for 1152 star clusters in a region of $\sim 7.3 \times 8.1$ kpc centered on the nucleus and extending into the outer spiral arms. In this paper we present the data set and exploit it to determine the age distribution and relationships among the fundamental parameters (i.e. age, mass, effective radius). Using this dataset we find: {\it i}) that the cluster formation rate seems to have had a large increase $\sim$ 50-70 Myr ago, which is coincident with the suggested {\it second passage} of its companion, NGC 5195, {\it ii}) a large number of extremely young ($<$ 10 Myr) star clusters, which we interpret as a population of unbound clusters of which a large majority will disrupt within the next $\sim$10 Myr, and {\it iii)} that the distribution of cluster sizescan be well approximated by a power-law with exponent, $ -η= -2.2 \pm 0.2$, which is very similar to that of Galactic globular clusters, indicating that cluster disruption is largely independent of cluster radius. In addition, we have used this dataset to search for correlations among the derived parameters. In particular, we do not find any strong trends between the age and mass, mass and effective radius, nor between the galactocentric distance and effective radius. There is, however, a strong correlation between the age of a cluster and its extinction, with younger clusters being more heavily reddened than older clusters.

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arXiv2004

CIRPASS near-infrared integral-field spectroscopy of massive star clusters in the starburst galaxy NGC 1140

[ABRIDGED] We analyse near-infrared integral field spectroscopy of the central starburst region of NGC 1140, obtained at the Gemini-South telescope equipped with CIRPASS. Our ~1.45-1.67 um wavelength coverage includes the bright [Fe II] emission line, as well as high-order Brackett (hydrogen) lines. While strong [Fe II] emission, thought to originate in the thermal shocks associated with supernova remnants, is found throughout the galaxy, both Br 12-4 and Br 14-4 emission, and weak CO(6,3) absorption, is predominantly associated with the northern starburst region. The Brackett lines originate from recombination processes occurring on smaller scales in (young) HII regions. The time-scale associated with strong [Fe II] emission implies that most of the recent star-formation activity in NGC 1140 was induced in the past 35-55 Myr. Based on the spatial distributions of the [Fe II] versus Brackett line emission, we conclude that a galaxy-wide starburst was induced several tens of Myr ago, with more recent starburst activity concentrated around the northern starburst region. This scenario is (provisionally) confirmed by our analysis of the spectral energy distributions of the compact, young massive star clusters (YMCs) detected in new and archival broad-band HST images. The YMC ages in NGC 1140 are all <= 20 Myr, consistent with independently determined estimates of the galaxy's starburst age, while there appears to be an age difference between the northern and southern YMC complexes in the sense expected from our CIRPASS analysis. Our photometric mass estimates of the NGC 1140 YMCs, likely upper limits, are comparable to those of the highest-mass Galactic globular clusters and to spectroscopically confirmed masses of (compact) YMCs in other starburst galaxies.

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