75 research outputs found
Dynamics and stellar populations of small stellar systems
Dit proefschrift beschrijft onderzoek naar elliptische dwergmelkwegstelsels. Vanwege de voor veel mensen enigzins esoterische aard van dit soort objecten, is het goedomuit te leggenwat precies een melkwegstelsel is. Melkwegstelsels zijn wolken van miljoenen tot duizenden miljarden sterren. In zo’n wolk bewegen de sterren, grotendeels gedreven door hun onderlinge aantrekkingskracht, langs elkaar heen. Twee soorten melkwegstelsels zijn algemeen bekend. Het eerste type melkwegstelsel staat bekend als spiraalstelsel. Spiraalstelsels bevatten, naast sterren, een grote hoeveelheid interstellair gas en stof. Uit dit gas en stof kunnen nieuwe sterren worden gevormd en de aanwezigheid van veel jonge sterren geeft deze melkwegstelsels een enigzins blauwe kleur. Omdat wij spiraalstelsels onder verschillende inclinatiehoeken zien en soms zelfs van de zijkant, weten wij dat de sterren en het gas zich ophouden in een platte, schijfachtige vorm. Onze zon bevindt zich ook in een spiraalstelsel, dat door de schijfvorm als een band aan de hemel verschijnt: de melkweg. Metingen aan de snelheden van sterren en gas in spiraalstelsels tonen aan dat sterren en gas in spiraalstelsels op hoge snelheid om het centrum van het melkwegstelsel roteren. In het midden van de schijf van spiraalstelsels ziet men vaak een verdikking. De sterren in deze verdikking, in het Engels aangeduid met de naam bulge, hebben minder impulsmoment dan de sterren in de schijf, en bewegen zich daardoor in chaotischer banen. De sterren in de bulge zijn gemiddeld wat ouder dan de sterren in de schijf, waardoor de bulge vaak een wat gelige kleur heeft
Young stellar populations in early-type dwarf galaxies; occurrence, radial extent and scaling relations
To understand the stellar population content of dwarf early-type galaxies
(dEs) and its environmental dependence, we compare the slopes and intrinsic
scatter of color-magnitude relations (CMRs) for three nearby clusters, Fornax,
Virgo and Coma. Additionally we present and compare internal color profiles of
these galaxies to identify central blue regions with younger stars.
We use the imaging of the HST/ACS Fornax cluster in the magnitude range of
-18.7 <= M_g' <= -16.0, to derive magnitudes, colors and color profiles, which
we compare with literature measurements.
Based on analysis of the color profiles, we report a large number of dEs with
young stellar populations in their center in all three clusters. While for
Virgo and Coma the number of blue-cored dEs is found to be 85 +/- 2% and 53 +/-
3% respectively, for Fornax, we find that all galaxies have a blue core. We
show that bluer cores reside in fainter dEs, similar to the trend seen in
nucleated dEs. We find no correlation between the luminosity of the galaxy and
the size of its blue core. Moreover, a comparison of the CMRs of the three
clusters shows that the scatter in Virgo's CMR is considerably larger than in
the Fornax and Coma clusters. Presenting adaptive smoothing we show that the
galaxies on the blue side of the CMR often show evidence for dust extinction,
which strengthens the interpretation that the bluer colors are due to young
stellar populations. We also find that outliers on the red side of the CMR are
more compact than expected for their luminosity. We find several of these red
outliers in Virgo, often close to more massive galaxies. No red outlying
compact early-types are found in Fornax and Coma in this magnitude range while
we find three in the Virgo cluster. We suggest that the large number of
outliers and larger scatter found for the Virgo cluster CMR is a result of
Virgo's different assembly history.Comment: 24 pages, accepted for publication in Astronomy and Astrophysic
Detection of Enhanced Central Mass-to-Light Ratios in Low-Mass Early-Type Galaxies: Evidence for Black Holes?
We present dynamical measurements of the central mass-to-light ratio ()
of a sample of 27 low-mass early-type ATLAS galaxies. We consider all
ATLAS galaxies with 9.7log(MM10.5 in our
analysis, selecting out galaxies with available high-resolution Hubble Space
Telescope (HST) data, and eliminating galaxies with significant central color
gradients or obvious dust features. We use the HST images to derive mass models
for these galaxies and combine these with the central velocity dispersion
values from ATLAS data to obtain a central dynamical estimate.
These central dynamical s are higher than dynamical s derived at
larger radii and stellar population estimates of the galaxy centers in
80\% of galaxies, with a median enhancement of 14\% and a
statistical significance of 3.3. We show that the enhancement in the
central is best described either by the presence of black holes in these
galaxies or by radial IMF variations. Assuming a black hole model, we derive
black hole masses for the sample of galaxies. In two galaxies, NGC 4458 and NGC
4660, the data suggests significantly over-massive BHs, while in most others
only upper limits are obtained. We also show that the level of
enhancements we see in these early-type galaxy nuclei are consistent with the
larger enhancements seen in ultracompact dwarf galaxies (UCDs), supporting the
scenario where massive UCDs are created by stripping galaxies of these masses.Comment: Accepted to ApJ, 17 pages, 13 figure
The central mass and mass-to-light profile of the Galactic globular cluster M15
We analyze line-of-sight velocity and proper motion data of stars in the
Galactic globular cluster M15 using a new method to fit dynamical models to
discrete kinematic data. Our fitting method maximizes the likelihood for
individual stars and, as such, does not suffer the same loss of spatial and
velocity information incurred when spatially binning data or measuring velocity
moments. In this paper, we show that the radial variation in M15 of the
mass-to-light ratio is consistent with previous estimates and theoretical
predictions, which verifies our method. Our best-fitting axisymmetric Jeans
models do include a central dark mass of , which
can be explained by a high concentration of stellar remnants at the cluster
center. This paper shows that, from a technical point of view and with current
computing power, spatial binning of data is no longer necessary. This not only
leads to more accurate fits, but also avoids biased mass estimates due to the
loss of resolution. Furthermore, we find that the mass concentration in M15 is
significantly higher than previously measured, and is in close agreement with
theoretical predictions for core-collapsed globular clusters without a central
intermediate-mass black hole.Comment: Accepted by MNRAS; 8 pages, 7 figure
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