[Show abstract][Hide abstract] ABSTRACT: Direct N-body simulations of globular clusters in a realistic Milky Way-like
potential are carried out using the code NBODY6 to determine the impact of the
host galaxy disk mass and geometry on the survival of star clusters. A
relationship between disk mass and star cluster dissolution timescale is
derived. These N-body models show that doubling the mass of the disk from
5x10^10 solar masses to 10x10^10 solar masses halves the dissolution time of a
satellite star cluster orbiting the host galaxy at 6 kpc from the galactic
center. Different geometries in a disk of identical mass can determine either
the survival or dissolution of a star cluster orbiting within the inner 6 kpc
of the galactic center. Furthermore, disk geometry has measurable effects on
the mass loss of star clusters up to 15 kpc from the galactic center. N-body
simulations performed with a fine output time step show that at each disk
crossing the outer layers of star clusters experience an increase in velocity
dispersion of ~5% of the average velocity dispersion in the outer section of
star clusters. This leads to an enhancement of mass-loss -- a clearly
discernable effect of disk shocking. By running models with different
inclinations we determine that star clusters with an orbit perpendicular to the
Galactic plane have larger mass loss rates than both clusters evolving in the
Galactic plane or in an inclined orbit.
Preview · Article · Feb 2014 · The Astrophysical Journal
[Show abstract][Hide abstract] ABSTRACT: A spectroscopic follow up of Ultra-Compact Dwarf (UCD) candidates in the
fossil group NGC 1132 is undertaken with the Gemini Multi Object Spectrograph
(GMOS). These new Gemini spectra prove the presence of six UCDs in the fossil
group NGC 1132 at a distance of D~100 Mpc and a recessional velocity of v_r =
6935 +/- 11 km/s. The brightest and largest member of the UCD population is an
M32 analog with a size of 77.1 pc and a magnitude of M_V=-14.8 mag with the
characteristics in between those of the brightest UCDs and compact elliptical
galaxies. The ensemble of UCDs have an average radial velocity of = 6966
+/- 208 km/s and a velocity dispersion of sigma_v = 169 +/-18 km/s similar to
the one of poor galaxy groups. This work shows that UCDs can be used as test
particles to determine the dynamical properties of galaxy groups. The presence
of UCDs in the fossil group environment is confirmed and thus the fact that
UCDs can form across diverse evolutionary conditions.
Full-text · Article · May 2013 · The Astrophysical Journal
[Show abstract][Hide abstract] ABSTRACT: Size differences of ≈20 per cent between red (metal-rich) and blue (metal-poor) subpopulations of globular clusters have been
observed, generating an ongoing debate as to whether these originate from projection effects or the difference in metallicity.
We present direct N-body simulations of metal-rich and metal-poor stellar populations evolved to study the effects of metallicity on cluster
evolution. The models start with N = 100 000 stars and include primordial binaries. We also take metallicity-dependent stellar evolution and an external tidal
field into account. We find no significant difference for the half-mass radii of those models, indicating that the clusters
are structurally similar. However, utilizing observational tools to fit half-light (or effective) radii confirms that metallicity effects related to stellar evolution combined with dynamical effects such
as mass segregation produce an apparent size difference of 17 per cent on average. The metallicity effect on the overall cluster
luminosity also leads to higher mass-to-light ratios for metal-rich clusters.
Preview · Article · Aug 2012 · Monthly Notices of the Royal Astronomical Society
[Show abstract][Hide abstract] ABSTRACT: Direct N-body simulations of star clusters in a realistic Milky Way-like
potential are carried out using the code NBODY6. Based on these simulations a
new relationship between scale size and galactocentric distance is derived: the
scale size of star clusters is proportional to the hyperbolic tangent of the
galactocentric distance. The half-mass radius of star clusters increases
systematically with galactocentric distance but levels off when star clusters
orbit the galaxy beyond ~40 kpc. These simulations show that the half-mass
radius of individual star clusters varies significantly as they evolve over a
Hubble time, more so for clusters with shorter relaxation times, and remains
constant through several relaxation times only in certain situations when
expansion driven by the internal dynamics of the star cluster and the influence
of the host galaxy tidal field balance each other. Indeed, the radius of a star
cluster evolving within the inner 20 kpc of a realistic galactic gravitational
potential is severely truncated by tidal interactions and does not remain
constant over a Hubble time. Furthermore, the half-mass radius of star clusters
measured with present day observations bears no memory of the original cluster
size. Stellar evolution and tidal stripping are the two competing physical
mechanisms that determine the present day size of globular clusters. These
simulations also show that extended star clusters can form at large
galactocentric distances while remaining fully bound to the host galaxy. There
is thus no need to invoke accretion from an external galaxy to explain the
presence of extended clusters at large galactocentric distances in a Milky
Preview · Article · Aug 2012 · The Astrophysical Journal
[Show abstract][Hide abstract] ABSTRACT: The giant radio galaxy M 87 with its proximity (16 Mpc), famous jet, and very massive black hole ((3 – 6) × 109 M
☉) provides a unique opportunity to investigate the origin of very high energy (VHE; E > 100 GeV) γ-ray emission generated in relativistic outflows and the surroundings of supermassive black holes. M 87 has been established as a VHE γ-ray emitter since 2006. The VHE γ-ray emission displays strong variability on timescales as short as a day. In this paper, results from a joint VHE monitoring campaign on M 87 by the MAGIC and VERITAS instruments in 2010 are reported. During the campaign, a flare at VHE was detected triggering further observations at VHE (H.E.S.S.), X-rays (Chandra), and radio (43 GHz Very Long Baseline Array, VLBA). The excellent sampling of the VHE γ-ray light curve enables one to derive a precise temporal characterization of the flare: the single, isolated flare is well described by a two-sided exponential function with significantly different flux rise and decay times of τrised = (1.69 ± 0.30) days and τdecayd = (0.611 ± 0.080) days, respectively. While the overall variability pattern of the 2010 flare appears somewhat different from that of previous VHE flares in 2005 and 2008, they share very similar timescales (~day), peak fluxes (Φ>0.35 TeV (1-3) × 10–11 photons cm–2 s–1), and VHE spectra. VLBA radio observations of 43 GHz of the inner jet regions indicate no enhanced flux in 2010 in contrast to observations in 2008, where an increase of the radio flux of the innermost core regions coincided with a VHE flare. On the other hand, Chandra X-ray observations taken ~3 days after the peak of the VHE γ-ray emission reveal an enhanced flux from the core (flux increased by factor ~2; variability timescale <2 days). The long-term (2001-2010) multi-wavelength (MWL) light curve of M 87, spanning from radio to VHE and including data from Hubble Space Telescope, Liverpool Telescope, Very Large Array, and European VLBI Network, is used to further investigate the origin of the VHE γ-ray emission. No unique, common MWL signature of the three VHE flares has been identified. In the outer kiloparsec jet region, in particular in HST-1, no enhanced MWL activity was detected in 2008 and 2010, disfavoring it as the origin of the VHE flares during these years. Shortly after two of the three flares (2008 and 2010), the X-ray core was observed to be at a higher flux level than its characteristic range (determined from more than 60 monitoring observations: 2002-2009). In 2005, the strong flux dominance of HST-1 could have suppressed the detection of such a feature. Published models for VHE γ-ray emission from M 87 are reviewed in the light of the new data.
Full-text · Article · Feb 2012 · The Astrophysical Journal
[Show abstract][Hide abstract] ABSTRACT: During the last decade, M87's jet has been the site of an extraordinary variability event, with one knot (HST-1) increasing by over a factor 100 in brightness. Variability has also been seen on timescales of months in the nuclear flux. Here we discuss the optical-UV polarization and spectral variability of these components, which show vastly different behavior. HST-1 shows a highly significant correlation between flux and polarization, with P increasing from ~20% at minimum to >40% at maximum, while the orientation of its electric vector stayed constant. HST-1's optical-UV spectrum is very hard (αUV-O ~ 0.5, F
ν∝ν–α), and displays "hard lags" during epochs 2004.9-2005.5, including the peak of the flare, with soft lags at later epochs. We interpret the behavior of HST-1 as enhanced particle acceleration in a shock, with cooling from both particle aging and the relaxation of the compression. We set 2σ upper limits of 0.5δ pc and 1.02c on the size and advance speed of the flaring region. The slight deviation of the electric vector orientation from the jet position angle (P.A.) makes it likely that on smaller scales the flaring region has either a double or twisted structure. By contrast, the nucleus displays much more rapid variability, with a highly variable electric vector orientation and "looping" in the (I, P) plane. The nucleus has a much steeper spectrum (αUV-O ~ 1.5) but does not show UV-optical spectral variability. Its behavior can be interpreted as either a helical distortion to a steady jet or a shock propagating through a helical jet.
Full-text · Article · Sep 2011 · The Astrophysical Journal
[Show abstract][Hide abstract] ABSTRACT: (Abridged) We have derived detailed R band luminosity profiles and structural
parameters for a total of 430 brightest cluster galaxies (BCGs), down to a
limiting surface brightness of 24.5 mag/arcsec^2. Light profiles were initially
fitted with a Sersic's R^(1/n) model, but we found that 205 (~48) BCGs require
a double component model to accurately match their light profiles. The best fit
for these 205 galaxies is an inner Sersic model, with indices n~1-7, plus an
outer exponential component.
Thus, we establish the existence of two categories of the BCGs luminosity
profiles: single and double component profiles. We found that double profile
BCGs are brighter ~0.2 mag than single profile BCG. In fact, the
Kolmogorov-Smirnov test applied to these subsamples indicates that they have
different total magnitude distributions, with mean values M_R=-23.8 +/- 0.6 mag
for single profile BCGs and M_R=-24.0 +/- 0.5 mag for double profile BCGs. We
find that partial luminosities for both subsamples are indistinguishable up to
r = 15 kpc, while for r > 20 kpc the luminosities we obtain are on average 0.2
mag brighter for double profile BCGs. This result indicates that extra-light
for double profile BCGs does not come from the inner region but from the outer
regions of these galaxies.
The best fit slope of the Kormendy relation for the whole sample is a = 3.13
+/- 0.04$. However, when fitted separately, single and double profile BCGs show
different slopes: a_(single) = 3.29 +/- 0.06 and a_(double)= 2.79 +/- 0.08.
On the other hand, we did not find differences between these two BCGs
categories when we compared global cluster properties such as the BCG-projected
position relative to the cluster X-ray center emission, X-ray luminosity, or
BCG orientation with respect to the cluster position angle.
Full-text · Article · Sep 2011 · The Astrophysical Journal Supplement Series
[Show abstract][Hide abstract] ABSTRACT: BCG images used in this work were provided by M. Postman (STScI) who kindly gave us access to the raw data. They were obtained under photometric conditions using the KPNO 2.1m and 4m telescopes, and the CTIO 1.5m telescope between 1989 November and 1995 April over a total of 13 observing runs. All the images were acquired in the Kron-Cousins Rc band and have typically exposure times of 200-600s. (1 data file).
[Show abstract][Hide abstract] ABSTRACT: Eleven ultra-compact dwarf (UCD) and 39 extended star cluster candidates are found to be associated with the galaxy NGC 1132. This giant elliptical galaxy is the remnant of a fossil group. UCD and extended star cluster candidates are identified through the analysis of their structural parameters, colors, spatial distribution, and luminosity using deep Hubble Space Telescope observations in two filters: the F475W (Sloan g) and F850LP (Sloan z). The median effective radius of these UCDs is rh = 13.0 pc. Two types of UCDs are identified in the vicinity of NGC 1132: one type shares the same color and luminosity as the brightest globular clusters and traces the onset of the mass-size relation. The second kind of UCD is represented by the brightest UCD candidate, an M32-type object, with an effective radius of rh = 77.1 pc, located at ~6.6 kpc from the nucleus of NGC 1132. This UCD candidate is likely the remaining nucleus of a minor merger with the host galaxy. With the exception of a particularly blue UCD candidate, UCDs are found to extend the mass-metallicity relation found in globular clusters to higher luminosities. The results of this work support the growing body of evidence showing that UCDs are not circumscribed to galaxy clusters as previously thought. UCDs are likely to be a common occurrence in all environments. The milder tidal field of a fossil group, when compared to a galaxy cluster, allows UCDs and extended star clusters to survive up to the present time at small galactocentric distances. Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS5-26555. These observations are associated with program 10558.
Preview · Article · Aug 2011 · The Astrophysical Journal Letters
[Show abstract][Hide abstract] ABSTRACT: We have discovered both a red and a blue subpopulation of ultra-compact dwarf (UCD) galaxy candidates in the Coma galaxy cluster. We analyzed deep F475W (Sloan g) and F814W (I) Hubble Space Telescope images obtained with the Advanced Camera for Surveys Wide Field Channel as part of the Coma Cluster Treasury Survey and have fitted the light profiles of ~5000 point-like sources in the vicinity of NGC 4874, one of the two central dominant galaxies of the Coma Cluster. Although almost all of these sources are globular clusters that remain unresolved, we found that 52 objects have effective radii between ~10 and 66 pc, in the range spanned by dwarf globular transition objects (DGTOs) and UCDs. Of these 52 compact objects, 25 are brighter than MV ~ –11 mag, a magnitude conventionally thought to separate UCDs and globular clusters. The UCD/DGTO candidates have the same color and luminosity distribution as the most luminous globular clusters within the red and blue subpopulations of the immensely rich NGC 4874 globular cluster system. Unlike standard globular clusters, blue and red UCD/DGTO subpopulations have the same median effective radius. The spatial distribution of UCD/DGTO candidates reveals that they congregate toward NGC 4874 and are not uniformly distributed. We find a relative deficit of UCD/DGTOs compared with globular clusters in the inner 15 kpc around NGC 4874; however, at larger radii UCD/DGTO and globular clusters follow the same spatial distribution.
Full-text · Article · Oct 2010 · The Astrophysical Journal
[Show abstract][Hide abstract] ABSTRACT: We compare the near-infrared (NIR) H-band photometric and morphological properties of low-z (z < 0.3) 3CR radio galaxies with samples of BL Lac objects and quasar host galaxies, merger remnants, quiescent elliptical galaxies, and brightest cluster galaxies drawn from the literature. In general, the 3CR host galaxies are consistent with luminous (~L
) elliptical galaxies. The vast majority of FR II's (~80%) occupy the most massive ellipticals and form a homogeneous population that is comparable to the population of radio-loud quasar (RLQ) host galaxies in the literature. However, a significant minority (~20%) of the 3CR FR II's appears under-luminous with respect to quasar host galaxies. All FR II objects in this faint tail are either unusually red, or appear to be the brightest objects within a group. We discuss the apparent differences between the radio galaxy and RLQ host galaxy populations. RLQs appear to require 1011M
☉ host galaxies (and ~109M
☉ black holes), whereas radio galaxies and radio-quiet quasars can exist in galaxies down to ~3 × 1010M
☉. This may be due to biases in the measured quasar host galaxy luminosities or populations studied, or due to a genuine difference in host galaxy. If due to a genuine difference, it would support the idea that radio and optical active galactic nuclei are two separate populations with a significant overlap.
Full-text · Article · Mar 2010 · The Astrophysical Journal
[Show abstract][Hide abstract] ABSTRACT: We derive structural parameters for ~2000 globular clusters in the giant Virgo elliptical Messier 87 (M87) using extremely deep Hubble Space Telescope images in F606W (V) and F814W (I) taken with the ACS/WFC. The cluster scale sizes (half-light radii rh ) and ellipticities are determined from point-spread-function -convolved King-model profile fitting. We find that the rh distribution closely resembles the inner Milky Way clusters, peaking at rh 2.5 pc and with virtually no clusters more compact than rh 1 pc. The metal-poor clusters have on average an rh 24% larger than the metal-rich ones. The cluster scale size shows a gradual and noticeable increase with galactocentric distance. Clusters are very slightly larger in the bluer waveband V, a possible hint that we may be beginning to see the effects of mass segregation within the clusters. We also derived a color magnitude diagram for the M87 globular cluster system which shows a striking bimodal distribution.
Preview · Article · Oct 2009 · The Astrophysical Journal
[Show abstract][Hide abstract] ABSTRACT: In HST cycle 13 (2004-2005) we embarked on a Near-Infrared Camera and
Multi-Object Spectrometer (NICMOS) H-band snapshot imaging campaign of
z<0.3 3CR sources (Madrid et al., 2006, Cat. J/ApJS/164/307, Paper
I). The SNAP program is now complete, having continued through cycle 14
(2005-2006) at reduced priority. Since publication of Paper I (which
presented images for the first 69 targets, observed during cycle 13) an
additional 22 sources have been successfully observed, and images for
these new targets are presented here. Finally, archival data for an
additional 11 objects previously observed with NICMOS2 in F160W were
obtained from the Multimission Archive at Space Telescope (MAST).
(6 data files).
[Show abstract][Hide abstract] ABSTRACT: Metal-poor globular clusters (GCs) are our local link to the earliest epochs of star formation and galaxy building. Studies of extragalactic GC systems using deep, high-quality imaging have revealed a small but significant slope to the color-magnitude relation for metal-poor GCs in a number of galaxies. We present a study of the M87 GC system using deep, archival HST/ACS imaging with the F606W and F814W filters, in which we find a significant color-magnitude relation for the metal-poor GCs. The slope of this relation in the I versus V-I color-magnitude diagram (γ I = –0.024 ± 0.006) is perfectly consistent with expectations based on previously published results using data from the ACS Virgo Cluster Survey. The relation is driven by the most luminous GCs, those with MI –10, and its significance is largest when fitting metal-poor GCs brighter than MI = –7.8, a luminosity which is ~1 mag fainter than our fitted Gaussian mean for the luminosity function (LF) of blue, metal-poor GCs (~0.8 mag fainter than the mean for all GCs). These results indicate that there is a mass scale at which the correlation begins, and is consistent with a scenario where self-enrichment drives a mass-metallicity relationship. We show that previously measured half-light radii of M87 GCs from best-fit PSF-convolved King models are consistent with the more accurate measurements in this study, and we also explain how the color-magnitude relation for metal-poor GCs is real and cannot be an artifact of the photometry. We fit Gaussian and evolved Schechter functions to the luminosity distribution of GCs across all colors, as well as divided into blue and red subpopulations, finding that the blue GCs have a brighter mean luminosity and a narrower distribution than the red GCs. Finally, we present a catalog of astrometry and photometry for 2250 M87 GCs.
Full-text · Article · Aug 2009 · The Astrophysical Journal
[Show abstract][Hide abstract] ABSTRACT: HST-1, a knot along the M87 jet located 0.85 arcsec from the nucleus of the galaxy has experienced dramatic and unexpected flaring activity since early 2000. We present analysis of Hubble Space Telescope Near-Ultraviolet (NUV) imaging of the M87 jet from 1999 May to 2006 December that reveals that the NUV intensity of HST-1 has increased 90 times over its quiescent level and outshines the core of the galaxy. The NUV light curve that we derive is synchronous with the light curves derived in other wavebands. The correlation of X-ray and NUV light curves during the HST-1 flare confirms the synchrotron origin of the X-ray emission in the M87 jet. The outburst observed in HST-1 is at odds with the common definition of AGN variability usually linked to blazars and originating in close proximity of the central black hole. In fact, the M87 jet is not aligned with our line of sight and HST-1 is located at one million Schwarzchild radii from the super-massive black hole in the core of the galaxy. Comment: April issue of AJ, 137, 3864 STScI press release http://hubblesite.org/newscenter/archive/releases/2009/16/
Preview · Article · Apr 2009 · The Astronomical Journal
[Show abstract][Hide abstract] ABSTRACT: We derive the ranking of the astronomical observatories with the highest impact in astronomy based on the citation analysis of papers published in 2006. We also present a description of the methodology we use to derive this ranking. The current ranking is lead by the Sloan Digital Sky Survey, followed by Swift and the Hubble Space Telescope.
[Show abstract][Hide abstract] ABSTRACT: We present newly acquired images of the near-infrared counterpart of 3CR radio sources. All the sources were selected to have a redshift of less than 0.3 to allow us to obtain the highest spatial resolution. The observations were carried out as a snapshot program using the Near-Infrared Camera and Multiobject Spectrograph (NICMOS) on-board the Hubble Space Telescope (HST). In this paper we describe 69 radio galaxies observed for the first time with NICMOS during HST cycle 13. All the objects presented here are elliptical galaxies. However, each of them has unique characteristics such as close companions, dust lanes, unresolved nuclei, arclike features, globular clusters, and jets clearly visible from the images or with basic galaxy subtraction.
Full-text · Article · Dec 2008 · The Astrophysical Journal Supplement Series
[Show abstract][Hide abstract] ABSTRACT: We present the second part of an H-band (1.6 μm) "atlas" of z < 0.3 3CR radio galaxies, using the Hubble Space Telescope Near Infrared Camera and Multi-Object Spectrometer (HST NICMOS2). We present new imaging for 21 recently acquired sources and host galaxy modeling for the full sample of 101 (including 11 archival)—an 87% completion rate. Two different modeling techniques are applied, following those adopted by the galaxy morphology and the quasar host galaxy communities. Results are compared and found to be in excellent agreement, although the former breaks down in the case of sources with strong active galactic nuclei (AGNs). Companion sources are tabulated, and the presence of mergers, tidal features, dust disks, and jets are cataloged. The tables form a catalog for those interested in the structural and morphological dust-free host galaxy properties of the 3CR sample, and for comparison with morphological studies of quiescent galaxies and quasar host galaxies. Host galaxy masses are estimated and found to typically lie at around 2 × 1011 M☉. In general, the population is found to be consistent with the local population of quiescent elliptical galaxies, but with a longer tail to low Sérsic index, mainly consisting of low-redshift (z < 0.1) and low-radio-power (FR I) sources. A few unusually disky FR II host galaxies are picked out for further discussion. Nearby external sources are identified in the majority of our images, many of which we argue are likely to be companion galaxies or merger remnants. The reduced NICMOS data are now publicly available from our Web site.
Full-text · Article · Dec 2008 · The Astrophysical Journal Supplement Series
[Show abstract][Hide abstract] ABSTRACT: Among radio galaxies containing nuclear dust disks, the bipolar jet axis is generally observed to be perpendicular to the disk major axis. The FR I radio source 3C 449 is an outlier to this statistical majority, as it possesses a nearly parallel jet/disk orientation on the sky. We examine the 600 pc dusty disk in this galaxy with images from the Hubble Space Telescope. We find that a 1.6 μm/0.7 μm color map of the disk exhibits a twist in its isocolor contours (isochromes). We model the color map by integrating galactic starlight through an absorptive disk and find that the anomalous twist in the isochromes can be reproduced in the model with a vertically thin, warped disk. The model predicts that the disk is nearly perpendicular to the jet axis within 100 pc of the nucleus. We discuss physical mechanisms capable of causing such a warp. We show that precessional models or a torque on the disk arising from a possible binary black hole in the AGN causes precession on a timescale that is too long to account for the predicted disk morphology. However, we estimate that the pressure in the X-ray-emitting interstellar medium is large enough to perturb the disk, and we argue that jet-driven anisotropy in the excited ISM may be the cause of the warp. In this way, the warped disk in 3C 449 may be a new manifestation of feedback from an active galactic nucleus.
Full-text · Article · Dec 2008 · The Astrophysical Journal
[Show abstract][Hide abstract] ABSTRACT: We report the discovery of a new optical-IR synchrotron jet in the radio galaxy 3C 133 from our HST NICMOS snapshot survey. The jet and eastern hot spot are well resolved and visible at both optical and IR wavelengths. The IR jet follows the morphology of the inner part of the radio jet, with three distinct knots identified with features in the radio. The radio-IR SEDs of the knots are examined, along with those of two more distant hot spots at the eastern extreme of the radio feature. The detected emission appears to be synchrotron, with peaks in the NIR for all except one case, which exhibits a power-law spectrum throughout.
Full-text · Article · Dec 2008 · The Astrophysical Journal