36 research outputs found
The Centaurus Group and the Outer Halo of NGC 5128: Are they Dynamically Connected?
NGC 5128, a giant elliptical galaxy only Mpc away, is the dominant
member of a galaxy group of over 80 probable members. The Centaurus group
provides an excellent sample for a kinematic comparison between the halo of NGC
5128 and its surrounding satellite galaxies. A new study, presented here, shows
no kinematic difference in rotation amplitude, rotation axis, and velocity
dispersion between the halo of NGC 5128, determined from over of its
globular clusters, and those of the Centaurus group as a whole. These results
suggest NGC 5128 could be behaving in part as the inner component to the galaxy
group, and could have begun as a large initial seed galaxy, gradually built up
by minor mergers and satellite accretions, consistent with simple cold dark
matter models. The mass and mass-to-light ratios in the B-band, corrected for
projection effects, are determined to be
M_{\sun} and M_{\sun}/L_{\sun} for NGC 5128 out to a
galactocentric radius of 45 kpc, and M_{\sun}
and M_{\sun}/L_{\sun} for the Centaurus group, consistent with
previous studies.Comment: 14 pages, 3 tables, 7 figures, Accepted for publication in A
Radial Velocities of Newly Discovered Globular Clusters in NGC 5128
We present radial velocity measurements for 74 globular clusters (GCs) in the
nearby giant elliptical NGC 5128, of which 31 are newly discovered clusters.
All the GC candidates were taken from the list of possible new clusters given
in the Harris, Harris, & Geisler (2004) photometric survey. In addition to the
newly confirmed clusters, we identified 24 definite foreground stars and 31
probable background galaxies. From a combined list of 299 known GCs in NGC 5128
with measured radial velocities and metallicity-sensitive (C - T_1) photometric
indices, we construct a new metallicity distribution function (MDF) for the
cluster system. The MDF shows an approximately bimodal form, with centroids at
[Fe/H] = -1.46 and -0.53, and with nearly equal numbers of metal-poor and
metal-rich clusters in the two modes. However, there are many
intermediate-color clusters in the distribution, and the fainter clusters tend
to have a higher proportion of red clusters. These features of the MDF may
indicate a widespread age range within the cluster system as well as an
intrinsically broad metallicity spread.Comment: 10 pages, 7 figures, 4 tables - accepted in Astronomical Journa
New g'r'i'z' Photometry of the NGC 5128 Globular Cluster System
We present new photometry for 323 of the globular clusters in NGC 5128
(Centaurus A), measured for the first time in the filter system. The
color indices are calibrated directly to standard stars in the
system and are used to establish the fiducial mean colors for the blue and red
(low and high metallicity) globular cluster sequences. We also use
spectroscopically measured abundances to establish the conversion between the
most metallicity-sensitive colors (, ) and metallicity,
[Fe/H].Comment: 14 pages, 9 figures, accepted in A
Globular Cluster Scale Sizes in Giant Galaxies: Orbital Anisotropy and Tidally Under-filling Clusters in M87, NGC 1399, and NGC 5128
We investigate the shallow increase in globular cluster half-light radii with
projected galactocentric distance observed in the giant galaxies M87,
NGC 1399, and NGC 5128. To model the trend in each galaxy, we explore the
effects of orbital anisotropy and tidally under-filling clusters. While a
strong degeneracy exists between the two parameters, we use kinematic studies
to help constrain the distance beyond which cluster orbits become
anisotropic, as well as the distance beyond which clusters are
tidally under-filling. For M87 we find kpc and kpc and kpc.
The connection of with each galaxy's mass profile indicates the
relationship between size and may be imposed at formation, with only
inner clusters being tidally affected. The best fitted models suggest the
dynamical histories of brightest cluster galaxies yield similar present-day
distributions of cluster properties. For NGC 5128, the central giant in a small
galaxy group, we find kpc and kpc. While we
cannot rule out a dependence on , NGC 5128 is well fitted by a tidally
filling cluster population with an isotropic distribution of orbits, suggesting
it may have formed via an initial fast accretion phase. Perturbations from the
surrounding environment may also affect a galaxy's orbital anisotropy profile,
as outer clusters in M87 and NGC 1399 have primarily radial orbits while outer
NGC 5128 clusters remain isotropic.Comment: 16 pages, 7 figures, 4 tables, Accepted for publication in MNRA
The Globular Cluster Systems around NGC 3311 and NGC 3309
We present extensive new photometry in (g',i') of the large globular cluster
(GC) system around NGC 3311, the central cD galaxy in the Hydra cluster. Our
GMOS data cover a 5.5' field of view and reach a limiting magnitude i' = 26,
about 0.5 magnitude fainter than the turnover point of the GC luminosity
function. We find that NGC 3311 has a huge population of ~16, 000 GCs, closely
similar to the prototypical high specific frequency Virgo giant M87. The
color-magnitude distribution shows that the metal-poor blue GC sequence and the
metal-richer red sequence are both present, with nearly equal numbers of
clusters. Bimodal fits to the color distributions confirm that the blue
sequence shows the same trend of progressively increasing metallicity with GC
mass that has previously been found in many other large galaxies; the
correlation we find corresponds to a scaling of GC metallicity with mass of Z ~
M^0.6 . By contrast, the red sequence shows no change of mean metallicity with
mass, but it shows an upward extension to much higher than normal luminosity
into the UCD-like range, strengthening the potential connections between
massive GCs and UCDs. The GC luminosity function, which we measure down to the
turnover point at M_I = -8.4, also has a normal form like those in other giant
ellipticals. Within the Hydra field, another giant elliptical NGC 3309 is
sitting just 100" from the cD NGC 3311. We use our data to solve simultaneously
for the spatial structure and total GC populations of both galaxies at once.
Their specific frequencies are S_N (NGC 3311) = 12.5 +/- 1.5 and S_N (NGC 3309)
= 0.6 +/-0.4. NGC 3311 is completely dominant and entirely comparable with
other cD-type systems such as M87 in Virgo.Comment: 15 pages, 15 figures. Accepted to the Astrophysical Journal. Version
with higher resolution figures is available at
http://www.thewehners.net/astro/papers/wehner_n3311_highres.pd
The Kinematics and Dynamics of the Globular Clusters and the Planetary Nebulae of NGC 5128
A new kinematic and dynamic study of the halo of the giant elliptical galaxy,
NGC 5128, is presented. From a spectroscopically confirmed sample of 340
globular clusters and 780 planetary nebulae, the rotation amplitude, rotation
axis, velocity dispersion, and the total dynamical mass are determined for the
halo of NGC 5128. The globular cluster kinematics were searched for both radial
dependence and metallicity dependence by subdividing the globular cluster
sample into 158 metal-rich ([Fe/H] > -1.0) and 178 metal-poor ([Fe/H] < -1.0)
globular clusters. Our results show the kinematics of the metal-rich and
metal-poor subpopulations are quite similar. The kinematics are compared to the
planetary nebula population where differences are apparent in the outer regions
of the halo. The total mass of NGC 5128 is found using the Tracer Mass
estimator (Evans et al. 2003), to determine the mass supported by internal
random motions, and the spherical component of the Jeans equation to determine
the mass supported by rotation. We find a total mass of (1.0+/-0.2) x 10^(12)
Msun from the planetary nebulae data out to a projected radius of 90 kpc and
(1.3+/-0.5) x 10^(12) Msun from the globular clusters out to a projected radius
of 50 kpc. Lastly, we present a new and homogeneous catalog of known globular
clusters in NGC 5128. This catalog combines all previous definitive cluster
identifications from radial velocity studies and HST imaging studies, as well
as 80 new globular clusters from a study of M.A. Beasley et al. (2007, in
preparation).Comment: Accepted in the Astronomical Journal,52 pages, 13 figures, 6 tables -
Changes made to Table 1 from originally submitted 0704.118
Globular Clusters and X-ray Point Sources in Centaurus A (NGC 5128)
We detect 353 X-ray point sources, mostly low-mass X-ray binaries (LMXBs), in
four Chandra observations of Centaurus A (NGC 5128), the nearest giant
early-type galaxy, and correlate this point source population with the largest
available ensemble of confirmed and likely globular clusters associated with
this galaxy. Of the X-ray sources, 31 are coincident with 30 globular clusters
that are confirmed members of the galaxy by radial velocity measurement (2
X-ray sources match one globular cluster within our search radius), while 1
X-ray source coincides with a globular cluster resolved by HST images. Another
36 X-ray point sources match probable, but spectroscopically unconfirmed,
globular cluster candidates. The color distribution of globular clusters and
cluster candidates in Cen A is bimodal, and the probability that a red, metal
rich GC candidate contains an LMXB is at least 1.7 times that of a blue, metal
poor one. If we consider only spectroscopically confirmed GCs, this ratio
increases to ~3. We find that LMXBs appear preferentially in more luminous
(massive) GCs. These two effects are independent, and the latter is likely a
consequence of enhanced dynamical encounter rates in more massive clusters
which have on average denser cores. The X-ray luminosity functions of the LMXBs
found in GCs and of those that are unmatched with GCs reveal similar underlying
populations, though there is some indication that fewer X-ray faint LMXBs are
found in globular clusters than X-ray bright ones. Our results agree with
previous observations of the connection of GCs and LMXBs in early-type galaxies
and extend previous work on Centaurus A.Comment: 34 pages, 10 figures, 2 tables, Accepted for Publication in The
Astrophysical Journa
The Ages, Metallicities and Alpha Element Enhancements of Globular Clusters in the Elliptical NGC 5128: A Homogeneous Spectroscopic Study with Gemini/GMOS
We present new integrated light spectroscopy of globular clusters (GCs) in
NGC 5128 in order to measure radial velocities and derive ages, metallicities,
and alpha-element abundance ratios. Using Gemini-S 8-m/GMOS, we obtained
spectroscopy in the range of ~3400-5700 AA for 72 GCs with S/N > 30 /AA and we
have also discovered 35 new GCs within NGC 5128 from our radial velocity
measurements. We measured and compared the Lick indices from HdeltaA through
Fe5406 with the single stellar population (SSP) models of Thomas et
al.(2003,2004). We also measure Lick indices for 41 Milky Way GCs from Puzia et
al. (2002) and Schiavon et al. (2005) with the same methodology for direct
comparison. Our results show that 68% of the NGC 5128 GCs have old ages (> 8
Gyr), 14% have intermediate ages (5-8 Gyr), and 18% have young ages (< 5 Gyr).
However, when we look at the metallicity of the GCs as a function of age, we
find 92% of metal-poor GCs and 56% of metal-rich GCs in NGC 5128 have ages > 8
Gyr, indicating that the majority of both metallicity subpopulations of GCs
formed early, with a significant population of young and metal-rich GCs forming
later. Our metallicity distribution function generated directly from
spectroscopic Lick indices is clearly bimodal, as is the color distribution of
the same set of GCs. Thus the metallicity bimodality is real and not an
artifact of the color to metallicity conversion. The [alpha/Fe] values are
supersolar with a mean value of 0.14pm0.04, indicating a fast formation
timescale. However, the GCs in NGC 5128 are not as [alpha/Fe] enhanced as the
Milky Way GCs also examined in this study. Our results support a rapid, early
formation of the GC system in NGC 5128, with subsequent major accretion and/or
GC and star forming events in more recent times (abridged).Comment: Accepted to The Astrophysical Journal, 36 pages, 14 figures, 7 table
The Kinematics of the Globular Cluster System of NGC 5128 with a New, Large Sample of Radial Velocity Measurements
New radial velocity measurements for previously known and newly confirmed
globular clusters (GCs) in the nearby massive galaxy NGC 5128 are presented. We
have obtained spectroscopy from LDSS-2/Magellan, VIMOS/VLT, and Hydra/CTIO from
which we have measured the radial velocities of 218 known, and identified 155
new, GCs. The current sample of confirmed GCs in NGC 5128 is now 605 with 564
of these having radial velocity measurements. We have performed a new kinematic
analysis of the GC system that extends out to 45 arcmin in galactocentric
radius. We have examined the systemic velocity, projected rotation amplitude
and axis, and the projected velocity dispersion of the GCs as functions of
galactocentric distance and metallicity. Our results indicate that the
metal-poor GCs have a very mild rotation signature of (26 pm 15) km/s. The
metal-rich GCs are rotating with a higher, though still small signature of (43
pm 15) km/s around the isophotal major axis of NGC 5128 within 15 arcmin. Their
velocity dispersions are consistent within the uncertainties and the profiles
appear flat or declining within 20 arcmin. We note the small sample of
metal-rich GCs with ages less than 5 Gyr in the literature appear to have
different kinematic properties than the old, metal-rich GC subpopulation. The
mass and mass-to-light ratios have also been estimated using the GCs as tracer
particles for NGC 5128. Out to a distance of 20 arcmin, we have obtained a mass
of (5.9 pm 2.0) x 10^(11) M_(sun) and a mass-to-light ratio in the B-band of 16
M_(sun)/L_(B,sun). Combined with previous work on the ages and metallicities of
its GCs, as well as properties of its stellar halo, our findings suggest NGC
5128 formed via hierarchical merging over other methods of formation, such as
major merging at late times.Comment: Accepted for The Astronomical Journal, 14 pages plus 12 figures and 7
table
A Transient Sub-Eddington Black Hole X-ray Binary Candidate in the Dust Lanes of Centaurus A
We report the discovery of a bright X-ray transient, CXOU J132527.6-430023,
in the nearby early-type galaxy NGC 5128. The source was first detected over
the course of five Chandra observations in 2007, reaching an unabsorbed
outburst luminosity of 1-2*10^38 erg/s in the 0.5-7.0 keV band before returning
to quiescence. Such luminosities are possible for both stellar-mass black hole
and neutron star X-ray binary transients. Here, we attempt to characterize the
nature of the compact object. No counterpart has been detected in the optical
or radio sky, but the proximity of the source to the dust lanes allows for the
possibility of an obscured companion. The brightness of the source after a >100
fold increase in X-ray flux makes it either the first confirmed transient
non-ULX black hole system in outburst to be subject to detailed spectral
modeling outside the Local Group, or a bright (>10^38 erg/s) transient neutron
star X-ray binary, which are very rare. Such a large increase in flux would
appear to lend weight to the view that this is a black hole transient. X-ray
spectral fitting of an absorbed power law yielded unphysical photon indices,
while the parameters of the best-fit absorbed disc blackbody model are typical
of an accreting ~10 Msol black hole in the thermally dominant state.Comment: 8 pages, 6 figures, accepted for publication in Ap