19 research outputs found
The Surface Mass Density and Structure of the Outer Disk of NGC 628
We study the kinematics of GALEX-selected H_alpha knots in the outer disk
(beyond R25) of NGC 628 (M74), a galaxy representative of large, undisturbed,
extended UV (Type 1 XUV) disks. Our spectroscopic target sample of 235 of the
bluest UV knots surrounding NGC 628 yielded 15 H_alpha detections (6%), roughly
the number expected given the different mean ages of the two populations. The
measured vertical velocity dispersion of the H_alpha knots between 1 - 1.8 R25
(13.5 - 23.2 kpc) is < 11 km/s. We assume that the H_alpha knots trace an
'intermediate' vertical mass density distribution (between the isothermal
sech(z)^2 and exponential distributions) with a constant scaleheight across the
outer disk (h_z = 700 pc) and estimate a total surface mass density of 7.5
solar masses/pc^2. This surface mass density can be accounted for by the
observed gas and stars in the outer disk (little or no dark matter in the disk
is required). The vertical velocity dispersion of the outer disk H_alpha knots
nearly matches that measured from older planetary nebulae near the outskirts of
the optical disk by Herrmann et al., suggesting a low level of scattering in
the outer disk. A dynamically cold stellar component extending nearly twice as
far as the traditional optical disk poses interesting constraints on the
accretion history of the galaxy.Comment: Accepted for publication in The Astrophysical Journal
A photometric search for transients in galaxy clusters
We have begun a program to search for supernovae and other transients in the
fields of galaxy clusters with the 2.3m Bok Telescope on Kitt Peak. We present
our automated photometric methods for data reduction, efficiency
characterization, and initial spectroscopy. With this program, we aim to
ultimately identify 25-35 cluster SN Ia (10 of which will be
intracluster, hostless events) and constrain the SN Ia rate associated with
old, passive stellar populations. With these measurements we will constrain the
relative contribution of hostless and hosted SN Ia to the metal enrichment of
the intracluster medium. In the current work, we have identified a central
excess of transient events within in our cluster fields after
statistically subtracting out the 'background' transient rate taken from an
off-cluster CCD chip. Based on the published rate of SN Ia for cluster
populations we estimate that 20 percent of the excess cluster transients
are due to cluster SN Ia, a comparable fraction to core collapse (CC)
supernovae and the remaining are likely to be active galactic nuclei.
Interestingly, we have identified three intracluster SN candidates, all of
which lay beyond . These events, if truly associated with the
cluster, indicate a large deficit of intracluster (IC) SN at smaller radii, and
may be associated with the IC stars of infalling groups or indicate that the
intracluster light (ICL) in the cluster outskirts is actively forming stars
which contribute CC SN or prompt SN Ia.Comment: Updated to match accepted version; 26 pages, 14 figures, AJ accepte
Super-Solar Super Lyman Limit Systems
We present abundance measurements for two super Lyman Limit systems (SLLS;
quasar absorption line systems with 10^19 cm^-2 < N_HI < 10^20.3 cm^-2)
selected from a set of metal-strong absorbers in the Sloan Digital Sky Survey
quasar database. After applying estimate corrections for photoionization
effects, we derive gas-phase metallicities of [M/H]=+0.7 +/- 0.2 dex for the
SLLS at z=1.7749 toward SDSS0927+5621 and [M/H]=+0.05 +/- 0.1 dex for the SLLS
at z=1.7678 toward SDSS0953+5230. The former exhibits among the highest gas
metallicity of any astrophysical environment and its total metal surface
density exceeds that of nearly every known damped Lya system. The properties of
these absorbers -- high metallicity and large velocity width (> 300 km/s) --
resemble those of gas observed in absorption in the spectra of bright,
star-forming galaxies at high redshift. We discuss the metal mass density of
the SLLS based on these observations and our ongoing SLLS survey and argue that
a conservative estimate to the total metal budget at z=2 is greater than 15% of
the total, suggesting that the metal-rich LLS may represent the dominant metal
reservoir in the young universe.Comment: 6 pages, 2 figures. Submitted to ApJL; Revised June 22, 200
Star Cluster Populations in the Outer Disks of Nearby Galaxies
We present a Large Binocular Telescope (LBT) imaging study that characterizes
the star cluster component of nearby galaxy outer disks (beyond the optical
radius R_25). Expanding on the pilot project of Herbert-Fort et al. (2009), we
present deep (~ 27.5 mag V-band point-source limiting magnitude) U- and V-band
imaging of six galaxies: IC 4182, NGC 3351, NGC 4736, NGC 4826, NGC 5474, and
NGC 6503. We find that the outer disk of each galaxy is populated with
marginally-resolved star clusters with masses ~10^3 M_sun and ages up to ~ 1
Gyr (masses and ages are limited by the depth of our imaging and uncertainties
are large given how photometry can be strongly affected by the presence or
absence of a few stars in such low mass systems), and that they are typically
found out to at least 2 R_25 but sometimes as far as 3 to 4 R_25- even beyond
the apparent HI disk. The mean rate of cluster formation for 1 R_25<= R <=
1.5R_25 is at least one every ~2.5 Myr and the clusters are spatially
correlated with the HI, most strongly with higher density gas near the
periphery of the optical disk and with lower density neutral gas at the HI disk
periphery. We hypothesize that the clusters near the edge of the optical disk
are formed in the extension of spiral structure from the inner disk and are a
fairly consistent phenomenon and that the clusters formed at the periphery of
the HI disk are the result of accretion episodes.Comment: 23 pages, accepted for publication in Ap
Spatially Correlated Cluster Populations in the Outer Disk of NGC 3184
We use deep (~27.5 mag V-band point-source limiting magnitude) V- and U-band
LBT imaging to study the outer disk (beyond the optical radius R_25) of the
non-interacting, face-on spiral galaxy NGC 3184 (D = 11.1 Mpc; R_25 = 11.1 kpc)
and find that this outer disk contains >1000 objects (or marginally-resolved
'knots') resembling star clusters with masses ~10^2 - 10^4 M_sun and ages up to
~1 Gyr. We find statistically significant numbers of these cluster-like knots
extending to ~1.4 R_25, with the redder knots outnumbering bluer at the largest
radii. We measure clustering among knots and find significant correlation to
galactocentric radii of 1.5 R_25 for knot separations <1 kpc. The effective
integrated surface brightness of this outer disk cluster population ranges from
30 - 32 mag arcsec^-2 in V. We compare the HI extent to that of the correlated
knots and find that the clusters extend at least to the damped Lyman-alpha
threshold of HI column density (2e20 cm^-2; 1.62 R_25). The blue knots are
correlated with HI spiral structure to 1.5 R_25, while the red knots may be
correlated with the outer fringes of the HI disk to 1.7 R_25. These results
suggest that outer disks are well-populated, common, and long-lasting features
of many nearby disk galaxies.Comment: Accepted for publication in The Astrophysical Journal. 12 pages, 10
figure
The Multi-Epoch Nearby Cluster Survey: type Ia supernova rate measurement in z~0.1 clusters and the late-time delay time distribution
We describe the Multi-Epoch Nearby Cluster Survey (MENeaCS), designed to
measure the cluster Type Ia supernova (SN Ia) rate in a sample of 57 X-ray
selected galaxy clusters, with redshifts of 0.05 < z < 0.15. Utilizing our real
time analysis pipeline, we spectroscopically confirmed twenty-three cluster SN
Ia, four of which were intracluster events. Using our deep CFHT/Megacam
imaging, we measured total stellar luminosities in each of our galaxy clusters,
and we performed detailed supernova detection efficiency simulations. Bringing
these ingredients together, we measure an overall cluster SN Ia rate within
R_{200} (1 Mpc) of 0.042^{+0.012}_{-0.010}^{+0.010}_{-0.008} SNuM
(0.049^{+0.016}_{-0.014}^{+0.005}_{-0.004} SNuM) and a SN Ia rate within red
sequence galaxies of 0.041^{+0.015}_{-0.015}^{+0.005}_{-0.010} SNuM
(0.041^{+0.019}_{-0.015}^{+0.005}_{-0.004} SNuM). The red sequence SN Ia rate
is consistent with published rates in early type/elliptical galaxies in the
`field'. Using our red sequence SN Ia rate, and other cluster SNe measurements
in early type galaxies up to , we derive the late time (>2 Gyr) delay
time distribution (DTD) of SN Ia assuming a cluster early type galaxy star
formation epoch of z_f=3. Assuming a power law form for the DTD, \Psi(t)\propto
t^s, we find s=-1.62\pm0.54. This result is consistent with predictions for the
double degenerate SN Ia progenitor scenario (s\sim-1), and is also in line with
recent calculations for the double detonation explosion mechanism (s\sim-2).
The most recent calculations of the single degenerate scenario delay time
distribution predicts an order of magnitude drop off in SN Ia rate \sim 6-7 Gyr
after stellar formation, and the observed cluster rates cannot rule this out.Comment: 35 pages, 14 figures, ApJ accepte
Intracluster supernovae in the Multi-epoch Nearby Cluster Survey
The Multi-Epoch Nearby Cluster Survey (MENeaCS) has discovered twenty-three
cluster Type Ia supernovae (SNe) in the 58 X-ray selected galaxy clusters (0.05
< z < 0.15) surveyed. Four of our SN Ia events have no host galaxy on close
inspection, and are likely intracluster SNe. Deep image stacks at the location
of the candidate intracluster SNe put upper limits on the luminosities of faint
hosts, with M_{r} > -13.0 mag and M_{g} > -12.5 mag in all cases. For such
limits, the fraction of the cluster luminosity in faint dwarfs below our
detection limit is <0.1%, assuming a standard cluster luminosity function. All
four events occurred within ~600 kpc of the cluster center (projected), as
defined by the position of the brightest cluster galaxy, and are more centrally
concentrated than the cluster SN Ia population as a whole. After accounting for
several observational biases that make intracluster SNe easier to discover and
spectroscopically confirm, we calculate an intracluster stellar mass fraction
of 0.16^{+0.13}_{-0.09} (68% CL) for all objects within R_{200}. If we assume
that the intracluster stellar population is exclusively old, and the cluster
galaxies themselves have a mix of stellar ages, we derive an upper limit on the
intracluster stellar mass fraction of <0.47 (84% one-sided CL). When combined
with the intragroup SNe results of McGee & Balogh, we confirm the declining
intracluster stellar mass fraction as a function of halo mass reported by
Gonzalez and collaborators. (Abridged)Comment: 24 pages, 8 figures, ApJ publishe
The Orbital Distribution of Satellite Galaxies
We measure the distribution of velocities for prograde and retrograde
satellite galaxies using a combination of published data and new observations
for 78 satellites of 63 extremely isolated disc galaxies (169 satellites
total). We find that the velocity distribution is non-Gaussian (>99.9%
confidence), but that it can be described as the sum of two Gaussians, one of
which is broad (sigma = 176 \pm 15 km/s), has a mean prograde velocity of 86
\pm 30 km/s, and contains ~55% of the satellites, while the other is slightly
retrograde with a mean velocity of -21 \pm 22 km/s and sigma = 74 \pm 18 km/s
and contains ~45% of the satellites. Both of these components are present over
all projected radii and found in the sample regardless of cuts on primary
inclination or satellite disc angle. The double-Gaussian shape, however,
becomes more pronounced among satellites of more luminous primaries. We remove
the potential dependence of satellite velocity on primary luminosity using the
Tully-Fisher relation and still find the velocity distribution to be asymmetric
and even more significantly non-Gaussian. The asymmetric velocity distribution
demonstrates a connection between the inner, visible disc galaxy and the
kinematics of the outer, dark halo. The reach of this connection, extending
even beyond the virial radii, suggests that it is imprinted by the satellite
infall pattern and large-scale effects, rather than by higher-level dynamical
processes in the formation of the central galaxy or late-term evolution of the
satellites.Comment: 14 pages, 13 figures; Accepted for publication in MNRA