Publications (9)0 Total impact
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ABSTRACT: The process by which the mass density profile of certain galaxy clusters
becomes centrally concentrated enough to produce high strong lensing (SL)
cross-sections is not well understood. It has been suggested that the baryonic
condensation of the intra-cluster medium (ICM) due to cooling may drag dark
matter to the cores and thus steepen the profile. In this work, we search for
evidence of ongoing ICM cooling in the first large, well-defined sample of
strong lensing selected galaxy clusters in the range 0.1 < z < 0.6. Based on
known correlations between the ICM cooling rate and both optical emission line
luminosity and star formation, we measure, for a sample of 89 strong lensing
clusters, the fraction of clusters that have [OII]3727 emission in their
brightest cluster galaxy (BCG). We find that the fraction of line-emitting BCGs
is constant as a function of redshift for z > 0.2 and shows no statistically
significant deviation from the total cluster population. Specific star
formation rates, as traced by the strength of the 4000 angstrom break, D_4000,
are also consistent with the general cluster population. Finally, we use
optical imaging of the SL clusters to measure the angular separation, R_arc,
between the arc and the center of mass of each lensing cluster in our sample
and test for evidence of changing [OII] emission and D_4000 as a function of
R_arc, a proxy observable for SL cross-sections. D_4000 is constant with all
values of R_arc, and the [OII] emission fractions show no dependence on R_arc
for R_arc > 10" and only very marginal evidence of increased weak [OII]
emission for systems with R_arc < 10". These results argue against the ability
of baryonic cooling associated with cool core activity in the cores of galaxy
clusters to strongly modify the underlying dark matter potential, leading to an
increase in strong lensing cross-sections.
05/2013;
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ABSTRACT: We present Hubble Space Telescope Wide Field Camera 3 observations of the
core of the Phoenix Cluster SPT-CLJ2344-4243 in five broadband filters spanning
rest-frame 1000--5500A. These observations reveal complex, filamentary blue
emission, extending for >40kpc from the brightest cluster galaxy. We observe an
underlying, diffuse population of old stars, following an r^1/4 distribution,
confirming that this system is somewhat relaxed. The spectral energy
distribution in the inner part of the galaxy, as well as along the extended
filaments, is a smooth continuum and is consistent with that of a star-forming
galaxy, suggesting that the extended, filamentary emission is not due to the
central AGN, either from a large-scale ionized outflow or scattered polarized
UV emission, but rather a massive population of young stars. We estimate an
extinction-corrected star formation rate of 798 +/- 42 Msun/yr, consistent with
our earlier work based on low spatial resolution ultraviolet, optical, and
infrared imaging. The lack of tidal features and multiple bulges, combine with
the need for an exceptionally massive (>10^11 Msun) cold gas reservoir, suggest
that this star formation is not the result of a merger of gas-rich galaxies.
Instead, we propose that the high X-ray cooling rate of ~2700 Msun/yr is the
origin of the cold gas reservoir. The combination of such a high cooling rate
and the relatively weak radio source in the cluster core suggests that feedback
has been unable to halt cooling in this system, leading to this tremendous
burst of star formation.
11/2012;
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ABSTRACT: We present the results of interferometric observations of the cool core of
Abell 1795 at CO(1-0) using the Combined Array for Research in Millimeter-Wave
Astronomy. In agreement with previous work, we detect a significant amount of
cold molecular gas (3.9 +/- 0.4 x10^9 Msun) in the central ~10 kpc. We report
the discovery of a substantial clump of cold molecular gas at clustercentric
radius of 30 kpc (2.9 +/- 0.4 x10^9 Msun), coincident in both position and
velocity with the warm, ionized filaments. We also place an upper limit on the
H_2 mass at the outer edge of the star-forming filament, corresponding to a
distance of 60 kpc (<0.9 x10^9 Msun). We measure a strong gradient in the
HII/H_2 ratio as a function of radius, suggesting different ionization
mechanisms in the nucleus and filaments of Abell1795. The total mass of cold
molecular gas (\sim7x10^9 Msun) is roughly 30% of the classical cooling
estimate at the same position, assuming a cooling time of 10^9 yr. Combining
the cold molecular gas mass with the UV-derived star formation rate and the
warm, ionized gas mass, the spectroscopically-derived X-ray cooling rate is
fully accounted for and in good agreement with the cooling byproducts over
timescales of \sim10^9 yr. The overall agreement between the cooling rate of
the hot intracluster medium and the mass of the cool gas reservoir suggests
that, at least in this system, the cooling flow problem stems from a lack of
observable cooling in the more diffuse regions at large radii.
07/2012;
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ABSTRACT: We have obtained deep, high spatial and spectral resolution, long-slit
spectra of the Halpha nebulae in the cool cores of 9 galaxy clusters. This
sample provides a wealth of information on the ionization state, kinematics,
and reddening of the warm gas in the cool cores of galaxy clusters. We find
evidence for only small amounts of reddening in the extended, line-emitting
filaments, with the majority of filaments having E(B-V) < 0.2. The combination
of [O III]/Hb, [N II]/Ha, [S II]/Ha, and [O I]/Ha allow us to rule out
collisional ionization by cosmic rays, thermal conduction, and photoionization
by ICM X-rays and AGN as strong contributors to the ionization of the warm gas
in both nuclei and filaments. The data are adequately described by a composite
model of slow shocks and star formation. This model is further supported by an
observed correlation between the linewidths and low ionization line ratios
which becomes stronger in systems with more modest star formation activity
based on far ultraviolet observations. We find that the more extended, narrow
filaments tend to have shallower velocity gradients and narrower linewidths
than the compact filamentary complexes. We confirm that the widths of the
emission lines decrease with radius, from FWHM \sim 600 km/s in the nuclei to
FWHM ~ 100 km/s in the most extended filaments. We suggest that this radial
dependence of the velocity width may in fact be linked to ICM turbulence and,
thus, may provide a glimpse into the amount of turbulence in cool cores. In the
central regions (r < 10 kpc) of several systems the warm gas shows kinematic
signatures consistent with rotation. We find that the kinematics of the most
extended filaments in this sample are broadly consistent with both infall and
outflow, and recommend further studies linking the warm gas kinematics to both
radio and X-ray maps in order to further understand the observed kinematics.
10/2011;
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ABSTRACT: There are few observational constraints on how the escape of ionizing photons
from starburst galaxies depends on galactic parameters. Here, we report on the
first major detection of an ionization cone in NGC 5253, a nearby starburst
galaxy. This high-excitation feature is identified by mapping the emission-line
ratios in the galaxy using [S III] lambda 9069, [S II] lambda 6716, and H_alpha
narrow-band images from the Maryland-Magellan Tunable Filter at Las Campanas
Observatory. The ionization cone appears optically thin, which is suggestive of
the escape of ionizing photons. The cone morphology is narrow with an estimated
solid angle covering just 3% of 4pi steradians, and the young, massive clusters
of the nuclear starburst can easily generate the radiation required to ionize
the cone. Although less likely, we cannot rule out the possibility of an
obscured AGN source. An echelle spectrum along the minor axis shows complex
kinematics that are consistent with outflow activity. The narrow morphology of
the ionization cone supports the scenario that an orientation bias contributes
to the difficulty in detecting Lyman continuum emission from starbursts and
Lyman break galaxies.
09/2011;
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ABSTRACT: We have assembled a sample of high spatial resolution far-UV (Hubble Space
Telescope Advanced Camera for Surveys Solar Blind Channel) and Halpha
(Maryland-Magellan Tunable Filter) imaging for 15 cool core galaxy clusters.
These data provide a detailed view of the thin, extended filaments in the cores
of these clusters. Based on the ratio of the far-UV to Halpha luminosity, the
UV spectral energy distribution, and the far-UV and Halpha morphology, we
conclude that the warm, ionized gas in the cluster cores is photoionized by
massive, young stars in all but a few (Abell 1991, Abell 2052, Abell 2580)
systems. We show that the extended filaments, when considered separately,
appear to be star-forming in the majority of cases, while the nuclei tend to
have slightly lower far-UV luminosity for a given Halpha luminosity, suggesting
a harder ionization source or higher extinction. We observe a slight offset in
the UV/Halpha ratio from the expected value for continuous star formation which
can be modeled by assuming intrinsic extinction by modest amounts of dust
(E(B-V) ~ 0.2), or a top-heavy IMF in the extended filaments. The measured star
formation rates vary from ~ 0.05 Msun/yr in the nuclei of non-cooling systems,
consistent with passive, red ellipticals, to ~ 5 Msun/yr in systems with
complex, extended, optical filaments. Comparing the estimates of the star
formation rate based on UV, Halpha and infrared luminosities to the
spectroscopically-determined X-ray cooling rate suggests a star formation
efficiency of 14(+18)(-8)%. This value represents the time-averaged fraction,
by mass, of gas cooling out of the intracluster medium which turns into stars,
and agrees well with the global fraction of baryons in stars required by
simulations to reproduce the stellar mass function for galaxies. This result
provides a new constraint on the efficiency of star formation in accreting
systems.
04/2011;
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ABSTRACT: We present the results of a combined X-ray and Halpha study of 10 galaxy
groups and 17 galaxy clusters using the Chandra X-ray Observatory and the
Maryland Magellan Tunable Filter. We find no difference in the morphology or
detection frequency of Halpha filaments in groups versus clusters, over the
mass range 10^13 < M_500 < 10^15 Msun. The detection frequency of Halpha
emission is shown to be only weakly dependent on the total mass of the system,
at the 52% confidence level. In contrast, we find that the presence of Halpha
filaments is strongly correlated with both the global (89% confidence level)
and core (84%) ICM entropy, as well as the X-ray cooling rate (72%). The Halpha
filaments are therefore an excellent proxy for the cooling ICM. The Halpha
filaments are more strongly correlated with the cooling properties of the ICM
than with the radio properties of the BCG; this further supports the scenario
where these filaments are directly associated with a thermally-unstable,
rapidly cooling ICM, rather than radio bubbles. The ICM cooling efficiency,
defined as the X-ray cooling rate per unit gas mass, is shown to correlate with
the total system mass, indicating that groups are more efficient at cooling
than clusters. This result implies that, in systems with cool cores, AGN
feedback scales with the total mass of the system, in agreement with earlier
suggestions.
02/2011;
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ABSTRACT: We present a high spatial resolution Halpha survey of 23 cooling flow clusters using the Maryland Magellan Tunable Filter (MMTF), covering 1-2 orders of magnitude in cooling rate, dM/dt, temperature and entropy. We find 8/23 (35%) of our clusters have complex, filamentary morphologies at Halpha, while an additional 7/23 (30%) have marginally extended or nuclear Halpha emission, in general agreement with previous studies of line emission in cooling flow cluster BCGs. A weak correlation between the integrated near-UV luminosity and the Halpha luminosity is also found for our complete sample, with a large amount of scatter about the expected relation for photoionization by young stars. We detect Halpha emission out to the X-ray cooling radius, but no further, in several clusters and find a strong correlation between the Halpha luminosity contained in filaments and the X-ray cooling flow rate of the cluster, suggesting that the warm ionized gas is linked to the cooling flow. Furthermore, we detect a strong enhancement in the cooling properties of the ICM coincident with the Halpha emission, compared to the surrounding ICM at the same radius. While the filaments in a few clusters may be entrained by buoyant radio bubbles, in general, the radially-infalling cooling flow model provides a better explanation for the observed trends. The correlation of the Halpha and X-ray properties suggests that conduction may be important in keeping the filaments ionized. The thinness of the filaments suggests that magnetic fields are an important part of channeling the gas and shielding it from the surrounding hot ICM. Comment: Accepted for publication in ApJ, 52 pages, 16 figures, 2 tables
08/2010;
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ABSTRACT: We have obtained deep, high spatial resolution images of the central region of Abell 1795 at Halpha and [NII] (6583A) with the Maryland Magellan Tunable Filter (MMTF), and in the far-ultraviolet (FUV) with the Advanced Camera for Surveys Solar Blind Channel on the Hubble Space Telescope (HST). The superb image quality of the MMTF data has made it possible to resolve the known SE filament into a pair of thin, intertwined filaments extending for ~50 kpc, with a width < 1 kpc. The presence of these thin, tangled strands is suggestive of a cooling wake where runaway cooling is taking place, perhaps aided by an enhanced magnetic field in this region. The HST data further resolve these strands into chains of FUV-bright stellar clusters, indicating that these filaments are indeed sites of on-going star formation, but at a rate ~2 orders of magnitude smaller than the mass-deposition rates predicted from the X-ray data. The elevated [NII]/Halpha ratio and large spatial variations of the FUV/Halpha flux ratio across the filaments indicate that O-star photoionization is not solely responsible for the ionization. The data favor collisional heating by cosmic rays either produced in-situ by magnetohydrodynamical processes or conducted in from the surrounding intracluster medium. Comment: Accepted for publication in ApJ Letters, 6 pages, 4 figures, 1 table; Corrected typos in Fig 1. & Table 1
09/2009;
