715 research outputs found
An X-ray/optical study of the geometry and dynamics of MACS J0140.0-0555, a massive post-collision cluster merger
We investigate the physical properties, geometry and dynamics of the massive
cluster merger MACS J0140.0-0555 (z=0.451) using X-ray and optical diagnostics.
Featuring two galaxy overdensities separated by about 250 kpc in projection on
the sky, and a single peak in the X-ray surface brightness distribution located
between them, MACS J0140.0-0555 shows the tell-tale X-ray/optical morphology of
a binary, post-collision merger. Our spectral analysis of the X-ray emission,
as measured by our Chandra ACIS-I observation of the system, finds the
intra-cluster medium to be close to isothermal (~8.5 keV) with no clear signs
of cool cores or shock fronts. Spectroscopic follow-up of galaxies in the field
of MACS J0140.0-0555 yields a velocity dispersion of 875 (+70/-100) km/s
(n_z=66) and no significant evidence of bimodality or substructure along the
line of sight. In addition, the difference in radial velocity between the
brightest cluster galaxies of the two sub-clusters of 144+/-25 km/s is small
compared to typical collision velocities of several 1000 km/s. A strongly
lensed background galaxy at z=0.873 (which features variable X-ray emission
from an active nucleus) provides the main constraint on the mass distribution
of the system. We measure M(<75 kpc) = (5.6+/- 0.5)*10^13 M_sun for the
north-western cluster component and a much less certain estimate of
(1.5-3)*10^13 M_sun for the south-eastern subcluster. These values are in good
agreement with our X-ray mass estimates which yield a total mass of MACS
J0140.0-0555 of M(<r_500) ~ (6.8-9.1)*10^14 M_sun. ......Comment: 11 pages, 8 figures, and 2 tables. Accepted for publication in MNRA
A Machine Learning Artificial Neural Network Calibration of the Strong-Line Oxygen Abundance
The HII region oxygen abundance is a key observable for studying chemical
properties of galaxies. Deriving oxygen abundances using optical spectra often
relies on empirical strong-line calibrations calibrated to the direct method.
Existing calibrations usually adopt linear or polynomial functions to describe
the non-linear relationships between strong line ratios and Te oxygen
abundances. Here, I explore the possibility of using an artificial neural
network model to construct a non-linear strong-line calibration. Using about
950 literature HII region spectra with auroral line detections, I build
multi-layer perceptron models under the machine learning framework of training
and testing. I show that complex models, like the neural network, are preferred
at the current sample size and can better predict oxygen abundance than simple
linear models. I demonstrate that the new calibration can reproduce metallicity
gradients in nearby galaxies and the mass-metallicity relationship. Finally, I
discuss the prospects of developing new neural network calibrations using
forthcoming large samples of HII region and also the challenges faced.Comment: 12 pages, 15 figures. Accepted to MNRA
Multiple Radial Cool Molecular Filaments in NGC 1275
We have extended our previous observation (Lim et al. 2008) of NGC1275
covering a central radius of ~10kpc to the entire main body of cool molecular
gas spanning ~14kpc east and west of center. We find no new features beyond the
region previously mapped, and show that all six spatially-resolved features on
both the eastern and western sides (three on each side) comprise radially
aligned filaments. Such radial filaments can be most naturally explained by a
model in which gas deposited "upstream" in localized regions experiencing an
X-ray cooling flow subsequently free falls along the gravitational potential of
PerA, as we previously showed can explain the observed kinematics of the two
longest filaments. All the detected filaments coincide with locally bright
Halpha features, and have a ratio in CO(2-1) to Halpha luminosity of ~1e-3; we
show that these filaments have lower star formation efficiencies than the
nearly constant value found for molecular gas in nearby normal spiral galaxies.
On the other hand, some at least equally luminous Halpha features, including a
previously identified giant HII region, show no detectable cool molecular gas
with a corresponding ratio at least a factor of ~5 lower; in the giant HII
region, essentially all the pre-existing molecular gas may have been converted
to stars. We demonstrate that all the cool molecular filaments are
gravitationally bound, and without any means of support beyond thermal pressure
should collapse on timescales ~< 1e6yrs. By comparison, as we showed previously
the two longest filaments have much longer dynamical ages of ~1e7yrs. Tidal
shear may help delay their collapse, but more likely turbulent velocities of at
least a few tens km/s or magnetic fields with strengths of at least several
~10uG are required to support these filaments.Comment: 52 pages, 11 figures. Accepted to Ap
The chemical evolution of local star forming galaxies: Radial profiles of ISM metallicity, gas mass, and stellar mass and constraints on galactic accretion and winds
The radially averaged metallicity distribution of the ISM and the young
stellar population of a sample of 20 disk galaxies is investigated by means of
an analytical chemical evolution model which assumes constant ratios of
galactic wind mass loss and accretion mass gain to star formation rate. Based
on this model the observed metallicities and their gradients can be described
surprisingly well by the radially averaged distribution of the ratio of stellar
mass to ISM gas mass. The comparison between observed and model predicted
metallicity is used to constrain the rate of mass loss through galactic wind
and accretion gain in units of the star formation rate. Three groups of
galaxies are found: galaxies with either mostly winds and only weak accretion,
or mostly accretion and only weak winds, and galaxies where winds are roughly
balanced by accretion. The three groups are distinct in the properties of their
gas disks. Galaxies with approximately equal rates of mass-loss and accretion
gain have low metallicity, atomic hydrogen dominated gas disks with a flat
spatial profile. The other two groups have gas disks dominated by molecular
hydrogen out to 0.5 to 0.7 isophotal radii and show a radial exponential
decline, which is on average steeper for the galaxies with small accretion
rates. The rates of accretion (<1.0 x SFR) and outflow (<2.4 x SFR) are
relatively low. The latter depend on the calibration of the zero point of the
metallicity determination from the use of HII region strong emission lines.Comment: 19 pages, 17 figure, accepted to MNRA
IFU observations of luminous type II AGN - I. Evidence for ubiquitous winds
We present observations of 17 luminous (log(L[O III]/L_Sun) > 8.7) local (z <
0.11) type II AGN. Our aim is to investigate the prevalence and nature of AGN
driven outflows in these galaxies by combining kinematic and ionization
diagnostic information. We use non-parametric methods (e.g. W80, the width
containing 80% of the line flux) to assess the line widths in the central
regions of our targets. The maximum values of W80 in each galaxy are in the
range 400 - 1600 km/s, with a mean of 790 +- 90 km/s. Such high velocities are
strongly suggestive that these AGN are driving ionized outflows. Multi-Gaussian
fitting is used to decompose the velocity structure in our galaxies. 14/17 of
our targets require 3 separate kinematic components in the ionized gas in their
central regions. The broadest components of these fits have FWHM = 530 - 2520
km/s, with a mean value of 920 +- 50 km/s. By simultaneously fitting both the
H{\beta}/[O III] and H{\alpha}/[N II] complexes we construct ionization
diagnostic diagrams for each component. 13/17 of our galaxies show a
significant (> 95 %) correlation between the [N II]/H{\alpha} ratio and the
velocity dispersion of the gas. Such a correlation is the natural consequence
of a contribution to the ionization from shock excitation and we argue that
this demonstrates that the outflows from these AGN are directly impacting the
surrounding ISM within the galaxies.Comment: 37 pages, 30 figures. Accepted for publication in MNRA
Starburst-AGN mixing: TYPHOON observations of NGC 1365, NGC 1068, and the effect of spatial resolution on the AGN fraction
We demonstrate a robust method of resolving the star-formation and AGN
contributions to emission lines using two very well known AGN systems: NGC
1365, and NGC 1068, using the high spatial resolution data from the
TYPHOON/PrISM survey. We expand the previous method of calculating the AGN
fraction by using theoretical-based model grids rather than empirical points.
The high spatial resolution of the TYPHOON/PrISM observations show evidence of
both star formation and AGN activity occurring in the nuclei of the two
galaxies. We rebin the data to the lower resolutions, typically found in other
integral field spectroscopy surveys such as SAMI, MaNGA, and CALIFA. The
results show that when rebinned from the native resolution of TYPHOON (< 200
pc/pixel) to 1 kpc/pixel, the effects include a roughly 3 kpc increase in the
radius of measured AGN activity, and a factor of 2 to 7 increase in the
detection of low surface brightness features such as shocks. All of this
information is critical, because information on certain physical processes may
be lost at varying resolutions. We make recommendations for analysing data at
current IFU survey resolutions.Comment: 30 pages, 28 figures, accepted for publication by MNRA
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