681 research outputs found
The SAMI Galaxy Survey: Shocks and Outflows in a normal star-forming galaxy
We demonstrate the feasibility and potential of using large integral field
spectroscopic surveys to investigate the prevalence of galactic-scale outflows
in the local Universe. Using integral field data from SAMI and the Wide Field
Spectrograph, we study the nature of an isolated disk galaxy, SDSS
J090005.05+000446.7 (z = 0.05386). In the integral field datasets, the galaxy
presents skewed line profiles changing with position in the galaxy. The skewed
line profiles are caused by different kinematic components overlapping in the
line-of-sight direction. We perform spectral decomposition to separate the line
profiles in each spatial pixel as combinations of (1) a narrow kinematic
component consistent with HII regions, (2) a broad kinematic component
consistent with shock excitation, and (3) an intermediate component consistent
with shock excitation and photoionisation mixing. The three kinematic
components have distinctly different velocity fields, velocity dispersions,
line ratios, and electron densities. We model the line ratios, velocity
dispersions, and electron densities with our MAPPINGS IV shock and
photoionisation models, and we reach remarkable agreement between the data and
the models. The models demonstrate that the different emission line properties
are caused by major galactic outflows that introduce shock excitation in
addition to photoionisation by star-forming activities. Interstellar shocks
embedded in the outflows shock-excite and compress the gas, causing the
elevated line ratios, velocity dispersions, and electron densities observed in
the broad kinematic component. We argue from energy considerations that, with
the lack of a powerful active galactic nucleus, the outflows are likely to be
driven by starburst activities. Our results set a benchmark of the type of
analysis that can be achieved by the SAMI Galaxy Survey on large numbers of
galaxies.Comment: 17 pages, 15 figures. Accepted to MNRAS. References update
Source finding, parametrization and classification for the extragalactic Effelsberg-Bonn HI Survey
Context. Source extraction for large-scale HI surveys currently involves
large amounts of manual labor. For data volumes expected from future HI surveys
with upcoming facilities, this approach is not feasible any longer.
Aims. We describe the implementation of a fully automated source finding,
parametrization, and classification pipeline for the Effelsberg-Bonn HI Survey
(EBHIS). With future radio astronomical facilities in mind, we want to explore
the feasibility of a completely automated approach to source extraction for
large-scale HI surveys.
Methods. Source finding is implemented using wavelet denoising methods, which
previous studies show to be a powerful tool, especially in the presence of data
defects. For parametrization, we automate baseline fitting, mask optimization,
and other tasks based on well-established algorithms, currently used
interactively. For the classification of candidates, we implement an artificial
neural network which is trained on a candidate set comprised of false positives
from real data and simulated sources. Using simulated data, we perform a
thorough analysis of the algorithms implemented.
Results. We compare the results from our simulations to the parametrization
accuracy of the HI Parkes All-Sky Survey (HIPASS) survey. Even though HIPASS is
more sensitive than EBHIS in its current state, the parametrization accuracy
and classification reliability match or surpass the manual approach used for
HIPASS data.Comment: 13 Pages, 13 Figures, 1 Table, accepted for publication in A&
Dissecting Galaxies: Separating Star Formation, Shock Excitation and AGN Activity in the Central Region of NGC 613
The most rapidly evolving regions of galaxies often display complex optical
spectra with emission lines excited by massive stars, shocks and accretion onto
supermassive black holes. Standard calibrations (such as for the star formation
rate) cannot be applied to such mixed spectra. In this paper we isolate the
contributions of star formation, shock excitation and active galactic nucleus
(AGN) activity to the emission line luminosities of individual spatially
resolved regions across the central 3 3 kpc region of the active
barred spiral galaxy NGC613. The star formation rate and AGN luminosity
calculated from the decomposed emission line maps are in close agreement with
independent estimates from data at other wavelengths. The star formation
component traces the B-band stellar continuum emission, and the AGN component
forms an ionization cone which is aligned with the nuclear radio jet. The
optical line emission associated with shock excitation is cospatial with strong
and [Fe II] emission and with regions of high ionized gas velocity
dispersion ( km s). The shock component also traces the
outer boundary of the AGN ionization cone and may therefore be produced by
outflowing material interacting with the surrounding interstellar medium. Our
decomposition method makes it possible to determine the properties of star
formation, shock excitation and AGN activity from optical spectra, without
contamination from other ionization mechanisms.Comment: 16 pages, 12 figures. Accepted for publication in MNRA
Automated Source Extraction for the Next Generation of Neutral Hydrogen Surveys
This thesis is a first step to develop the necessary tools to automatically extract and parameterize sources from future HI surveys with ASKAP, WSRT/Apertif, and SKA. The current approach to large-scale HI surveys, that is, automated source finding followed by manual classification and parametrization, is no longer feasible in light of the data volumes expected for future surveys. We use data from EBHIS to develop and test a completely automated source extraction pipeline for extragalactic HI surveys. We apply a 2D-1D wavelet de-noising technique to HI data and show that it is well adapted to the typical shapes of sources encountered in HI surveys. This technique allows to reliably extract sources even from data containing defects commonly encountered in single-dish HI surveys. Automating the task of false-positive rejection requires reliable parameters for all source candidates generated by the source-finding step. For this purpose, we develop a reliable, automated parametrization pipeline that combines time-tested algorithms with new approaches to baseline estimation, spectral filtering, and mask optimization. The accuracy of the algorithms is tested by performing extensive simulations. By comparison with the uncertainty estimates from HIPASS we show that our automated pipeline gives equal or better accuracy than manual parametrization. We implement the task of source classification using artificial neural networks using the automatically determined parameters of the source candidates as inputs. The viability of this approach is verified on a training data set comprised of parameters measured from simulated sources and false positives extracted from real EBHIS data. Since the number of true positives from real data is small compared to the number of false positives, we explore various methods of training artificial neural networks from imbalanced data sets. We show that the artificial neural networks trained in this way do not achieve sufficient completeness and reliability when applied to the source candidates detected from the extragalactic EBHIS survey. We use the trained artificial neural networks in a semi-supervised manner to compile the first extragalactic EBHIS source catalog. The use of artificial neural networks reduces the number of source candidates that require manual inspection by more than an order of magnitude. We compare the results from EBHIS to HIPASS and show that the number of sources in the compiled catalog is approximately half of the sources expected. The main reason for this detection inefficiency is identified to be mis-classification by the artificial neural networks. This is traced back to the limited training data set, which does not cover the parameter space of real detections sufficiently, and the similarity of true and false positives in the parameter space spanned by the measured parameters. We conclude that, while our automated source finding and parametrization algorithms perform satisfactorily, the classification of sources is the most challenging task for future HI surveys. Classification based on the measured source parameters does not provide sufficient discriminatory power and we propose to explore methods based on machine vision which learns features of real sources from the data directly
Galaxy-Wide Shocks in Late-Merger Stage Luminous Infrared Galaxies
We present an integral field spectroscopic study of two nearby Luminous
Infrared Galaxies (LIRGs) that exhibit evidence of widespread shock excitation
induced by ongoing merger activity, IC 1623 and NGC 3256. We show the
importance of carefully separating excitation due to shocks vs. excitation by
HII regions and the usefulness of IFU data in interpreting the complex
processes in LIRGs. Our analysis focuses primarily on the emission line gas
which is extensive in both systems and is a result of the abundant ongoing star
formation as well as widespread LINER-like excitation from shocks. We use
emission-line ratio maps, line kinematics, line-ratio diagnostics and new
models as methods for distinguishing and analyzing shocked gas in these
systems. We discuss how our results inform the merger sequence associated with
local U/LIRGs and the impact that widespread shock excitation has on the
interpretation of emission-line spectra and derived quantities of both local
and high-redshift galaxies.Comment: 14 pages, 11 figures, Accepted to Ap
Radio Frequency Interference Mitigation
Radio astronomy observational facilities are under constant upgradation and
development to achieve better capabilities including increasing the time and
frequency resolutions of the recorded data, and increasing the receiving and
recording bandwidth. As only a limited spectrum resource has been allocated to
radio astronomy by the International Telecommunication Union, this results in
the radio observational instrumentation being inevitably exposed to undesirable
radio frequency interference (RFI) signals which originate mainly from
terrestrial human activity and are becoming stronger with time. RFIs degrade
the quality of astronomical data and even lead to data loss. The impact of RFIs
on scientific outcome is becoming progressively difficult to manage. In this
article, we motivate the requirement for RFI mitigation, and review the RFI
characteristics, mitigation techniques and strategies. Mitigation strategies
adopted at some representative observatories, telescopes and arrays are also
introduced. We also discuss and present advantages and shortcomings of the four
classes of RFI mitigation strategies, applicable at the connected causal
stages: preventive, pre-detection, pre-correlation and post-correlation. The
proper identification and flagging of RFI is key to the reduction of data loss
and improvement in data quality, and is also the ultimate goal of developing
RFI mitigation techniques. This can be achieved through a strategy involving a
combination of the discussed techniques in stages. Recent advances in high
speed digital signal processing and high performance computing allow for
performing RFI excision of large data volumes generated from large telescopes
or arrays in both real time and offline modes, aiding the proposed strategy.Comment: 26 pages, 10 figures, Chinese version accepted for publication in
Acta Astronomica Sinica; English version to appear in Chinese Astronomy and
Astrophysic
Obscuration in AGNs: near-infrared luminosity relations and dust colors
We combine two approaches to isolate the AGN luminosity at near-infrared
wavelengths and relate the near-IR pure AGN luminosity to other tracers of the
AGN. Using integral-field spectroscopic data of an archival sample of 51 local
AGNs, we estimate the fraction of non-stellar light by comparing the nuclear
equivalent width of the stellar 2.3 micron CO absorption feature with the
intrinsic value for each galaxy. We compare this fraction to that derived from
a spectral decomposition of the integrated light in the central arc second and
find them to be consistent with each other. Using our estimates of the near-IR
AGN light, we find a strong correlation with presumably isotropic AGN tracers.
We show that a significant offset exists between type 1 and type 2 sources in
the sense that type 1 sources are 7 (10) times brighter in the near-IR at log
L_MIR = 42.5 (log L_X = 42.5). These offsets only becomes clear when treating
infrared type 1 sources as type 1 AGNs.
All AGNs have very red near-to-mid-IR dust colors. This, as well as the range
of observed near-IR temperatures, can be explained with a simple model with
only two free parameters: the obscuration to the hot dust and the ratio between
the warm and hot dust areas. We find obscurations of A_V (hot) = 5 - 15 mag for
infrared type 1 sources and A_V (hot) = 15 - 35 mag for type 2 sources. The
ratio of hot dust to warm dust areas of about 1000 is nicely consistent with
the ratio of radii of the respective regions as found by infrared
interferometry.Comment: 17 pages, 10 Figures, 3 Tables, accepted by A&
The Shape of LITTLE THINGS Dwarf Galaxies DDO 46 and DDO 168: Understanding the stellar and gas kinematics
We present the stellar and gas kinematics of DDO 46 and DDO 168 from the
LITTLE THINGS survey and determine their respective Vmax/sigma_z,0 values. We
used the KPNO's 4-meter telescope with the Echelle spectrograph as a long-slit
spectrograph. We acquired spectra of DDO 168 along four position angles by
placing the slit over the morphological major and minor axes and two
intermediate position angles. However, due to poor weather conditions during
our observing run for DDO 46, we were able to extract only one useful data
point from the morphological major axis. We determined a central stellar
velocity dispersion perpendicular to the disk, sigma_z,0, of 13.5+/-8 km/s for
DDO 46 and of 10.7+/-2.9 km/s for DDO 168. We then derived the
maximum rotation speed in both galaxies using the LITTLE THINGS HI data. We
separated bulk motions from non-circular motions using a double Gaussian
decomposition technique and applied a tilted-ring model to the bulk velocity
field. We corrected the observed HI rotation speeds for asymmetric drift and
found a maximum velocity, Vmax, of 77.4 +/- 3.7 and 67.4 +/- 4.0 km/s for DDO
46 and DDO 168, respectively. Thus, we derived a kinematic measure,
Vmax/sigma_z,0, of 5.7 +/- 0.6 for DDO 46 and 6.3 +/- 0.3 for DDO 168.
Comparing these values to ones determined for spiral galaxies, we find that DDO
46 and DDO 168 have Vmax/sigma_z,0 values indicative of thin disks, which is in
contrast to minor-to-major axis ratio studies
Deep learning denoising by dimension reduction: Application to the ORION-B line cubes
Context. The availability of large bandwidth receivers for millimeter radio
telescopes allows the acquisition of position-position-frequency data cubes
over a wide field of view and a broad frequency coverage. These cubes contain
much information on the physical, chemical, and kinematical properties of the
emitting gas. However, their large size coupled with inhomogenous
signal-to-noise ratio (SNR) are major challenges for consistent analysis and
interpretation.Aims. We search for a denoising method of the low SNR regions of
the studied data cubes that would allow to recover the low SNR emission without
distorting the signals with high SNR.Methods. We perform an in-depth data
analysis of the 13 CO and C 17 O (1 -- 0) data cubes obtained as part of the
ORION-B large program performed at the IRAM 30m telescope. We analyse the
statistical properties of the noise and the evolution of the correlation of the
signal in a given frequency channel with that of the adjacent channels. This
allows us to propose significant improvements of typical autoassociative neural
networks, often used to denoise hyperspectral Earth remote sensing data.
Applying this method to the 13 CO (1 -- 0) cube, we compare the denoised data
with those derived with the multiple Gaussian fitting algorithm ROHSA,
considered as the state of the art procedure for data line cubes.Results. The
nature of astronomical spectral data cubes is distinct from that of the
hyperspectral data usually studied in the Earth remote sensing literature
because the observed intensities become statistically independent beyond a
short channel separation. This lack of redundancy in data has led us to adapt
the method, notably by taking into account the sparsity of the signal along the
spectral axis. The application of the proposed algorithm leads to an increase
of the SNR in voxels with weak signal, while preserving the spectral shape of
the data in high SNR voxels.Conclusions. The proposed algorithm that combines a
detailed analysis of the noise statistics with an innovative autoencoder
architecture is a promising path to denoise radio-astronomy line data cubes. In
the future, exploring whether a better use of the spatial correlations of the
noise may further improve the denoising performances seems a promising avenue.
In addition
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