35 research outputs found
Inefficient jet-induced star formation in Centaurus A:High resolution ALMA observations of the northern filaments
NGC 5128 is one of the best targets to study AGN-feedback in the local Universe. At 13.5 kpc from the galaxy, optical filaments with recent star formation lie along the radio-jet direction. It is a testbed region for positive feedback (jet-induced star formation). APEX revealed strong CO emission in star-forming regions but also in regions with no detected tracers of star formation. When observed, star formation appears to be inefficient compared to the Kennicutt-Schmidt relation. We used ALMA to map the 12CO(1-0) emission all along the filaments at 1.3"~ 23.8 pc resolution. The CO emission is clumpy and distributed in two main structures: (i) the Horseshoe complex, outside the HI cloud, where gas is mostly excited by shocks and no star formation is observed; (ii) the Vertical filament, at the edge of the HI shell, which is a region of moderate star formation. We identified 140 molecular clouds. A statistical study reveals that they have very similar physical properties that in the inner Milky Way. However, the range of radius available with the present observations does not enable to investigate whether the clouds follow the Larson relation or not. The large virial parameter of the clouds suggests that gravity is not dominant. Finally, the total energy injection in the filaments is of the same order as in the inner part of the Milky Way. The strong CO emission detected in the filaments is an indication that the energy injected by the jet acts positively in the formation of dense molecular gas. The relatively high virial parameter of the molecular clouds suggests that the injected kinetic energy is too strong for star formation to be efficient. This is particularly the case in the Horseshoe complex where the virial parameter is the largest and where strong CO is detected with no associated star formation. This is the first evidence of inefficient AGN positive feedback
Mapping a lower limit on the mass fraction of the cold neutral medium using Fourier transformed HI 21cm emission line spectra: Application to the DRAO Deep Field from DHIGLS and the HI4PI survey
We develop a new method for spatially mapping a lower limit on the mass
fraction of the cold neutral medium by analyzing the amplitude structure of
, the Fourier transform of , the spectrum of the
brightness temperature of HI 21cm line emission with respect to the radial
velocity . This advances a broader effort exploiting 21cm emission line data
alone (without absorption line data, ) to extract integrated properties
of the multiphase structure of the HI gas and to map each phase separately.
Using toy models, we illustrate the origin of interference patterns seen in
. Building on this, a lower limit on the cold gas mass fraction
is obtained from the amplitude of at high . Tested on a
numerical simulation of thermally bi-stable turbulence, the lower limit from
this method has a strong linear correlation with the "true" cold gas mass
fraction from the simulation for relatively low cold gas mass fraction. At
higher mass fraction, our lower limit is lower than the "true" value, because
of a combination of interference and opacity effects. Comparison with
absorption surveys shows a similar behavior, with a departure from linear
correlation at cm. Application to
the DRAO Deep Field (DF) from DHIGLS reveals a complex network of cold
filaments in the Spider, an important structural property of the thermal
condensation of the HI gas. Application to the HI4PI survey in the velocity
range km/s produces a full sky map of a lower limit on the mass
fraction of the cold neutral medium at 16'.2 resolution. Our new method has the
ability to extract a lower limit on the cold gas mass fraction for massive
amounts of emission line data alone with low computing time and memory,
pointing the way to new approaches suitable for the new generation of radio
interferometers.Comment: 29 pages, 35 figures, Accepted for publication in Ap
Inefficient jet-induced star formation in Centaurus A:High resolution ALMA observations of the northern filaments
International audienceNGC 5128 (Centaurus A) is one of the best targets to study AGN feedback in the local Universe. At 13.5 kpc from the galaxy, optical filaments with recent star formation lie along the radio jet direction. This region is a testbed for positive feedback, here through jet-induced star formation. Atacama Pathfinder EXperiment (APEX) observations have revealed strong CO emission in star-forming regions and in regions with no detected tracers of star formation activity. In cases where star formation is observed, this activity appears to be inefficient compared to the Kennicutt-Schmidt relation. We used the Atacama Large Millimeter/submillimeter Array (ALMA) to map the 12CO(1â0) emission all along the filaments of NGC 5128 at a resolution of 1.3âČâČ ~ 23.8pc. We find that the CO emission is clumpy and is distributed in two main structures: (i) the Horseshoe complex, located outside the HI cloud, where gas is mostly excited by shocks and where no star formation is observed, and (ii) the Vertical filament, located at the edge of the HI shell, which is a region of moderate star formation. We identified 140 molecular clouds using a clustering method applied to the CO data cube. A statistical study reveals that these clouds have very similar physical properties, such as size, velocity dispersion, and mass, as in the inner Milky Way. However, the range of radius available with the present ALMA observations does not enable us to investigate whether or not the clouds follow the Larson relation. The large virial parameter αvir of the clouds suggests that gravity is not dominant and clouds are not gravitationally unstable. Finally, the total energy injection in the northern filaments of Centaurus A is of the same order as in the inner part of the Milky Way. The strong CO emission detected in the northern filaments is an indication that the energy injected by the jet acts positively in the formation of dense molecular gas. The relatively high virial parameter of the molecular clouds suggests that the injected kinetic energy is too strong for star formation to be efficient. This is particularly the case in the horseshoe complex, where the virial parameter is the largest and where strong CO is detected with no associated star formation. This is the first evidence of AGN positive feedback in the sense of forming molecular gas through shocks, associated with low star formation efficiency due to turbulence injection by the interaction with the radio jet
Separation of dust emission from the Cosmic Infrared Background in Herschel observations with Wavelet Phase Harmonics
The low brightness dust emission at high Galactic latitude is of interest to
study the interplay between physical processes in shaping the structure of the
interstellar medium (ISM), as well as to statistically characterize dust
emission as a foreground to the Cosmic Microwave Background (CMB). Progress in
this avenue of research have been hampered by the difficulty of separating the
dust emission from the Cosmic Infrared Background (CIB). We demonstrate that
dust and CIB may be effectively separated based on their different structure on
the sky and use the separation to characterize the structure of diffuse dust
emission on angular scales where CIB is a significant component in terms of
power. We use scattering transform statistics, the Wavelet Phase Harmonics
(WPH), to perform a statistical component separation using Herschel SPIRE
observations. This component separation is done only from observational data
using non-Gaussian properties as a lever arm, and is done at a single 250
microns frequency. This method, that we validate on mock data, gives us access
to non-Gaussian statistics of the interstellar dust and an output dust map
essentially free from CIB contamination. Our statistical modelling
characterizes the non-Gaussian structure of the diffuse ISM down to the
smallest scales observed by Herschel. We recover the power-law shape of the
dust power spectrum up to a wavenumber of 2 arcmin where the dust signal
represents 2 percent of the total power. The output dust map reveals coherent
structures at the smallest scales which were hidden by the CIB anisotropies. It
opens new observational perspectives on the formation of structure in the
diffuse ISM which we discuss with reference to past work. We have succeeded to
perform a statistical separation from observational data only at a single
frequency by using non-Gaussian statistics.Comment: Accepted in A&A on October 23, 202
BFORE: The B-mode Foreground Experiment
The B-mode Foreground Experiment (BFORE) is a proposed NASA balloon project
designed to make optimal use of the sub-orbital platform by concentrating on
three dust foreground bands (270, 350, and 600 GHz) that complement
ground-based cosmic microwave background (CMB) programs. BFORE will survey ~1/4
of the sky with 1.7 - 3.7 arcminute resolution, enabling precise
characterization of the Galactic dust that now limits constraints on inflation
from CMB B-mode polarization measurements. In addition, BFORE's combination of
frequency coverage, large survey area, and angular resolution enables science
far beyond the critical goal of measuring foregrounds. BFORE will constrain the
velocities of thousands of galaxy clusters, provide a new window on the cosmic
infrared background, and probe magnetic fields in the interstellar medium. We
review the BFORE science case, timeline, and instrument design, which is based
on a compact off-axis telescope coupled to >10,000 superconducting detectors.Comment: 7 pages, 4 figures, conference proceedings published in Journal of
Low Temperature Physic
BLAST05: Power Spectra of Bright Galactic Cirrus at Submillimeter Wavelengths
We report multi-wavelength power spectra of diffuse Galactic dust emission
from BLAST observations at 250, 350, and 500 microns in Galactic Plane fields
in Cygnus X and Aquila. These submillimeter power spectra statistically
quantify the self-similar structure observable over a broad range of scales and
can be used to assess the cirrus noise which limits the detection of faint
point sources. The advent of submillimeter surveys with the Herschel Space
Observatory makes the wavelength dependence a matter of interest. We show that
the observed relative amplitudes of the power spectra can be related through a
spectral energy distribution (SED). Fitting a simple modified black body to
this SED, we find the dust temperature in Cygnus X to be 19.9 +/- 1.3 K and in
the Aquila region 16.9 +/- 0.7 K. Our empirical estimates provide important new
insight into the substantial cirrus noise that will be encountered in
forthcoming observations.Comment: Submitted to the Astrophysical Journal. Maps and other data are
available at http://blastexperiment.info
The Galactic dynamics revealed by the filamentary structure in atomic hydrogen emission
Funding: J.D.S., R.K.S., and S.C.O.G. are funded by the European Research Council via the ERC Synergy Grant âECOGAL â Understanding our Galactic ecosystem: From the disk of the Milky Way to the formation sites of stars and planetsâ (project ID 855130). R.J.S. acknowledges funding from an STFC ERF (grant ST/N00485X/1) and HPC from the DiRAC facility (ST/P002293/1).We present a study of the filamentary structure in the atomic hydrogen (HI) emission at the 21 cm wavelength toward the Galactic plane using the observations in the HI4PI survey. Using the Hessian matrix method across radial velocity channels, we identified the filamentary structures and quantified their orientations using circular statistics. We found that the regions of the Milky Way's disk beyond 10 kpc and up to roughly 18 kpc from the Galactic center display HI filamentary structures predominantly parallel to the Galactic plane. For regions at lower Galactocentric radii, we found that the HI filaments are mostly perpendicular or do not have a preferred orientation with respect to the Galactic plane. We interpret these results as the imprint of supernova feedback in the inner Galaxy and Galactic rotation in the outer Milky Way. We found that the HI filamentary structures follow the Galactic warp and that they highlight some of the variations interpreted as the effect of the gravitational interaction with satellite galaxies. In addition, the mean scale height of the filamentary structures is lower than that sampled by the bulk of the HI emission, thus indicating that the cold and warm atomic hydrogen phases have different scale heights in the outer galaxy. Finally, we found that the fraction of the column density in HI filaments is almost constant up to approximately 18 kpc from the Galactic center. This is possibly a result of the roughly constant ratio between the cold and warm atomic hydrogen phases inferred from the HI absorption studies. Our results indicate that the HI filamentary structures provide insight into the dynamical processes shaping the Galactic disk. Their orientations record how and where the stellar energy input, the Galactic fountain process, the cosmic ray diffusion, and the gas accretion have molded the diffuse interstellar medium in the Galactic plane.Peer reviewe
Constraints on galaxy formation from the cosmic-far-infrared-background â optical-imaging cross-correlation using Herschel and UNIONS
Using -SPIRE imaging and the Canada-France Imaging Survey (CFIS) Low Surface Brightness data products from the Ultraviolet Near-Infrared Optical Northern Survey (UNIONS), we present a cross-correlation between the cosmic infrared background and cosmic optical background fluctuations. With a combined sky area of and a minimum resolved scale of arcsec, the cross-spectrum is measured for two cases: all galaxies are kept in the images; or all individually-detected galaxies are masked to produce `background' maps. We report the detection of the cross-correlation signal at ( for the background map). The part of the optical brightness variations that are correlated with the submm emission translates to an rms brightness of in the band, a level normally unreachable for individual sources. A critical issue is determining what fraction of the cross-power spectrum might be caused by emission from Galactic cirrus. For one of the fields, the Galactic contamination is approximately a factor of 10 higher than the extragalactic signal, with the contamination being estimated using a linear regression from several external survey maps; however, for the other fields, the contamination is typically around 20 per cent. An additional discriminant is that the cross-power spectrum is of the approximate form , much shallower than that of Galactic cirrus. We interpret the results in a halo-model framework, which shows good agreement with independent measurements for the scalings of star-formation rates in galaxies. The approach presented in this study holds great promise for future surveys such as FYST/CCAT-prime combined with or the Vera Rubin Observatory (LSST), which will enable a detailed exploration of the evolution of star formation in galaxies
Evidence for Environmental Changes in the Submillimeter Dust Opacity
The submillimeter opacity of dust in the diffuse Galactic interstellar medium
(ISM) has been quantified using a pixel-by-pixel correlation of images of
continuum emission with a proxy for column density. We used three BLAST bands
at 250, 350, and 500 \mu m and one IRAS at 100 \mu m. The proxy is the
near-infrared color excess, E(J-Ks), obtained from 2MASS. Based on observations
of stars, we show how well this color excess is correlated with the total
hydrogen column density for regions of moderate extinction. The ratio of
emission to column density, the emissivity, is then known from the
correlations, as a function of frequency. The spectral distribution of this
emissivity can be fit by a modified blackbody, whence the characteristic dust
temperature T and the desired opacity \sigma_e(1200) at 1200 GHz can be
obtained. We have analyzed 14 regions near the Galactic plane toward the Vela
molecular cloud, mostly selected to avoid regions of high column density (N_H >
10^{22} cm^-2) and small enough to ensure a uniform T. We find \sigma_e(1200)
is typically 2 to 4 x 10^{-25} cm^2/H and thus about 2 to 4 times larger than
the average value in the local high Galactic latitude diffuse atomic ISM. This
is strong evidence for grain evolution. There is a range in total power per H
nucleon absorbed (re-radiated) by the dust, reflecting changes in the
interstellar radiation field and/or the dust absorption opacity. These changes
affect the equilibrium T, which is typically 15 K, colder than at high
latitudes. Our analysis extends, to higher opacity and lower T, the trend of
increasing opacity with decreasing T that was found at high latitudes. The
recognition of changes in the emission opacity raises a cautionary flag because
all column densities deduced from dust emission maps, and the masses of compact
structures within them, depend inversely on the value adopted.Comment: Original version (22 Dec 2011): 14 pages, 8 figures. Revised version
(24 February 2012) accepted for publication in the Astrophysical Journal (14
March 2012): elaborated details of analysis, extended discussion including
new Appendix; abstract, results, conclusions unchanged. 16 pages, 9 figure