45 research outputs found
GHIGLS: HI mapping at intermediate Galactic latitude using the Green Bank Telescope
This paper introduces the data cubes from GHIGLS, deep Green Bank Telescope
surveys of the 21-cm line emission of HI in 37 targeted fields at intermediate
Galactic latitude. The GHIGLS fields together cover over 1000 square degrees at
9.55' spatial resolution. The HI spectra have an effective velocity resolution
about 1.0 km/s and cover at least -450 < v < +250 km/s. GHIGLS highlights that
even at intermediate Galactic latitude the interstellar medium is very complex.
Spatial structure of the HI is quantified through power spectra of maps of the
column density, NHI. For our featured representative field, centered on the
North Ecliptic Pole, the scaling exponents in power-law representations of the
power spectra of NHI maps for low, intermediate, and high velocity gas
components (LVC, IVC, and HVC) are -2.86 +/- 0.04, -2.69 +/- 0.04, and -2.59
+/- 0.07, respectively. After Gaussian decomposition of the line profiles, NHI
maps were also made corresponding to the narrow-line and broad-line components
in the LVC range; for the narrow-line map the exponent is -1.9 +/- 0.1,
reflecting more small scale structure in the cold neutral medium (CNM). There
is evidence that filamentary structure in the HI CNM is oriented parallel to
the Galactic magnetic field. The power spectrum analysis also offers insight
into the various contributions to uncertainty in the data. The effect of 21-cm
line opacity on the GHIGLS NHI maps is estimated.Comment: Accepted for publication in The Astrophysical Journal, 2015 July 16.
32 pages, 21 figures (Fig. 10 new). Minor revisions from review, particularly
Section 8 and Appendix C; results unchanged. Additional surveys added and
made available; new Appendix B. Added descriptions of available FITS files
and links to four illustrative movies on enhanced GHIGLS archive
(www.cita.utoronto.ca/GHIGLS/
Structure formation in a colliding flow: The Herschel view of the Draco nebula
The Draco nebula is a high Galactic latitude interstellar cloud likely to
have been formed by the collision of a Galactic halo cloud entering the disk of
the Milky Way. Such conditions are ideal to study the formation of cold and
dense gas in colliding flows of warm gas. We present Herschel-SPIRE
observations that reveal the fragmented structure of the interface between the
infalling cloud and the Galactic layer. This front is characterized by a
Rayleigh-Taylor instability structure. From the determination of the typical
length of the periodic structure (2.2 pc) we estimated the gas kinematic
viscosity and the turbulence dissipation scale (0.1 pc) that is compatible with
that expected if ambipolar diffusion is the main mechanism of energy
dissipation in the WNM. The small-scale structures of the nebula are typical of
that seen in some molecular clouds. The gas density has a log-normal
distribution with an average value of cm. The size of the
structures is 0.1-0.2 pc but this estimate is limited by the resolution of the
observations. The mass ranges from 0.2 to 20 M and the distribution
of the more massive clumps follows a power law . We
identify a mass-size relation with the same exponent as that found in GMCs
() but only 15% of the mass of the cloud is in gravitationally
bound structures. We conclude that the increase of pressure in the collision is
strong enough to trigger the WNM-CNM transition caused by the interplay between
turbulence and thermal instability as self-gravity is not dominating the
dynamics.Comment: 16 pages, A&A, in pres
On the O II ground configuration energy levels
The most accurate way to measure the energy levels for the O II 2p^3 ground
configuration has been from the forbidden lines in planetary nebulae. We
present an analysis of modern planetary nebula data that nicely constrain the
splitting within the ^2D term and the separation of this term from the ground
^4S_{3/2} level. We extend this method to H II regions using high-resolution
spectroscopy of the Orion nebula, covering all six visible transitions within
the ground configuration. These data confirm the splitting of the ^2D term
while additionally constraining the splitting of the ^2P term. The energies of
the ^2P and ^2D terms relative to the ground (^4S) term are constrained by
requiring that all six lines give the same radial velocity, consistent with
independent limits placed on the motion of the O+ gas and the planetary nebula
data.Comment: 20 pages, 3 figures. To be published in Ap
Temperature Variations from \u3cem\u3eHubble Space Telescope\u3c/em\u3e Spectroscopy of the Orion Nebula
We present Hubble Space Telescope (HST)/STIS long-slit spectroscopy of NGC 1976. Our goal is to measure the intrinsic line ratio [O III] 4364/5008 and thereby evaluate the electron temperature (Te) and the fractional mean-square Te variation (t2A)across the nebula. We also measure the intrinsic line ratio [N II] 5756/6585 in order to estimate Te and t2A in the N+region. The interpretation of the [N II] data is not as clear cut as the [O III] data because of a higher sensitivity to knowledge of the electron density as well as a possible contribution to the [N II] 5756 emission by recombination (and cascading). We present results from binning the data along the various slits into tiles that are 0.5 arcsec square (matching the slit width). The average [O III] temperature for our four HST/STIS slits varies from 7678 K to 8358 K; t2A varies from 0.00682 to at most 0.0176. For our preferred solution, the average [N II] temperature for each of the four slits varies from 9133 to 10 232 K; t2A varies from 0.00584 to 0.0175. The measurements of Te reported here are an average along each line of sight. Therefore, despite finding remarkably low t2A, we cannot rule out significantly larger temperature fluctuations along the line of sight. The result that the average [N II]Te exceeds the average [O III]Te confirms what has been previously found for Orion and what is expected on theoretical grounds. Observations of the proplyd P159-350 indicate: large local extinction associated; ionization stratification consistent with external ionization by Ξ1 Ori C; and indirectly, evidence of high electron density
Deviations from He I Case B Recombination Theory and Extinction Corrections in the Orion Nebula
We are engaged in a comprehensive program to find reliable elemental
abundances in and to probe the physical structure of the Orion Nebula, the
brightest and best-resolved H II region. In the course of developing a robust
extinction correction covering our optical and ultraviolet FOS and STIS
observations, we examined the decrement within various series of He I lines.
The decrements of the 2^3S-n^3P, 2^3P-n^3S and 3^3S-n^3P series are not in
accord with caseB recombination theory. None of these anomalous He I decrements
can be explained by extinction, indicating the presence of additional radiative
transfer effects in He I lines ranging from the near-IR to the near-UV. CLOUDY
photoionization equilibrium models including radiative transfer are developed
to predict the observed He I decrements and the quantitative agreement is quite
remarkable. Following from these results, select He I lines are combined with H
I and [O II] lines and stellar extinction data to validate a new normalizable
analytic expression for the wavelength dependence of the extinction. In so
doing, the He+/H+ abundance is also derived.Comment: 42 pages, 10 figures. To be published in Ap
A photoionized Herbig-Haro object in the Orion nebula
The spectra of Herbig Haro objects are usually characteristic of ionization
and excitation in shock-heated gas, whether an internal shock in an unsteady
outflow or a bow shock interface with the interstellar medium. We examine the
eastern-most shock -- the leading optically visible shock -- of a Herbig Haro
outflow (HH 529) seen projected on the face of the Orion Nebula, using deep
optical echelle spectroscopy, showing that the spectrum of this gas is
consistent with photoionization by Ori C. By modeling the emission
lines, we determine a gas-phase abundance of Fe which is consistent with the
depleted (relative to solar) abundance found in the Orion nebula -- evidence
for the presence of dust in the nebula and therefore in the Herbig Haro
outflow. The spectrum also allows for the calculation of temperature
fluctuations, , in the nebula and the shock. These fluctuations have been
used to explain discrepancies between abundances obtained from recombination
lines versus those obtained from collisionally-excited lines, although to date
there has not been a robust theory for how such large fluctuations () can exist.Comment: 50 pages, 8 figures, To be published in Ap
Temperature Variations from HST Spectroscopy of the Orion Nebula
We present HST/STIS long-slit spectroscopy of NGC 1976. Our goal is to
measure the intrinsic line ratio [O III] 4364/5008 and thereby evaluate the
electron temperature (T_e) and the fractional mean-square T_e variation (t_A^2)
across the nebula. We also measure the intrinsic line ratio [N II] 5756/6585 in
order to estimate T_e and t_A^2 in the N^+ region. The interpretation of the [N
II] data is not as clear cut as the [O III] data because of a higher
sensitivity to knowledge of the electron density as well as a possible
contribution to the [N II] 5756 emission by recombination (and cascading). We
present results from binning the data along the various slits into tiles that
are 0.5" square (matching the slit width). The average [O III] temperature for
our four HST/STIS slits varies from 7678 K to 8358 K; t_A^2 varies from 0.00682
to at most 0.0176. For our preferred solution, the average [N II] temperature
for each of the four slits varies from 9133 K to 10232 K; t_A^2 varies from
0.00584 to 0.0175. The measurements of T_e reported here are an average along
each line of sight. Therefore, despite finding remarkably low t_A^2, we cannot
rule out significantly larger temperature fluctuations along the line of sight.
The result that the average [N II] T_e exceeds the average [O III] T_e confirms
what has been previously found for Orion and what is expected on theoretical
grounds. Observations of the proplyd P159-350 indicate: large local extinction
associated; ionization stratification consistent with external ionization by
theta^1 Ori C; and indirectly, evidence of high electron density.Comment: MNRAS accepted: 30 pages, 3 Figures, 2 Table
Planck Early Results XVIII: The power spectrum of cosmic infrared background anisotropies
Using Planck maps of six regions of low Galactic dust emission with a total area of about 140 deg2, we determine the angular power spectra of
cosmic infrared background (CIB) anisotropies from multipole â = 200 to â = 2000 at 217, 353, 545 and 857 GHz. We use 21-cm observations of
Hi as a tracer of thermal dust emission to reduce the already low level of Galactic dust emission and use the 143 GHz Planck maps in these fields
to clean out cosmic microwave background anisotropies. Both of these cleaning processes are necessary to avoid significant contamination of the
CIB signal. We measure correlated CIB structure across frequencies. As expected, the correlation decreases with increasing frequency separation,
because the contribution of high-redshift galaxies to CIB anisotropies increases with wavelengths. We find no significant difference between the
frequency spectrum of the CIB anisotropies and the CIB mean, with âI/I=15% from 217 to 857 GHz. In terms of clustering properties, the
Planck data alone rule out the linear scale- and redshift-independent bias model. Non-linear corrections are significant. Consequently, we develop
an alternative model that couples a dusty galaxy, parametric evolution model with a simple halo-model approach. It provides an excellent fit to
the measured anisotropy angular power spectra and suggests that a different halo occupation distribution is required at each frequency, which
is consistent with our expectation that each frequency is dominated by contributions from different redshifts. In our best-fit model, half of the
anisotropy power at â=2000 comes from redshifts z 2 at 353
and 217 GHz, respectively