Single Gaussian Fit Research Articles

Emission-line properties of the most luminous AGNs in massive galaxies at intermediate redshifts

ABSTRACT We have analysed the emission-line properties of 6019 Type II active galactic nuclei (AGNs) at redshifts in the range 0.4–0.8 with [O iii] luminosities greater than $3 \times 10^8 \, \mathrm{L}_{\odot }$, characteristic of the Type II quasars first identified in population studies by Zakamska et al. The AGNs are drawn from the CMASS sample of galaxies with stellar masses greater than $10^{11} \, \mathrm{M}_{\odot }$ that were studied as part of the Baryon Oscillation Spectroscopic Survey (BOSS) and comprise 0.5 per cent of the total population of these galaxies. Individual spectra have low S/N, so the analysis is carried out on stacked spectra in bins of [O iii] luminosity and estimated stellar age. The emission line ratios of the stacks are well fit with simple uniform-density photoionization models with metallicities between solar and twice solar. In the stacks, a number of emission lines are found to have distinct broad components requiring a double Gaussian rather than a single Gaussian fit, indicative of outflowing ionized gas. These are: [O iii] λ4959, [O iii] λ5007, [O ii] λ3727,3729, and H αλ6563. Higher ionization lines such as [Ne iii] λ3869 and [Ne v] λ3345 are detected in the stacks, but are well fit by single Gaussians. The broad components typically contain a third of the total line flux and have widths of 600 km s−1 for the oxygen lines and 900 km s−1 for H α. The fraction of the flux in the broad component and its width are independent of [O iii] luminosity, stellar age, radio, and mid-IR luminosity. The stellar mass of the galaxy is the only parameter we could identify that influences the width of the broad line component.

Finding binary star fractions in any distribution

Abstract Candidate visual binary systems are often found by identifying two stars that are closer together than would be expected by chance. However, in regions with non-trivial density distributions, the ‘random’ distances between stars varies because of the background distribution, as well as the presence of binaries. We show that when no binaries are present, the distribution of the ratios of the distances to the nearest and tenth nearest neighbours, d1/d10, is always well approximated by a Gaussian with mean 0.2–0.3 and variance 0.16–0.19 for any underlying density distribution. The introduction of binaries causes some (or all) nearest neighbours to become closer than expected by random chance, introducing a component to the distribution where d1/d10 is much lower than expected. We show how a simple single or double Gaussian fit to the distribution of d1/d10 can be used to find the binary fraction in any underlying density distribution quickly and simply.

Two Solar Tornadoes Observed with the Interface Region Imaging Spectrograph

Abstract The barbs or legs of some prominences show an apparent motion of rotation, which are often termed solar tornadoes. It is under debate whether the apparent motion is a real rotating motion, or caused by oscillations or counter-streaming flows. We present analysis results from spectroscopic observations of two tornadoes by the Interface Region Imaging Spectrograph. Each tornado was observed for more than 2.5 hr. Doppler velocities are derived through a single Gaussian fit to the Mg ii k 2796 Å and Si iv 1393 Å line profiles. We find coherent and stable redshifts and blueshifts adjacent to each other across the tornado axes, which appears to favor the interpretation of these tornadoes as rotating cool plasmas with temperatures of 104 K–105 K. This interpretation is further supported by simultaneous observations of the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory, which reveal periodic motions of dark structures in the tornadoes. Our results demonstrate that spectroscopic observations can provide key information to disentangle different physical processes in solar prominences.

Interaction between an emerging flux region and a pre-existing fan-spine dome observed by IRIS and SDO

Abstract We present multiwavelength observations of a fan-spine dome in the active region NOAA 11996 with the Interface Region Imaging Spectrograph (IRIS) and the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory (SDO) on 2014 March 9. The destruction of the fan-spine topology owing to the interaction between its magnetic fields and a nearby emerging flux region (EFR) is observed for the first time. The line-of-sight magnetograms from the Helioseismic and Magnetic Imager on board the SDO reveal that the dome is located on the mixed magnetic fields, with its rim rooted in the redundant positive polarity surrounding the minority parasitic negative fields. The fan surface of the dome consists of a filament system and recurring jets are observed along its spine. The jet occurring around 13:54 UT is accompanied by a quasi-circular ribbon that brightens in the clockwise direction along the bottom rim of the dome, which may indicate an occurrence of slipping reconnection in the fan-spine topology. The EFR emerges continuously and meets with the magnetic fields of the dome. Magnetic cancellations take place between the emerging negative polarity and the outer positive polarity of the dome's fields, which lead to the rise of the loop connecting the EFR and brightenings related to the dome. A single Gaussian fit to the profiles of the IRIS Si IV 1394 Å line is used in the analysis. It appears that there are two rising components along the slit, in addition to the rise in the line-of-sight direction. The cancellation process repeats again and again. Eventually the fan-spine dome is destroyed and a new connectivity is formed. We suggest that magnetic reconnection between the EFR and the magnetic fields of the fan-spine dome is responsible for the destruction of the dome.

Solar Coronal Plumes and the Fast Solar Wind

The spectral profiles of the coronal Ne viii line at 77 nm have different shapes in quiet-Sun regions and Coronal Holes (CHs). A single Gaussian fit of the line profile provides an adequate approximation in quiet-Sun areas, whereas, a strong shoulder on the long-wavelength side is a systematic feature in CHs. Although this has been noticed since 1999, no physical reason for the peculiar shape could be given. In an attempt to identify the cause of this peculiarity, we address three problems that could not be conclusively resolved, in a review article by a study team of the International Space Science Institute (ISSI) (Wilhelm et al. 2011): (1) The physical processes operating at the base and inside of plumes, as well as their interaction with the Solar Wind (SW). (2) The possible contribution of plume plasma to the fast SW streams. (3) The signature of the First-Ionization Potential (FIP) effect between plumes and inter-plume regions (IPRs). Before the spectroscopic peculiarities in IPRs and plumes in Polar Coronal Holes (PCHs) can be further investigated with the instrument Solar Ultraviolet Measurements of Emitted Radiation (SUMER) aboard the Solar and Heliospheric Observatory (SOHO), it is mandatory to summarize the results of the review to place the spectroscopic observations into context. Finally, a plume model is proposed that satisfactorily explains the plasma flows up and down the plume field lines and leads to the shape of the neon line in PCHs.

HIGH-RESOLUTION OBSERVATIONS OF THE SHOCK WAVE BEHAVIOR FOR SUNSPOT OSCILLATIONS WITH THE INTERFACE REGION IMAGING SPECTROGRAPH

We present the first results of sunspot oscillations from observations by the Interface Region Imaging Spectrograph. The strongly nonlinear oscillation is identified in both the slit-jaw images and the spectra of several emission lines formed in the transition region and chromosphere. We first apply a single Gaussian fit to the profiles of the Mgii 2796.35 {\AA}, Cii 1335.71 {\AA}, and Si iv 1393.76 {\AA} lines in the sunspot. The intensity change is about 30%. The Doppler shift oscillation reveals a sawtooth pattern with an amplitude of about 10 km/s in Si iv. In the umbra the Si iv oscillation lags those of Cii and Mgii by about 3 and 12 s, respectively. The line width suddenly increases as the Doppler shift changes from redshift to blueshift. However, we demonstrate that this increase is caused by the superposition of two emission components. We then perform detailed analysis of the line profiles at a few selected locations on the slit. The temporal evolution of the line core is dominated by the following behavior: a rapid excursion to the blue side, accompanied by an intensity increase, followed by a linear decrease of the velocity to the red side. The maximum intensity slightly lags the maximum blueshift in Si iv, whereas the intensity enhancement slightly precedes the maximum blueshift in Mgii. We find a positive correlation between the maximum velocity and deceleration, a result that is consistent with numerical simulations of upward propagating magnetoacoustic shock waves.

Temperature dependence of ultraviolet line parameters in network and internetwork regions of the quiet Sun and coronal holes

Aims. We study the temperature dependence of the average Doppler shift and the non-thermal line width in network and internetwork regions for both the quiet Sun (QS) and the coronal hole (CH), by using observations of the Solar Ultraviolet Measurements of Emitted Radiation instrument onboard the Solar and Heliospheric Observatory spacecraft. Methods. We obtain the average Doppler shift and non-thermal line width in the network regions of QS, internetwork regions of QS, network regions of CH, and internetwork regions of CH by applying a single-Gaussian fit to the line profiles averaged in each of the four regions. The formation temperatures of the lines we use cover the range from 10 4 to 1.2 × 10 6 K. Two simple scenarios are proposed to explain the temperature dependence of the line parameters in the network regions. In one of the scenarios, the spectral line consists of three components: a rapid, weak upflow generated in the lower atmosphere, a nearly static background, and a slow cooling downflow. In the other scenario, there are just two components, which include a bright core component and a faint wide tail one. Results. An enhancement of the Doppler shift magnitude and the non-thermal line width in network regions compared to the internetwork regions is reported. We also report that most transition region lines are less redshifted (by 0− 8k m s −1 ) and broader (by 0− 5k m s −1 ) in CH compared to the counterparts of QS. In internetwork regions, the difference in the Doppler shifts between the coronal hole and the QS is slightly smaller, especially for the lines with formation temperatures lower than 2 × 10 5 K. And the two simple scenarios can reproduce the variation in the line parameters with the temperature very well. Conclusions. Our results suggest that the physical processes in network and internetwork regions are different and that one needs to separate network and internetwork when discussing dynamics and physical properties of the solar atmosphere. The agreement between the results of the observation and our scenarios suggests that the temperature dependence of Doppler shifts and line widths might be caused by the different relative contributions of the three components at different temperatures. The results may shed new light on our understanding of the complex chromosphere-corona mass cycle. However, the existing observational results do not allow us to distinguish between the two scenarios. At this stage, a high-resolution instrument Interface Region Imaging Spectrograph is highly desirable.

On spectral line profiles in Type Ia supernova spectra

We present a detailed analysis of spectral line profiles in Type Ia supernova (SN Ia) spectra. We focus on the feature at ~3500 - 4000 A, which is commonly thought to be caused by blueshifted absorption of Ca H&K. Unlike some other spectral features in SN Ia spectra, this feature often has two overlapping (blue and red) components. It is accepted that the red component comes from photospheric calcium. However, it has been proposed that the blue component is caused by either high-velocity calcium (from either abundance or density enhancements above the photosphere of the SN) or Si II 3858. By looking at multiple data sets and model spectra, we conclude that the blue component of the Ca H&K feature is caused by Si II 3858 for most SNe Ia. The strength of the Si II 3858 feature varies strongly with the light-curve shape of a SN. As a result, the velocity measured from a single-Gaussian fit to the full line profile correlates with light-curve shape. The velocity of the Ca H&K component of the profile does not correlate with light-curve shape, contrary to previous claims. We detail the pitfalls of assuming that the blue component of the Ca H&K feature is caused by calcium, with implications for our understanding of SN Ia progenitors, explosions, and cosmology.

ON THE DOPPLER VELOCITY OF EMISSION LINE PROFILES FORMED IN THE “CORONAL CONTRAFLOW” THAT IS THE CHROMOSPHERE-CORONA MASS CYCLE

This analysis begins to explore the complex chromosphere-corona mass cycle using a blend of imaging and spectroscopic diagnostics. Single Gaussian fits to hot emission line profiles (formed above 1MK) at the base of coronal loop structures indicate material blue-shifts of 5-10km/s while cool emission line profiles (formed below 1MK) yield red-shifts of a similar magnitude - indicating, to zeroth order, that a temperature-dependent bifurcating flow exists on coronal structures. Image sequences of the same region reveal weakly emitting upward propagating disturbances in both hot and cool emission with apparent speeds of 50-150km/s. Spectroscopic observations indicate that these propagating disturbances produce a weak emission component in the blue wing at commensurate speed, but that they contribute only a few percent to the (ensemble) emission line profile in a single spatio-temporal resolution element. Subsequent analysis of imaging data shows material "draining" slowly (~10km/s) out of the corona, but only in the cooler passbands. We interpret the draining as the return-flow of coronal material at the end of the complex chromosphere-corona mass cycle. Further, we suggest that the efficient radiative cooling of the draining material produces a significant contribution to the red wing of cool emission lines that is ultimately responsible for their systematic red-shift as derived from a single Gaussian fit when compared to those formed in hotter (conductively dominated) domains. The presence of counter-streaming flows complicates the line profiles, their interpretation, and asymmetry diagnoses, but allows a different physical picture of the lower corona to develop.

WHAT CAN WE LEARN ABOUT SOLAR CORONAL MASS EJECTIONS, CORONAL DIMMINGS, AND EXTREME-ULTRAVIOLET JETS THROUGH SPECTROSCOPIC OBSERVATIONS?

We analyze several data sets obtained by Hinode/EIS and find various types of flows during CMEs and EUV jet eruptions. CME-induced dimming regions are found to be characterized by significant blueshift and enhanced line width by using a single Gaussian fit. While a red-blue (RB) asymmetry analysis and a RB-guided double Gaussian fit of the coronal line profiles indicate that these are likely caused by the superposition of a strong background emission component and a relatively weak (~10%) high-speed (~100 km s-1) upflow component. This finding suggests that the outflow velocity in the dimming region is probably of the order of 100 km s-1, not ~20 km s-1 as reported previously. Density and temperature diagnostics suggest that dimming is primarily an effect of density decrease rather than temperature change. The mass losses in dimming regions as estimated from different methods are roughly consistent with each other and they are 20%-60% of the masses of the associated CMEs. With the guide of RB asymmetry analysis, we also find several temperature-dependent outflows (speed increases with temperature) immediately outside the (deepest) dimming region. In an erupted CME loop and an EUV jet, profiles of emission lines formed at coronal and transition region temperatures are found to exhibit two well-separated components, an almost stationary component accounting for the background emission and a highly blueshifted (~200 km s-1) component representing emission from the erupting material. The two components can easily be decomposed through a double Gaussian fit and we can diagnose the electron density, temperature and mass of the ejecta. Combining the speed of the blueshifted component and the projected speed of the erupting material derived from simultaneous imaging observations, we can calculate the real speed of the ejecta.

TWO COMPONENTS OF THE SOLAR CORONAL EMISSION REVEALED BY EXTREME-ULTRAVIOLET SPECTROSCOPIC OBSERVATIONS

Recent spectroscopic observations have revealed the ubiquitous presence of blueward asymmetries of emission lines formed in the solar corona and transition region. These asymmetries are most prominent in loop footpoint regions, where a clear correlation of the asymmetry with the Doppler shift and line width determined from the single Gaussian fit is found. Such asymmetries suggest at least two emission components: a primary component accounting for the background emission and a secondary component associated with high-speed upflows. The latter has been proposed to play a vital role in the coronal heating process and there is no agreement on its properties. Here we slightly modify the initially developed technique of Red-Blue (RB) asymmetry analysis and apply it to both artificial spectra and spectra observed by the EUV Imaging Spectrometer onboard Hinode, and demonstrate that the secondary component usually contributes a few percent of the total emission, has a velocity ranging from 50 to 150 km s-1 and a Gaussian width comparable to that of the primary one in loop footpoint regions. The results of the RB asymmetry analysis are then used to guide a double Gaussian fit and we find that the obtained properties of the secondary component are generally consistent with those obtained from the RB asymmetry analysis. Through a comparison of the location, relative intensity, and velocity distribution of the blueward secondary component with the properties of the upward propagating disturbances revealed in simultaneous images from the Atmospheric Imaging Assembly onboard the Solar Dynamics Observatory, we find a clear association of the secondary component with the propagating disturbances.

ON THE NATURE OF THE SPECTRAL LINE BROADENING IN SOLAR CORONAL DIMMINGS

We analyze the profiles of iron emission lines observed in solar coronal dimmings associated with coronal mass ejections, using the EUV Imaging Spectrometer on board Hinode. We quantify line profile distortions with empirical coefficients (asymmetry and peakedness) that compare the fitted Gaussian to the data. We find that the apparent line broadenings reported in previous studies are likely to be caused by inhomogeneities of flow velocities along the line of sight, or at scales smaller than the resolution scale, or by velocity fluctuations during the exposure time. The increase in the amplitude of Alfv\'en waves cannot, alone, explain the observed features. A double-Gaussian fit of the line profiles shows that, both for dimmings and active region loops, one component is nearly at rest while the second component presents a larger Doppler shift than that derived from a single-Gaussian fit.

QUASI-PERIODIC PROPAGATING SIGNALS IN THE SOLAR CORONA: THE SIGNATURE OF MAGNETOACOUSTIC WAVES OR HIGH-VELOCITY UPFLOWS?

Since the discovery of quasi-periodic propagating oscillations with periods of order three to ten minutes in coronal loops with TRACE and EIT (later with EUVI and EIS), they have been almost universally interpreted as evidence for propagating slow-mode magnetoacoustic (MA) waves in the low-beta coronal environment. We show that this interpretation is not unique. We focus instead on the ubiquitous faint upflows, associated with blue asymmetries of spectral line profiles in footpoint regions of coronal loops, and as faint disturbances propagating along coronal loops in EUV/XR imaging timeseries. The two scenarios are difficult to differentiate using only imaging data, but careful analysis of spectral line profiles indicates that faint upflows are likely responsible for some of the observed quasi-periodic oscillatory signals in the corona. We show that EIS measurements of intensity and velocity oscillations in coronal lines (previously interpreted as direct evidence for propagating waves) are actually accompanied by significant oscillations in the line width that are driven by a quasi-periodically varying component of emission in the blue wing of the line. The faint blue-shifted emission component quasi-periodically modulates the peak intensity and line-centroid of a single Gaussian fit to the profile with the same small amplitudes (respectively a few percent of background intensity, and a few km/s) used to infer the presence of MA waves. Our results indicate that a significant fraction of the quasi-periodicities observed with coronal imagers and spectrographs, previously interpreted as propagating MA waves, are caused by these upflows. The different physical cause for coronal oscillations would significantly impact the prospects of successful coronal seismology using propagating disturbances in coronal loops.

MULTIPLE COMPONENT OUTFLOWS IN AN ACTIVE REGION OBSERVED WITH THE EUV IMAGING SPECTROMETER ONHINODE

We have used the Extreme Ultraviolet Imaging Spectrometer (EIS) on the Hinode spacecraft to observe large areas of outflow near an active region. These outflows are seen to persist for at least 6 days. The emission line profiles suggest that the outflow region is composed of multiple outflowing components, Doppler-shifted with respect to each other. We have modeled this scenario by imposing a double-Gaussian fit to the line profiles. These fits represent the profile markedly better than a single Gaussian fit for Fe XII and XIII emission lines. For the fastest outflowing components, we find velocities as high as 200 km/s. However, there remains a correlation between the fitted line velocities and widths, suggesting that the outflows are not fully resolved by the double-Gaussian fit and that the outflow may be comprised of further components.

Radial velocity observations of the extended lunar sodium tail

We report the first velocity resolved sodium 5889.950 Å line profile observations of the lunar sodium tail observed in the anti‐lunar direction near new Moon. These observations were made on 29 March 2006, 27 April 2006 and 28 April 2006 from Pine Bluff (WI) observatory with a double etalon Fabry‐Perot spectrometer at a resolving power of ∼80,000. The observations were made within 2–14 hours from new Moon, pointing near the anti‐lunar point. The average observed radial velocity of the lunar sodium tail in the vicinity of the anti‐lunar point for the three nights reported was 12.4 km s−1 (from geocentric zero). The average Doppler width of a single Gaussian fit to the emission line was 7.6 km s−1. In some cases the line profile appears asymmetric, with excess lunar sodium emission at higher velocity (∼18 km s−1 from geocentric zero) that is not accounted for by our single Gaussian fit to the emission.

Offset Pointing Calibrators for Large Radio Telescopes

ABSTRACTWe present a catalog of pointing calibrators suitable for offset pointing and for determining the pointing constants of large radio telescopes. It contains 3399 strong, compact, and unconfused radio sources with accurate (σαcos δ ≈ σδ ≈ 0.″5) positions from the NRAO Very Large Array Sky Survey (NVSS) uniformly covering the sky north of J2000 δ = -40°. The NVSS images, restored with a θ = 45′′ FWHM Gaussian beam, were also convolved to larger beam sizes θ = 90′′, 180″, 360″, 540″, and 720″. The catalog lists the maximum beam size θm for which each calibration source remains unconfused and a single Gaussian fit yields an rms position error σ≤ θm/100. For all δ> - 40°, the average angular distance to the nearest calibrator is only 〈ϕ〉 ≈ 0.03 rad, so offset pointing from these calibrators may reduce slowly varying pointing errors (caused by incorrect values for the traditional pointing constants, gravitational deformations, differential thermal expansion, refraction, etc.) by factors up to 〈ϕ〉-1 ≈ 30.

A distinct 14 residue site triggers coiled-coil formation in cortexillin I.

We have investigated the process of the assembly of the Dictyostelium discoideum cortexillin I oligomerization domain (Ir) into a tightly packed, two-stranded, parallel coiled-coil structure using a variety of recombinant polypeptide chain fragments. The structures of these Ir fragments were analyzed by circular dichroism spectroscopy, analytical ultracentrifugation and electron microscopy. Deletion mapping identified a distinct 14 residue site within the Ir coiled coil, Arg311-Asp324, which was absolutely necessary for dimer formation, indicating that heptad repeats alone are not sufficient for stable coiled-coil formation. Moreover, deletion of the six N-terminal heptad repeats of Ir led to the formation of a four- rather than a two-helix structure, suggesting that the full-length cortexillin I coiled-coil domain behaves as a cooperative folding unit. Most interestingly, a 16 residue peptide containing the distinct coiled-coil 'trigger' site Arg311-Asp324 yielded approximately 30% helix formation as monomer, in aqueous solution. pH titration and NaCl screening experiments revealed that the peptide's helicity depends strongly on pH and ionic strength, indicating that electrostatic interactions by charged side chains within the peptide are critical in stabilizing its monomer helix. Taken together, these findings demonstrate that Arg311-Asp324 behaves as an autonomous helical folding unit and that this distinct Ir segment controls the process of coiled-coil formation of cortexillin I.