Emission-line Equivalent Widths Research Articles

An Objective Approach to Spectral Classification

The next generation of spectroscopic surveys, both Galactic and extra-galactic (e.g., SDSS, 2dF), present the challenge of classifying spectra in an efficient and objective manner. The standard approach to this problem has been to visually classify spectra based on a number of spectral features (e.g., the equivalent widths of emission lines). The size of new spectral surveys (> 106 galaxies) and the desire to compare the luminosity and environments of galaxies with their spectral properties make these techniques infeasible. We describe here an automated classification scheme that is being developed for the SDSS.

Temporal Changes in Quasar Broad Emission Line Profiles and the Gravitationally Lensed Nature of Q1634+267A,B and Q2345+007A,B

(abridged) Steidel & Sargent (1991a) found that the spectra of the components of the close quasar pairs Q1634+267A,B and Q2345+007A,B were in each case remarkably similar but that individual broad emission lines due to Ly_alpha, Si IV, C IV, and C III] exhibited differences in profile or equivalent width between the A and B components. If the A and B components of these objects result from gravitational lensing of a single QSO, the difference in light travel time for the two images is roughly 1 year for both pairs. Accordingly, the authors suggested that observations of the spectra of single QSOs would reveal similar changes in emission line profiles and equivalent widths on a time scale of a year or less. In order to test Steidel & Sargent's hypothesis, we obtained in 1992 high quality spectra of 30 quasars with z(em) ~ 1 selected from the brighter QSOs in the sample of Steidel & Sargent (1991b), which those authors had observed in 1989-90. We find that the spectra from the two epochs are remarkably similar for all QSOs, except for overall changes in fluxes and minor differences in continuum shape which are probably artifacts introduced by the observing procedures. However, there are also changes in the equivalent widths or shapes of the stronger emission lines on rest-frame time scales of 1-1.5 years in about two thirds of the QSOs. These observations, therefore, support the notion that the spectra of the components in lensed QSOs need not be exactly identical. Moreover, it should be expected that any differences between the components in such systems should change with time.

Observations of the SW Sextantis star UU Aquarii

We present 14 nights of medium resolution (1-2\AA) spectroscopy of the eclipsing cataclysmic variable UU Aquarii obtained during a high accretion state in 1995 August-October. UU Aqr appears to be an SW Sextantis star, as noted by Baptista, Steiner, & Horne (1996), and we discuss its spectroscopic behavior in the context of the SW Sex phenomenon. Emission line equivalent width curves, Doppler tomography, and line profile simulation provide evidence for the presence of a bright spot at the impact site of the accretion stream with the edge of the disk, and a non-axisymmetric, vertically- and azimuthally-extended absorbing structure in the disk. The absorption has maximum depth in the emission lines around orbital phase 0.8, but is present from $\phi\approx0.4$ to $\phi\approx0.95$. An origin is explored for this absorbing structure (as well as the other spectroscopic behavior of UU Aqr) in terms of the explosive impact of the accretion stream with the disk.

A Search for Ultraviolet Emission from LINERs

We have obtained Hubble Space Telescope Wide Field and Planetary Camera 2 ultraviolet (UV) 2200 A and optical V-band images of 20 low-luminosity active galactic nuclei, most of which are spectroscopically classified as LINERs, in order to search for a possible photoionizing continuum. Six (30%) of the galaxies are detected in the UV. Two of the detected galaxies (NGC 3642 and NGC 4203) have compact, unresolved nuclear UV sources, while the remaining four UV sources (in NGC 4569, NGC 5005, NGC 6500, and NGC 7743) are spatially extended. Combining our data set with the earlier sample of Maoz et al., we find that the probability of detection of a nuclear UV source is greatest for galaxies having low internal reddening and low inclination, and we conclude that dust obscuration is the dominant factor determining whether or not a UV source is detected. Large emission-line equivalent widths and the presence of broad-line emission also increase the likelihood of detection of nuclear UV emission. Our results suggest that the majority of LINERs harbor obscured nuclear UV sources, which may be either accretion-powered active nuclei or young star clusters. Under the assumption that the compact UV sources in NGC 3642 and NGC 4203 have nonstellar spectra of slope fν ∝ ν-1 extending into the extreme ultraviolet, the extrapolated ionizing fluxes are sufficiently strong to photoionize the narrow-line regions of these objects. The V-band images of many galaxies in our sample reveal remarkably strong dust lanes that may be responsible for obscuring some UV sources.

Discovery of Interstellar Hydrogen Fluoride

We report the first detection of interstellar hydrogen fluoride. Using the Long Wavelength Spectrometer of the Infrared Space Observatory (ISO), we have detected the 121.6973 micron J = 2-1 line of HF in absorption toward the far-infrared continuum source Sagittarius B2. The detection is statistically significant at the 13 sigma level. On the basis of our model for the excitation of HF in Sgr B2, the observed line equivalent width of 1.0 nm implies a hydrogen fluoride abundance of approximately 3 x 10(exp -10) relative to H2. If the elemental abundance of fluorine in Sgr B2 is the same as that in the solar system, then HF accounts for approximately 2% of the total number of fluorine nuclei. We expect hydrogen fluoride to be the dominant reservoir of gas-phase fluorine in Sgr B2, because it is formed rapidly in exothermic reactions of atomic fluorine with either water or molecular hydrogen; thus, the measured HF abundance suggests a substantial depletion of fluorine onto dust grains. Similar conclusions regarding depletion have previously been reached for the case of chlorine in dense interstellar clouds. We also find evidence at a lower level of statistical significance (approximately 5 sigma) for an emission feature at the expected position of the 4(sub 32)-4(sub 23) 121.7219 micron line of water. The emission-line equivalent width of 0.5 nm for the water feature is consistent with the water abundance of 5 x 10(exp -6) relative to H2 that has been inferred previously from observations of the hot core of Sgr B2.

Origin of the Spread in theB−KColor of Quasars

Recently, Webster et al. have shown that there is excess reddening in radio-selected quasars relative to the optically selected population. If this reddening is universal to all quasars, then the optical surveys that depend on the UV-excess selection criteria, say, would be seriously incomplete. Using data compiled from the literature and various statistical tests, we show that there is no significant correlation between B - K colors and other reddening indicators in the case of optically selected quasars. The distribution of emission-line equivalent widths and line ratios of radio-loud and radio-quiet quasars in the optically selected sample are consistent with their being drawn from the same parent population. Our results suggest that there is an intrinsic spread in B - K color of ~2 mag in optically selected bright quasars due to effects other than dust.

The Spatial Distribution of Stellar CO Absorption in M83

Long-slit H- and K-band near-infrared spectra have been acquired through the bright nuclear starburst region in the nearby galaxy M83. The spatial distribution of the stellar CO 2-0 absorption band broadly resembles that of the 2 μm continuum emission and has a maximum depth consistent with a pure starburst population. The band strength deceases monotonically away from this location to a value consistent with that seen in the giant-dominated populations in ellipticals and the bulges of spiral galaxies. There is no significant variation in the ratio of H (CO 6-3) and K (CO 2-0) band equivalent widths, which argues against any significant dilution of the bands by hot dust. The CO depth and Brγ emission-line equivalent width are anticorrelated. Comparison with starburst evolutionary models indicates an age gradient across this central region.

Emission Lines in the Far-UV and Extreme-UV Region

AbstractRecent observations with HST and HUT have been made of the spectral region shortward of Lyα in AGN. Emission lines in this region, such as O VIλ1035 and Ne VIIIλ774, are mainly produced by collisional excitation. Their strengths and the lack of other significant emission lines of higher excitation energy indicate a temperature of ~ 6 × 104 K for the highly ionized gas in line-emitting clouds. Model calculations suggest that the strength of O VI and Ne vIII emission is correlated to the intensity of the high-energy photons above 100 eV. Therefore, these lines are useful diagnostics of the continuum shape in the soft X-ray range. The luminosity dependence of emission-line equivalent widths is more prominent for lines of higher ionization level, suggesting that the UV bump in AGN spectra may be weaker at higher luminosities.

The Relationship between the High-Energy Continuum and Emission Lines in Quasars: A Low-Redshift Sample

view Abstract Citations (52) References (60) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Relationship between the High-Energy Continuum and Emission Lines in Quasars: A Low-Redshift Sample Green, Paul J. Abstract Photoionization models dictate that many prominent quasar emission lines are sensitive to both the luminosity and shape of the quasars high- energy continuum-primarily the extreme ultraviolet (EUV) and soft X-ray continuum. Unfortunately, the EUV band is severely obscured by Galactic absorption. Using data from the adjacent UV and soft X-ray bandpasses, we initiate the first large-scale, multiline investigation of correlations between the QSO soft X-ray continuum and line emission in a sample of QSOs observed by Einstein and IUE. We present a new error analysis for objective, automated line measurements, which enables us to include the information contained in weak or undetected lines. We tabulate more than 300 UV emission-line equivalent widths from IUE spectra of 85 QSOs in the atlas of Lanzetta, Turnshek, & Sandoval, then characterize the distributions of line equivalent and velocity widths (Wlambda_ and FWHM). We then compare these line parameters to the QSO continuum spectral energy distributions from optical through soft X-ray wavelengths, using survival analysis to incorporate any nondetections for X-ray flux and/or UV emission lines. Several correlations noted in previous studies are not reproduced here. However, we illustrate that the exclusion of undetected lines from such studies may spuriously enhance apparent correlations. We find significant correlations between Wlambda_ and UV luminosity (e.g., the well-studied Baldwin effect) for Lyα, C IV, He II, and C III]. Wlambda_(C III]) and Wlambda_(He II) also show previously unreported correlations with X-ray luminosity that, for C III], appears to be primary. The line ratios C III]/Lyα and He II/Lyα both show strongest dependence on l_x_. Wlambda_(Lyα) correlates strongly with spectral slopes α_UV_ and α_OX_ (between 2500 A and 2 keV), but not with X-ray luminosity. Using these results, we argue that one simple geometrical interpretation of the Baldwin effect (BEff) as a result of a distribution of disk inclinations is not plausible. We also provide evidence that the BEff weakens or disappears when the line emission is correctly compared to the luminosity in the continuum bandpass relevant to its production. We thus support the interpretation of the BEff as a change in spectral energy distribution with luminosity, and we predict that no BEff relative to X-ray luminosity should be found for Fe II or Mg II emission lines. Extensions of our method to samples of a wider redshift/ luminosity range will be presented in a later paper, which will test these predictions. Publication: The Astrophysical Journal Pub Date: August 1996 DOI: 10.1086/177584 arXiv: arXiv:astro-ph/9603099 Bibcode: 1996ApJ...467...61G Keywords: GALAXIES: ACTIVE; GALAXIES: QUASARS: EMISSION LINES; GALAXIES: QUASARS: GENERAL; ULTRAVIOLET: GALAXIES; X-RAYS: GALAXIES; Astrophysics E-Print: 31 pages including 3 figures, AAS latex, plus 6 tables totaling 8 pages, to appear in ApJ Vol. 468, September 1, 1996 full text sources arXiv | ADS | data products NED (85) SIMBAD (10) MAST (1) INES (1)

ITAL]Hubble Space Telescope[/ITAL] Observations of the Gravitational Lens System B1422+231

Hubble Space Telescope observations of the gravitational lens system B1422+231 are presented. The spectra of the four components are virtually identical, although there are differences among the Lyα emission-line equivalent widths that might be due to microlensing. There are also significant differences between the component flux density ratios measured at radio and optical wavelengths. High-resolution imaging reveals the lens close to the faintest quasar image. The lens has photometric properties consistent with an elliptical galaxy at z ~ 0.4. The addition of new constraints to simple lensing models affirms the B1422+231 system as a suitable tool for an eventual direct measure of the Hubble constant.

Keck Spectroscopy and Hubble Space Telescope Imaging of Field Galaxies at Moderate Redshift

We present 18 spectra, obtained with the Keck 10m telescope, of faint field galaxies (19 $<$ I $<$ 22, 0.2 $<$ z $<$ 0.84) previously imaged by {\it HST's} WFPC2. Though small, our sample appears to be representative of field spirals with a magnitude--limit of I $\le$ 22. Combining the results from the spectral and imaging data, we have derived various quantitative parameters for the galaxies, including colors, inclinations, emission line equivalent widths, redshifts, luminosities, internal velocity information and physical scale lengths. In particular, disk scale lengths (with sizes ranging from $\sim$1--5 kpc) have been measured from fits to the surface brightness profiles. We have also measured internal velocities with a rest frame resolution of $\sigma$ = 55 to 80 km s$^{-1}$ by fitting to the emission lines. The luminosity--disk size and luminosity--internal velocity (Tully--Fisher) relations for our moderate redshift galaxies are similar to the scaling relations seen for local galaxies, albeit with a modest brightening of $\sim$1 magnitude. The one bulge--dominated galaxy in our sample (at z = 0.324) has a relatively blue color, reveals weak emission lines and is $\sim$0.5 magnitude brighter in the rest frame than expected for a passive local elliptical. Our data suggest that galaxies at about half the age of the Universe have undergone mild luminosity evolution to the present epoch, but are otherwise quantitatively similar to galaxies seen locally.

The X-Ray Nova GRO J0422+32 in Decline and Quiescence

We have followed the X-ray nova GRO J0422+32, spectroscopically and photometrically, throughout the decline to quiescence.In the final stages of decay (days 430…880 after the outburst, see Callanan et al. (1995) for the epoch 1995), the equivalent width (EW) of the Hα emission increases monotonically and the R magnitude decreases monotonically. This suggests that the flux in the Hα line is constant, while the continuum fades. The Hα flux is the product of the R band flux (F(R), arbitrarily scaled to 100 at R = 19 mag) and the EW, and is shown in the last column of the table below. The Hα flux varies by only ~ 30% while the continuum fades by a factor of eight (from R = 19 mag to R = 21.3 mag). So, to first order, the Hα luminosity is constant in the final stages of decay. While it is generally the case that the emission line EWs in individual dwarf novae also increase during the decay, the exact behavior seen in GRO J0422+32 is not what is seen for dwarf novae (on average). Using the relation between EW[Hβ] and Mv given in figure 6 of Patterson (1984), we would expect a factor of ~ 5 variation in the Hα flux during days 430…880. The stability of the Hα flux implies that somehow the emission line region is ‘disconnected’ from the continuum (R–band) emission region.

The Evolution of QSO Spectra: Evidence for Microlensing?

We find that the spectra of QSOs evolve: high redshift QSOs (z &gt; 1.5) have lower equivalent width emission lines than low redshift QSOs (z &lt; 0.5) with the same luminosities and radio properties.We propose that microlensing by compact objects may account for the apparent evolution. If Ω ∼ 0.05 in compact objects, the continuum emission from many high redshift QSOs will be amplified, but not the line emission, leading to the observed decrease in the apparent equivalent widths.

The evolution of QSO spectra

We find that the spectra of QSOs evolve: high redshift QSOs (z > 1.5) have lower equivalent width emission lines than low redshift QSOs (z < 0.5) with the same luminosities and radio properties.

Ultraviolet to optical spectral distributions of northern star-forming galaxies

We report spectral energy distribution from the UV to the optical for a sample of 31 northern star-forming galaxies. We also present measurements for emission-line fluxes, continuum levels, and equivalent widths of absorption features for each individual spectrum as well as averages for the eight galactic activity classes, including normal, starburst, Seyfert 2, blue compact dwarf, blue compact, Low-Inonization Nuclear Emission Regions (LINER), H II, and combination LINER-H II galaxies.

Optically thin broad-line clouds in active galactic nuclei

The broad-line region (BLR) in Seyfert galaxy nuclei exhibits correlated variations in continuum and emission-line luminosity that are qualitatively consistent with photoionization of ionization-bounded (optically thick) clouds. However, evidence is growing that a nonnegligible fraction of the BLR cloud population is optically thin to the Lyman continuum and fully ionized in hydrogen. We consider the implications of this nebular component for observed line emission and find that inclusion of thin clouds in photoionization calculations can resolve several outstanding puzzles of Seyfert variability, notably the behavior of the C IV lambda 1549/Ly-alpha ratio as a function of continuum luminosity. A similar population of thin clouds located along our line of sight can account for observed ultraviolet absorption features and 'warm absorber' behavior at X-ray energies. The Baldwin effect for active galaxies, a negative correlation between ultraviolet emission-line equivalent width and continuum luminosity, can also be explained in detail by a decrease in the covering factor of an optically thin component with increasing source luminosity. The luminosity dependence of covering factor may result from outflows of thin clouds that proceed more efficiently in intrinsically brighter sources. The presence of absorption features in active galactic nuclei (AGNs) that are blueshifted and attain the highest velocities in broad absorption line features associated with luminous QSOs would be consistent with this interpretation.

EVOLUTION OF EMISSION LINE GALAXIES

ABSTRACT This thesis compares two samples of emission line galaxies, selected on the basis of the strength of their O III and/or H-beta emission lines. The distant sample has been drawn from the 4-Shooter transit survey undertaken by Schmidt, Schneider and Gunn (1994, AJ, 107, 1245 and references therein), and consists of 370 galaxies with emission line equivalent widths in excess of 50A and fluxes above well-defined wavelength-dependent limits. This survey the moderate resolution spectra taken to identify the emission line. The nearby sample has been taken from the first CfA Northern Sky Redshift Survey, and consists of 81 galaxies from Burg (1987, Ph.D. thesis, MIT, Cambridge, MA) with O III EW ≥ 23.75A and an additional 26 Seyfert galaxies fbeen observed on the 1.5m Oscar Meyer telescope using the Echelle Spectrograph in a low-resolution, long slit mode (McCarthy, 1988, Ph.D. thesis, Caltech, Pascovers the emission lines of interest such as H-beta, O II. These data have been used both to classify the 107 galaxies from their line ratio diagnostics as well as to model the spatial and spectral light distributions on the plane of the sky for a comparison of how each would appear in the distant survey as a function of redshift. Maximum redshifts in both the nearby and the distant survey have been determined for each CfA galaxy, and predicted number counts, based on both a no-evolution model as well as a model incorporating density evolution, have been made from the corresponding ratio of accessible volumes in the two surveys. Corrections were made to the predicted counts to account for sample incompletenesses and the overdensity of the CfA survey relative to the average density of galaxies in the "local" universe. These predicted counts were comclass. The results from this cono-evolution model for emission line galaxies out to z~0.5, and do not support the conjecture (Broadhurst et al., 1988, MNRAS, 235, 827; Colless et al., 1990, MNRAS, 244, 408) that there is an evolving population of dwarf star-forming emission line galaxies.

Continuum and emission line observations of Wolf-Rayet stars: single and “WR+abs” objects

The UV — IR properties of the continuous energy distribution of single WR stars in the Galaxy and LMC are reviewed. The observed, dereddened distributions may be approximated as a power law Fλ ∼ λ−α with a non-Rayleigh Jeans range of α directly associated with the emission of excess flux over the full wavelength range and across all spectral subtypes. The observational results are compared to published NLTE models, where we find an important discrepancy in the frequency distribution of α for Galactic WN stars. A dependency of emission line equivalent widths on the continuum flux at line center is discussed; the systematic offset of the absorption line (but otherwise presumed single) objects is examined, and the observed behavior is compared to models and explained with simple theory.

Spin-resolved optical CCD spectroscopy and photometry of BG Canis Minoris

We report spin-resolved optical photometry and spectroscopy of the intermediate polar BG Canis Minoris. New 913-s pulsation ephemerides are derived. We find that a cubic fit provides a better description of the O-C values than the hitherto assumed quadratic ephemeris at a confidence level of 97.3 per cent. From the spectroscopy, we find that the violet-to-red (V/R) ratio of the emission lines varies with the 913-s period but not at the alternative period of 847 s detected in the X-ray band. This suggests that 913 s is the most probable spin period of the white dwarf. However, we do find evidence that the emission line intensities and equivalent widths are modulated at 847 s

A spectrophotometric study of BQS quasars

Flux-calibrated spectra of 39 low-redshift quasars from the Bright Quasar Survey (BQS) have been used to study a near-complete data set of optically selected quasars. Statistical partial rank analysis shows that the forbidden O III luminosity is more intimately connected to the H-beta luminosity, L(H-beta), than to observed continuum luminosity, L(cont). L(H-beta) is thus considered to be a better measure than L(cont) of the photoionizing luminosity. The effects of an orientation-dependent L(cont) on the rest-frame emission-line equivalent widths, EW(H-beta), and EW(forbidden O III), have been studied. It is found that scatter in the L(H-beta) - L(cont) relation is not due to a range in broad-line covering factor, and is inconsistent with one simple orientation scheme unless objects with very high disk inclination angles are excluded by the BQS selection criteria. It is demonstrated that there exists a correlation between the full width at half maximum of the H-beta line and L(H-beta), providing evidence for a gravitationally bound H-beta-emitting region and a scaling M(central) approximately proportional to L.