Laura Ellen Kay

The Frequency of Polarized Broad Emission Lines in Type 2 Seyfert Galaxies

We have discovered polarized broad emission lines in five type 2 Seyfert galaxies (NGC 424, NGC 591, NGC 2273, NGC 3081, and NGC 4507), establishing that these objects are type 1 Seyferts obscured by dense circumnuclear material. The galaxies are part of a distance-limited sample of 31 Seyfert 2s, for which spectropolarimetric observations are now complete. Combined with published reports, our results indicate that at least 11 of the galaxies in this sample, or ≥35%, possess hidden broad-line regions. As the first reliable estimate of the frequency of polarized broad emission lines in type 2 Seyfert galaxies, this has important implications for the general applicability of Seyfert unification models.

Detection of obscured broad-line regions in four Seyfert 2 galaxies

We present spectropolarimetric observations of the nuclei of four Seyfert 2 galaxies, Mrk 477, Mrk 1210, NGC 7212, and Was 49b. The data, covering the spectral range 4600-7400 A at moderate resolution, were obtained with the CCD spectrograph/polarimeter attached to the 3 m Shane telescope at Lick Observatory. They show clear evidence for broad components of Hα and Hβ emission lines in the polarized flux spectra of these objects, similar to those observed in NGC 1068 and four other previously known obscured Seyfert 1 galaxies reported by Miller & Goodrich

A Composite Seyfert 2 X-Ray Spectrum: Implications for the Origin of the Cosmic X-Ray Background

We present a composite 1-10 keV Seyfert 2 X-ray spectrum derived from ASCA observations of a distance-limited sample of nearby galaxies. All 29 observed objects were detected. Above ~3 keV, the composite spectrum is inverted, confirming that Seyfert 2 galaxies as a class have the spectral properties necessary to explain the flat shape of the cosmic X-ray background spectrum. Integrating the composite spectrum over redshift, we find that the total emission from Seyfert 2 galaxies, combined with the expected contribution from unabsorbed type 1 objects, provides an excellent match to the spectrum and intensity of the hard X-ray background. The principal uncertainty in this procedure is the cosmic evolution of the Seyfert 2 X-ray luminosity function. Separate composite spectra for objects in our sample with and without polarized broad optical emission lines are also presented.

3.3 and 11.3 micron images of HD 44179 : evidence for an optically thick polycyclic aromatic hydrocarbon disk

Images of HD 44179 (the Red Rectangle) obtained in the 3.3 and 11.3 micron emission bands show two different spatial distributions. The 3.3 micron band image is centrally peaked and slightly extended N-S while the 11.3 micron image shows a N-S bipolar shape with no central peak. If the 3.3 micron band image shows the intrinsic emission of the 11.3 micron band, then the data suggest absorption of the 11.3 micron emission near the center of HD 44179 by a disk with an optical depth of about one, making HD 44179 the first object in which the IR emission bands have been observed to be optically thick. Since there is no evidence of absorption of the 3.3 micron emission band by the disk, the absorption cross section of the 3.3 micron band must be substantially less than for the 11.3 micron band. Since the 3.3 and 11.3 micron bands are thought to arise from different size PAHs, the similar N-S extents of the two images implies that the ratio of small to large PAHs does not change substantially with distance from the center.

The Optical Polarization and Warm Absorber in IRAS 17020+4544

We report the detection of ionized absorption in the ASCA spectrum of the narrow-line Seyfert 1 galaxy IRAS 17020+4544. Subsequent optical spectropolarimetry revealed high polarization increasing from 3% in the red to 5% in the blue, which indicates electron or dust scattering as a likely origin. The broad emission line Hα is somewhat less polarized than the continuum, which supports a location of the polarizing material within the active galactic nucleus. The Balmer line decrement and reddened optical spectrum support the presence of a dusty warm absorber in this object. We compared the broadband optical polarization and ionized X-ray absorption of a collection of Seyfert 1 and 1.5 galaxies, excluding classes of objects that are likely to have significant neutral X-ray absorption. Warm absorber objects are generally more likely to have high optical polarization than objects with no detected ionized absorption. This result lends additional support to the idea that the warm absorber is associated with dust and implies either that dust transmission is responsible for at least part of the polarization or that the polarization is revealed because of the dimming of the optical spectrum. Spectropolarimetry of Seyfert 1 galaxies generally locates the scattering material inside the narrow-line region and often close to or within the broad-line region, consistent with estimates of the location of the dusty warm absorber.

Ultraviolet Imaging Polarimetry of the Seyfert 2 Galaxy Markarian 3

We present ultraviolet (UV) imaging polarimetry data of the Seyfert 2 galaxy Mrk 3, taken by the Hubble Space Telescope. The polarized flux is found to be extended to ~1 kpc from the nucleus, and the position angles of polarization are centrosymmetric, confirming that the polarization is caused by scattering. We determine the location of the hidden nucleus as the center of this centrosymmetric pattern. From the polarization images taken in two broad bands, we have obtained the color distribution of the polarized flux. Some regions have blue polarized flux, consistent with optically thin dust scattering, but some bright knots have a color similar to that of a Seyfert 1 nucleus. Also, the recent Chandra X-ray observation suggests that the ratio of scattered UV flux to scattered X-ray flux is rather similar to the intrinsic UV/X-ray ratio in a Seyfert 1 nucleus, if the observed extended X-ray continuum is scattered light. While the scattered X-rays would essentially be from electron scattering, the similarity of both the UV slope and UV/X-ray ratio to those of a Seyfert 1 nucleus would lead to two possibilities as to the nature of the UV scatterers. One is that the UV may also be scattered by electrons, in which case the scattering gas is somehow dust free. The other is that the UV is scattered by dust grains, but the wavelength-independent UV scattering with low efficiency, indicated by the UV slope and UV/X-ray ratio, would suggest that the grains reside in UV-opaque clouds, or that the dust might be mainly composed of large grains and lack small-grain population.

A Hidden Broad‐Line Region in the Weak Seyfert 2 Galaxy NGC 788

We have detected a broad Hα emission line in the polarized flux spectrum of the Seyfert 2 galaxy NGC 788, indicating that it contains an obscured Seyfert 1 nucleus. Although such features have been observed in ~15 other Seyfert 2 galaxies, this example is unusual because it has a higher fraction of galaxy starlight in its spectrum, a lower average measured polarization, and a significantly lower radio luminosity than other hidden Seyfert 1 galaxies discovered to date. This demonstrates that polarized broad-line regions can be detected in relatively weak classical Seyfert 2 galaxies and illustrates why well-defined, reasonably complete spectropolarimetric surveys at Hα are necessary in order to assess whether or not all Seyfert 2 galaxies are obscured Seyfert 1 galaxies.

Transient and Highly Polarized Double-Peaked Hα Emission in the Seyfert 2 Nucleus of NGC 2110

We have discovered an extremely broad, double-peaked Hα emission line in the polarized flux spectrum of NGC 2110, establishing that this well-studied Seyfert 2 galaxy contains a disk-like hidden broad-line region (BLR). Several properties of NGC 2110 suggest that it is an obscured twin of Arp 102B, the prototypical double-peaked emission-line active galactic nucleus (AGN). A comparison between our data and previous spectra of NGC 2110 indicates that the double-peaked Hα feature is transient. The presence of a disk-like BLR in NGC 2110 has important implications for AGNs: it expands the range of properties exhibited by Seyfert 2 galaxies, and the fact that the BLR is obscured by a torus-like structure provides the first evidence that double-peaked emitters and classical Seyfert nuclei may have the same basic parsec-scale geometry.

“Hidden” Seyfert 2 Galaxies in the Chandra Deep Field North

We have compared the X-ray-to-optical flux ratios (FX/Fopt) of absorbed active galactic nuclei (AGNs) in the Chandra Deep Field North (CDF-N) with those of nearby, optically classified Seyfert 2 galaxies. The comparison provides an opportunity to explore the extent to which the local population of absorbed AGNs can account for the properties of the distant, spectroscopically ambiguous sources that produce the hard X-ray background. Our nearby sample consists of 38 objects that well represent the local Seyfert 2 luminosity function. Integrated UBVRI photometry and broadband X-ray observations are presented. Using these data, we have simulated the FX/Fopt ratios that local Seyfert 2s would exhibit if they were observed in the redshift range 0.2 ≤ z ≤ 1.3 as part of the CDF-N. In the simulations we account for the effects of redshift on flux measurements in fixed observed-frame bands and the way the luminosity function of a given population is sampled in a flux-limited survey like the CDF-N. Overall, we find excellent agreement between our simulations and the observed distribution of FX/Fopt ratios for absorbed AGNs in the CDF-N. Our analysis has thus failed to reveal any physical differences between the local population of Seyfert 2s and CDF-N sources with similar X-ray properties. These results support the hypothesis that the nuclear emission lines of many distant hard X-ray galaxies are hidden in ground-based spectra due to a combination of observational effects: signal-to-noise ratio, wavelength coverage, and dilution by host-galaxy light.

Spectropolarimetry of the Luminous Narrow-Line Seyfert Galaxies IRAS 20181-2244 and IRAS 13224-3809

We observed the narrow-line Seyfert 1 galaxies IRAS 20181-2244 and IRAS 13224-3809 with a new spectropolarimeter on the Ritchey-Chretien spectrograph at the CTIO 4 m telescope. Previously it had been suggested that IRAS 20181-2244 was a type 2 QSO and thus might contain an obscured broad-line region that could be detected by the presence of broad Balmer lines in the polarized flux. We found the object to be polarized at about 2% and constant with wavelength (unlike most narrow-line Seyfert 1 galaxies), but with no evidence of broad Balmer lines in polarized flux. The spectropolarimetry indicates that the scattering material is inside the broad-line region. IRAS 13224-3809, notable for its high variability in X-ray and UV wavelengths, has a low polarization consistent with a Galactic interstellar origin.