Showing papers by "Dominion Astrophysical Observatory published in 2000"

Hubble Space Telescope imaging of the CFRS and LDSS redshift surveys—IV. Influence of mergers in the evolution of faint field galaxies from z∼1

Abstract: ABSTRA C T Hubble Space Telescope images of a sample of 285 galaxies with measured redshifts from the Canada‐France Redshift Survey (CFRS) and Autofib‐Low Dispersion Spectrograph Survey (LDSS) redshift surveys are analysed to derive the evolution of the merger fraction out to redshifts z , 1. We have performed visual and machine-based merger identifications, as well as counts of bright pairs of galaxies with magnitude differences dm # 1:5 mag. We find that the pair fraction increases with redshift, with up to ,20 per cent of the galaxies being in physical pairs at z , 0:75‐1. We derive a merger fraction varying with redshift as /O1a zU 3:2^0:6 , after correction for line-of-sight contamination, in excellent agreement with the merger fraction derived from the visual classification of mergers for which ma 3:4 ^ 0:6. After correcting for seeing effects on the ground-based selection of survey galaxies, we conclude that the pair fraction evolves as/O1a zU 2:7^0:6 . This implies that an average L* galaxy will have undergone 0.8‐1.8 merger events from za 1t oza 0, with 0.5 to 1.2 merger events occuring in a 2-Gyr time-span at around z , 0:9. This result is consistent with predictions from semi-analytical models of galaxy formation. From the simple coaddition of the observed luminosities of the galaxies in pairs, physical mergers are computed to lead to a brightening of 0.5 mag for each pair on average, and a boost in star formation rate of a factor of 2, as derived from the average [O ii] equivalent widths. Mergers of galaxies are therefore contributing significantly to the evolution of both the luminosity function and luminosity density of the Universe out to z , 1.

The Hubble Space Telescope Key Project on the Extragalactic Distance Scale. XXVI. The Calibration of Population II Secondary Distance Indicators and the Value of the Hubble Constant

Abstract: A Cepheid-based calibration is derived for four distance indicators that utilize stars in old stellar populations: the tip of the red giant branch (TRGB), the planetary nebula luminosity function (PNLF), the globular cluster luminosity function (GCLF), and the surface brightness fluctuation method (SBF) The calibration is largely based on the Cepheid distances to 18 spiral galaxies within cz = 1500 km s-1 obtained as part of the Hubble Space Telescope (HST) Key Project on the Extragalactic Distance Scale, but relies also on Cepheid distances from separate HST and ground-based efforts The newly derived calibration of the SBF method is applied to obtain distances to four Abell clusters in the velocity range 3800-5000 km s-1 Combined with cluster velocities corrected for a cosmological flow model, these distances imply a value of the Hubble constant of H0 = 69 ? 4 (random) ? 6 (systematic) km s-1 Mpc-1 This result assumes that the Cepheid PL relation is independent of the metallicity of the variable stars; adopting a metallicity correction as in Kennicutt et al would produce a 5% ? 3% decrease in H0 Finally, the newly derived calibration allows us to investigate systematics in the Cepheid, PNLF, SBF, GCLF, and TRGB distance scales

The Hubble Space Telescope Key Project on the Extragalactic Distance Scale. XXIV. The Calibration of Tully-Fisher Relations and the Value of the Hubble Constant

Abstract: This paper presents the calibration of BVRIH-0.5 Tully-Fisher relations based on Cepheid distances to 21 galaxies within 25 Mpc and 23 clusters within 10,000 km s-1. These relations have been applied to several distant cluster surveys in order to derive a value for the Hubble constant, H0, mainly concentrating on an I-band all-sky survey by Giovanelli and collaborators, consisting of total I magnitudes and 50% line width data for ~550 galaxies in 16 clusters. For comparison, we also derive the values of H0 using surveys in the B and V bands by Bothun and collaborators, and in H band by Aaronson and collaborators. Careful comparisons with various other databases from the literature suggest that the H-band data, which have isophotal magnitudes extrapolated from aperture magnitudes rather than total magnitudes, are subject to systematic uncertainties. Taking a weighted average of the estimates of Hubble constants from four surveys, we obtain H0 = 71 ± 4 (random) ± 7 (systematic). We have also investigated how the value of H0 is affected by various systematic uncertainties, such as the internal extinction correction method used, Tully-Fisher slopes and shapes, a possible metallicity dependence of the Cepheid period-luminosity relation, and cluster population incompleteness bias.

A Database of Cepheid Distance Moduli and Tip of the Red Giant Branch, Globular Cluster Luminosity Function, Planetary Nebula Luminosity Function, and Surface Brightness Fluctuation Data Useful for Distance Determinations

Abstract: We present a compilation of Cepheid distance moduli and data for four secondary distance indicators that employ stars in the old stellar populations: the planetary nebula luminosity function (PNLF), the globular cluster luminosity function (GCLF), the tip of the red giant branch (TRGB), and the surface brightness fluctuation (SBF) method. The database includes all data published as of 1999 July 15. The main strength of this compilation resides in the fact that all data are on a consistent and homogeneous system: all Cepheid distances are derived using the same calibration of the period-luminosity relation, the treatment of errors is consistent for all indicators, and measurements that are not considered reliable are excluded. As such, the database is ideal for comparing any of the distance indicators considered, or for deriving a Cepheid calibration to any secondary distance indicator, such as the Tully-Fisher relation, the Type Ia supernovae, or the fundamental plane for elliptical galaxies. This task has already been undertaken by Ferrarese et al., Sakai et al., Kelson et al., and Gibson et al. Specifically, the database includes (1) Cepheid distances, extinctions, and metallicities; (2) reddened apparent λ5007 A magnitudes of the PNLF cutoff; (3) reddened apparent magnitudes and colors of the turnover of the GCLF (in both the V and B bands); (4) reddened apparent magnitudes of the TRGB (in the I band) and V-I colors at 0.5 mag fainter than the TRGB; and (5) reddened apparent surface brightness fluctuation magnitudes measured in Kron-Cousin I, K', and Kshort, and using the F814W filter with the Hubble Space Telescope (HST) WFPC2. In addition, for every galaxy in the database we give reddening estimates from IRAS/DIRBE as well as H I maps, J2000 coordinates, Hubble and T-type morphological classification, apparent total magnitude in B, and systemic velocity.

A Kinematic Model for the Narrow-Line Region in NGC 4151

Abstract: We present a simple kinematic model for the narrow-line region (NLR) of the Seyfert 1 galaxy NGC 4151, based on our previous observations of extended [O III] emission with the Space Telescope Imaging Spectrograph. The model is similar to a biconical radial outflow model developed for the Seyfert 2 galaxy NGC 1068, except that the bicone axis is tilted much more into our line of sight (40? out of the plane of the sky instead of 5?), and the maximum space velocities are lower (750 km s-1 instead of 1300 km s-1). We find evidence for radial acceleration of the emission-line knots to a distance of 160 pc, followed by deceleration that approaches the systemic velocity at a distance of 290 pc (for a distance to NGC 4151 of 13.3 Mpc). Other similarities to the kinematics of NGC 1068 are (1) there are a number of high-velocity clouds that are not decelerated, suggesting that the medium responsible for the deceleration is patchy, and (2) the bicone in NGC 4151 is at least partially evacuated along its axis. Together, these two Seyfert galaxies provide strong evidence for radial outflow (e.g., due to radiation and/or wind pressure) and against gravitational motion or expansion away from the radio jets as the principal kinematic component in the NLR.

The Properties of the X-Ray-selected EMSS Sample of BL Lacertae Objects

Abstract: We present updated and complete radio, optical, and X-ray data for BL Lac objects in the Einstein Medium Sensitivity Survey (EMSS). The complete "M91" sample first presented in Morris et al. is updated to include 26 BL Lac objects in total, and we define a new, virtually complete sample consisting of 41 EMSS BL Lac objects (the "D40" sample). New high signal-to-noise ratio, arcsecond-resolution VLA observations are also presented for 11 EMSS BL Lac objects, completing VLA observations of the M91 sample. The addition of four new objects, as well as updated X-ray flux and redshift information, has increased the Ve/Va value for the M91 sample to 0.399 ± 0.057 and Ve/Va = 0.427 ± 0.045 for the newly defined D40 sample. In conjunction with other studies of X-ray–selected BL Lac (XBL) samples, these results solidify negative evolution for XBLs, especially for more extreme high-energy–peaked BL Lac objects, for which we find Ve/Va = 0.271 ± 0.077. The observed Ve/Va, spectral, and radio properties of XBLs are completely consistent with being the beamed population of low-luminosity, FR-1 radio galaxies. However, our VLA observations do confirm that XBLs are too core dominated to be consistent with a beamed population of FR-1's seen at intermediate angles, as suggested by the unified model, if XBLs have moderate outflow velocities (γ ~ 5).

Space Telescope Imaging Spectrograph Long-Slit Spectroscopy of the Narrow-Line Region of NGC 4151. II. Physical Conditions along Position Angle 221° *

Abstract: We have examined the physical conditions in the narrow-line region of the well-studied Seyfert galaxy NGC 4151, using long-slit spectra obtained with the Hubble Space Telescope Space Telescope Imaging Spectrograph. The data were taken along a position angle of 221°, centered on the optical nucleus. We have generated photoionization models for a contiguous set of radial zones, out to 23 in projected position to the southwest of the nucleus and 27 to the northeast. Given the uncertainties in the reddening correction, the calculated line ratios successfully matched nearly all the dereddened ratios. We find that the narrow-line region consists of dusty atomic gas photoionized by a power-law continuum that has been modified by transmission through a mix of low- and high-ionization gas, specifically, UV-absorbing and X-ray-absorbing components. The physical characteristics of the absorbers resemble those observed along our line of sight to the nucleus, although the column density of the X-ray absorber is a factor of 10 less than observed. The large inferred covering factor of the absorbing gas is in agreement with the results of our previous study of UV absorption in Seyfert 1 galaxies. We find evidence, specifically the suppression of Lyα, that we are observing the back end of dusty ionized clouds in the region southwest of the nucleus. Since these clouds are blueshifted, this supports the interpretation of the cloud kinematics as being due to radial outflow from the nucleus. We find that the narrow-line gas at each radial position is inhomogeneous and can be modeled as consisting of a radiation-bounded component and a more tenuous, matter-bounded component. The density of the narrow-line gas drops with increasing radial distance, which confirms our earlier results and may be a result of the expansion of radially outflowing emission-line clouds.

Hubble Space Telescope observations of X-ray-selected active galactic nuclei

Abstract: We report on the initial results of a comprehensive Hubble Space Telescope (HST) snapshot imaging survey of 76 low-redshift (z<0.15) X-ray-selected active galactic nuclei (AGN) in the Einstein Extended Medium Sensitivity Survey. This survey is expected to show no bias with respect to host galaxy types, and so is arguably one of the best available samples with HST imaging for the study of the host galaxies. The HST observations in the F814W band are complemented by deeper ground-based observations in the B and R bands for most AGN. The absolute magnitudes for AGN in this sample lie in the range −24

The Resolved Narrow-Line Region in NGC 4151

Abstract: We present slitless spectra of the narrow-line region (NLR) in NGC 4151 from the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope and investigate the kinematics and physical conditions of the emission-line clouds in this region. Using medium resolution (~0.5 A) slitless spectra at two roll angles and narrow-band undispersed images, we have mapped the NLR velocity field from 1.2 kpc to within 13 pc (H0 = 75 km s-1 Mpc-1) of the nucleus. The inner biconical cloud distribution exhibits recessional velocities relative to the nucleus to the NE and approaching velocities to the SW of the nucleus. We find evidence for at least two kinematic components in the NLR. One kinematic component is characterized by low velocities and low velocity dispersions (LVLVD clouds: v < 400 km s-1, and Δv < 130 km s-1). This population extends through the NLR, and their observed kinematics may be gravitationally associated with the host galaxy. Another component is characterized by high velocities and high velocity dispersions (HVHVD clouds: 400 < v 1700 km s-1, Δv ≥ 130 km s-1). This set of clouds is located within 11 (~70 pc) of the nucleus and has radial velocities that are too high to be gravitational in origin but show no strong correlation between velocity or velocity dispersion and the position of the radio knots. Outflow scenarios will be discussed as the driving mechanism for these HVHVD clouds. We also find clouds characterized by low velocities and high velocity dispersions (LVHVD clouds: v < 400 km s-1, Δv ≥ 130 km s-1). These clouds are located within 32 (~200 pc) of the nucleus. It is not clear if the LVHVD clouds are HVHVD clouds whose low velocities are the result of projection effects. Within 32 (~200 pc) of the nucleus, the [O III]/Hβ ratio declines roughly linearly for both the high-velocity-dispersion (HVD) and LVLVD clouds. Since the ionization parameter is proportional to r-2n-1, it appears that the density, n, must decrease as ~r-1 for the clouds within the inner ~32. At distances further from the nucleus, the [O III]/Hβ ratio is roughly constant.

The Hubble Space Telescope Key Project on the Extragalactic Distance Scale. XXVII. A Derivation of the Hubble Constant Using the Fundamental Plane and Dn-sigma; Relations in Leo I, Virgo and Fornax

Abstract: Using published photometry and spectroscopy, we construct the fundamental plane and Dn-? relations in Leo I, Virgo, and Fornax. The published Cepheid period-luminosity (PL) relations to spirals in these clusters fixes the relation between angular size and metric distance for both the fundamental plane and Dn-? relations. Using the locally calibrated fundamental plane, we infer distances to a sample of clusters with a mean redshift of cz ? 6000 km s-1, and derive a value of H0 = 78 ? 5 ? 9 km s-1 Mpc-1 (random and systematic errors, respectively) for the local expansion rate. This value includes a correction for depth effects in the Cepheid distances to the nearby clusters, which decreased the deduced value of the expansion rate by 5% ? 5%. If one further adopts the metallicity correction to the Cepheid PL relation as derived by the Key Project, the value of the Hubble constant would decrease by a further 6% ? 4%. These two sources of systematic error, when combined with a ?6% error due to the uncertainty in the distance to the Large Magellanic Cloud, a ?4% error due to uncertainties in the WFPC2 calibration, and several small sources of uncertainty in the fundamental plane analysis, yield a total systematic uncertainty of ?11%. We find that the values obtained using either the cosmic microwave background (CMB) or a flow-field model, for the reference frame of the distant clusters, agree to within 1%. The Dn-? relation also produces similar results, as expected from the correlated nature of the two scaling relations. A complete discussion of the sources of random and systematic error in this determination of the Hubble constant is also given, in order to facilitate comparison with the other secondary indicators being used by the Key Project.

Far Ultraviolet Spectroscopic Explorer Observations of Intrinsic Absorption in the Seyfert 1 Galaxy Markarian 509

Abstract: We present far-ultraviolet spectra of the Seyfert 1 galaxy Mrk 509 obtained in 1999 November with the Far Ultraviolet Spectroscopic Explorer. Our data span the observed wavelength range 915-1185 A at a resolution of ~20 km s-1. The spectrum shows a blue continuum, broad O VI λλ1032, 1038 emission, and a broad C III λ977 line. We resolve associated absorption lines of O VI λλ1032, 1038, C III λ977, and Lyman lines through Lyζ, superposed on these emission components. Seven distinct kinematic components are present, spanning a velocity range from -440 to 170 km s-1 relative to the systemic velocity. The absorption is clustered in two groups, one centered at -370 km s-1 and another at the systemic velocity. The blueshifted cluster may be associated with the extended line emission visible in deep images of Mrk 509 obtained by Phillips et al. Although several components appear to be saturated, they are not black at their centers. Partial covering or scattering permits ~7% of the broad-line or continuum flux to be unaffected by absorption. Of the multiple components, only one has the same ionization state and column density as highly ionized gas that produces the O VII and O VIII ionization edges in X-ray spectra of Mrk 509.

The Luminous Type Ic Supernova 1992ar at z = 0.145

Abstract: We present spectroscopic and photometric observations of SN 1992ar, the most distant supernova (SN) in the Calan/Tololo Survey. We compare its spectrum with those of nearby Type Ia and Ic SNe and conclude that the latter type is a better match to SN 1992ar. Using K-corrections based on the spectra of well-observed Type Ic and Ia SNe, we compute different possible rest-frame light curves of SN 1992ar and compare them with those of representative SNe of each type observed in the nearby universe. From the photometry and the spectra, we are able to conclude that SN 1992ar cannot be matched by any known example of a Type Ia SN. Even though the data set collected is fairly complete (one spectrum and 10 photometric points), it is not possible to decide whether SN 1992ar was a fast Type Ic SN, like SN 1994I, or a slow one, like SN 1983V. The absolute V magnitudes at maximum implied by each of these possibilities are -19.2 and -20.2, respectively. The latter would make SN 1992ar one of the brightest SNe on record. SN 1992ar, hence, illustrates the problem of contamination faced by the high-z Type Ia SNe samples whose luminosity distances are used to determine the cosmological parameters of the universe. We present observational criteria to distinguish the two SN types when the Si II 6355 A line is redshifted out of the sensitivity range of typical CCD detectors and discuss the effect that these luminous Type Ic SNe would have on the measured cosmological parameters, if not removed from the high-z Type Ia SN samples.

Space Telescope Imaging Spectrograph Long-Slit Spectroscopy of the Narrow-Line Region of NGC 4151. I. Kinematics and Emission-Line Ratios

Abstract: Long-slit spectra of the Seyfert galaxy NGC 4151 from the UV to the near-infrared have been obtained with the Space Telescope Imaging Spectrograph (STIS) to study the kinematics and physical conditions in the narrow-line region (NLR). The kinematics shows evidence for three components, a low-velocity system in normal disk rotation, a high-velocity system in radial outflow at a few hundred kilometers per second relative to the systemic velocity, and an additional high-velocity system also in outflow with velocities up to 1400 km s-1, in agreement with results from STIS slitless spectroscopy. We have explored two simple kinematic models and suggest that radial outflow in the form of a wind is the most likely explanation. We also present evidence indicating that the wind may be decelerating with distance from the nucleus. We find that the emission-line ratios along our slits are all entirely consistent with photoionization from the nuclear continuum source. A decrease in the ratios [O III] λ5007/Hβ and [O III] λ5007/[O II] λ3727 suggests that the density decreases with distance from the nucleus. This trend is borne out by the [S II] ratios as well. We find no strong evidence for interaction between the radio jet and the NLR gas in either the kinematics or the emission-line ratios, in agreement with the recent results of Kaiser et al., who found no spatial coincidence of NLR clouds and knots in the radio jet. These results are in contrast to other recent studies of nearby active galactic nuclei that find evidence for significant interaction between the radio source and the NLR gas.

Modelling the spectra of colliding winds in the Wolf-Rayet WC7+O binaries WR 42 and WR 79

Abstract: We have obtained complete phase coverage of the WC7+O binaries WR 42 = HD 97152 and WR 79 = HD 152270 with high signal-to-noise ratio (S/N), moderate-resolution spectra. Remarkable orbital phase-locked profile variations of the C iiiλ5696 line are observed and interpreted as arising from colliding wind effects. Within this scenario, we have modelled the spectra using a purely geometrical model that assumes a cone-shaped wind–wind interaction region which partially wraps around the O star. Such modelling holds the exciting promise of revealing a number of interesting parameters for WR+O binaries, such as the orbital inclination, the streaming velocity of material in the interaction region and the ratio of wind momentum flux. Knowledge of these parameters in turn leads to the possibility of a better understanding of WR star masses, mass-loss rates and wind region characteristics.

Some Musings on Globular Cluster Systems

Abstract: Globular cluster systems exhibit a bewildering variety of characteristics. No single scenario appears to be able to account for the wide range of specific globular cluster frequencies that are observed in galaxies of various types. The fraction of all star formation that produces massive bound clusters differs from galaxy to galaxy and (in some cases) appears to vary with time. Currently available data strongly suggest that the specific cluster forming frequency is highest during violent bursts of star formation. Globular cluster systems show a wide variety of color (metallicity) distributions, with many luminous galaxies having double (or even multiple) metallicity peaks that were probably produced during distinct episodes of cluster formation. The bulk of the globulars in the main body of the Galactic halo appear to have formed during a single short-lived burst that took place ~13 Gyr ago. This was followed by a second, perhaps more extended, burst associated with the formation of the Galactic bulge. A few metal-rich clusters may also be associated with the formation of the Galactic thick disk. Finally, many of the clusters beyond RGc 15 kpc might have been formed in dwarf spheroidal galaxies that, after a few Gyr, were accreted by the main body of the Galactic halo. Some of these outer clusters are younger, and less luminous, than the majority of globular clusters formed at smaller Galactocentric distances.

Space Telescope Imaging Spectrograph Echelle Observations of NGC 4151: Variable Ionization of the Intrinsic UV Absorbers

Abstract: We present echelle observations of the intrinsic UV absorption lines in the Seyfert galaxy NGC 4151; these observations were obtained with the Space Telescope Imaging Spectrograph aboard the Hubble Space Telescope on 1999 July 19. The UV-continuum flux at 1450 A decreased by factor of about 4 over the previous 2 years, and there was a corresponding dramatic increase in the column densities of the low-ionization absorption lines (e.g., Si II, Fe II, and Al II), presumably as a result of a decrease in the ionizing continuum. In addition to the absorption lines seen in previous low states, we identify a large number of Fe II absorption lines that arise from metastable levels as high as 4.1 eV above the ground state, indicating high densities (>106 cm-3). We find that the transient absorption feature in the blue wing of the broad C IV emission, seen in a Goddard High Resolution Spectrograph spectrum and thought to be a high-velocity C IV component, is actually a Si II fine-structure absorption line at a radial velocity of -560 km s-1 (relative to systemic). We also demonstrate that the "satellite" emission lines of C IV found in IUE spectra are actually regions of unabsorbed continuum plus broad emission that become prominent when the UV continuum of NGC 4151 is in a low state.

The Ionization Source in the Nucleus of M84

Abstract: We have obtained new Hubble Space Telescope (HST) observations of M84, a nearby massive elliptical galaxy whose nucleus contains a approximately 1.5 X 10(exp 9) solar mass dark compact object, which presumably is a supermassive black hole. Our Space Telescope Imaging Spectrograph (STIS) spectrum provides the first clear detection of emission lines in the blue (e.g., [0 II] lambda 3727, HBeta and [0 III] lambda lambda4959,5007), which arise from a compact region approximately 0".28 across centered on the nucleus. Our Near Infrared Camera and MultiObject Spectrometer (NICMOS) images exhibit the best view through the prominent dust lanes evident at optical wavelengths and provide a more accurate correction for the internal extinction. The relative fluxes of the emission lines we have detected in the blue together with those detected in the wavelength range 6295 - 6867 A by Bower et al. indicate that the gas at the nucleus is photoionized by a nonstellar process, instead of hot stars. Stellar absorption features from cool stars at the nucleus are very weak. We update the spectral energy distribution of the nuclear point source and find that although it is roughly flat in most bands, the optical to UV continuum is very red, similar to the spectral energy distribution of BL Lac. Thus, the nuclear point source seen in high-resolution optical images is not a star cluster but is instead a nonstellar source. Assuming isotropic emission from this source, we estimate that the ratio of bolometric luminosity to Eddington luminosity is about 5 x 10(exp -7). However, this could be underestimated if this source is a misaligned BL Lac object, which is a possibility suggested by the spectral energy distribution and the evidence of optical variability we describe.

Abundances and Kinematics of Field Halo and Disk Stars I: Observational Data and Abundance Analysis

Abstract: We describe observations and abundance analysis of a high-resolution, high-S/N survey of 168 stars, most of which are metal-poor dwarfs. We follow a self-consistent LTE analysis technique to determine the stellar parameters and abundances, and estimate the effects of random and systematic uncertainties on the resulting abundances. Element-to-iron ratios are derived for key alpha, odd, Fe-peak, r- and s-process elements. Effects of Non-LTE on the analysis of Fe I lines are shown to be very small on the average. Spectroscopically determined surface gravities are derived that are generally close to those obtained from Hipparcos parallaxes.

High-efficiency sol-gel antireflection coatings for astronomical optics

Abstract: The antireflective properties of silica sol-gel coatings have been known for some time, and such coatings have bene used to reduce losses in the optical elements of high- powered lasers used in fusion experiments. Research at DAO has developed the technology to the pont where optical elements in three 4-meter class telescopes at CFHT, KPNO, and CTIO have benefitted from coatings made form high- efficiency sol-gel films. The process is attractive because it is simple to apply, is inexpensive, and is as effective as multi-layer vacuum-deposited coatings which it can replace. A description is given of the basic chemistry involved and the techniques used to apply the coatings. Techniques used to improve the durability of the films by hardening and waterproofing are described, as well as a two- layer coating in combination with magnesium fluoride.

Markarian 421'S Unusual Satellite Galaxy

Abstract: We present Hubble Space Telescope (HST) imagery, and ground-based spectroscopy and CCD photometry of the active galaxy Markarian 421 and its companion galaxy 14 arcsec to the ENE.

Analyses of the Currently Noneclipsing Binary SS Lacertae or SS Lacertae’s Eclipses*

Abstract: Confirmatory evidence for changing light-curve amplitude of the former eclipsing and current SB2 system SS Lac in the Open Cluster NGC 7209 has been uncovered. Remeasured Harvard plate data and published and compiled data sets reveal that the depth of the primary minimum increased between the 1890s and 1902 and decreased in the 1920s and 1930s. A parabolic fitting of the amplitude with phase predicts a maximum at 1911.5, with an eclipse onset at 1885.3 and eclipse cessation at 1937.8. We confirm the finding of Lehmann, that the system's inclination varies with time and that a central eclipse occurred ~1912, and we concur with Mossakovskaya that eclipses effectively ceased ~1940. Estimates of SS Lac on plates taken at Tashkent between 1937 and 1940 further serve to confirm the result. Thus, SS Lac belongs to a small but elite class of triple systems in which changes due to dynamical effects can be seen over a single human lifetime. In order to explore the properties of the SS Lac system, recent radial velocity curves and archival photographic and visual light curves have been analyzed with versions of the Wilson-Devinney code, augmented with a simplex routine to test solution uniqueness. The modeling solutions for the Dugan-Wright light curves ostensibly indicate that the former eclipsing system is composed of two early A stars of only slightly differing masses (2.57 ± 0.16 and 2.59 ± 0.19 M⊙) and effective surface temperatures (8750 ± 300 [assumed for component 1] and 8542 ± 309 K), but significantly different radii (2.38 ± 0.02 and 3.63 ± 0.07 R⊙) and luminosities (30 ± 4 and 63 ± 9 L⊙) for the hotter and cooler components, respectively. The light-curve solutions are compromised somewhat by variable eclipse depths over the ranges of dates of the data sets. This is especially true of the most complete light curve, that of Dugan & Wright; the others also suffer from incompleteness (that of Wachmann) and high scatter (that of Kordylewski, Pagaczewski, & Szafraniec). As a consequence, small, temporal variations in such system properties as the eccentricity, argument of periastron, modified Roche potentials, luminosities, and third light level, cannot be ruled out from currently available data. However, solutions with WD95, a self-iterating, damped–least squares version of the Wilson-Devinney program, reveal optimized inclinations for the data sets that project an inclination variation of 016 yr-1, but no evidence of apsidal motion. We find a distance for the system of 898 ± 95 pc, consistent with the value of Vansevicius et al. of 1040 ± 10 pc, and finally, on the bases of location on the sky, proper motion, radial velocity, photometry, and properties deduced in the present study, we confirm its membership in the cluster NGC 7209.

Spectroscopic Gravitational Lens Candidates in the CNOC2 Field Galaxy Redshift Survey

Abstract: We present five candidate gravitational lenses discovered spectroscopically in the Canadian Network for Observational Cosmology Field Galaxy Redshift Survey (CNOC2), along with one found in followup observations. Each has a secure redshift based on several features, plus a discrepant emission line which does not match any known or plausible feature and is visible in multiple direct spectral images. We identify these lines as Lyman-alpha or [OII] emission from galaxies lensed by, or projected onto, the CNOC2 target galaxies. Einstein radii estimated from the candidate deflector galaxy luminosities indicate that for two candidates the lines are probably [OII] from projected z 3 galaxies. We estimate that only 1.9+-0.7 [OII]-emitting galaxies are expected to project onto target galaxies in the original CNOC2 sample, consistent with three or four of the six candidates being true gravitational lenses.

The space density of low-redshift active galactic nuclei

Abstract: We present a new determination of the optical luminosity function (OLF) of active galactic nuclei (AGN) at low redshifts (z −20. Although the estimate of the OLF is best fitted by a power-law slope at MB 0.35, the binned estimate of the low-redshift OLF is still consistent with an extrapolation of the z>0.35 OLF based on pure luminosity evolution.

Spectroscopic Gravitational Lens Candidates in the CNOC2 Field Galaxy Redshift Survey

Abstract: We present five candidate gravitational lenses discovered spectroscopically in the Canadian Network for Observational Cosmology Field Galaxy Redshift Survey (CNOC2), along with one found in follow-up observations. Each has a secure redshift based on several features, plus a discrepant emission line that does not match any known or plausible feature and is visible in multiple direct spectral images. We identify these lines as Lyα λ1216 or [O II] λ3727 emission from galaxies lensed by or projected onto the CNOC2 target galaxies. Einstein radii estimated from the candidate deflector galaxy luminosities indicate that for two candidates the lines are probably [O II] from projected z < 1 galaxies (consistent with the detection of Hβ as well as [O II] in one of them), but that in the remaining four cases the lines could be Lyα from lensed z < 3 galaxies. We estimate that only 1.9 ± 0.7 [O II]–emitting galaxies are expected to project onto target galaxies in the original CNOC2 sample, consistent with three or four of the six candidates being true gravitational lenses.

FUSE: fine error sensor optical performance

Abstract: The Far Ultraviolet Spectroscopic Explorer mission imposes stringent requirements on the satellite attitude control system. Target acquisition accuracy and target tracking stability must each be no greater than 0.5 arcseconds FWHM. The data required by the attitude control system to meet these requirements are provided by two redundant Fine Error Sensors. Each Fine Error Sensor operates as a slit-jaw camera that provides either complete images of the star-field around the line of sight of the telescope, or centroided positions of selected guide stars in the field of view. The satellite pointing requirements must be met over a wide dynamic range of target or guide star brightness, for both sparse and crowded starfields, and for targets that may be either point sources or extended objects. We will describe the operational characteristics of the FES and present data on its performance. We also discuss the optical, mechanical, thermal, and electronic design challenges encountered in meeting the mission requirements, and how they were addressed in the context of a very tight development schedule.

Kinematics and Morphology of the Resolved Narrow Line Region in NGC 4151

Abstract: A complement of HST narrow band images, STIS slitless and long-slit spectra have been employed to probe the morphology, kinematics, dynamics, and ionization structure of the near-nuclear region in NGC 4151. The images illustrate the inner biconical cloud distribution which exhibits recessional velocities relative to the nucleus to the NE and approaching velocities to the SW of the nucleus. The STIS spectra of the NLR map the velocity field from 1.2 kpc to within 13 pc (Ho = 75 km s-1 Mpc-1) of the nucleus. We find evidence for at least two kinematic cloud populations. One is characterized by Low Velocities and Low Velocity Dispersions (LVLVD clouds: |v| < 400km s-1,and Δ v <130 km s-1). These clouds extend through the NLR and may be gravitationally associated with the host galaxy. Another component is characterized by High Velocities and High Velocity Dispersions (HVHVD clouds: 400 < |v| ≲1700 km s-1, Δ v ≥ 130 km s-1). This set of clouds is located within 1.1”(~70 pc) of the nucleus and has radial velocities which are too high to be gravitational in origin, show no strong correlation with the knots in the radio jet, but are consistent with wind driven outflow.

A Kinematic Model for the Narrow-Line Region in NGC 4151

Abstract: We present a simple kinematic model for the narrow-line region (NLR) of the Seyfert 1 galaxy NGC 4151, based on our previous observations of extended [O III] emission with the Space Telescope Imaging Spectrograph (STIS). The model is similar to a biconical radial outflow model developed for the Seyfert 2 galaxy NGC 1068, except that the bicone axis is tilted much more into our line of sight (40 degrees out of the plane of the sky instead of 5 degrees), and the maximum space velocities are lower (750 km/s instead of 1300 km/s. We find evidence for radial acceleration of the emission-line knots to a distance of 160 pc, followed by deceleration that approaches the systemic velocity at a distance of 290 pc (for a distance to NGC 4151 of 13.3 Mpc). Other similarities to the kinematics of NGC 1068 are: 1) there are a number of high-velocity clouds that are not decelerated, suggesting that the medium responsible for the deceleration is patchy, and 2) the bicone in NGC 4151 is at least partially evacuated along its axis. Together, these two Seyfert galaxies provide strong evidence for radial outflow (e.g., due to radiation and/or wind pressure) and against gravitational motion or expansion away from the radio jets as the principal kinematic component in the NLR.

The Galaxies of the Local Group: The Triangulum galaxy (M33)

Abstract: Introduction The galaxy M33 (= NGC598) is the third-brightest member of the Local Group. It is a late-type spiral of type Sc II–III. The large angular size of the Triangulum galaxy, and its intermediate inclination i = 56° (Zaritsky, Elston & Hill 1989), make it particularly suitable for studies of spiral structure and stellar content (see Figure 5.1). Only M31, the Magellanic Clouds, and the tidally disrupted Sagittarius dwarf have larger angular diameters than the Triangulum galaxy. The spiral nature of this galaxy was first hinted at by visual observations made by the Earl of Rosse (1850). The modern era of exploration of M33 began with the independent discovery of variable stars in this object by Duncan (1922) and by Wolf (1923). Neither of these papers show any indication that these authors anticipated the revolutionary impact that the discovery of variable stars in nearby “nebulae” would soon have on Man's view of the Universe. In the words of Hubble (1926) “The nature of nebulae and their place in the scheme of the universe have been favorite subjects of controversy since the very dawn of telescopic observations.” Hubble writes that his investigation “followed naturally upon the partial resolution of Messier 33 into swarms of actual stars.” He concluded that “The data lead to a conception of the object as an isolated system of stars and nebulae, lying far outside the limits of the [G]alactic system.” In his paper Hubble was able to show that 35 of the variables in M33 were classical Cepheids, thus demonstrating, beyond reasonable doubt, that galaxies were “island universes, ” and ending the reat debate (Heatherington 1972, Hoskin 1976) on the nature of spiral nebulae.

Some Musings on Globular Cluster Systems

Abstract: Globular cluster systems exhibit a bewildering variety of characteristics. No single scenario appears to be able to account for the wide range of specific globular cluster frequencies that are observed in galaxies of various types. The fraction of all star formation that produces massive bound clusters differs from galaxy to galaxy, and (in some cases) appears to vary with time. Presently available data strongly suggest that the specific cluster forming frequency is highest during violent bursts of star formation. Globular cluster systems show a wide variety of color (metallicity) distributions, with many luminous galaxies having double (or even multiple) metallicity peaks that were probably produced during distinct episodes of cluster formation. The bulk of the globulars in the main body of the Galactic halo appear to have formed during a single short-lived burst that took place ~ 13 Gyr ago. This was followed by a second, perhaps more extended, burst associated with the formation of the Galactic bulge. A few metal-rich clusters may also be associated with the formation of the Galactic Thick Disk. Finally many of the clusters beyond RGc ? 15 kpc might have been formed in dwarf spheroidal galaxies that, after a few Gyr, were accreted by the main body of the Galactic halo. Some of these outer clusters are younger, and less luminous, than the majority of globular clusters formed at smaller Galactocentric distances.