Absence Of Emission Lines Research Articles

XMM–NuSTAR Observation and Multiwavelength Spectral Energy Distribution Modeling of Blazar 4FGL J1520.8–0348

Active galactic nuclei (AGNs) can power relativistic jets, which are called blazars when pointed close to our line of sight. Depending on the presence or absence of emission lines in their optical spectra, blazars are categorized into flat spectrum radio quasars (FSRQs) or BL Lacertae (BL Lac) objects. According to the “blazar sequence,” as synchrotron peak frequency ( νpksy ) shifts to higher energies, the synchrotron peak luminosity decreases. This means that BL Lac objects as luminous as FSRQs, and with synchrotron peak frequencies νpksy>1015 Hz, should not exist. Detected as a high-synchrotron peak (HSP; νpksy>1015 Hz) BL Lac object, 4FGL J1520.8-0348 shows high γ-ray luminosity (L γ > 1046 erg s−1), being at a high redshift of z = 1.46. Since it is an outlier in the “blazar sequence,” the process of its jet acceleration and power may be different from bona fide BL Lac objects. In this work, we constrain its spectral energy distribution (SED) by modeling the multiwavelength data from infrared to γ-ray regime. Simultaneous X-ray data were obtained from X-ray Multi-Mirror Mission and Nuclear Spectroscopic Telescope Array to constrain the synchrotron emission and underlying electron distribution. On undertaking the SED modeling of the source, including the effect of extragalactic background light, we conclude that the source is more likely to be a “blue FSRQ” or “masquerading BL Lac” where the BL Lac object is actually an FSRQ in disguise.

Spectroscopic Confirmation of CEERS NIRCam-selected Galaxies at z ≃ 8–10

We present JWST/NIRSpec prism spectroscopy of seven galaxies selected from Cosmic Evolution Early Release Science (CEERS) survey NIRCam imaging with photometric redshifts z phot > 8. We measure emission line redshifts of z = 7.65 and 8.64 for two galaxies. For two other sources without securely detected emission lines we measure and by fitting model spectral templates to the prism data, from which we detect continuum breaks consistent with Lyα opacity from a mostly neutral intergalactic medium. The presence of strong breaks and the absence of strong emission lines give high confidence that these two galaxies have redshifts z > 9.6, but the redshift values derived from the breaks alone have large uncertainties given the low spectral resolution and relatively low S/N of the CEERS NIRSpec prism data. The two z ∼ 10 sources observed are relatively luminous (M UV < −20), with blue continua (−2.3 ≲ β ≲ −1.9) and low dust attenuation (); and at least one of them has a high stellar mass for a galaxy at that redshift (). Considered together with spectroscopic observations of other CEERS NIRCam-selected high-z galaxy candidates in the literature, we find a high rate of redshift confirmation and low rate of confirmed interlopers (8%). Ten out of 35 z > 8 candidates with CEERS NIRSpec spectroscopy do not have secure redshifts, but the absence of emission lines in their spectra is consistent with redshifts z > 9.6. We find that z > 8 photometric redshifts are generally in agreement (within their uncertainties) with the spectroscopic values, but also that the photometric redshifts tend to be slightly overestimated (〈Δz〉 = 0.45 ± 0.11), suggesting that current templates do not fully describe the spectra of very-high-z sources. Overall, the spectroscopy solidifies photometric redshift evidence for a high spatial density of bright galaxies at z > 8 compared to theoretical model predictions, and further disfavors an accelerated decline in the integrated UV luminosity density at z > 8.

AGN-driven Cold Gas Outflow of NGC 1068 Characterized by Dissociation-sensitive Molecules

Recent developments in (sub)millimeter facilities have drastically changed the amount of information obtained from extragalactic spectral scans. In this paper, we present a feature extraction technique using principal component analysis (PCA) applied to arcsecond-resolution (1.″0–2.″0 = 72–144 pc) spectral scan data sets for the nearby type-2 Seyfert galaxy NGC 1068, using Band 3 of the Atacama Large Millimeter/submillimeter Array. We apply PCA to 16 well-detected molecular line intensity maps convolved to a common 150 pc resolution. In addition, we include the [S iii]/[S ii] line ratio and [C i] 3 P 1–3 P 0 maps in the literature, both of whose distributions show a remarkable resemblance to that of a kiloparsec-scale biconical outflow from the central active galactic nucleus. We identify two prominent features: (1) central concentration at the circumnuclear disk (CND) and (2) two peaks across the center that coincide with the biconical outflow peaks. The concentrated molecular lines in the CND are mostly high-dipole molecules (e.g., H13CN, HC3N, and HCN). Line emissions from molecules known to be enhanced in an irradiated interstellar medium, CN, C2H, and HNC, show similar concentrations and extended components along the bicone, suggesting that molecule dissociation is a dominant chemical effect of the cold molecular outflow of this galaxy. Although further investigation should be made, this scenario is consistent with the faintness or absence of emission lines from CO isotopologues, CH3OH, and N2H+ in the outflow, which are easily destroyed by dissociating photons and electrons.

Correlation between plasma parameters and structure of thin TiO2 films deposited by conventional and pulsed magnetron sputtering methods

The aim of the work was to diagnose plasmas excited by two magnetron sputtering techniques: conventional reactive magnetron sputtering (RMS) and magnetron sputtering realized under gas impulse conditions (GIMS) with use of light emission spectroscopy and Langmuir probing. The plasmas were studied during deposition of TiO2 coatings by magnetron sputtering of Ti target in O2 atmosphere. The obtained deposits were investigated with use of infrared spectroscopy (FTIR) and X-ray diffraction (GIXRD).Two major conclusions were drawn from the investigations. One concerned the mechanism of TiO2 formation during reactive sputtering. An absence of emission lines corresponding to plain Ti atoms proves that their oxidation takes place at the surface of the Ti target without any gas phase reactions. The second conclusion concerned deposition thermodynamics. Although the diagnostics of the GIMS plasma revealed a greater plasma density and electron temperature than in the RMS one, it was the latter technique that produced more high temperature polymorph of TiO2. It appears that the pulse technique offers a way to control thermodynamic conditions of deposition in a broad range up to a conversion to the RMS mode of deposition.

The optoelectronic properties of Eu/F-codoped tin oxide, an experimental and DFT study

Tin oxide (TO), F-doped SnO2 (FTO), and Eu/F-codoped SnO2 (EFTO) films were deposited by spray pyrolysis. Additionally, for the DFT calculations, different superstructures were considered as models for TO (SnO2), FTO (Sn36O71F), and EFTO (EuSn35O70F2). Electrical and optical measurements showed that the EFTO films maintain the characteristics of a transparent conducting oxide (TCOs), i.e., high optical transparency and low electrical resistivity (ρ). The plasma wavelength for EFTO thin films was red-shifted when compared to FTO, which is related to the diminishing of free electron density (ne) and the increase of ρ; such effects were promoted by the incorporation of Eu impurities which act as electron acceptors. Photoluminescence (PL) studies revealed that the EFTO films did not show electronic transitions related to Eu states in the visible part of the spectra (vis). On the other hand, the DFT calculations confirmed the experimental results related to the incorporation of Eu/F impurities in TO, such as cell expansion, the creation of direct bandgap TCOs, the appearance of the Burstein-Moss effect, the absence of emission lines in the vis, and the diminishing of ne due to the presence of Eu acceptor states.

Resolving Quiescent Galaxies at z ≳ 2. I. Search for Gravitationally Lensed Sources and Characterization of Their Structure, Stellar Populations, and Line Emission

Abstract Quiescent galaxies at z ≳ 2 are compact and have weak or absent emission lines, making it difficult to spatially resolve their kinematics and stellar populations using ground-based spectroscopy. Gravitationally lensed examples provide a promising route forward, but such objects are very rare. We describe a search in the fields of 232 galaxy clusters that has uncovered five bright (H AB &lt; 20) lensed galaxies with red near-infrared colors. These include MRG-M0138, which is the brightest lensed galaxy known in the near-infrared. Analysis of near-infrared spectra and multiband photometry confirms that all are quiescent galaxies at z = 1.95–2.64 with stellar ages of 0.5–1.4 Gyr (corresponding to formation epochs z form ≃ 3–4) and stellar masses of , where μ is the magnification. In three cases, we derive lens models and reconstruct the source structure; these galaxies are massive ( ) and follow the mass–size relation defined by unlensed samples. In two of these three galaxies, the main structural component is an inclined disk. Weak emission lines are detected in four of five galaxies with high ratios [N ii]/Hα ≃ 2–6 that are inconsistent with a star formation origin. Based on the line ratios, the Hα equivalent widths, and the distribution and kinematics of the gas, we infer that shocks are likely to be present in at least two galaxies and could be present in all of the line emitters. We speculate that these could be analogs of local galaxies in which AGN jet-driven outflows are thought to heat the interstellar medium and suppress star formation. In further papers, we will present spatially resolved measurements of the stellar populations and kinematics of this unique sample.

Observations of the planetary nebula RWT 152 with OSIRIS/GTC

RWT 152 is one of the few known planetary nebulae with an sdO central star. We present subarcsecond red tunable filter Halpha imaging and intermediate-resolution, long-slit spectroscopy of RWT 152 obtained with OSIRIS/GTC with the goal of analyzing its properties. The Halpha image reveals a bipolar nebula with a bright equatorial region and multiple bubbles in the main lobes. A faint circular halo surrounds the main nebula. The nebular spectra reveal a very low-excitation nebula with weak emission lines from H^+, He^+, and double-ionized metals, and absence of emission lines from neutral and single-ionized metals, except for an extremely faint [NII]6584 emission line. These spectra may be explained if RWT 152 is a density-bounded planetary nebula. Low nebular chemical abundances of S, O, Ar, N, and Ne are obtained in RWT 152, which, together with the derived high peculiar velocity ($\sim$ 92-131 km s$^{-1}$), indicate that this object is a halo planetary nebula. The available data are consistent with RWT 152 evolving from a low-mass progenitor ($\sim$ 1 M$_{\odot}$) formed in a metal-poor environment.

A CONSTANT LIMITING MASS SCALE FOR FLAT EARLY-TYPE GALAXIES FROMz∼ 1 TOz= 0: DENSITY EVOLVES BUT SHAPES DO NOT

We measure the evolution in the intrinsic shape distribution of early-type galaxies from z~1 to z~0 by analyzing their projected axis-ratio distributions. We extract a low-redshift sample (0.04 < z < 0.08) of early-type galaxies with very low star-formation rates from the SDSS, based on a color-color selection scheme and verified through the absence of emission lines in the spectra. The inferred intrinsic shape distribution of these early-type galaxies is strongly mass dependent: the typical short-to-long intrinsic axis-ratio of high-mass early-type galaxies (>1e11 M_sun) is 2:3, where as at masses below 1e11 M_sun this ratio narrows to 1:3, or more flattened galaxies. In an entirely analogous manner we select a high-redshift sample (0.6 < z < 0.8) from two deep-field surveys: GEMS and COSMOS. We find a seemingly universal mass of ~1e11 M_sun for highly flatted early-type systems at all redshifts. This implies that the process that grows an early-type galaxy above this ceiling mass involves forming round systems. Using both parametric and non-parametric tests, we find no evolution in the projected axis-ratio distribution for galaxies with masses >3e10 M_sun with redshift. At the same time, our samples imply an increase of 2-3x in co-moving number density for early-type galaxies at masses >3e10 M_sun, in agreement with previous studies. Given the direct connection between the axis-ratio distribution and the underlying bulge-to-disk ratio distribution, our findings imply that the number density evolution of early-type galaxies is not exclusively driven by the emergence of either bulge- or disk-dominated galaxies, but rather by a balanced mix that depends only on the stellar mass of the galaxy. The challenge for galaxy formation models is to reproduce this overall non-evolving ratio of flattened to round early-type galaxies in the context of a continually growing population.

FR II radio galaxies in the Sloan Digital Sky Survey: observational facts

Starting from the Cambridge Catalogues of radio sources, we have created a sample of 401 FRII radio sources that have counterparts in the main galaxy sample of the 7th Data release of the SDSS and analyse their radio and optical properties. We find that the luminosity in the Halpha line - which we argue gives a better measure of the total emission-line flux than the widely used O[III] luminosity - is strongly correlated with the radio luminosity P1.4GHz. We show that the absence of emission lines in about one third of our sample is likely due to a detection threshold and not to a lack of optical activity. We find that the properties of FRII galaxies are mainly driven by the Eddington parameter LHa/"MBH" or, equivalently, P1.4GHz/"MBH". Radio galaxies with hot spots are found among the ones with the highest values of P1.4GHz/"MBH". Compared to classical AGN hosts in the main galaxy sample of the SDSS, our FRII galaxies show a larger proportion of objects with very hard ionizing radiation field and large ionization parameter. A few objects are, on the contrary, ionized by a softer radiation field. We find that the black hole masses and stellar masses in FRII galaxies are very closely related. A comparison sample of line-less galaxies in the SDSS follows the same relation, although on average the masses are smaller. This suggests that the FRII radio phenomenon occurs in normal elliptical galaxies, preferentially in the most massive ones. Although most FRII galaxies are old, some contain traces of young stellar populations. Such young populations are not seen in normal line-less galaxies, suggesting that the activity in some FRII galaxies may be triggered by recent star formation. The "MBH"-Mgal relation in a comparison sample of radio-quiet AGNs from the SDSS is very different, suggesting that galaxies which are still forming stars are also still building their central black holes.

The orbital velocity anisotropy of cluster galaxies: evolution

<i>Context. <i/>In nearby clusters early-type galaxies follow isotropic orbits. The orbits of late-type galaxies are instead characterized by slightly radial anisotropy. Little is known about the orbits of the different populations of cluster galaxies at redshift .<i>Aims. <i/>We investigate the redshift evolution of the orbits of cluster galaxies.<i>Methods. <i/>We use two samples of galaxy clusters spanning similar (evolutionary corrected) mass ranges at different redshifts. The sample of low-redshift (<i>z<i/> ~ 0.0-0.1) clusters is extracted from the ESO Nearby Abell Cluster Survey (ENACS) catalog. The sample of high-redshift (<i>z<i/> ~ 0.4-0.8) clusters is mostly made of clusters from the ESO Distant Cluster Survey (EDisCS). For each of these samples, we solve the Jeans equation for hydrostatic equilibrium separately for two cluster galaxy populations, characterized by the presence and, respectively, absence of emission-lines in their spectra (“ELGs” and “nELGs” hereafter). Using two tracers of the gravitational potential allows to partially break the mass-anisotropy degeneracy which plagues these kinds of analyses.<i>Results. <i/>We confirm earlier results for the nearby cluster sample. The mass profile is well fitted by a Navarro, Frenk & White (NFW) profile with concentration . The mass profile of the distant cluster sample is also well fitted by a NFW profile, but with a slightly lower concentration, as predicted by cosmological simulations of cluster-sized halos. While the mass density profile becomes less concentrated with redshift, the number density profile of nELGs becomes more concentrated with redshift. In nearby clusters, the velocity anisotropy profile of nELGs is close to isotropic, while that of ELGs is increasingly radial with clustercentric radius. In distant clusters the projected phase-space distributions of both nELGs and ELGs are best-fitted by models with radial velocity anisotropy.<i>Conclusions. <i/>No significant evolution is detected for the orbits of ELGs, while the orbits of nELGs evolve from radial to isotropic with time. We speculate that this evolution may be driven by the secular mass growth of galaxy clusters during their fast accretion phase. Cluster mass density profiles and their evolution with redshift are consistent with predictions for cluster-sized halos in <i>Λ<i/> Cold Dark Matter cosmological simulations. The evolution of the nELG number density profile is opposite to that of the mass density profile, becoming less concentrated with time, probably a result of the transformation of ELGs into nELGs.

L-Band Spectra of 13 Outbursting Be Stars

ABSTRACTWe present new L-band spectra of 13 outbursting Be stars obtained with ISAAC at the ESO Paranal Observatory. These stars can be classified in three groups depending on the presence or absence of emission lines and the strength of Brα and Pfγ emission lines relative to those of Humphreys lines from transitions 6–14 to the end of the series. These groups are representative of circumstellar envelopes with different optical depths. For the group showing Brα and Pfγ lines stronger than Humphreys lines, the Humphreys decrement roughly follows the Menzel case-B for optically thin conditions. For the group showing comparable Brα, Pfγ, and Humphreys emission-line strengths, the Humphreys decrements moves from an optically thin to an optically thick regime at a transition wavelength that is characteristic for each star but typically is located around 3.65–3.75 μm (transitions 6–19 and 6–17). Higher-order Humphreys lines probe optically thin inner regions even in the optically thicker envelopes. We find evidence of larger broadening in the infrared emission lines compared with optical lines, probably reflecting larger vertical velocity fields near the star. The existence of the aforementioned groups is in principle consistent with the description recently proposed by de Wit et al. for Be star outbursts in terms of the ejection of an optically thick disk that expands and becomes optically thin before dissipation into the interstellar medium. Time-resolved L-band spectroscopy sampling the outburst cycle promises to be an unique tool for testing Be star disk evolution.

A population of high-redshift type 2 quasars – II. Radio properties

Abstract We present multi-frequency radio observations of a sample of z∼ 2 obscured (type 2) quasars in the Spitzer extragalactic First Look Survey area. We combine the public data at 1.4 GHz, used in the selection of these sources, with new observations at 610 MHz (Giant Meterwave Radio Telescope) and at 4.9 GHz (Very Large Array). We find that the sample includes sources with steep, flat and gigahertz-peaked spectra. There are no strong correlations between the presence or absence of emission lines in the optical spectra and the radio spectral properties of the sample. However, there are no secure flat-spectrum type 2 quasars with narrow emission lines which would be problematic for unified schemes. Most of the population have straight radio spectra with spectral index α∼ 1 as is expected for developed, potentially Fanaroff–Riley type I-like jets in which continuous injection of relativistic electrons is accompanied by inverse Compton losses against the cosmic microwave background.

Analysis of [formula omitted] non-LTE emissions at [formula omitted] in the Martian atmosphere as observed by PFS/Mars Express and SWS/ISO

Daytime nadir spectra of the Martian atmosphere taken by the planetary Fourier spectrometer/Mars Express (PFS) and by the infrared space observatory (ISO) in the 4.3 μ m region are analyzed using a theoretical non-LTE model of the CO 2 infrared emissions in that atmosphere. Averaged spectra from these instruments show a double peak structure which can be explained by the strong solar pumping at 2.7 μ m of the ( 1 0 0 1 ) and ( 0 2 0 1 ) vibrational states of the major CO 2 isotope. After the initial absorption, these CO 2 states relax radiatively by emission in the 4.3 μ m second hot bands, whose R- and P-branches produce such structure. It is shown that an LTE atmosphere could not produce such structure. A good fit to the measured spectra is obtained, particularly with PFS, using the currently assumed non-LTE parameters without special adjustments. Analysis of individual bands contributions, their dependence with the thermal structure and solar zenith angle, and the simulation of the altitude variation of the different non-LTE features are presented and discussed. These results confirm the prediction of the non-LTE model and have allowed to identify the main spectral features observed in PFS and ISO data. The dominant double maxima are due to the second hot bands while the individual lines come from the fundamental band. Also the Q-branch of the first hot band and the absence of emission lines in the center of the fundamental band in the PFS spectrum are well reproduced by the simulations.

The 2QZ BL Lac survey

We have optically identified a sample of 56 featureless continuum objects without significant proper motion from the 2dF quasi-stellar object (QSO) redshift survey (2QZ). The steep number-magnitude relation of the sample, n(bj) oc 10 0 . 7 b J, is similar to that derived for QSOs in the 2QZ and inconsistent with any population of Galactic objects. Follow-up high-resolution, high signal-to-noise spectroscopy of five randomly selected objects confirms the featureless nature of these sources. Assuming the objects in the sample to be largely featureless active galactic nuclei (AGN), and using the QSO evolution model derived for the 2QZ, we predict the median redshift of the sample to be z = 1.1. This model also reproduces the observed number-magnitude relation of the sample using a renormalization of the QSO luminosity function, Φ* = Φ* Q S O / 6 6 ≃ 1.65 x 10 - 8 mag - 1 Mpc - 3 . Only ∼20 per cent of the objects have a radio flux density of S 1 . 4 > 3 mJy, and further Very Large Array observations at 8.4 GHz place a 5σ limit of S 8 . 4 < 0.2 mJy on the bulk of the sample. We postulate that these objects could form a population of radio-weak AGN with weak or absent emission lines, whose optical spectra are indistinguishable from those of BL Lac objects.

Optical Imaging of the Central Kiloparsecs of a Sample of Low Luminosity AGN

Selected with the primary objective of studying the differences between BL Lacs and their closest relatives, a sample of 55 optically bright, low luminosity core dominated radio sources has been studied. The spectroscopy carried out for this sample, however, yielded not only BL Lacs but also a completely unexpected number of sources with strong emission lines (much like those found in Sy galaxies), and almost 50% of galaxies with weak (EW ≤ 30Å), or absent emission lines. Such optical diversity is at odds with the traditional unification schemes for low luminosity AGN and it has prompted an optical imaging campaign. High resolution optical images obtained at NOT are discussed, in particular the central colours and profiles of the different types of sources found in the sample.

Optical Spectroscopy of the Unusual Galaxy J2310-43

We present and discuss new spectroscopic observations of the unusual galaxy J2310-43. The observations cover a wide wavelength range, from 3700 A to 9800 A allowing the study of both the regions where H alpha and the Ca II ``contrast'' are expected. No evidence for H alpha in emission is found and we thus confirm the absence of emission lines in the spectrum of J2310-43, ruling out the possibility that it may host a Seyfert nucleus. The CaII break is clearly detected and the value of the contrast (38 +/-4 %) is intermediate between that of a typical elliptical galaxy (about 50 %) and that of a BL Lac object (<25 %). This result imposes limits on the intensity of a possible non-stellar continuum and, in the light of the radio and X-ray loudness of the source, draws further attention to the problem of the recognition of a BL Lac object. Objects like J2310-43 may be more common than previously recognized, and begin to emerge in surveys of radio-emitting X-ray sources.

The Unusual Galaxy J2310−43: An Active Nucleus without Optical Emission Lines and without a Substantial Optical Continuum

X-ray, optical, and radio observations show the galaxy J2310-43 (Einstein Observatory Source Catalog number 4689, z = 0.0886) to be a very unusual object. Our analysis of ROSAT PSPC data for 2310-43 (23h10m417, -43°47'38'', J2000) indicates that the X-ray emission is extended spatially out to ~35. Spectral analysis of the PSPC data finds that a simple power-law model with line-of-sight Galactic absorption can fit the observed PSPC energy distribution, as can two-component Raymond-Smith models, and composite power-law and Raymond-Smith models. Analysis of CCD optical images indicates that 2310-43 is located within a cluster that is probably Abell richness class 0, and a search of the NASA Extragalactic Database finds a 5 GHz radio source of 62 mJy associated with 2310-43. While the data support a variety of interpretations, we note that 2310-43 differs significantly from clusters and groups of galaxies as well as from normal elliptical galaxies. We can interpret ~20% of the observed X-ray emission as an extended component emitting ~3.4 × 1043 ergs s-1 (0.1-2.4 keV), associated with the cluster around 2310-43. The bulk of the X-ray emission, corresponding to ~1.45 × 1044 ergs s-1, can then be associated with a nuclear point source consistent with the PSPC spatial and spectral data. However, the (B - V) color of 2310-43 is typical of a normal elliptical galaxy, making a BL Lac interpretation questionable, and the absence of emission lines in the optical spectrum rules out a standard quasar/Seyfert identification for the source. Our data show that 2310-43 is most similar to the dull, X-ray, and radio-loud galaxy 3C 264 (first characterized as optically dull by Elvis et al.). The location of these two objects in the αro - αox, color-color diagram is at the extreme for BL Lac objects, implying a substantial deficiency in the optical continuum emission. We assume that the nuclear emissions from 2310-43 and 3C 264 are primarily nonthermal radiation, possibly associated with a nuclear jet. Then, we examine our observations in the context of several current models with the goal of understanding how a process (such as jet-disk symbiosis) can result in depressed optical continuum compared to most active galactic nuclei (AGNs) and BL Lac objects. We also discuss future observations (some already underway) that can shed additional light on these unusual sources.

The X-Ray Spectrum of a Metal-poor Corona

We present an ASCA SIS spectrum of the Population II binary star HD 89499. This is the most metal poor star ([Fe/H] = -2.1) known to have a corona. The ASCA data confirm initial results from ROSAT which suggested a very hot, single-component plasma temperature, with the temperature derived from the ASCA data being slightly higher (2.6 × 107 K) than the one determined by ROSAT. The ASCA spectrum clearly shows the absence of emission lines and can be best modeled as thermal bremsstrahlung, although strong absorption at photon energies less than 0.5 keV is indicated. We suggest that such an extremely high coronal temperature results from the lack of an efficient cooling mechanism due to the absence of metallic (mostly Fe) emission lines.

Can BL Lac Nuclei and Broad Line Regions Coexist? or When is an Object a BL Lac?

The problem of BL Lac classification is a long standing one and it is mainly due to the subjectiveness of selection criteria used in the definition of BL Lac samples. For instance, an object will undoubtedly be classified as a BL Lac if it shows flat radio spectrum, high optical and radio polarization, featureless optical continuum with weak or absent emission lines, and variable flux and polarization. However, the problem arises when the object shows some but not all of these properties. In face of this difficulty, different authors (Stickel et al. 1991, Stocke et al. 1991) have tried to make a systematic analysis of the data and it has been common to classify as BL Lacs those objects whose strongest emission lines have equivalent width (EW) ≤ 5 Å. Another common criterion is to require the 4000 Å break contrast to be ≤ 0.25. Nevertheless, both of these criteria are rather arbitrary and more directly related to practical observational considerations, than they are to any physical distinction between objects. What is proposed here is a slightly different approach; it is proposed that we take a step back from common classification and that instead of imposing strict selection criteria, we create a multi-observational parameter space to investigate any breaks in the distribution of observed properties that will help clarify the distinction between BL Lacs and other flat radio spectrum sources.

Arcsecond Scale Polarization of BL Lac Objects

BL Lac objects are an enigmatic class of active galactic nuclei. They are characterized by high luminosity, a flat radio spectrum that steepens at higher energies, relatively high optical and radio polarization, rapid variability and an optical continuum with weak or absent emission lines (see Urry and Padovani, 1995 for a recent review).These properties have been interpreted in terms of a relativistic jet closely aligned to the line of sight (Blandford and Rees, 1978, Ghisellini et al., 1993). This model, known as the beaming model, implies that there must be a so called “parent population” of radio sources intrinsically identical to BL Lac objects, but with the jets oriented at large angles to the line of sight. Browne (1983) was the first to propose the low luminosity FR I radio galaxies as the most likely candidates for the “parent population” of the core dominated BL Lac objects. An outcome of the beaming model is that all the properties not depending on orientation should be shared by the BL Lac objects and the FR I radio galaxies.