Archival Hubble Space Telescope Data Research Articles

Radio--optical alignments in a low radio luminosity sample

We present an optically-based study of the alignment between the radio axes and the optical major axes of eight z approximately 0.7 radio galaxies in a 7C sample. The radio galaxies in this sample are approximately 20-times less radio luminous than 3C galaxies at the same redshift, and are significantly less radio-luminous than any other well-defined samples studied to date. Using Nordic Optical Telescope images taken in good seeing conditions at rest-frame wavelengths just longward of the 4000A break, we find a statistically significant alignment effect in the 7C sample. Furthermore, in two cases where the aligned components are well separated from the host we have been able to confirm spectroscopically that they are indeed at the same redshift as the radio galaxy. However, a quantitative analysis of the alignment in this sample and in a corresponding 3C sample from HST (Hubble Space Telescope) archival data indicates that the percentage of aligned flux may be lower and of smaller spatial scale in the 7C sample. Our study suggests that alignments on the 50-kpc scale are probably closely related to the radio luminosity, whereas those on the 15 kpc scale are not. We discuss these results in the context of popular models for the alignment effect.

The Central Regions of M81

High–angular resolution near-infrared images obtained with the Canada-France-Hawaii Telescope adaptive optics system are combined with archival Hubble Space Telescope data to investigate the central regions of the nearby Sb galaxy M81 (NGC 3031). The spectral-energy distribution of the circumnuclear region, which extends out to 15 (∼24 pc if μ0 = 27.5) from the nucleus, can be modeled as a combination of an old metal-rich population and emission from hot dust. Thermal emission, which has been attributed to hot dust, has been detected near other active galactic nuclei, and simple models indicate that hot dust can account for ∼20% of the light in K within 05 of the M81 nucleus. An elongated structure with MV ∼ -7, which may be an area of active star formation, is detected 045 from the nucleus. At distances in excess of 15 from the nucleus, the J-K color of the M81 bulge is not significantly different from what is seen in M31. The HST data are also used to search for bright globular clusters within 2 kpc of the center of M81. The area within 0.26 kpc of the M81 nucleus is largely devoid of bright globular clusters, in agreement with what is seen in the central regions of the Galaxy and M31. However, our survey indicates that there may be ∼45 ± 12 globular cluster candidates with MV ≤ -7 within 2 kpc of the galaxy center, which is consistent with what would be infrared from the Milky Way cluster system after adjusting for differences in the total number of clusters.

Stellar Populations in the Dwarf Spheroidal Galaxy Leo I

We present a detailed study of the color-magnitude diagram (CMD) of the dwarf spheroidal galaxy Leo I, based on archival Hubble Space Telescope data. Our photometric analysis, confirming previous results on the brighter portion of the CMD, also allow us to obtain an accurate sampling of the stellar populations at the faint magnitudes corresponding to the main sequence. By adopting a homogeneous and consistent theoretical scenario for both hydrogen and central helium–burning evolutionary phases, the various features observed in the CMD are interpreted and reliable estimates for both the distance modulus and the age(s) of the main stellar components of Leo I are derived. In more detail, from the upper luminosity of the red giant branch and the lower luminosity of the subgiant branch we simultaneously constrain the galaxy distance and the age of the oldest stellar population in Leo I. In this way we obtain a distance modulus (m - M)V = 22.00 ± 0.15 mag and an age of 10–15 Gyr or 9–13 Gyr, adopting a metallicity of Z = 0.0001 or 0.0004, respectively. The reliability of this distance modulus has been tested by comparing the observed distribution of the Leo I anomalous Cepheids in the period-magnitude diagram with the predicted boundaries of the instability strip as given by convective pulsating models. A detailed investigation of the age(s) of the Leo I stellar populations is then performed by comparing the CMD with a suitable set of theoretical isochrones and central helium–burning models. By taking into account all the various features, including the lack of RR Lyrae variables, we conclude that the star formation process in Leo I started ∼10 Gyr (with Z = 0.0001) or ∼13 Gyr (with Z = 0.0004) ago, and stopped about 1 Gyr ago. Some evidence is reported supporting the mild metal deficiency (Z = 0.0004), whereas no clear indication has been found supporting a star formation history characterized by episodic bursts. The adoption of updated physics, including the inward diffusion of elements as recently presented for globular cluster stars, would yield a slightly larger distance modulus (∼0.10 mag) and a slightly lower age for the most ancient stellar component (∼1 Gyr).

AHubble Space TelescopeFaint Object Spectrograph Survey for Broad Absorption Lines in a Sample of Low‐Redshift Weak [Oiii] Quasi‐stellar Objects

The study by Boroson & Meyers led to the suggestion that radio-quiet QSOs with weak [O III] and strong Fe II emission spectra form a class of QSOs that has a high probability of exhibiting broad absorption lines (BALs) in their spectra. Furthermore, they argued that since narrow-line [O III] emission is almost certainly emitted isotropically, this indicates that such objects have relatively large BAL region covering factors. Low covering factor models are consistent with scenarios in which most QSOs have BAL regions, while higher covering factor models are consistent with scenarios in which there are special classes of QSOs with large BAL region outflows. By making Hubble Space Telescope (HST) FOS observations and using IUE or HST archival data when available, the details of the Boroson & Meyers suggestion have been explored by directly searching for classical C IV BALs in a sample of 18 QSOs with weak [O III] and often strong Fe II emission. Six of the 18 QSOs are found to exhibit C IV BALs. (In the archival sample, four of six objects have BALs, while two of the 12 new objects observed with the HST FOS have BALs.) However, there is evidence that the sample is heterogeneous, with IRAS-selected objects and high-luminosity objects having a greater tendency to exhibit BALs. If an isotropic model for [O III] emission equivalent width is considered, the results suggest that for the 18 object sample as a whole the average BAL region covering factor is ≈ 0.33 -->+ 0.20−0.09, which is significantly larger (with a more than 99% probability) than the overall fraction of QSOs observed to have BALs (normally taken as ≈ 0.1). Given possible selection effects, in the context of an isotropic model the results may indicate that some of the sample objects have covering factors 0.33, while others have covering factors 0.33. At the same time, it is impossible to rule out in a non-model-dependent way a scenario in which orientation effects are important and covering factors are generally small. Results such as these ultimately provide constraints on QSO geometries, intrinsic QSO properties, and evolutionary processes in QSOs.