Variable Extended Objects in Selected Area 57

Astronomy and Astrophysics, 2000

Two nearby galaxies, IC 1613 and NGC 6822, were observed over four years to detect short period Cepheids and to obtain good light curves for Fourier decomposition. Since the program was carried out with a relatively small telescope, the Dutch 0.9 m at ESO-La Silla, the observations were performed without filter (white light), or Wh-band; the advantage of this technique is that the photon statistics correspond to those of V-band observations of similar exposure time obtained with a 2-m class telescope. In this paper we discuss some properties of Wh photometry and present the results of the observations of Field B in IC 1613 (3.8'x3.8'). A total of 65 images were obtained and the reduction was performed with DAOPHOT. The light curves of 4037 stars were measured, and the analysis using various techniques allowed the detection of 121 variable stars. We detected 51 population I Cepheids, while previously only 12 Cepheids were known in the same field. Most of these have short periods and some are first overtone mode pulsators. The other variable stars include one eclipsing binary, two suspected W Vir stars, 57 irregular and semi-regular red variables, 3 periodic red variables and 6 irregular blue variables. Based on observations collected at ESO-La Silla

2000

We present 8.4 GHz VLA A-configuration observations of 87 sources from the mid-infrared-selected AGN sample of . These 0.25 arcsec resolution observations allow elongated radio structures tens of parsecs in size to be resolved and enable radio components smaller than 3.5 arcsec to be isolated from diffuse galactic disc emission. When combined with previous data, matched radio observations covering ninety percent of the sample have been collected and these represent the largest sub-arcsecond-resolution radio imaging survey of a homogeneously-selected sample of Seyfert galaxies to date.

Astrophysics, 1991

The investigations have resulted in the discovery of 25 QSOs, two type 1Seyfert galaxies, one type 2 Seyfert galaxy, and two emission galaxies. The remaining objects are white dwarfs and hot subdwarfs of various types. Five continuum objects were found. The main parameters of the emission lines and other data are given for all the quasistellar objects, and also scans of the overwhelming majority of the QSOs. Scans of some typical white dwarfs and subdwarfs are also given.

Monthly Notices of the Royal Astronomical Society, 2001

We have used the ROSAT Bright Source Catalogue to extract a complete sample of sources selected in the band from 0.1-0.4 keV. This 1/4 keV-selected sample is comprised of 54 Seyfert galaxies, 25 BL Lacertae objects, 4 clusters and 27 Galactic stars or binaries. Seyfert-type galaxies with "ultrasoft" X-ray spectra can very often be classed optically as Narrow-line Seyfert 1s (NLS1s). Such objects are readily detected in 1/4 keV surveys; the sample reported here contains 20 NLS1s, corresponding to a 40% fraction of the Seyferts. Optical spectra of the Seyfert galaxies were gathered for correlative analysis, which confirmed the well-known relations between X-ray slope and optical spectral properties (e.g., [O III]/Hβ ratio; Fe II strength, Hβ width). The various intercorrelations are most likely driven, fundamentally, by the shape of the photoionising continuum in Seyfert nuclei. We argue that a steep X-ray spectrum is a better indicator of an "extreme" set of physical properties in Seyfert galaxies than is the narrowness of the optical Hβ line. The correlation studies were also used to isolate a number of Seyfert galaxies with apparently "anomalous" properties. Of particular interest are the six objects with relatively weak permitted line emission (Hβ and Fe II) and weak optical continua. Such objects are rare in most surveys, but two of these (IC 3599 and NGC 5905) are known to be transient active galactic nuclei in which the X-ray flux has faded by factors ∼100. If the other four objects also turn out to be transient, this would demonstrate that 1/4 keV surveys provide an efficient way of finding an interesting class of object. Finally, the luminosity function of the 1/4 keV-selected Seyfert galaxies was determined and broken down into subsamples to investigate the relative space densities of Seyferts when separated on the basis of either X-ray slope or Hβ line width.

2000

We present 8.4 GHz VLA A-configuration observations of 87 sources from the mid-infrared-selected AGN sample of Rush et al (1993). These 0.25 arcsec resolution observations allow elongated radio structures tens of parsecs in size to be resolved and enable radio components smaller than 3.5 arcsec to be isolated from diffuse galactic disc emission. When combined with previous data, matched radio observations covering ninety percent of the sample have been collected and these represent the largest sub-arcsecond-resolution radio imaging survey of a homogeneously-selected sample of Seyfert galaxies to date. We use our observations to identify 5 radio-loud AGN in the sample. The nature of the radio emission from Seyfert nuclei will be discussed in subsequent papers.

Monthly Notices of the Royal Astronomical Society, 2012

We report the discovery of two new Galactic candidate luminous blue variable (cLBV) stars via detection of circular shells (typical of known confirmed and cLBVs) and follow-up spectroscopy of their central stars. The shells were detected at 22 µm in the archival data of the Mid-Infrared All Sky Survey carried out with the Wide-field Infrared Survey Explorer (WISE). Follow-up optical spectroscopy of the central stars of the shells conducted with the renewed Southern African Large Telescope (SALT) showed that their spectra are very similar to those of the well-known LBVs P Cygni and AG Car, and the recently discovered cLBV MN112, which implies the LBV classification for these stars as well. The LBV classification of both stars is supported by detection of their significant photometric variability: one of them brightened in the R-and I-bands by 0.68 ± 0.10 mag and 0.61 ± 0.04 mag, respectively, during the last 13-18 years, while the second one (known as Hen 3-1383) varies its B, V, R, I and K s brightnesses by ≃ 0.5 − 0.9 mag on timescales from 10 days to decades. We also found significant changes in the spectrum of Hen 3-1383 on a timescale of ≃ 3 months, which provides additional support for the LBV classification of this star. Further spectrophotometric monitoring of both stars is required to firmly prove their LBV status. We discuss a connection between the location of massive stars in the field and their fast rotation, and suggest that the LBV activity of the newly discovered cLBVs might be directly related to their possible runaway status.

Monthly Notices of the Royal Astronomical Society, 2009

We study the nature of the extended near-UV emission in the inner kiloparsec of a sample of 15 Seyfert (Sy) galaxies which have both near-UV (F330W) and narrow-band [O III] highresolution Hubble images. For the majority of the objects, we find a very similar morphology in both bands. From the [O III] images, we construct synthetic images of the nebular continuum plus the emission-line contribution expected through the F330W filter, which can be subtracted from the F330W images. We find that the emission of the ionized gas dominates the near-UV extended emission in half of the objects. A further broad-band photometric study, in the bands F330W (U), F547M (V) and F160W (H), shows that the remaining emission is dominated by the underlying galactic bulge contribution. We also find a blue component whose nature is not clear in four out of 15 objects. This component may be attributed to scattered light from the active galactic nuclei, to a young stellar population in unresolved star clusters, or to early disrupted clusters. Star-forming regions and/or bright off nuclear star clusters are observed in 4/15 galaxies of the sample.

Monthly Notices of the Royal Astronomical Society, 2001

We present 8.4 GHz VLA A-configuration observations of 87 sources from the mid-infrared-selected AGN sample of . These 0.25 arcsec resolution observations allow elongated radio structures tens of parsecs in size to be resolved and enable radio components smaller than 3.5 arcsec to be isolated from diffuse galactic disc emission. When combined with previous data, matched radio observations covering ninety percent of the sample have been collected and these represent the largest sub-arcsecond-resolution radio imaging survey of a homogeneously-selected sample of Seyfert galaxies to date.

2010

We present the results of a new search for variable stars in the Local Group (LG) isolated dwarf galaxy IC 1613, based on 24 orbits of F475W and F814W photometry from the Advanced Camera for Surveys onboard the Hubble Space Telescope. We detected 259 candidate variables in this field, of which only 13 (all of them bright Cepheids) were previously known. Out of the confirmed variables, we found 90 RR Lyrae stars, 49 classical Cepheids (including 36 new discoveries), and 38 eclipsing binary stars for which we could determine a period. The RR Lyrae include 61 fundamental (RRab) and 24 first-overtone (RRc) pulsators, and 5 pulsating in both modes simultaneously (RRd). As for the majority of LG dwarfs, the mean periods of the RRab and RRc (0.611 and 0.334 day, respectively) as well as the fraction of overtone pulsators (f c =0.28) place this galaxy in the intermediate regime between the Oosterhoff types. From their position on the period-luminosity diagram and light-curve morphology, we can unambiguously classify 25 and 14 Cepheids as fundamental and firstovertone mode pulsators, respectively. Another two are clearly second-overtone Cepheids, the first ones to be discovered beyond the Magellanic Clouds. Among the remaining candidate variables, five were classified as δ-Scuti and five as long-period variables. Most of the others are located on the main-sequence, the majority of them likely eclipsing binary systems, although some present variations similar to pulsating stars. We estimate the distance to IC 1613 using various methods based on the photometric and pulsational properties of the Cepheids and RR Lyrae stars. The values we find are in very good agreement with each other and with previous estimates based on independent methods. When corrected to a common reddening of E(B−V)=0.025 and true LMC distance modulus of (m−M) LMC,0 =18.515±0.085, we find that all the distance determinations from the literature converge to a common value of (m−M) 0 =24.400±0.014 (statistical), or 760 kpc. The parallel WFPC2 field, which lies within three core radii, was also searched for variable stars. We discovered nine RR Lyrae stars (4 RRab, 4 RRc and 1 RRd) and two Cepheids, even though the lower signal-to-noise ratio of the observations did not allow us to measure their periods as accurately as for the variables in the ACS field-of-view. We provide their coordinates and approximate properties for completeness.

The Astrophysical Journal, 2018

We present mosaicked long-slit spectral maps of 18 nearby Active Galactic Nuclei (AGNs), 2 LINERs, and 4 starforming galaxies. With the resulting data cubes taken using the Kast dual spectrograph on the 3 m Shane telescope of the Lick Observatory, we measure the aperture effects on the spectroscopic classification of AGNs. With more starlight included in a larger aperture, the nuclear spectrum that is Seyfert-like may become contaminated. We generated standard spectroscopic classification diagrams in different observing apertures. These show quantitatively how the ensemble of Seyferts migrates toward the H II region classification when being observed with increasing aperture sizes. But the effect ranges widely in individual active galaxies. Some of the less luminous Seyferts shift by a large amount, while some others barely move or even shift in different directions. We find that those Seyfert galaxies with the fraction of nuclear Hα emission lower than 0.2 of the host galaxy, 2-10 keV hard X-ray luminosity lower than 10 43 erg s −1 , and the observed nuclear [O III] luminosity lower than 10 40.5 erg s −1 , are more likely to change activity classification type when the entire host galaxy is included. Overall, 4 of our 24 galaxies (18 Seyferts) change their spectral activity classification type when observed with a very large aperture.