AM CVn systems are a rare (about a dozen previously known) class of cataclysmic variables, arguably encompassing the shortest orbital periods (down to about 10 minutes) of any known binaries. Both ...binary components are thought to be degenerate (or partially so), likely with mass transfer from a helium-rich donor onto a white dwarf, driven by gravitational radiation. Although rare, AM CVn systems are of high interest as possible Type Ia supernova progenitors and because they are predicted to be common sources of gravity waves in upcoming experiments such as the Laser Interferometer Space Antenna. We have identified four new AM CVn candidates from the Sloan Digital Sky Survey (SDSS) spectral database. All four show hallmark spectroscopic characteristics of the AM CVn class: each is devoid of hydrogen features and instead shows a spectrum dominated by helium. All four show double-peaked emission, indicative of helium-dominated accretion disks. Limited time series CCD photometric follow-up data have been obtained for three of the new candidates from the Astrophysical Research Consortium 3.5 m telescope; most notably, a 28.3 minute binary period with sharp, deep eclipses is discovered in one case, SDSS J0926+3624. This is the first confirmed eclipsing AM CVn, and our data allow initial estimates of binary parameters for this ultracompact system. The four new SDSS objects also provide a substantial expansion of the current critically small sample of AM CVn systems.
We present here a new and homogeneous sample of 3340 galaxies selected from the Sloan Digital Sky Survey (SDSS) based solely on the observed strength of their
$\mathrm{H}\delta$
hydrogen Balmer ...absorption line. The presence of a strong
$\mathrm{H}\delta$
line within the spectrum of a galaxy indicates that the galaxy has undergone a significant change in its star-formation history within the last Gigayear. Therefore, such galaxies have received considerable attention in recent years, because they provide an opportunity to study galaxy evolution in action. These galaxies are commonly known as “post-starburst”, “E
$+$
A”, “k
$+$
a”, and
$\mathrm{H}\delta$
-strong galaxies. Their study has been severely hampered by the lack of a large, statistical sample of such galaxies. In this paper, we rectify this problem by selecting a sample of galaxies which possess an absorption
$\mathrm{H}\delta$
equivalent width of
$\mathrm{EW}(\mathrm{H}\delta_{\mathrm{max}})-\Delta \mathrm{EW}(\mathrm{H}\delta_{\mathrm{max}}) \gt 4 \,{Å}$
$\mathrm{EW}(\mathrm{H}\delta_{\mathrm{max}})-\Delta \mathrm{EW}(\mathrm{H}\delta_{\mathrm{max}}) \gt 4 \,$
Å from 106682 galaxies in the SDSS. The measured abundance of our
$\mathrm{H}\delta$
-strong (HDS) galaxies is
$2.6 \pm 0.1 \%$
of all galaxies within a volume-limited sample of
$0.05 < z < 0.1$
and
$M_{r^{*}} <-20.5$
, which is consistent with previous studies of such galaxies described in the literature. We find that only 25 of our HDS galaxies in this volume-limited sample (
$3.5 \pm 0.7\%$
) show no, or little, evidence for O II and
$\mathrm{H}\alpha$
emission lines, thus indicating that true E
$+$
A galaxies (as originally defined by Dressler and Gunn) are extremely rare objects at low redshift, i.e., only
$0.09 \pm 0.02\%$
of all galaxies in this volume-limited sample are true E
$+$
A galaxies. In contrast,
$89 \pm 5\%$
of our HDS galaxies in the volume-limited sample have significant detections of the O II and
$\mathrm{H}\alpha$
emission lines. Of these, only 131 galaxies are robustly classified as Active Galactic Nuclei (AGNs) and, therefore, a majority of these emission-line HDS galaxies are star-forming galaxies. We find that
$52 \pm 12\%$
(
$27/52$
) of the galaxies in our volume-limited HDS sample that possess no detectable O II emission, do however possess detectable
$\mathrm{H}\alpha$
emission lines. These may be dusty star-forming galaxies. We provide the community with this new catalog of
$\mathrm{H}\delta$
-strong galaxies to aid in our understanding of these galaxies, via detailed follow-up observations, as well as providing a low-redshift sample for comparison with higher redshift studies of HDS galaxies.
We report the discoveries of two two-image gravitationally lensed quasars selected from the Sloan Digital Sky Survey: SDSS J0806+2006 at zs = 1.540 and SDSS J1353+1138 at zs = 1.629 with image ...separations of plus or minus = 140 and plus or minus = 141, respectively. Spectroscopic and optical/near-infrared imaging follow-up observations show that the quasar images have identical redshifts and possess extended objects between the images that are likely to be lens galaxies at zl 0.6 in SDSS J0806+2006 and zl 0.3 in SDSS J1353+1138. The field of SDSS J0806+2006 contains several nearby galaxies that may significantly perturb the system, and SDSS J1353+1138 has an extra component near its Einstein ring that is probably a foreground star. Simple mass models with reasonable parameters reproduce the quasar positions and fluxes of both systems.
We present a new public archive of light-motion curves in Sloan Digital Sky Survey (SDSS) Stripe 82, covering 99 deg in right ascension from RA = 20.7 h to 3.3 h and spanning 2.52 deg in declination ...from Dec = -1.26 to 1.26 deg, for a total sky area of ~249 sq deg. Stripe 82 has been repeatedly monitored in the u, g, r, i and z bands over a seven-year baseline. Objects are cross-matched between runs, taking into account the effects of any proper motion. The resulting catalogue contains almost 4 million light-motion curves of stellar objects and galaxies. The photometry are recalibrated to correct for varying photometric zeropoints, achieving ~20 mmag and ~30 mmag root-mean-square (RMS) accuracy down to 18 mag in the g, r, i and z bands for point sources and extended sources, respectively. The astrometry are recalibrated to correct for inherent systematic errors in the SDSS astrometric solutions, achieving ~32 mas and ~35 mas RMS accuracy down to 18 mag for point sources and extended sources, respectively. For each light-motion curve, 229 photometric and astrometric quantities are derived and stored in a higher-level catalogue. On the photometric side, these include mean exponential and PSF magnitudes along with uncertainties, RMS scatter, chi^2 per degree of freedom, various magnitude distribution percentiles, object type (stellar or galaxy), and eclipse, Stetson and Vidrih variability indices. On the astrometric side, these quantities include mean positions, proper motions as well as their uncertainties and chi^2 per degree of freedom. The here presented light-motion curve catalogue is complete down to r~21.5 and is at present the deepest large-area photometric and astrometric variability catalogue available.
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