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.
Discoveries of two new white dwarf plus M star binaries with striking optical cyclotron emission features from the Sloan Digital Sky Survey (SDSS) brings to six the total number of X-ray-faint, ...magnetic accretion binaries that accrete at rates M 10 super(-13) M sub( )yr super(-1), or <1% of the values normally encountered in cataclysmic variables. This fact, coupled with donor stars that underfill their Roche lobes and very cool white dwarfs, brand the binaries as post-common-envelope systems whose orbits have not yet decayed to the point of Roche Iobe contact. They are premagnetic cataclysmic variables, or pre-Polars. The systems exhibit spin-orbit synchronism and apparently accrete by efficient capture of the stellar wind from the secondary star, a process that has been dubbed a "magnetic siphon." Because of this, period evolution of the binaries will occur solely by gravitational radiation, which is very slow for periods >3 hr. Optical surveys for the cyclotron harmonics appear to be the only means of discovery, so the space density of pre-Polars could rival that of Polars, and the binaries provide an important channel of progenitors (in addition to the asynchronous intermediate Polars). Both physical and SDSS observational selection effects are identified that may help to explain the clumping of all six systems in a narrow range of magnetic field strength around 60 MG.
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 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.
In this Letter, we announce the discovery of a new dwarf satellite of the Milky Way, located in the constellation Canes Venatici. It was found as a stellar overdensity in the north Galactic cap using ...Sloan Digital Sky Survey Data Release 5 (SDSS DR5). The satellite's color-magnitude diagram shows a well-defined red giant branch as well as a horizontal branch. As judged from the tip of the red giant branch, it lies at a distance of 6220 kpc. Based on the SDSS data, we estimate an absolute magnitude of M sub(v) 6 - 7.9, a central surface brightness of k sub(0,V) 6 28 mag arcsec super(-2), and a half-light radius of 6'.5 (6550 pc at the measured distance). The outer regions of Canes Venatici appear extended and distorted. The discovery of such a faint galaxy in proximity to the Milky Way strongly suggests that more such objects remain to be found.
We have examined the radial velocity data for stars spectroscopically observed by the Sloan Digital Sky Survey (SDSS) more than once to investigate the incidence of spectroscopic binaries, and to ...evaluate the accuracy of the SDSS stellar radial velocities. We find agreement between the fraction of stars with significant velocity variations and the expected fraction of binary stars in the halo and thick disk populations. The observations produce a list of 675 possible new spectroscopic binary stars and orbits for eight of them.
PDF
