44 research outputs found
DASCH Discovery of A Possible Nova-like Outburst in A Peculiar Symbiotic Binary
We present photometric and spectroscopic observations of a peculiar variable
(designated DASCH J075731.1+201735 or J0757) discovered from our DASCH project
using the digitized Harvard College Observatory archival photographic plates.
It brightened by about 1.5 magnitudes in B within a year starting in 1942, and
then slowly faded back to its pre-outburst brightness from 1943 to the 1950s.
The mean brightness level was stable before and after the outburst, and
ellipsoidal variations with a period of days are seen,
suggesting that the star is tidally distorted. Radial-velocity measurements
indicate that the orbit is nearly circular () with a
spectroscopic period that is the same as the photometric period. The binary
consists of a M0III star, and a
companion, very likely a white dwarf (WD). Unlike other symbiotic binaries,
there is no sign of emission lines or a stellar wind in the spectra. With an
outburst timescale of ~10 years and estimated B band peak luminosity M_B~0.7,
J0757 is different from any other known classic or symbiotic novae. The most
probable explanation of the outburst is Hydrogen shell-burning on the WD,
although an accretion-powered flare cannot be ruled out.Comment: 12 pages, 6 figures, accepted for publication in Ap
The type IIn supernova 1994W: evidence for the explosive ejection of a circumstellar envelope
We present and analyse spectra of the Type IIn supernova 1994W obtained
between 18 and 203 days after explosion. During the luminous phase (first 100
d) the line profiles are composed of three major components: (i) narrow P-Cygni
lines with the absorption minima at -700 km/s; (ii) broad emission lines with
BVZI ~4000 km/s; and (iii) broad, smooth wings, most apparent in H-alpha. These
components are identified with an expanding circumstellar (CS) envelope,
shocked cool gas in the forward post-shock region, and multiple Thomson
scattering in the CS envelope, respectively. The absence of broad P-Cygni lines
from the supernova is the result of the formation of an optically thick, cool,
dense shell at the interface of the ejecta and the CS envelope. We model the
supernova deceleration and Thomson scattering wings to recover the density,
radial extent and Thomson optical depth of the CS envelope during the first
month. We reproduce the light curve with a hydrodynamical model and find it to
be powered by a combination of internal energy leakage after the explosion of
an extended pre-supernova (~10^15 cm) and luminosity from circumstellar
interaction. We recover the pre-explosion kinematics of the CS envelope: it is
close to homologous expansion with outer velocity ~1100 km/s and a kinematic
age of ~1.5 yr. The CS envelope's high mass and kinetic energy, combined with
its small age, strongly suggest that the CS envelope was explosively ejected
about 1.5 yr before the supernova explosion.Comment: 22 pages, 21 figures. Accepted for publication in Monthly Notices of
the Royal Astronomical Societ
Shining Light on Merging Galaxies I: The Ongoing Merger of a Quasar with a `Green Valley' Galaxy
Serendipitous observations of a pair z = 0.37 interacting galaxies (one
hosting a quasar) show a massive gaseous bridge of material connecting the two
objects. This bridge is photoionized by the quasar (QSO) revealing gas along
the entire projected 38 kpc sightline connecting the two galaxies. The emission
lines that result give an unprecedented opportunity to study the merger process
at this redshift. We determine the kinematics, ionization parameter (log U ~
-2.5 +- 0.03), column density (N_H ~ 10^{21} cm^{-2}), metallicity ([M/H] ~
-0.20 +- 0.15), and mass (~ 10^8 Msun) of the gaseous bridge. We simultaneously
constrain properties of the QSO-host (M_DM>8.8x 10^{11} Msun) and its companion
galaxy (M_DM>2.1 x 10^{11} Msun; M_star ~ 2 x 10^{10} Msun; stellar burst
age=300-800 Myr; SFR~6 Msun/yr; and metallicity 12+log (O/H)= 8.64 +- 0.2). The
general properties of this system match the standard paradigm of a
galaxy-galaxy merger caught between first and second passage while one of the
galaxies hosts an active quasar. The companion galaxy lies in the so-called
`green valley', with a stellar population consistent with a recent starburst
triggered during the first passage of the merger and has no detectable AGN
activity. In addition to providing case-studies of quasars associated with
galaxy mergers, quasar/galaxy pairs with QSO-photoionized tidal bridges such as
this one offer unique insights into the galaxy properties while also
distinguishing an important and inadequately understood phase of galaxy
evolution.Comment: 23 pages, 12 figures, 5 tables, Submitted to ApJ, revised to address
referee's comment
Galaxy Zoo: Dust in Spirals
We investigate the effect of dust on spiral galaxies by measuring the
inclination-dependence of optical colours for 24,276 well-resolved SDSS
galaxies visually classified in Galaxy Zoo. We find clear trends of reddening
with inclination which imply a total extinction from face-on to edge-on of 0.7,
0.6, 0.5 and 0.4 magnitudes for the ugri passbands. We split the sample into
"bulgy" (early-type) and "disky" (late-type) spirals using the SDSS fracdeV (or
f_DeV) parameter and show that the average face-on colour of "bulgy" spirals is
redder than the average edge-on colour of "disky" spirals. This shows that the
observed optical colour of a spiral galaxy is determined almost equally by the
spiral type (via the bulge-disk ratio and stellar populations), and reddening
due to dust. We find that both luminosity and spiral type affect the total
amount of extinction, with "disky" spirals at M_r ~ -21.5 mags having the most
reddening. This decrease of reddening for the most luminous spirals has not
been observed before and may be related to their lower levels of recent star
formation. We compare our results with the latest dust attenuation models of
Tuffs et al. We find that the model reproduces the observed trends reasonably
well but overpredicts the amount of u-band attenuation in edge-on galaxies. We
end by discussing the effects of dust on large galaxy surveys and emphasize
that these effects will become important as we push to higher precision
measurements of galaxy properties and their clustering.Comment: MNRAS in press. 25 pages, 22 figures (including an abstract comparing
GZ classifications with common automated methods for selecting disk/early
type galaxies in SDSS data). v2 corrects typos found in proof
The clustering of galaxies in the SDSS-III Baryon Oscillation Spectroscopic Survey: measurements of the growth of structure and expansion rate at z=0.57 from anisotropic clustering
We analyze the anisotropic clustering of massive galaxies from the Sloan
Digital Sky Survey III Baryon Oscillation Spectroscopic Survey (BOSS) Data
Release 9 (DR9) sample, which consists of 264,283 galaxies in the redshift
range 0.43 < z < 0.7 spanning 3,275 square degrees. Both peculiar velocities
and errors in the assumed redshift-distance relation ("Alcock-Paczynski
effect") generate correlations between clustering amplitude and orientation
with respect to the line-of-sight. Together with the sharp baryon acoustic
oscillation (BAO) standard ruler, our measurements of the broadband shape of
the monopole and quadrupole correlation functions simultaneously constrain the
comoving angular diameter distance (2190 +/- 61 Mpc) to z=0.57, the Hubble
expansion rate at z=0.57 (92.4 +/- 4.5 km/s/Mpc), and the growth rate of
structure at that same redshift (d sigma8/d ln a = 0.43 +/- 0.069). Our
analysis provides the best current direct determination of both DA and H in
galaxy clustering data using this technique. If we further assume a LCDM
expansion history, our growth constraint tightens to d sigma8/d ln a = 0.415
+/- 0.034. In combination with the cosmic microwave background, our
measurements of DA, H, and growth all separately require dark energy at z >
0.57, and when combined imply \Omega_{\Lambda} = 0.74 +/- 0.016, independent of
the Universe's evolution at z<0.57. In our companion paper (Samushia et al.
prep), we explore further cosmological implications of these observations.Comment: 19 pages, 11 figures, submitted to MNRAS, comments welcom
Cosmological parameters from SDSS and WMAP
We measure cosmological parameters using the three-dimensional power spectrum
P(k) from over 200,000 galaxies in the Sloan Digital Sky Survey (SDSS) in
combination with WMAP and other data. Our results are consistent with a
``vanilla'' flat adiabatic Lambda-CDM model without tilt (n=1), running tilt,
tensor modes or massive neutrinos. Adding SDSS information more than halves the
WMAP-only error bars on some parameters, tightening 1 sigma constraints on the
Hubble parameter from h~0.74+0.18-0.07 to h~0.70+0.04-0.03, on the matter
density from Omega_m~0.25+/-0.10 to Omega_m~0.30+/-0.04 (1 sigma) and on
neutrino masses from <11 eV to <0.6 eV (95%). SDSS helps even more when
dropping prior assumptions about curvature, neutrinos, tensor modes and the
equation of state. Our results are in substantial agreement with the joint
analysis of WMAP and the 2dF Galaxy Redshift Survey, which is an impressive
consistency check with independent redshift survey data and analysis
techniques. In this paper, we place particular emphasis on clarifying the
physical origin of the constraints, i.e., what we do and do not know when using
different data sets and prior assumptions. For instance, dropping the
assumption that space is perfectly flat, the WMAP-only constraint on the
measured age of the Universe tightens from t0~16.3+2.3-1.8 Gyr to
t0~14.1+1.0-0.9 Gyr by adding SDSS and SN Ia data. Including tensors, running
tilt, neutrino mass and equation of state in the list of free parameters, many
constraints are still quite weak, but future cosmological measurements from
SDSS and other sources should allow these to be substantially tightened.Comment: Minor revisions to match accepted PRD version. SDSS data and ppt
figures available at http://www.hep.upenn.edu/~max/sdsspars.htm
The Milky Way Tomography with SDSS: I. Stellar Number Density Distribution
Abridged: We estimate the distances to ~48 million stars detected by the
Sloan Digital Sky Survey and map their 3D number density distribution in 100 <
D < 20 kpc range over 6,500 deg^2 of sky. The data show strong evidence for a
Galaxy consisting of an oblate halo, a disk component, and a number of
localized overdensities with exponential disk parameters (bias-corrected for an
assumed 35% binary fraction) H_1 = 300 pc, L_1 = 2600 pc, H_2 = 900 pc, L_2 =
3600 pc, and local density normalization of 12%. We find the halo to be oblate,
with best-fit axis ratio c/a = 0.64, r^{-2.8} profile, and the local
halo-to-thin disk normalization of 0.5%. We estimate the errors of derived
model parameters to be no larger than ~20% (disk scales) and ~10% (thick disk
normalization). While generally consistent with the above model, the density
distribution shows a number of statistically significant localized deviations.
We detect two overdensities in the thick disk region at (R, Z) ~ (6.5, 1.5)kpc
and (R, Z) ~ (9.5, 0.8) kpc, and a remarkable density enhancement in the halo
covering >1000deg^2 of sky towards the constellation of Virgo, at distances of
~6-20 kpc. Compared to a region symmetric with respect to the l=0 line, the
Virgo overdensity is responsible for a factor of 2 number density excess and
may be a nearby tidal stream or a low-surface brightness dwarf galaxy merging
with the Milky Way. After removal of the resolved overdensities, the remaining
data are consistent with a smooth density distribution; we detect no evidence
of further unresolved clumpy substructure at scales ranging from ~50pc in the
disk, to ~1 - 2 kpc in the halo.Comment: 60 pages, 46 figures (reduced resolution; see the ApJ for hi-res
version