255 research outputs found
Evidence for a Hard Ionizing Spectrum from a z=6.11 Stellar Population
We present the Magellan/FIRE detection of highly-ionized CIV 1550 and OIII]
1666 in a deep infrared spectrum of the z=6.11 gravitationally lensed low-mass
galaxy RXC J2248.7-4431-ID3, which has previously-known Lyman-alpha. No
corresponding emission is detected at the expected location of HeII 1640. The
upper limit on HeII paired with detection of OIII] and CIV constrains possible
ionization scenarios. Production of CIV and OIII] requires ionizing photons of
2.5-3.5 Ryd, but once in that state their multiplet emission is powered by
collisional excitation at lower energies (~0.5 Ryd). As a pure recombination
line, HeII emission is powered by 4 Ryd ionizing photons. The data therefore
require a spectrum with significant power at 3.5 Ryd but a rapid drop toward
4.0 Ryd. This hard spectrum with a steep drop is characteristic of
low-metallicity stellar populations, and less consistent with soft AGN
excitation, which features more 4 Ryd photons and hence higher HeII flux. The
conclusions based on ratios of metal line detections to Helium non-detection
are strengthened if the gas metallicity is low. RXJ2248-ID3 adds to the growing
handful of reionization-era galaxies with UV emission line ratios distinct from
the general z=2-3 population, in a way that suggests hard ionizing spectra that
do not necessarily originate in AGN.Comment: 7 pages, 4 figures, 1 table. Accepted for publication to ApJ
Dissecting the Gaseous Halos of z~2 Damped Ly Systems with Close Quasar Pairs
We use spectroscopy of close pairs of quasars to study diffuse gas in the
circumgalactic medium (CGM) surrounding a sample of 40 Damped Lya systems
(DLAs). The primary sightline in each quasar pair probes an intervening DLA in
the redshift range 1.6 < z_DLA < 3.6, such that the second quasar sightline
then probes Lya, CII, SiII, and CIV absorption in the CGM transverse to the DLA
to projected distances kpc. Analysis of the Lya profiles in
these CGM sightlines constrains the covering fraction (f_C) of optically thick
HI (having column density N_HI > 10^17.2 cm^-2) to be greater than ~30% within
kpc of DLAs. Strong SiII 1526 absorption with equivalent
width W_1526 > 0.2 Ang occurs with an incidence f_C (W_1526 > 0.2 Ang) =
20(+12/-8)% within kpc, indicating that low-ionization metal
absorption associated with DLAs probes material at a physical distance R_3D <
30 kpc. However, we find that strong CIV 1548 absorption is ubiquitous in these
environments (f_C (W_1548 > 0.2 Ang) = 57(+12/-13)% within
kpc), and in addition exhibits a high degree of kinematic coherence on scales
up to ~175 kpc. We infer that this high-ionization material arises
predominantly in large, quiescent structures extending beyond the scale of the
DLA host dark matter halos rather than in ongoing galactic winds. The Lya
equivalent width in the DLA-CGM is anticorrelated with at >98%
confidence, suggesting that DLAs arise close to the centers of their host halos
rather than on their outskirts. Finally, the average Lya, CII and CIV
equivalent widths are consistent with those measured around z~2 Lyman Break
Galaxies. Assuming that DLAs trace a galaxy population with lower masses and
luminosities, this finding implies that the absorption strength of cool
circumgalactic material has a weak dependence on dark matter halo mass for M_h
< 10^12 M_sun.Comment: Submitted to ApJ. 30 pages, 13 figures, 3 tables, 1 appendix. Uses
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An ultrahigh-speed digitizer for the Harvard College Observatory astronomical plates
A machine capable of digitizing two 8 inch by 10 inch (203 mm by 254 mm)
glass astrophotographic plates or a single 14 inch by 17 inch (356 mm by 432
mm) plate at a resolution of 11 microns per pixel or 2309 dots per inch (dpi)
in 92 seconds is described. The purpose of the machine is to digitize the
\~500,000 plate collection of the Harvard College Observatory in a five year
time frame. The digitization must meet the requirements for scientific work in
astrometry, photometry, and archival preservation of the plates. This paper
describes the requirements for and the design of the subsystems of the machine
that was developed specifically for this task.Comment: 12 pages, 9 figures, 1 table; presented at SPIE (July, 2006) and
published in Proceeding
Spectral shape of the UV ionizing background and HeII absorption at redshifts 1.8 < z < 2.9
The shape of the UV ionizing background is reconstructed from optically thin
metal absorption-line systems identified in spectra of HE2347-4342, Q1157+3143,
and HS1700+6416 in the redshift interval 1.8 < z < 2.9. The systems are
analyzed by means of the Monte Carlo Inversion method completed with the
spectral shape recovering procedure. The UVB spectral shape fluctuates at 2.4 <
z < 2.9 mostly due to radiative transfer processes in the clumpy IGM. At z <
1.8, the IGM becomes almost transparent both in the HI and HeII Lyman continua
and the variability of the spectral shape comes from diversity of spectral
indices describing the QSO/AGN intrinsic radiation. At z > 2.4, the recovered
spectral shapes show intensity depression between 3 and 4 Ryd due to HeII
Ly-alpha absorption in the IGM clouds (line blanketing) and continuous medium
(true Gunn-Petersen effect). The mean HeII Ly-alpha opacity estimated from the
depth of this depression corresponds within 1-2sigma to the values directly
measured from the HI/HeII Ly-alpha forest towards the quasars studied. The
observed scatter in eta = N(HeII)/N(HI) and anti-correlation between N(HI) and
eta can be explained by the combined action of variable spectral softness and
differences in the mean gas density between the absorbing clouds. Neither of
the recovered spectral shapes show features which can be attributed to the
putative input of radiation from soft sources like starburst galaxies.Comment: 20 pages, 20 figures. Accepted for publication in A&
Massive Lyman Break Galaxies at z~3 in the Spitzer Extragalactic First Look Survey
We investigate the properties of 1088 Lyman Break Galaxies (LBGs) at z~3
selected from a ~2.63M/L$ in
rest-frame near-infrared. Most infrared-luminous LBGs (S_{24um} > 100 uJy) are
dusty star-forming galaxies with star formation rates of 100--1000 Msun/yr,
total infrared luminosity of > 10^12 Lsun. By constructing the UV luminosity
function of massive LBGs, we estimate that the lower limit for the star
formation rate density from LBGs more massive than 10^11 Msun at z~3 is > 3.3 x
10^-3 Msun/yr/Mpc^3, showing for the first time that the UV-bright population
of massive galaxies alone contributes significantly to the global star
formation rate density at z~3. When combined with the star formation rate
densities at z < 2, our result reveals a steady increase in the contribution of
massive galaxies to the global star formation from z=0 to z=3, providing strong
support to the downsizing of galaxy formation.Comment: 15 pages, 13 figures. Accepted for publication in Ap
Discovery of three z>6.5 quasars in the VISTA Kilo-degree Infrared Galaxy (VIKING) survey
Studying quasars at the highest redshifts can constrain models of galaxy and
black hole formation, and it also probes the intergalactic medium in the early
universe. Optical surveys have to date discovered more than 60 quasars up to
z~6.4, a limit set by the use of the z-band and CCD detectors. Only one z>6.4
quasar has been discovered, namely the z=7.08 quasar ULAS J1120+0641, using
near-infrared imaging. Here we report the discovery of three new z>6.4 quasars
in 332 square degrees of the Visible and Infrared Survey Telescope for
Astronomy Kilo-degree Infrared Galaxy (VIKING) survey, thus extending the
number from 1 to 4. The newly discovered quasars have redshifts of z=6.60,
6.75, and 6.89. The absolute magnitudes are between -26.0 and -25.5, 0.6-1.1
mag fainter than ULAS J1120+0641. Near-infrared spectroscopy revealed the MgII
emission line in all three objects. The quasars are powered by black holes with
masses of ~(1-2)x10^9 M_sun. In our probed redshift range of 6.44<z<7.44 we can
set a lower limit on the space density of supermassive black holes of
\rho(M_BH>10^9 M_sun) > 1.1x10^(-9) Mpc^(-3). The discovery of three quasars in
our survey area is consistent with the z=6 quasar luminosity function when
extrapolated to z~7. We do not find evidence for a steeper decline in the space
density of quasars with increasing redshift from z=6 to z=7.Comment: 14 pages, 9 figures. Published in Ap
The polar Catalysmic Variable 1RXS J173006.4+033813
We report the discovery of 1RXS J173006.4+033813, a polar cataclysmic
variable with a period of 120.21 min. The white dwarf primary has a magnetic
field of B = 42+6-5 MG, and the secondary is a M3 dwarf. The system shows
highly symmetric double peaked photometric modulation in the active state as
well as in quiescence. These arise from a combination of cyclotron beaming and
ellipsoidal modulation. The projected orbital velocity of the secondary is K2 =
390+-4 km/s. We place an upper limit of 830+-65 pc on the distance.Comment: ApJ Accepted. 12 Pages, 13 Figures, 6 table
FIRE Spectroscopy of Five Late-type T Dwarfs Discovered with the Wide-field Infrared Survey Explorer
We present the discovery of five late-type T dwarfs identified with the
Wide-field Infrared Survey Explorer (WISE). Low-resolution near-infrared
spectroscopy obtained with the Magellan Folded-port InfraRed Echellette (FIRE)
reveal strong water and methane absorption in all five sources, and spectral
indices and comparison to spectral templates indicate classifications ranging
from T5.5 to T8.5:. The spectrum of the latest-type source, WISE J1812+2721, is
an excellent match to that of the T8.5 companion brown dwarf Wolf 940B.
WISE-based spectrophotometric distance estimates place these T dwarfs at 12-13
pc from the Sun, assuming they are single. Preliminary fits of the spectral
data to the atmosphere models of Saumon & Marley indicate effective
temperatures ranging from 600 K to 930 K, both cloudy and cloud-free
atmospheres, and a broad range of ages and masses. In particular, two sources
show evidence of both low surface gravity and cloudy atmospheres, tentatively
supporting a trend noted in other young brown dwarfs and exoplanets. In
contrast, the high proper motion T dwarf WISE J2018-7423 exhibits a suppressed
K-band peak and blue spectrophotometric J-K colors indicative of an old,
massive brown dwarf; however, it lacks the broadened Y-band peak seen in
metal-poor counterparts. These results illustrate the broad diversity of
low-temperature brown dwarfs that will be uncovered with WISE.Comment: 19 pages, 13 figures; accepted for publication to Ap
Sizes, Shapes, and Correlations of Lyman Alpha Clouds and Their Evolution in the CDM Universe
This study analyzes the sizes, shapes and correlations of \lya clouds
produced by a hydrodynamic simulation of a spatially flat CDM universe with a
non-zero cosmological constant (, , ), over the redshift range . The \lya clouds range in
size from several kiloparsecs to about a hundred kiloparsecs in proper units,
and they range in shape from roundish, high column density regions with
\nhi\ge 10^{15} cm^{-2} to low column density sheet-like structures with
\nhi \le 10^{13} cm^{-2} at z=3. The most common shape found in the
simulation resembles that of a flattened cigar. The physical size of a typical
cloud grows with time roughly as while its shape hardly evolves
(except for the most dense regions ). Our result indicates that
any simple model with a population of spheres (or other shapes) of a uniform
size is oversimplified; if such a model agrees with observational evidence, it
is probably only by coincidence. We also illustrate why the use of double
quasar sightlines to set lower limits on cloud sizes is useful only when the
perpendicular sightline separation is small ( kpc).
Finally, we conjecture that high column density \lya clouds (\nhi\ge 10^{15}
cm^{-2}) may be the progenitors of the lower redshift faint blue galaxies.
This seems plausible because their correlation length, number density
(extrapolated to lower redshift) and their masses are in fair agreement with
those observed.Comment: ApJ, in press, 34 pages, 21 figures, figs (1a,b,c) can be at
http://astro.princeton.edu/~cen/LYASSC/lyassc.htm
Luhman 16AB: A Remarkable, Variable L/T Transition Binary 2 pc from the Sun
Luhman (2013) has reported the discovery of a brown dwarf binary system only 2.01±0.15 pc from the Sun. The binary is well-resolved with a projected separation of 1farcs 5, and spectroscopic observations have identified the components as late-L and early-T dwarfs. The system exhibits several remarkable traits, including a ``flux reversal'', where the T dwarf is brighter over 0.9-1.3 mu m but fainter at other wavelengths; and significant (˜10%) short-period (˜4.9 hr) photometric variability with a complex light curve. These observations suggest spatial variations in condensate cloud structure, which is known to evolve substantially across the L dwarf/T dwarf transition. Here we report preliminary results from a multi-site monitoring campaign aimed at probing the spectral and temporal properties of this source. Focusing on our spectroscopic observations, we report the first detections of NIR spectral variability, present detailed analysis of K I lines that confirm differences in condensate opacity between the components; and preliminary determinations of radial and rotational velocities based on high-resolution NIR spectroscopy.Peer reviewe
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