219 research outputs found
Emission line gas in early-type galaxies: Kinematics and physical conditions
Recent studies have found line emission gas in nearby early-type galaxies, but the properties of the emission-line gas in these 'normal' galaxies remain enigmatic. In terms of activity in the nucleus, these LINER-like galaxies form an important link between giant H 2 region galaxies and low-luminosity Seyferts. Despite their large numbers and evolutionary significance, we do not know whether these galaxies form a homogeneous class of objects; nor do we know how the distribution and kinematics of the line emission gas are affected by the host galaxy's environment or by the properties of the central engine, if present. To address these issues we are conducting a magnitude and volume limited survey of nearby early-type galaxies at Lick Observatory and the Michigan-Dartmouth-MIT Observatory. We have selected approximately 100 galaxies from radio catalogs. A large sample is necessary because while studies of individual 'LINERS' have led to a certain understanding of the phenomenon, these studies have not provided a global framework. Here we present results from our first run of medium resolution (approximately 5 A FWHM) spectroscopy. Kinematic data and line ratios determined along the major and minor axes of 6 galaxies are discussed. The information gleaned from spectroscopic data, when combined with data at other wavelengths, will enable a thorough investigation into the nature of low luminosity nuclear activity
Evidence for Cosmic Acceleration is Robust to Observed Correlations Between Type Ia Supernova Luminosity and Stellar Age
Type Ia Supernovae (SNe Ia) are powerful standardizable candles for
constraining cosmological models and provided the first evidence of the
accelerated expansion of the universe. Their precision derives from empirical
correlations, now measured from SNe Ia, between their luminosities,
light-curve shapes, colors and most recently with the stellar mass of their
host galaxy. As mass correlates with other galaxy properties, alternative
parameters have been investigated to improve SN Ia standardization though none
have been shown to significantly alter the determination of cosmological
parameters. We re-examine a recent claim, based on 34 SN Ia in nearby passive
host galaxies, of a 0.05 mag/Gyr dependence of standardized SN Ia luminosity on
host age which if extrapolated to higher redshifts, would be a bias up to 0.25
mag, challenging the inference of dark energy. We reanalyze this sample of
hosts using both the original method and a Bayesian hierarchical model and find
after a fuller accounting of the uncertainties the significance of a dependence
on age to be and after the removal of a single
poorly-sampled SN Ia. To test the claim that a trend seen in old stellar
populations can be applied to younger ages, we extend our analysis to a larger
sample which includes young hosts. We find the residual dependence of host age
(after all standardization typically employed for cosmological measurements) to
be consistent with zero for 254 SNe Ia from the Pantheon sample, ruling out the
large but low significance trend seen in passive hosts.Comment: 9 pages, 3 figures, 3 tables. Accepted for publication in ApJ
Cool white dwarfs as standards for infrared observations
In the era of modern digital sky surveys, uncertainties in the flux of stellar standards are commonly the dominant systematic error in photometric calibration and can often affect the results of higher level experiments. The Hubble Space Telescope (HST) spectrophotometry, which is based on computed model atmospheres for three hot (Teff>30000 K) pure hydrogen (DA) white dwarfs, is currently considered the most reliable and internally consistent flux calibration. However, many next-generation facilities (e.g. Harmoni on E-ELT, Euclid, and JWST) will focus on IR observations, a regime in which white dwarf calibration has not yet been robustly tested. Cool DA white dwarfs have energy distributions that peak close to the optical or near-infrared, do not have shortcomings from UV metal line blanketing, and have a reasonably large sky density (≃4 deg−2 at G < 20), making them, potentially, excellent calibrators. Here, we present a pilot study based on STIS + WFC3 observations of two bright DA white dwarfs to test whether targets cooler than current hot primary standards (Teff<20000 K) are consistent with the HST flux scale. We also test the robustness of white dwarf models in the IR regime from an X-shooter analysis of Paschen lines and by cross-matching our previously derived Gaia white dwarf catalogue with observations obtained with 2MASS, UKIDSS, VHS, and WISE
Photometry and spectroscopy of faint candidate spectrophotometric standard DA white dwarfs
We present precise photometry and spectroscopy for 23 candidate
spectrophotometric standard white dwarfs. The selected stars are distributed in
the Northern hemisphere and around the celestial equators and are all fainter
than r ~ 16.5 mag. This network of stars, when established as standards,
together with the three Hubble Space Telescope primary CALSPEC white dwarfs,
will provide a set of spectrophotometric standards to directly calibrate data
products to better than 1%. These new faint standard white dwarfs will have
enough signal-to-noise ratio in future deep photometric surveys and facilities
to be measured accurately while still avoiding saturation in such surveys. They
will also fall within the dynamic range of large telescopes and their
instruments for the foreseeable future. This paper discusses the provenance of
the observational data for our candidate standard stars. The comparison with
models, reconciliation with reddening, and the consequent derivation of the
full spectral energy density distributions for each of them is reserved for a
subsequent paper.Comment: 31 pages, 17 figures, 10 tables, ApJ in press (accepted on December
23rd, 2018
Generalized Second Law and phantom Cosmology: accreting black holes
The accretion of phantom fields by black holes within a thermodynamic context
is addressed. For a fluid violating the dominant energy condition, case of a
phantom fluid, the Euler and Gibbs relations permit two different possibilities
for the entropy and temperature: a situation in which the entropy is negative
and the temperature is positive or vice-versa. In the former case, if the
generalized second law (GSL) is valid, then the accretion process is not
allowed whereas in the latter, there is a critical black hole mass below which
the accretion process occurs. In a universe dominated by a phantom field, the
critical mass drops quite rapidly with the cosmic expansion and black holes are
only slightly affected by accretion. All black holes disappear near the big
rip, as suggested by previous investigations, if the GSL is violated.Comment: 8 pp., no figure
How to Tell a Jet from a Balloon: A Proposed Test for Beaming in Gamma Ray Bursts
If gamma ray bursts are highly collimated, the energy requirements of each
event may be reduced by several (~ 4-6) orders of magnitude, and the event rate
increased correspondingly. Extreme conditions in gamma ray bursters lead to
highly relativistic motions (bulk Lorentz factors Gamma > 100). This results in
strong forward beaming of the emitted radiation in the observer's rest frame.
Thus, all information on gamma ray bursts comes from those ejecta emitted in a
narrow cone (opening angle 1/Gamma) pointing towards the observer. We are at
present ignorant of whether there are ejecta outside that cone or not.
The recent detection of longer wavelength transients following gamma ray
bursts allows an empirical test of whether gamma ray bursts are collimated jets
or spherical fireballs. The bulk Lorentz factor of the burst ejecta will
decrease with time after the event, as the ejecta sweep up the surrounding
medium. Thus, radiation from the ejecta is beamed into an ever increasing solid
angle as the burst remnant evolves. It follows that if gamma ray bursts are
highly collimated, many more optical and radio transients should be observed
without associated gamma rays than with them. Published supernova searches may
contain enough data to test the most extreme models of gamma ray beaming. We
close with a brief discussion of other possible consequences of beaming,
including its effect on the evolution of burst remnants.Comment: Original replaced with accepted refereed manuscript. 11 pages, uses
AASTeX 4.0 LaTeX macros. To be published in The Astrophysical Journal
Letters, vol. 487, p. L1 (20 September 1997
A Calibration of NICMOS Camera 2 for Low Count-Rates
NICMOS 2 observations are crucial for constraining distances to most of the
existing sample of z > 1 SNe Ia. Unlike the conventional calibration programs,
these observations involve long exposure times and low count rates. Reciprocity
failure is known to exist in HgCdTe devices and a correction for this effect
has already been implemented for high and medium count-rates. However
observations at faint count-rates rely on extrapolations. Here instead, we
provide a new zeropoint calibration directly applicable to faint sources. This
is obtained via inter-calibration of NIC2 F110W/F160W with WFC3 in the low
count-rate regime using z ~ 1 elliptical galaxies as tertiary calibrators.
These objects have relatively simple near-IR SEDs, uniform colors, and their
extended nature gives superior signal-to-noise at the same count rate than
would stars. The use of extended objects also allows greater tolerances on PSF
profiles. We find ST magnitude zeropoints (after the installation of the NICMOS
cooling system, NCS) of 25.296 +- 0.022 for F110W and 25.803 +- 0.023 for
F160W, both in agreement with the calibration extrapolated from count-rates
1,000 times larger (25.262 and 25.799). Before the installation of the NCS, we
find 24.843 +- 0.025 for F110W and 25.498 +- 0.021 for F160W, also in agreement
with the high-count-rate calibration (24.815 and 25.470). We also check the
standard bandpasses of WFC3 and NICMOS 2 using a range of stars and galaxies at
different colors and find mild tension for WFC3, limiting the accuracy of the
zeropoints. To avoid human bias, our cross-calibration was "blinded" in that
the fitted zeropoint differences were hidden until the analysis was finalized.Comment: Accepted for Publication in the Astronomical Journal. New version
contains added referenc
Calibrating Redshift Distributions Beyond Spectroscopic Limits with Cross-Correlations
We describe a new method for measuring the true redshift distribution of any
set of objects studied only photometrically. The angular cross-correlation
between objects in a photometric sample with objects in some spectroscopic
sample as a function of the spectroscopic z, in combination with standard
correlation measurements, provides sufficient information to reconstruct the
true redshift distribution of the photometric sample. This technique enables
the robust calibration of photometric redshifts even beyond spectroscopic
limits. The spectroscopic sample need not resemble the photometric one in
galaxy properties, but must overlap in sky coverage and redshift range. We test
this new technique with Monte Carlo simulations using realistic error
estimates. RMS errors in recovering both the mean and sigma of the true,
Gaussian redshift distribution of a single photometric redshift bin are
1.4x10^(-3) (sigma_z/0.1) (Sigma_p/10)^(-0.3) (dN_s/dz / 25,000)^(-0.5), where
sigma_z is the true sigma of the redshift distribution, Sigma_p is the surface
density of the photometric sample in galaxies/arcmin^2, and dN_s/dz is the
number of galaxies with a spectroscopic redshift per unit z. We test the impact
of redshift outliers and of a variety of sources of systematic error; none
dominate measurement uncertainties in reasonable scenarios. With this method,
the true redshift distributions of even arbitrarily faint photometric redshift
samples may be determined to the precision required by proposed dark energy
experiments (errors in mean and sigma below 3x10^(-3) at z~1) using expected
extensions of current spectroscopic samples.Comment: 16 pages, including 8 figures and 2 tables; accepted for publication
by the Astrophysical Journa
K Corrections For Type Ia Supernovae and a Test for Spatial Variation of the Hubble Constant
Cross-filter K corrections for a sample of "normal" Type Ia supernovae (SNe)
have been calculated for a range of epochs. With appropriate filter choices,
the combined statistical and systematic K correction dispersion of the full
sample lies within 0.05 mag for redshifts z<0.7. This narrow dispersion of the
calculated K correction allows the Type Ia to be used as a cosmological probe.
We use the K corrections with observations of seven SNe at redshifts 0.3 < z
<0.5 to bound the possible difference between the locally measured Hubble
constant (H_L) and the true cosmological Hubble constant (H_0).Comment: 6 pages, 3 Postscript figures, uuencoded uses crckapb.sty and
psfig.sty. To appear in Thermonuclear Supernovae (NATO ASI), eds. R. Canal,
P. Ruiz-LaPuente, and J. Isern. Postscript version is also available at
http://www-supernova.lbl.gov
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