747 research outputs found
Star Formation Rate Distributions: Inadequacy of the Schechter Function
In this paper we posit that galaxy luminosity functions (LFs) come in two
fundamentally different types depending on whether the luminosity traces galaxy
stellar mass or its current star formation rate (SFR). Mass function types
reflect the older stars and therefore the stellar mass distribution, while SFR
function types arise from the young stars and hence the distribution of SFRs.
Optical and near-infrared LFs are of the mass function type, and are well fit
by a Schechter function (power law with an exponential cutoff at the bright
end). In contrast, LFs of the SFR function type are of a different form, one
that cannot be adequately described by a Schechter function. We demonstrate
this difference by generating SFR distributions for mock samples of galaxies
drawn from a Schechter stellar mass distribution along with established
empirical relations between the SFR and stellar mass. Compared with the
Schechter function, SFR distributions have a shallower decline at the bright
end, which can be traced to the large intrinsic scatter of SFRs at any given
stellar mass. A superior description of SFR distributions is given by the
"Saunders" function, which combines a power law with a Gaussian at the high
end. We show that the Schechter-like appearance of UV and H alpha LFs, although
they are LFs of SFR function type, results when luminosities are not corrected
for dust, or when average statistical corrections are used because individual
attenuation measurements are not available. We thus infer that the
non-Schechter form of the far-IR LFs is a true reflection of the underlying SFR
distribution, rather than the purported artifact of AGN contamination.Comment: Revised after a referee report. Submitted to ApJ. Compatible with B/W
printers. Comments are welcom
Dust Attenuation Curves in the Local Universe: Demographics and New Laws for Star-forming Galaxies and High-redshift Analogs
We study dust attenuation curves of 230,000 individual galaxies in the local
universe, ranging from quiescent to intensely star-forming systems, using
GALEX, SDSS, and WISE photometry calibrated on Herschel-ATLAS. We use a new
method of constraining SED fits with infrared luminosity (SED+LIR fitting), and
parameterized attenuation curves determined with the CIGALE SED fitting code.
Attenuation curve slopes and UV bump strengths are reasonably well constrained
independently from one another. We find that attenuation
curves exhibit a very wide range of slopes that are on average as steep as the
SMC curve slope. The slope is a strong function of optical opacity. Opaque
galaxies have shallower curves - in agreement with recent radiate transfer
models. The dependence of slopes on the opacity produces an apparent dependence
on stellar mass: more massive galaxies having shallower slopes. Attenuation
curves exhibit a wide range of UV bump amplitudes, from none to MW-like; with
an average strength 1/3 of the MW bump. Notably, local analogs of high-redshift
galaxies have an average curve that is somewhat steeper than the SMC curve,
with a modest UV bump that can be to first order ignored, as its effect on the
near-UV magnitude is 0.1 mag. Neither the slopes nor the strengths of the UV
bump depend on gas-phase metallicity. Functional forms for attenuation laws are
presented for normal star-forming galaxies, high-z analogs and quiescent
galaxies. We release the catalog of associated SFRs and stellar masses
(GSWLC-2).Comment: Accepted to ApJ. GSWLC-2 catalog of SED+LIR SFRs and M* to be
released Jun 1 at http://pages.iu.edu/~salims/gswlc
On the Mass-Metallicity-Star Formation Rate Relation for Galaxies at
Recent studies have shown that the local mass-metallicity (M-Z) relation
depends on the specific star formation rate (SSFR). Whether such a dependence
exists at higher redshifts, and whether the resulting M-Z-SFR relation is
redshift invariant, is debated. We re-examine these issues by applying the
non-parametric techniques of Salim et al. (2014) to ~130 galaxies
with N2 and O3 measurements from KBSS (Steidel et al. 2014). We find that the
KBSS M-Z relation depends on SSFR at intermediate masses, where such dependence
exists locally. KBSS and SDSS galaxies of the same mass and SSFR ("local
analogs") are similarly offset in the BPT diagram relative to the bulk of local
star-forming galaxies, and thus we posit that metallicities can be compared
self-consistently at different redshifts as long as the masses and SSFRs of the
galaxies are similar. We find that the M-Z-SFR relation of galaxies is
consistent with the local one at , but is offset up to -0.25 dex
at higher masses, so it is altogether not redshift invariant. This high-mass
offset could arise from a bias that high-redshift spectroscopic surveys have
against high-metallicity galaxies, but additional evidence disfavors this
possibility. We identify three causes for the reported discrepancy between N2
and O3N2 metallicities at : (1) a smaller offset that is also present
for SDSS galaxies, which we remove with new N2 calibration, (2) a genuine
offset due to differing ISM condition, which is also present in local analogs,
(3) an additional offset due to unrecognized AGN contamination.Comment: ApJ accepted. 14 pages. Comments welcom
Mach-Zehnder Interferometry in a Strongly Driven Superconducting Qubit
We demonstrate Mach-Zehnder-type interferometry in a superconducting flux
qubit. The qubit is a tunable artificial atom, whose ground and excited states
exhibit an avoided crossing. Strongly driving the qubit with harmonic
excitation sweeps it through the avoided crossing two times per period. As the
induced Landau-Zener transitions act as coherent beamsplitters, the accumulated
phase between transitions, which varies with microwave amplitude, results in
quantum interference fringes for n=1...20 photon transitions. The
generalization of optical Mach-Zehnder interferometry, performed in qubit phase
space, provides an alternative means to manipulate and characterize the qubit
in the strongly-driven regime.Comment: 14 pages, 6 figure
A Study of Two Dwarf Irregular Galaxies with Asymmetrical Star Formation Distributions
Two dwarf irregular galaxies, DDO 187 and NGC 3738, exhibit a striking pattern of star formation: intense star formation is taking place in a large region occupying roughly half of the inner part of the optical galaxy. We use data on the H i distribution and kinematics and stellar images and colors to examine the properties of the environment in the high star formation rate (HSF) halves of the galaxies in comparison with the low star formation rate halves. We find that the pressure and gas density are higher on the HSF sides by 30%–70%. In addition we find in both galaxies that the H i velocity fields exhibit significant deviations from ordered rotation and there are large regions of high-velocity dispersion and multiple velocity components in the gas beyond the inner regions of the galaxies. The conditions in the HSF regions are likely the result of large-scale external processes affecting the internal environment of the galaxies and enabling the current star formation there
Resonant readout of a superconducting persistent current qubit
Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2006.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Includes bibliographical references (p. 211-218).Superconducting Josephson junction devices rank among the best candidates for realizing a quantum computer. While the coherent control of quantum dynamics has been demonstrated in these solid-state, macroscopic quantum systems, a major challenge has been to increase the coherence times for these qubits. With an objective to reduce the level of readout-induced decoherence, this thesis work focuses on a resonant readout scheme developed for a niobium persistent-current (PC) qubit. This non-dissipative readout approach detects the flux state of the qubit by sensing a change in the Josephson inductance of a SQUID magnetometer. By incorporating the SQUID inductor in a high-Q resonant circuit, we distinguished the flux states of the qubit as a shift in the resonant frequency at 300 mK. The nonlinearity due to the Josephson inductance has characteristic effects on the resonant behavior of the readout circuit. We observed novel manifestation of this nonlinearity given the high quality factor of the resonance. The readout circuit was characterized in the linear as well as the nonlinear regime for its potential use as a bifurcation amplifier. Numerical simulations based on Josephson-junction circuits were also performed to understand the observed nonlinearity in the resonant behavior.by Janice C. Lee.Ph.D
A Deeper Look at Faint Hα Emission in Nearby Dwarf Galaxies
We present deep Hα imaging of three nearby dwarf galaxies, carefully selected to optimize observations with the Maryland-Magellan Tunable Filter (MMTF) on the Magellan 6.5 m telescope. An effective bandpass of ~13 Å is used, and the images reach 3σ flux limits of ~8 × 10^(−18) erg s^(−1) cm^(−2), which is about an order of magnitude lower than standard narrowband observations obtained by the most recent generation of local Hα galaxy surveys. The observations were originally motivated by the finding that the Hα/FUV flux ratio of galaxies systematically declines as global galactic properties such as the star formation rate (SFR) and stellar mass decrease. The three dwarf galaxies selected for study have SFRs that, when calculated from their Hα luminosities using standard conversion recipes, are ~50% of those based on the FUV. Follow-up studies of many of the potential causes for the trends in the Hα/FUV flux ratio have been performed, but the possibility that previous observations have missed a non-negligible fraction of faint ionized emission in dwarf galaxies has not been investigated. The MMTF observations reveal both diffuse and structured Hα emission (filaments, shells, possible single-star H ii regions) spanning extents up to 2.5 times larger relative to previous observations. However, only up to an additional ~5% of Hα flux is captured, which does not account for the trends in the Hα/FUV ratio. Beyond investigation of the Hα/FUV ratio, the impact of the newly detected extended flux on our understanding of star formation, the properties of H ii regions, and the propagation of ionizing photons warrant further investigation
- …