7,216 research outputs found
Push-Pull Control of Motor Output
Inhibition usually decreases inputâoutput excitability of neurons. If, however, inhibition is coupled to excitation in a pushâpull fashion, where inhibition decreases as excitation increases, neuron excitability can be increased. Although the presence of pushâpull organization has been demonstrated in single cells, its functional impact on neural processing depends on its effect on the system level. We studied pushâpull in the motor output stage of the feline spinal cord, a system that allows independent control of inhibitory and excitatory components. Pushâpull organization was clearly present in ankle extensor motoneurons, producing increased peak-to-peak modulation of synaptic currents. The effect at the system level was equally strong. Independent control of the inhibitory component showed that the stronger the background of inhibition, the greater the peak force production. This illustrates the paradox at the heart of pushâpull organization: increased force output can be achieved by increasing background inhibition to provide greater disinhibition
Local Starbursts in a Cosmological Context
In this contribution I introduce some of the major issues that motivate the
conference, with an emphasis on how starbursts fit into the ``big picture''. I
begin by defining starbursts in several different ways, and discuss the merits
and limitations of these definitions. I will argue that the most physically
useful definition of a starburst is its ``intensity'' (star formation rate per
unit area). This is the most natural parameter to compare local starbursts with
physically similar galaxies at high redshift, and indeed I will argue that
local starbursts are unique laboratories to study the processes at work in the
early universe. I will describe how NASA's GALEX mission has uncovered a rare
population of close analogs to Lyman Break Galaxies in the local universe. I
will then compare local starbursts to the Lyman-Break and sub-mm galaxies high
redshift populations, and speculate that the multidimensional ``manifold'' of
starbursts near and far can be understood largely in terms of the
Schmidt/Kennicutt law and galaxy mass-metallicity relation. I will briefly
summarize he properties of starburst-driven galactic superwinds and their
possible implications for the evolution of galaxies and the IGM. These complex
multiphase flows are best studied in nearby starbursts, where we can study the
the hot X-ray gas that contains the bulk of the energy and newly produced
metals.Comment: Proceedings of the Conference "Starbursts: Fropm 30 Doradus to Lyman
Break Galaxies
The Discovery of an Active Galactic Nucleus in the Late-type Galaxy NGC 3621: Spitzer Spectroscopic Observations
We report the discovery of an Active Galactic Nucleus (AGN) in the nearby SAd
galaxy NGC 3621 using Spitzer high spectral resolution observations. These
observations reveal the presence of [NeV] 14 um and 24 um emission which is
centrally concentrated and peaks at the position of the near-infrared nucleus.
Using the [NeV] line luminosity, we estimate that the nuclear bolometric
luminosity of the AGN is ~ 5 X 10^41 ergs s^-1, which corresponds based on the
Eddington limit to a lower mass limit of the black hole of ~ 4 X 10^3 Msun.
Using an order of magnitude estimate for the bulge mass based on the Hubble
type of the galaxy, we find that this lower mass limit does not put a strain on
the well-known relationship between the black hole mass and the host galaxy's
stellar velocity dispersion established in predominantly early-type galaxies.
Mutli-wavelength follow-up observations of NGC 3621 are required to obtain more
precise estimates of the bulge mass, black hole mass, accretion rate, and
nuclear bolometric luminosity. The discovery reported here adds to the growing
evidence that a black hole can form and grow in a galaxy with no or minimal
bulge.Comment: 5 pages, 7 figures, Accepted for publication in ApJ Letter
Uncovering Spiral Structure in Flocculent Galaxies
We present K'(2.1 micron) observations of four nearby flocculent spirals,
which clearly show low-level spiral structure and suggest that kiloparsec-scale
spiral structure is more prevalent in flocculent spirals than previously
supposed. In particular, the prototypical flocculent spiral NGC 5055 is shown
to have regular, two-arm spiral structure to a radius of 4 kpc in the near
infrared, with an arm-interarm contrast of 1.3. The spiral structure in all
four galaxies is weaker than that in grand design galaxies. Taken in unbarred
galaxies with no large, nearby companions, these data are consistent with the
modal theory of spiral density waves, which maintains that density waves are
intrinsic to the disk. As an alternative, mechanisms for driving spiral
structure with non-axisymmetric perturbers are also discussed. These
observations highlight the importance of near infrared imaging for exploring
the range of physical environments in which large-scale dynamical processes,
such as density waves, are important.Comment: 12 pages AASTeX; 3 compressed PS figures can be retrieved from
ftp://ftp.astro.umd.edu/pub/michele as file thornley.tar (1.6Mbytes).
Accepted to Ap.J. Letters.(Figures now also available here, and from
ftp://ftp.astro.umd.edu/pub/michele , in GIF format.
Spitzer Uncovers Active Galactic Nuclei Missed by Optical Surveys in 7 Late-type Galaxies
We report the discovery using Spitzers high resolution spectrograph of 7
Active Galactic Nuclei (AGN) in a sample of 32 late-type galaxies that show no
definitive signatures of AGN in their optical spectra. Our observations suggest
that the AGN detection rate in late-type galaxies is possibly 4 times larger
than what optical spectroscopic observations alone suggest. We demonstrate
using photoionization models with an input AGN and an extreme EUV-bright
starburst ionizing radiation field that the observed mid-infrared line ratios
cannot be replicated unless an AGN contribution, in some cases as little as 10%
of the total galaxy luminosity, is included. These models show that when the
fraction of the total luminosity due to the AGN is low, optical diagnostics are
insensitive to the presence of the AGN. In this regime of parameter space, the
mid-infrared diagnostics offer a powerful tool for uncovering AGN missed by
optical spectroscopy. The AGN bolometric luminosities in our sample range from
~3 X 10^41 - ~2 X 10^43 ergs s^-1, which, based on the Eddington limit,
corresponds to a lower mass limit for the black hole that ranges from ~3 X
10^3Mdot to as high as ~1.5 X 10^5Mdot. These lower mass limits however do not
put a strain on the well-known relationship between the black hole mass and the
host galaxy's stellar velocity dispersion established in predominantly
early-type galaxies. Our findings add to the growing evidence that black holes
do form and grow in low-bulge environments and that they are significantly more
common than optical studies indicate.Comment: 8 figures, 17 pages, astro-ph\0801.2766 (Abel & Satyapal 2008; ApJ
accepted) and this posting designed to form a two-part investigatio
Viewpoint: Estimating the causal effects of policies and programs
Estimation, inference and interpretation of the causal effects of programs and policies have all advanced dramatically over the past 25 years. We highlight three particularly important intellectual trends: an improved appreciation of the substantive importance of heterogeneous responses and of their methodological implications, a stronger focus on internal validity brought about by the âcredibility revolution,â and the scientific value that follows from grounding estimation and interpretation in economic theory. We discuss a menu of commonly employed partial equilibrium approaches to the identification of causal effects, emphasizing that the researcherâs central intellectual contribution always consists of making an explicit case for a specific causal interpretation given the relevant economic theory, the data, the institutional context and the economic question of interest. We also touch on the importance of general equilibrium effects and full costâbenefit analyses.RĂ©sumĂ©Point de vue: Sur lâestimation des effets causatifs des politiques et programmes. Dans le monde de lâestimation, lâinfĂ©rence et lâinterprĂ©tation des effets causatifs des programmes et des politiques, il y a eu des progrĂšs dramatiques au cours des derniers 25 ans. Les auteurs soulignent trois tendances intellectuelles particuliĂšrement importantes : une apprĂ©ciation amĂ©liorĂ©e de lâimportance substantielle des rĂ©ponses hĂ©tĂ©rogĂšnes et de leur importance mĂ©thodologique, une focalisation plus robuste sur la validitĂ© interne engendrĂ©e par la « rĂ©volution de la crĂ©dibilitĂ© », et la valeur scientifique qui dĂ©coule dâun ancrage de lâestimation et de lâinterprĂ©tation dans la thĂ©orie Ă©conomique. On discute un Ă©ventail dâapproches dâĂ©quilibre partiel Ă lâidentification des effets causatifs, mettant au premier plan que la contribution intellectuelle centrale du chercheur consiste Ă bĂątir un argumentaire explicite pour une interprĂ©tation causale spĂ©cifique compte tenu de la thĂ©orie Ă©conomique pertinente, des donnĂ©es, du contexte institutionnel, et de la question Ă©conomique dâintĂ©rĂȘt. On mentionne aussi lâimportance des effets dâĂ©quilibre gĂ©nĂ©ral et des analyses de tous les coĂ»ts et avantages.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/134884/1/caje12217.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/134884/2/caje12217_am.pd
Spectral Energy Distributions of starburst galaxies in the 900-1200 A range
We present the 970-1175 A spectral energy distributions (SEDs) of 12
starburst galaxies observed with the Far Ultraviolet Spectroscopic Explorer
FUSE. We take benefit of the high spectral resolution of FUSE to estimate a
continuum as much as possible unaffected by the interstellar lines. The
continuum is rather flat with, in few cases, a decrease at lambda <~1050 A, the
amplitude of which being correlated with various indicators of the dust
extinction. The far-UV SEDs are compared with synthetic population models. The
galaxies with almost no extinction have a SED consistent with an on-going star
formation over some Myrs. We derive a mean dust attenuation law in the
wavelength range 965-1140 A by comparing the SED of obscured galaxies to an
empirical dust-free SED. The extinction is nearly constant longward of 1040 A
but rises at shorter wavelengths. We compare our results with other studies of
the extinction for galaxies and stars in this wavelength range.Comment: 11 pages, 6 postscript figures, accepted for publication in Astronomy
& Astrophysic
Optical Spectroscopy of the IRAS 1-Jy Sample of Ultraluminous Infrared Galaxies
This paper discusses the optical spectroscopic properties of the IRAS 1-Jy
sample of ultraluminous infrared galaxies (ULIGs). One hundred and eight of the
118 1-Jy ULIGs have been observed at dlambda = 8.3 AA resolution over the
wavelength range ~4500 A -- 8900 A. These data are combined with large,
previously published sets of optical spectroscopic data of lower luminosity
infrared galaxies to look for systematic trends with infrared luminosity over
the luminosity range L_ir ~ 10^{10.5}-10^{13} L_sun. As found in previous
studies, the fraction of Seyfert galaxies among luminous infrared galaxies
increases abruptly above L_ir ~ 10^{12.3} L_sun --- about 50% of the galaxies
with L_ir > 10^{12.3} L_sun present Seyfert characteristics. Many of the
optical and infrared spectroscopic properties of the Seyfert galaxies are
consistent with the presence of a genuine active galactic nucleus (AGN). About
30% of these galaxies are Seyfert 1s with broad-line regions similar to those
of optical quasars. The percentage of Seyfert 1 ULIGs increases with infrared
luminosity, contrary to the predictions of the standard unification model for
Seyfert galaxies. Comparisons of the broad-line luminosities of optical and
obscured Seyfert 1 ULIGs with those of optically selected quasars of comparable
bolometric luminosity suggest that the dominant energy source in most of these
ULIGs is the same as in optical quasars, namely mass accretion onto a
supermassive black hole, rather than a starburst. These results are consistent
with recently published ISO, ASCA, and VLBI data. (abridged)Comment: Text and 23 figures (45 pages), Tables 1 - 6 (16 pages
The Role of Starbursts in the Formation of Galaxies & Active Galactic Nuclei
Starbursts are episodes of intense star-formation in the central regions of
galaxies, and are the sites of roughly 25% of the high-mass star-formation in
the local universe. In this contribution I review the role starbursts play in
the formation and evolution of galaxies, the intergalactic medium, and active
galactic nuclei. Four major conclusions are drawn. 1) Starburst galaxies are
good analogues (in fact, the only plausible local analogues) to the known
population of star-forming galaxies at high-redshift. 2) Integrated over cosmic
time, supernova-driven galactic-winds (`superwinds') play an essential role in
the evolution of galaxies and the inter-galactic medium. 3) Circumnuclear
starbursts are an energetically-significant component of the Seyfert
phenomenon. 4) The evolution of the population of the host galaxies of
radio-quiet quasars is significantly different than that of powerful radio
galaxies, and is at least qualitatively consistent with the standard picture of
the hierarchical assembly of massive galaxies at relatively late times.Comment: 16 pages, 4 figures, Royal Society discussion meeting `The formation
of galaxies
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