155 research outputs found
The Luminosity Function and Surface Brightness Distribution of HI Selected Galaxies
We measure the z=0 B-band optical luminosity function (LF) for galaxies
selected in a blind HI survey. The total LF of the HI selected sample is flat,
with Schechter parameters M*=-19.38_{-0.62}^{+1.02} + 5 log h mag and
alpha=-1.03_{-0.15}^{+0.25}, in good agreement with LFs of optically selected
late-type galaxies. Bivariate distribution functions of several galaxy
parameters show that the HI density in the local Universe is more widely spread
over galaxies of different size, central surface brightness, and luminosity
than is the optical luminosity density. The number density of very low surface
brightness (>24.0 mag/arcsec^2) gas-rich galaxies is considerably lower than
that found in optical surveys designed to detect dim galaxies. This suggests
that only a part of the population of LSB galaxies is gas rich and that the
rest must be gas poor. However, we show that this gas-poor population must be
cosmologically insignificant in baryon content. The contribution of gas-rich
LSB galaxies (>23.0 mag/arcsec^2) to the local cosmological gas and luminosity
density is modest (18_{-5}^{+6} and 5_{-2}^{+2} per cent respectively); their
contribution to Omega_matter is not well-determined, but probably < 11 per
cent. These values are in excellent agreement with the low redshift results
from the Hubble Deep Field.Comment: Accepted for publication in MNRAS, 12 pages 6 figure
Responses of Descending Visually-Sensitive Neurons in the Hawkmoth, Manduca sexta, to Three-Dimensional Flower-Like Stimuli.
Hawkmoths rely on vision to track moving flowers during hovering-feeding bouts. Visually guided flight behaviors require a sensorimotor transformation, where motion information processed by the optic ganglia ultimately modifies motor axon activity. While a great deal is known about motion processing in the optic lobes of insects, there has been far less exploration into the visual information available to flight motor axons. Visual information recorded at this stage has likely arisen from multiple visual pathways, and has potentially been modified by outside sensory information. As a first step, understanding the sensorimotor transformation from transduction of moving flower signals to active flower tracking behavior requires that the visual information available to the thoracic flight control centers be assayed. This paper investigated the response of descending visually sensitive neurons in the cervical connectives of the hawkmoth, Manduca sexta L. (Lepidoptera: Sphingidae), to flower-like stimuli. Because flower structure lends itself to oscillatory (vibratory) motion, the stimuli used in these experiments were discs oscillating in each axis of motion (horizontal, vertical, and looming). Object-sensitive descending-neurons (OSDNs) respond to multiple directions of object motion and do not clearly sort into classes of directional tuning. The broad spatial distribution of directional sensitivities exhibited by OSDNs indicates that the direction of object motion may be encoded on a population scale. Although OSDNs exhibit broad frequency response curves, over the range of frequencies that M. sexta are able to track (0–2 Hz) OSDNs exhibit monotonically increasing response. Additionally, OSDNs respond to discs oscillating at frequencies as high at 6 Hz, indicating that the visual information being sent to thoracic motor control centers is not likely the limiting factor in flower tracking ability
The Luminosity Function and Surface Brightness Distribution of HI Selected Galaxies
We measure the z=0 B-band optical luminosity function (LF) for galaxies
selected in a blind HI survey. The total LF of the HI selected sample is flat,
with Schechter parameters M*=-19.38_{-0.62}^{+1.02} + 5 log h mag and
alpha=-1.03_{-0.15}^{+0.25}, in good agreement with LFs of optically selected
late-type galaxies. Bivariate distribution functions of several galaxy
parameters show that the HI density in the local Universe is more widely spread
over galaxies of different size, central surface brightness, and luminosity
than is the optical luminosity density. The number density of very low surface
brightness (>24.0 mag/arcsec^2) gas-rich galaxies is considerably lower than
that found in optical surveys designed to detect dim galaxies. This suggests
that only a part of the population of LSB galaxies is gas rich and that the
rest must be gas poor. However, we show that this gas-poor population must be
cosmologically insignificant in baryon content. The contribution of gas-rich
LSB galaxies (>23.0 mag/arcsec^2) to the local cosmological gas and luminosity
density is modest (18_{-5}^{+6} and 5_{-2}^{+2} per cent respectively); their
contribution to Omega_matter is not well-determined, but probably < 11 per
cent. These values are in excellent agreement with the low redshift results
from the Hubble Deep Field.Comment: Accepted for publication in MNRAS, 12 pages 6 figure
Low Surface Brightness Galaxies in the Local Universe; 3, Implications for the Field Galaxy Luminosity Function
We present a luminosity function for low surface brightness (LSB) galaxies identified in the APM survey of Impey et al 1996. These galaxies have central surface brightnesses in B in the range 22 < mu < 25. Using standard maximum-likelihood estimators, we determine that the best-fit Schechter function parameters for this luminosity function (LF) are alpha = -1.42, M* = -18.34, and phi* = 0.0036, assuming H_0 = 100. We compare these results to those of other recent studies and find that surveys which do not take account of the observation selection bias imposed by surface brightness are missing a substantial fraction of the galaxies in the local universe. Under our most conservative estimates, our derivation of the LF for LSB galaxies suggests that the CfA redshift survey has missed at least one third of the local galaxy population. This overlooked fraction is not enough by itself to explain the large number of faint blue galaxies observed at moderate redshift under no-evolution models, but it does help close the gap between local and moderate-redshift galaxy counts
Neural mechanisms underlying target detection in a dragonfly centrifugal neuron
© The Company of Biologists Ltd 2007Visual identification of targets is an important task for many animals searching for prey or conspecifics. Dragonflies utilize specialized optics in the dorsal acute zone, accompanied by higher-order visual neurons in the lobula complex, and descending neural pathways tuned to the motion of small targets. While recent studies describe the physiology of insect small target motion detector (STMD) neurons, little is known about the mechanisms that underlie their exquisite sensitivity to target motion. Lobula plate tangential cells (LPTCs), a group of neurons in dipteran flies selective for wide-field motion, have been shown to take input from local motion detectors consistent with the classic correlation model developed by Hassenstein and Reichardt in the 1950s. We have tested the hypothesis that similar mechanisms underlie the response of dragonfly STMDs. We show that an anatomically characterized centrifugal STMD neuron (CSTMD1) gives responses that depend strongly on target contrast, a clear prediction of the correlation model. Target stimuli are more complex in spatiotemporal terms than the sinusoidal grating patterns used to study LPTCs, so we used a correlation-based computer model to predict response tuning to velocity and width of moving targets. We show that increasing target width in the direction of travel causes a shift in response tuning to higher velocities, consistent with our model. Finally, we show how the morphology of CSTMD1 allows for impressive spatial interactions when more than one target is present in the visual field.Bart R. H. Geurten, Karin Nordström, Jordanna D. H. Sprayberry, Douglas M. Bolzon and David C. O'Carrol
HI Observations of Giant Low Surface Brightness Galaxies
We have used the Nancay Radio Telescope to obtain new global HI data for 16
giant low surface brightness (LSB) galaxies. Our targets have optical
luminosities and disk scale lengths at the high end for spiral galaxies
(L_B~10^10 Lsun and h_r>~6 kpc for H_0=75 km/s/Mpc), but they have diffuse
stellar disks, with mean disk surface brightnesses ~1 magnitude or more fainter
than normal giant spirals. Thirteen of the galaxies previously had been
detected in HI by other workers, but the published HI observations were either
confused, resolved by the telescope beam, of low signal-to-noise, or showed
significant discrepancies between different authors. For the other 3 galaxies,
no HI data were previously available. Several of the galaxies were resolved by
the Nancay 3.6' E-W beam, so global parameters were derived from multiple-point
mapping observations. Typical HI masses for our sample are ~10^10 Msun, with
M_HI/L_B=0.3-1.7 (in solar units). All of the observed galaxies have published
optical surface photometry, and we have compiled key optical measurements for
these objects from the literature. We frequently find significant variations
among physical parameters of giant LSB galaxies reported by various workers.Comment: accepted to Astronomy and Astrophysics Supplements; 14 pages;
improved table formattin
Low and High Surface Brightness Galaxies at Void Walls
We study the relative fraction of low and high surface brightness galaxies
(LSBGs and HSBGs) at void walls in the SDSS DR7. We focus on galaxies in equal
local density environments. We assume that the host dark-matter halo mass (for
which we use SDSS group masses) is a good indicator of local density. This
analysis allows to examine the behavior of the abundance of LSBG and HSBG
galaxies at a fixed local density and distinguish the large-scale environment
defined by the void geometry. We compare galaxies in the field, and in the void
walls; the latter are defined as the volume of void shells of radius equal to
that of the void. We find a significant decrement, a factor , of the
relative fraction of blue, active star-forming LSBGs in equal mass groups at
the void walls and the field. This decrement is consistent with an increase of
the fraction of blue, active star-forming HSBGs. By contrast, red LSBGs and
HSBGs show negligible changes. We argue that these results are consistent with
a scenario where LSBGs with blue colors and strong star formation activity at
the void walls are fueled by gas from the expanding void regions. This process
could lead to LSBG to HSBG transformations.Comment: 5 pages, 4 figures, accepted for publication in MNRAS Letter
A search for Low Surface Brightness galaxies in the near-infrared I. Selection of the sample
A sample of about 3,800 Low Surface Brightness (LSB) galaxies was selected
using the all-sky near-infrared (J, H and K_s-band) 2MASS survey. The selected
objects have a mean central surface brightness within a 5 arcsec radius around
their centre fainter than 18 mag/sq.arcsec in the K_s band, making them the
lowest surface brightness galaxies detected by 2MASS. A description is given of
the relevant properties of the 2MASS survey and the LSB galaxy selection
procedure, as well as of basic photometric properties of the selected objects.
The latter properties are compared to those of other samples of galaxies, of
both LSBs and `classical' high surface brightness (HSB) objects, which were
selected in the optical. The 2MASS LSBs have a (B_T_c)-(K_T) colour which is on
average 0.9 mag bluer than that of HSBs from the NGC. The 2MASS sample does not
appear to contain a significant population of red objects.Comment: accepted for publication in Astronomy and Astrophysics on 24/2/2003;
62 page
Low Surface Brightness Galaxies in the Local Universe. III. Implications for the Field Galaxy Luminosity Function
We present a luminosity function for low surface brightness (LSB) galaxies
identified in the APM survey of Impey et al 1996. These galaxies have central
surface brightnesses in B in the range 22 < mu < 25. Using standard
maximum-likelihood estimators, we determine that the best-fit Schechter
function parameters for this luminosity function (LF) are alpha = -1.42, M* =
-18.34, and phi* = 0.0036, assuming H_0 = 100. We compare these results to
those of other recent studies and find that surveys which do not take account
of the observation selection bias imposed by surface brightness are missing a
substantial fraction of the galaxies in the local universe. Under our most
conservative estimates, our derivation of the LF for LSB galaxies suggests that
the CfA redshift survey has missed at least one third of the local galaxy
population. This overlooked fraction is not enough by itself to explain the
large number of faint blue galaxies observed at moderate redshift under
no-evolution models, but it does help close the gap between local and
moderate-redshift galaxy counts.Comment: 23 pages including 8 figures and 1 table, LaTeX, style file dspp4.sty
included, complete PostScript file available at
ftp://lplx06.ing.iac.es/pub/dspray/lsb/lumfunc/preprint.ps.gz , Accepted by
ApJ, scheduled for June 1 199
- …