1,201 research outputs found
Extensive near-infrared (H-band) photometry in Coma
We present extensive and accurate photometry in the near-infrared H band of a
complete sample of objects in an area of about 400 arcmin2 toward the Coma
cluster of galaxies. The sample, including about 300 objects, is complete down
to H~17 mag, the exact value depending on the type of magnitude (isophotal,
aperture, Kron) and the particular region studied. This is six magnitudes below
the characteristic magnitude of galaxies, well into the dwarfs' regime at the
distance of the Coma cluster. For each object (star or galaxy) we provide
aperture magnitudes computed within five different apertures, the magnitude
within the 22 mag arcsec2 isophote, the Kron magnitude and radius, magnitude
errors, as well as the coordinates, the isophotal area, and a stellarity index.
Photometric errors are 0.2 mag at the completness limit. This sample is meant
to be the zero-redshift reference for evolutionary studies of galaxies.Comment: A&AS in press, paper, with high resolution images & tables are
available at http://oacosf.na.astro.it/~andreon/listapub.htm
Proteoglycan-Mediated Axon Degeneration Corrects Pretarget Topographic Sorting Errors
SummaryProper arrangement of axonal projections into topographic maps is crucial for brain function, especially in sensory systems. An important mechanism for map formation is pretarget axon sorting, in which topographic ordering of axons appears in tracts before axons reach their target, but this process remains poorly understood. Here, we show that selective axon degeneration is used as a correction mechanism to eliminate missorted axons in the optic tract during retinotectal development in zebrafish. Retinal axons are not precisely ordered during initial pathfinding but become corrected later, with missorted axons selectively fragmenting and degenerating. We further show that heparan sulfate is required non-cell-autonomously to correct missorted axons and that restoring its synthesis at late stages in a deficient mutant is sufficient to restore topographic sorting. These findings uncover a function for developmental axon degeneration in ordering axonal projections and identify heparan sulfate as a key regulator of that process
Morphological classification and structural parameter of galaxies in the Coma and Perseus clusters
We present the results of an isophotal shape analysis of galaxies in the Coma
and Perseus clusters. These data, together with those of two previous papers,
provide two complete samples of galaxies with reliable Hubble types in rich
clusters: 1) all galaxies brighter than m_b = 16.5 falling within one degree
(=2.3 Mpc) from the center of the Coma cluster (187 galaxies), 2) all galaxies
brighter than m_{Zwicky}=15.7 in a region of 5 deg 3' times 5 deg 27' around
the center of the Perseus cluster (139 galaxies). These two complete samples
cover 5 orders of magnitude in galaxy density and span areas of 91 and 17
Mpc^2, clustercentric radii up to 2.3 and 6.4 Mpc, for Perseus and Coma
respectively. They will be used in subsequent papers to study the dependence of
galaxy types on cluster environment and as reference samples in comparisons
with distant clusters.Comment: A&AS, in press. 14 pages including 2 figures and 5 tables, A&A macro
The Star Clusters in the Irregular Galaxy NGC 4449
We examine the star clusters in the irregular galaxy NGC 4449. We use a
near-infrared spectrum and broad-band images taken with the HST to place a
limit of 8--15 Myrs on the age of the bright central ojbect in NGC 4449. Its
luminosity and size suggest that it is comparable to young super star clusters.
However, there is a peculiar nucleated-bar structure at the center of this star
cluster, and we suggest that this structure is debris from the interaction that
has produced the counter-rotating gas systems and extended gas streamers in the
galaxy.
From the images we identify 60 other candidate compact star clusters in NGC
4449. Fourteen of these could be background elliptical galaxies or old globular
star clusters. Of the star clusters, three, in addition to the central object,
are potentially super star clusters, and many others are comparable to the
populous clusters found in the LMC. The star clusters span a large range in
ages with no obvious peak in cluster formation that might be attributed to the
interaction that the galaxy has experienced.Comment: To be published in PASP, Feb. 2001; also attainable from
ftp.lowell.edu, cd pub/dah/n4449pape
How Dry Are Red Mergers?
The focus of current research in galaxy evolution has increasingly turned to
understanding the effect that mergers have on the evolution of systems on the
red sequence. For those interactions purported to occur dissipationlessly (so
called "dry mergers"), it would appear that the role of gas is minimal.
However, if these mergers are not completely dry, then even low levels of gas
may be detectable. The purpose of our study is to test whether early type
galaxies with HI in or around them, or "wet" ellipticals, would have been
selected as dry mergers by the criteria in van Dokkum (2005, AJ, 130, 2647). To
that end, we examine a sample of 20 early types from the HI Rogues Gallery with
neutral hydrogen in their immediate environs. Of these, the 15 brightest and
reddest galaxies match the optical dry merger criteria, but in each case, the
presence of HI means that they are not truly dry.Comment: 8 pages plus 1 table and 5 figures; accepted for publication in A
Morphological classification and structural parameters for early-type galaxies in the Coma cluster
We present the results of an isophotal shape analysis of three samples of
galaxies in the Coma cluster. Quantitative morphology, together with structural
and photometric parameters, is given for each galaxy. Special emphasis has been
placed on the detailed classification of early-type galaxies.
The three samples are: i) a sample of 97 early-type galaxies brighter than
m = 17.00 falling within one degree from the center of the Coma cluster;
these galaxies were observed with CCD cameras, mostly in good to excellent
resolution conditions; ii) a magnitude complete sample of 107 galaxies of all
morphological types down to m = 17.00 falling in a circular region of 50
arcmin diameter, slightly offcentered to the North-West of the cluster center;
the images for this and the next sample come from digitized photographic
plates; iii) a complete comparison sample of 26 galaxies of all morphological
types down to m = 16.05 (or m 17.5), also in a region of 50
arcmin diameter, but centered 2.6 degrees West of the cluster center.
The reliability of our morphological classifications and structural
parameters of galaxies, down to the adopted magnitude limits, is assessed by
comparing the results on those galaxies for which we had images taken with
different instrumentation and/or seeing conditions, and by comparing our
results with similar data from other observers.Comment: 22 pages, including 4 figures and 4 tables, uuencoded, gzipped
postscrip
The Age Difference between the Globular Cluster Sub-populations in NGC 4472
The age difference between the two main globular cluster sub-populations in
the Virgo giant elliptical galaxy, NGC 4472 (M 49), has been determined using
HST WFPC2 images in the F555W and F814W filters. Accurate photometry has been
obtained for several hundred globular clusters in each of the two main
sub-populations, down to more than one magnitude below the turn-over of their
luminosity functions. This allows precise determinations of both the mean
colors and the turn-over magnitudes of the two main sub-populations. By
comparing the data with various population synthesis models, the
age-metallicity pairs that fit both the observed colors and magnitudes have
been identified. The metal-poor and the metal-rich globular clusters are found
to be coeval within the errors ( Gyr). If one accepts the validity of
our assumptions, these errors are dominated by model uncertainties. A
systematic error of up to 4 Gyr could affect this result if the blue and the
red clusters have significantly different mass distributions. However, that one
sub-population is half as old as the other is excluded at the 99% confidence
level. The different globular cluster populations are assumed to trace the
galaxy's major star-formation episodes. Consequently, the vast majority of
globular clusters, and by implication the majority of stars, in NGC 4472 formed
at high redshifts but by two distinct mechanisms or in two episodes.Comment: 32 pages, including 12 postscript figures, accepted for publication
in the Astronomical Journal, December 1999 issu
Linear characterization and modeling of GaN-on-Si HEMT technologies with 100 nm and 60 nm gate lengths
Motivated by the growing interest towards low-cost, restriction-free MMIC processes suitable for multi-function, possibly space-qualified applications, this contribution reports the extraction of reliable linear models for two advanced GaN-on-Si HEMT technologies, namely OMMICâs D01GH (100 nm gate length) and D006GH (60 nm gate length). This objective is pursued by means of both classical and more novel approaches. In particular, the latter include a nondestructive method for determining the extrinsic resistances and an optimizaion-based approach to extracting the remaining parasitic elements: these support standard DC and RF measurements in order to obtain a scalable, bias-dependent equivalent-circuit model capturing the small-signal behavior of the two processes. As to the noise model, this is extracted by applying the well known noise-temperature approach to noise figure measurements performed in two different frequency ranges: a lower band, where a standard Y-factor test bench is used, and an upper band, where a custom cold-source test bench is set up and described in great detail. At 5 V drain-source voltage, minimum noise figures as low as 1.5 dB and 1.1 dB at 40 GHz have been extracted for the considered 100 nm and 60 nm HEMTs, respectively: this testifies the maturity of both processes and the effectiveness of the gate length reduction. The characterization and modeling campaign, here presented for the first time, has been repeatedly validated by published designs, a couple of which are reviewed for the Readerâs convenience
Trans-Axonal Signaling in Neural Circuit Wiring
The development of neural circuits is a complex process that relies on the proper navigation of axons through their environment to their appropriate targets. While axonâenvironment and axonâtarget interactions have long been known as essential for circuit formation, communication between axons themselves has only more recently emerged as another crucial mechanism. Trans-axonal signaling governs many axonal behaviors, including fasciculation for proper guidance to targets, defasciculation for pathfinding at important choice points, repulsion along and within tracts for pre-target sorting and target selection, repulsion at the target for precise synaptic connectivity, and potentially selective degeneration for circuit refinement. This review outlines the recent advances in identifying the molecular mechanisms of trans-axonal signaling and discusses the role of axonâaxon interactions during the different steps of neural circuit formation
- âŠ