1,086 research outputs found
AGC 226067: A possible interacting low-mass system
We present Arecibo, GBT, VLA and WIYN/pODI observations of the ALFALFA source
AGC 226067. Originally identified as an ultra-compact high velocity cloud and
candidate Local Group galaxy, AGC 226067 is spatially and kinematically
coincident with the Virgo cluster, and the identification by multiple groups of
an optical counterpart with no resolved stars supports the interpretation that
this systems lies at the Virgo distance (D=17 Mpc). The combined observations
reveal that the system consists of multiple components: a central HI source
associated with the optical counterpart (AGC 226067), a smaller HI-only
component (AGC 229490), a second optical component (AGC 229491), and extended
low surface brightness HI. Only ~1/4 of the single-dish HI emission is
associated with AGC 226067; as a result, we find M_HI/L_g ~ 6 Msun/Lsun, which
is lower than previous work. At D=17 Mpc, AGC 226067 has an HI mass of 1.5 x
10^7 Msun and L_g = 2.4 x 10^6 Lsun, AGC 229490 (the HI-only component) has
M_HI = 3.6 x 10^6 Msun, and AGC 229491 (the second optical component) has L_g =
3.6 x 10^5 Lsun. The nature of this system of three sources is uncertain: AGC
226067 and AGC 229490 may be connected by an HI bridge, and AGC 229490 and AGC
229491 are separated by only 0.5'. The current data do not resolve the HI in
AGC 229490 and its origin is unclear. We discuss possible scenarios for this
system of objects: an interacting system of dwarf galaxies, accretion of
material onto AGC 226067, or stripping of material from AGC 226067.Comment: Accepted for publication in A&A. 6 pages, 4 figure
Nr-CAM and neurofascin interactions regulate ankyrin G and sodium channel clustering at the node of Ranvier
AbstractVoltage-dependent sodium (Na+) channels are highly concentrated at nodes of Ranvier in myelinated axons and play a key role in promoting rapid and efficient conduction of action potentials by saltatory conduction. The molecular mechanisms that direct their localization to the node are not well understood but are believed to involve contact-dependent signals from myelinating Schwann cells [1] and interactions of Na+ channels with the cytoskeletal protein, ankyrin G [2]. Two cell adhesion molecules (CAMs) expressed at the axon surface, Nr-CAM and neurofascin, are also linked to ankyrin G and accumulate at early stages of node formation, suggesting that they mediate contact-dependent Schwann cell signals to initiate node development [3]. To examine the potential role of Nr-CAM in this process, we treated myelinating cocultures of DRG (dorsal root ganglion) neurons and Schwann cells with an Nr-CAM-Fc (Nr-Fc) fusion protein. Nr-Fc had no effect on initial axon-Schwann cell interactions, including Schwann cell proliferation, or on the extent of myelination, but it strikingly and specifically inhibited Na+ channel and ankyrin G accumulation at the node. Nr-Fc bound directly to neurons and clustered and coprecipitated neurofascin expressed on axons. These results provide the first evidence that neurofascin plays a major role in the formation of nodes, possibly via interactions with Nr-CAM
Dust Properties and Star-Formation Rates in Star-Forming Dwarf Galaxies
We have used the Spitzer Space Telescope to study the dust properties of a
sample of star-forming dwarf galaxies. The differences in the mid-infrared
spectral energy distributions for these galaxies which, in general, are low
metallicity systems, indicate differences in the physical properties, heating,
and/or distribution of the dust. Specifically, these galaxies have more hot
dust and/or very small grains and less PAH emission than either spiral or
higher luminosity starburst galaxies. As has been shown in previous studies,
there is a gradual decrease in PAH emission as a function of metallicity.
Because much of the energy from star formation in galaxies is re-radiated in
the mid-infrared, star-formation rate indicators based on both line and
continuum measurements in this wavelength range are coming into more common
usage. We show that the variations in the interstellar medium properties of
galaxies in our sample, as measured in the mid-infrared, result in over an
order of magnitude spread in the computed star-formation rates.Comment: 25 pages, 11 figures, 4 tables, ApJ accepte
HI study of extremely metal-deficient dwarf galaxies. I. The Nancay Radio Telescope observations of twenty-two objects
The goal of this study is to measure parameters of the integrated HI emission
for twenty-two dwarf galaxies with oxygen abundance 12+log(O/H) in the range of
7.42 to 7.65, which are representatives of the eXtremely Metal-Deficient (XMD)
galaxy group. Some of them are expected to be similar to the well-known
candidates for local young galaxies, IZw18 and SBS 0335-052 that have most of
their baryon mass in the form of neutral gas. Therefore, the HI 21-cm line
observations are crucial to understanding their group and individual
properties. The Nancay Radio Telescope (NRT) with the upgraded focal receiver
was used for observations of the 21-cm HI-line. This permitted the detection of
the faintest sources with rms of ~1 mJy per 10.5 km/s resolution element. For
eighteen detected galaxies we present the parameters of their integrated HI
line emission and describe the data on individual objects in more detail. For
four undetected XMD galaxies, we give upper limits on their M$(HI). For 70% of
the twenty studied non low surface brightness XMD galaxies, we find evidence
(both from HI and optical data) for their interaction with neighboring objects.
In the brief discussion of the group HI properties of the observed subsample
(the total O/H range is of 0.23 dex, or a factor of 1.7), we underline the
broad distributions of the HI mass (range is of 2 orders of magnitude), of the
ratio M(HI)/L_B (of 1 order of magnitude), and of the blue luminosity (range is
of 2 orders of magnitude). We also obtained HI parameters of six galaxies that
do not belong to the XMD sample. These data increase the number of XMD galaxies
with known integrated HI parameters (or upper limits) by a factor of two. This
allows us to address statistical properties of this group, which will be
presented in a forthcoming paper. (Abridged).Comment: 11 pages, including 2 tables and 2 postscript figures. Accepted to
Astronomy and Astrophysic
The Tully-Fisher Relation and H_not
The use of the Tully-Fisher (TF) relation for the determination of the Hubble
Constant relies on the availability of an adequate template TF relation and of
reliable primary distances. Here we use a TF template relation with the best
available kinematical zero-point, obtained from a sample of 24 clusters of
galaxies extending to cz ~ 9,000 km/s, and the most recent set of Cepheid
distances for galaxies fit for TF use. The combination of these two ingredients
yields H_not = 69+/-5 km/(s Mpc). The approach is significantly more accurate
than the more common application with single cluster (e.g. Virgo, Coma)
samples.Comment: 10 pages, including 2 figures and 1 table; uses AAS LaTex. Submitted
to ApJ Letter
Metal Abundances of KISS Galaxies. I. Coarse Metal Abundances and the Metallicity-Luminosity Relation
We derive metal abundance estimates for a large sample of starbursting
emission-line galaxies (ELGs). Our sample is drawn from the KPNO International
Spectroscopic Survey (KISS) which has discovered over 2000 ELG candidates to
date. Follow-up optical spectra have been obtained for ~900 of these objects. A
three step process is used to obtain metal abundances for these galaxies. We
first calculate accurate nebular abundances for 12 galaxies whose spectra cover
the full optical region from [OII]3726,29 to beyond [SII]6717,31 and include
detection of [OIII]4363. Using secondary metallicity indicators R_23 and p_3,
we calculate metallicities for an additional 59 galaxies with spectra that
cover a similar wavelength range but lack [OIII]4363. The results are used to
calibrate relations between metallicity and two readily observed emission-line
ratios, which allow us to estimate coarse metallicities for 519 galaxies in
total. The uncertainty in these latter abundance estimates is 0.16 dex. From
the large, homogeneously observed sample of star-forming galaxies we identify
low metallicity candidates for future study and investigate the
metallicity-luminosity relation. We find a linear metallicity-luminosity
relation of the following form: 12 + log(O/H) = 4.059 - 0.240 M_B, with an RMS
scatter of 0.252. This result implies that the slope of the metallicity-
luminosity relation is steeper than when dwarf galaxies are considered alone,
and may be evidence that the relationship is not linear over the full
luminosity range of the sample.Comment: 15 pages, including 7 figures and 4 tables. To appear in May, 2002 A
A Test of the Standard Hypothesis for the Origin of the HI Holes in Holmberg II
The nearby irregular galaxy Holmberg II has been extensively mapped in HI
using the Very Large Array (VLA), revealing intricate structure in its
interstellar gas component (Puche et al. 1992). An analysis of these structures
shows the neutral gas to contain a number of expanding HI holes. The formation
of the HI holes has been attributed to multiple supernova events occurring
within wind-blown shells around young, massive star clusters, with as many as
10-200 supernovae required to produce many of the holes. From the sizes and
expansion velocities of the holes, Puche et al. assigned ages of ~10^7 to 10^8
years. If the supernova scenario for the formation of the HI holes is correct,
it implies the existence of star clusters with a substantial population of
late-B, A and F main sequence stars at the centers of the holes. Many of these
clusters should be detectable in deep ground-based CCD images of the galaxy. In
order to test the supernova hypothesis for the formation of the HI holes, we
have obtained and analyzed deep broad-band BVR and narrow-band H-alpha images
of Ho II. We compare the optical and HI data and search for evidence of the
expected star clusters in and around the HI holes. We also use the HI data to
constrain models of the expected remnant stellar population. We show that in
several of the holes the observed upper limits for the remnant cluster
brightness are strongly inconsistent with the SNe hypothesis described in Puche
et al. Moreover, many of the HI holes are located in regions of very low
optical surface brightness which show no indication of recent star formation.
Here we present our findings and explore possible alternative explanations for
the existence of the HI holes in Ho II, including the suggestion that some of
the holes were produced by Gamma-ray burst events.Comment: 30 pages, including 6 tables and 3 images. To appear in Astron.
Journal (June 1999
Emission-Line Galaxy Surveys as Probes of the Spatial Distribution of Dwarf Galaxies. I. The University of Michigan Survey
Objective-prism surveys which select galaxies on the basis of line-emission
are extremely effective at detecting low-luminosity galaxies and constitute
some of the deepest available samples of dwarfs. In this study, we confirm that
emission-line galaxies (ELGs) in the University of Michigan (UM)
objective-prism survey (MacAlpine et al. 1977-1981) are reliable tracers of
large-scale structure, and utilize the depth of the samples to examine the
spatial distribution of low-luminosity (M -18.0) dwarfs relative to
higher luminosity giant galaxies (M -18.0) in the Updated Zwicky
Catalogue (Falco et al. 1999). New spectroscopic data are presented for 26 UM
survey objects. We analyze the relative clustering properties of the overall
starbursting ELG and normal galaxy populations, using nearest neighbor and
correlation function statistics. This allows us to determine whether the
activity in ELGs is primarily caused by gravitational interactions. We conclude
that galaxy-galaxy encounters are not the sole cause of activity in ELGs since
ELGs tend to be more isolated and are more often found in the voids when
compared to their normal galaxy counterparts. Furthermore, statistical analyses
performed on low-luminosity dwarf ELGs show that the dwarfs are less clustered
when compared to their non-active giant neighbors. The UM dwarf samples have
greater percentages of nearest neighbor separations at large values and lower
correlation function amplitudes relative to the UZC giant galaxy samples. These
results are consistent with the expectations of galaxy biasing.Comment: 17 pages, 4 tables, 10 figures. Accepted for publication in the Ap
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