772 research outputs found
Generators for the hyperelliptic Torelli group and the kernel of the Burau representation at t = -1
We prove that the hyperelliptic Torelli group is generated by Dehn twists about
separating curves that are preserved by the hyperelliptic involution. This verifies a
conjecture of Hain. The hyperelliptic Torelli group can be identified with the kernel
of the Burau representation evaluated at t = â1 and also the fundamental group of
the branch locus of the period mapping, and so we obtain analogous generating sets
for those. One application is that each component in Torelli space of the locus of
hyperelliptic curves becomes simply connected when curves of compact type are added
Reconstructing Deconstruction: High-Velocity Cloud Distance Through Disruption Morphology
We present Arecibo L-band Feed Array 21-cm observations of a sub-complex of
HVCs at the tip of the Anti-Center Complex. These observations show
morphological details that point to interaction with the ambient halo medium
and differential drag within the cloud sub-complex. We develop a new technique
for measuring cloud distances, which relies upon these observed morphological
and kinematic characteristics, and show that it is consistent with H-alpha
distances. These results are consistent with distances to HVCs and halo
densities derived from models in which HVCs are formed from cooling halo gas.Comment: 8 pages, 2 figures, 1 tabe, Accepted to Ap
FUSE Observations of the Magellanic Bridge Gas toward Two Early-Type Stars: Molecules, Physical Conditions, and Relative Abundance
We discuss FUSE observations of two early-type stars, DI1388 and DGIK975, in
the low density and low metallicity gas of Magellanic Bridge (MB). Toward
DI1388, the FUSE observations show molecular hydrogen, O VI, and numerous other
atomic or ionic transitions in absorption, implying the presence of multiple
gas phases in a complex arrangement. The relative abundance pattern in the MB
is attributed to varying degrees of depletion onto dust similar to that of halo
clouds. The N/O ratio is near solar, much higher than N/O in damped Ly-alpha
systems, implying subsequent stellar processing to explain the origin of
nitrogen in the MB. The diffuse molecular cloud in this direction has a low
column density and low molecular fraction. H2 is observed in both the
Magellanic Stream and the MB, yet massive stars form only in the MB, implying
significantly different physical processes between them. In the MB some of the
H2 could have been pulled out from the SMC via tidal interaction, but some also
could have formed in situ in dense clouds where star formation might have taken
place. Toward DGIK975, the presence of neutral, weakly and highly ionized
species suggest that this sight line has also several complex gas phases. The
highly ionized species of O VI, C IV, and Si IV toward both stars have very
broad features, indicating that multiple components of hot gas at different
velocities are present. Several sources (a combination of turbulent mixing
layer, conductive heating, and cooling flows) may be contributing to the
production of the highly ionized gas in the MB. Finally, this study has
confirmed previous results that the high-velocity cloud HVC 291.5-41.2+80 is
mainly ionized composed of weakly and highly ions. The high ion ratios are
consistent with a radiatively cooling gas in a fountain flow model.Comment: Accepted for publication in the ApJ (October 10, 2002). Added
reference (Gibson et al. 2000
Metallicity and Physical Conditions in the Magellanic Bridge
We present a new analysis of the diffuse gas in the Magellanic Bridge (RA>3h)
based on HST/STIS E140M and FUSE spectra of 2 early-type stars lying within the
Bridge and a QSO behind it. We derive the column densities of HI (from
Ly\alpha), NI, OI, ArI, SiII, SII, and FeII of the gas in the Bridge. Using the
atomic species, we determine the first gas-phase metallicity of the Magellanic
Bridge, [Z/H]=-1.02+/-0.07 toward one sightline, and -1.7<[Z/H]<-0.9 toward the
other one, a factor 2 or more smaller than the present-day SMC metallicity.
Using the metallicity and N(HI), we show that the Bridge gas along our three
lines of sight is ~70-90% ionized, despite high HI columns, logN(HI)=19.6-20.1.
Possible sources for the ongoing ionization are certainly the hot stars within
the Bridge, hot gas (revealed by OVI absorption), and leaking photons from the
SMC and LMC. From the analysis of CII*, we deduce that the overall density of
the Bridge must be low (<0.03-0.1 cm^-3). We argue that our findings combined
with other recent observational results should motivate new models of the
evolution of the SMC-LMC-Galaxy system.Comment: Accepted for publication in the Ap
High-velocity clouds as streams of ionized and neutral gas in the halo of the Milky Way
High-velocity clouds (HVC), fast-moving ionized and neutral gas clouds found
at high galactic latitudes, may play an important role in the evolution of the
Milky Way. The extent of this role depends sensitively on their distances and
total sky covering factor. We search for HVC absorption in HST high resolution
ultraviolet spectra of a carefully selected sample of 133 AGN using a range of
atomic species in different ionization stages. This allows us to identify
neutral, weakly ionized, or highly ionized HVCs over several decades in HI
column densities. The sky covering factor of UV-selected HVCs with |v_LSR|>90
km/s is 68%+/-4% for the entire Galactic sky. We show that our survey is
essentially complete, i.e., an undetected population of HVCs with extremely low
N(H) (HI+HII) is unlikely to be important for the HVC mass budget. We confirm
that the predominantly ionized HVCs contain at least as much mass as the
traditional HI HVCs and show that large HI HVC complexes have generally ionized
envelopes extending far from the HI contours. There are also large regions of
the Galactic sky that are covered with ionized high-velocity gas with little HI
emission nearby. We show that the covering factors of HVCs with 90<|v_LSR|<170
km/s drawn from the AGN and stellar samples are similar. This confirms that
these HVCs are within 5-15 kpc of the sun. The covering factor of these HVCs
drops with decreasing vertical height, which is consistent with HVCs being
decelerated or disrupted as they fall to the Milky Way disk. The HVCs with
|v_LSR|>170 km/s are largely associated with the Magellanic Stream at b<0 and
its leading arm at b>0 as well as other large known HI complexes. Therefore
there is no evidence in the Local Group that any galaxy shows a population of
HVCs extending much farther away than 50 kpc from its host, except possibly for
those tracing remnants of galaxy interaction.Comment: Submitted to MNRAS (19 pages, 11 figures). Comments are welcom
A Very Sensitive 21cm Survey for Galactic High-Velocity HI
Very sensitive HI 21cm observations have been made in 860 directions at dec
>= -43deg in search of weak, Galactic, high-velocity HI emission lines at
moderate and high Galactic latitudes. One-third of the observations were made
toward extragalactic objects. The median 4-sigma detection level is NHI =
8x10^{17} cm^-2 over the 21' telescope beam. High-velocity HI emission is
detected in 37% of the directions; about half of the lines could not have been
seen in previous surveys. The median FWHM of detected lines is 30.3 km/s. High-
velocity HI lines are seen down to the sensitivity limit of the survey implying
that there are likely lines at still lower values of NHI. The weakest lines
have a kinematics and distribution on the sky similar to that of the strong
lines, and thus do not appear to be a new population. Most of the emission
originates from objects which are extended over several degrees; few appear to
be compact sources. At least 75%, and possibly as many as 90%, of the lines are
associated with one of the major high-velocity complexes. The Magellanic Stream
extends at least 10 deg to higher Galactic latitude than previously thought and
is more extended in longitude as well. Although there are many lines with low
column density, their numbers do not increase as rapidly as NHI^-1, so most of
the HI mass in the high-velocity cloud phenomenon likely resides in the more
prominent clouds. The bright HI features may be mere clumps within larger
structures, and not independent objects.Comment: 88 pages includes 22 figures Accepted for Publication in ApJ Suppl.
June 200
Intergalactic HII Regions Discovered in SINGG
A number of very small isolated HII regions have been discovered at projected
distances up to 30 kpc from their nearest galaxy. These HII regions appear as
tiny emission line objects in narrow band images obtained by the NOAO Survey
for Ionization in Neutral Gas Galaxies (SINGG). We present spectroscopic
confirmation of four isolated HII regions in two systems, both systems have
tidal HI features. The results are consistent with stars forming in interactive
debris due to cloud-cloud collisions. The H-alpha luminosities of the isolated
HII regions are equivalent to the ionizing flux of only a few O stars each.
They are most likely ionized by stars formed in situ, and represent atypical
star formation in the low density environment of the outer parts of galaxies. A
small but finite intergalactic star formation rate will enrich and ionize the
surrounding medium. In one system, NGC 1533, we calculate a star formation rate
of 1.5e-3 msun/yr, resulting in a metal enrichment of ~1e-3 solar for the
continuous formation of stars. Such systems may have been more common in the
past and a similar enrichment level is measured for the `metallicity floor' in
damped Lyman-alpha absorption systems.Comment: accepted for publication in the Astronomical Journal, 19 pages,
including 5 figures, some low resolution. Paper with high resolution images
can be downloaded from
http://astro.ph.unimelb.edu.au/~eryan/publications/eldots.ps.g
Effelsberg HI observations of compact high-velocity clouds
We have mapped 11 compact high-velocity clouds (CHVCs) in the 21-cm line
emission of neutral, atomic hydrogen, using the Effelsberg 100-m radio
telescope. The aim of our observations was to study the overall distribution of
the warm neutral medium of CHVCs with high sensitivity. The achieved baseline
rms of about 50 mK at the original 2.6 km/s velocity resolution allows us to
search for evidence of ram-pressure interaction with the ambient medium. In
addition, we have obtained spectra along an appropriate axis across each CHVC
with longer integration times and denser angular sampling. These deep slices
with a spectral baseline rms of about 25...35 mK allow us to determine the
column density profile in greater detail as well as the velocity and line width
gradient across each cloud. The most outstanding result of our observations is
the complexity of the HI column density distribution and the line profiles of
the investigated CHVCs. We have found only one cloud with a
spherically-symmetric appearance. Among the remaining clouds we observe
head-tail structures, bow-shock shapes, and objects with irregular shapes.
These complex morphologies in combination with the obtained physical parameters
suggest that ram-pressure interactions with an ambient medium may play a
significant role in shaping some of the CHVCs from our sample. These results
are consistent with a circumgalactic distribution of CHVCs with typical
distances of the order of 100 kpc. The pressure of the ambient medium might
also stabilise CHVCs in addition to their own gravitational potential.Comment: 18 pages, 11 figures, accepted for publication by Astronomy &
Astrophysic
The Survey for Ionization in Neutral Gas Galaxies: I. Description and Initial Results
We introduce the Survey for Ionization in Neutral Gas Galaxies (SINGG), a
census of star formation in HI-selected galaxies. The survey consists of
H-alpha and R-band imaging of a sample of 468 galaxies selected from the HI
Parkes All Sky Survey (HIPASS). The sample spans three decades in HI mass and
is free of many of the biases that affect other star forming galaxy samples. We
present the criteria for sample selection, list the entire sample, discuss our
observational techniques, and describe the data reduction and calibration
methods. This paper focuses on 93 SINGG targets whose observations have been
fully reduced and analyzed to date. The majority of these show a single
Emission Line Galaxy (ELG). We see multiple ELGs in 13 fields, with up to four
ELGs in a single field. All of the targets in this sample are detected in
H-alpha indicating that dormant (non-star forming) galaxies with M(HI) > ~3e7
M_sun are very rare. A database of the measured global properties of the ELGs
is presented. The ELG sample spans four orders of magnitude in luminosity
(H-alpha and R-band), and H-alpha surface brightness, nearly three orders of
magnitude in R surface brightness and nearly two orders of magnitude in H-alpha
equivalent width (EW). The surface brightness distribution of our sample is
broader than that of the Sloan Digital Sky Survey spectroscopic sample, the
(EW) distribution is broader than prism-selected samples, and the morphologies
found include all common types of star forming galaxies (e.g. irregular,
spiral, blue compact dwarf, starbursts, merging and colliding systems, and even
residual star formation in S0 and Sa spirals). (abridged)Comment: 28 pages, ApJS, in press. Full resolution version with all panels of
Fig. 8 available at http://sungg.pha.jhu.edu/publications.html . On line data
available at http://sungg.pha.jhu.edu/PubData/ . Author list corrected. Wrong
value for f_ap used in eq. 7 now corrected; typos corrected, non-used
references replaced, others update
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