105 research outputs found
The Closest Damped Lyman Alpha System
A difficulty of studying damped Lyman alpha systems is that they are distant,
so one knows little about the interstellar medium of the galaxy. Here we report
upon a damped Lyman alpha system in the nearby galaxy NGC 4203, which is so
close (v_helio = 1117 km/s) and bright (B_o = 11.62) that its HI disk has been
mapped. The absorption lines are detected against Ton 1480, which lies only
1.9' (12 h_50 kpc) from the center of NGC 4203. Observations were obtained with
the Faint Object Spectrograph on HST (G270H grating) over the 2222-3277
Angstrom region with 200 km/s resolution. Low ionization lines of Fe, Mn, and
Mg were detected, leading to metallicities of -2.29, -2.4, which
are typical of other damped Lyman alpha systems, but well below the stellar
metallicity of this type of galaxy. Most notably, the velocity of the lines is
1160 +- 10 km/s, which is identical to the HI rotational velocity of 1170 km/s
at that location in NGC 4203, supporting the view that these absorption line
systems can be associated with the rotating disks of galaxies. In addition, the
line widths of the Mg lines give an upper limit to the velocity dispersion of
167 km/s, to the 99% confidence level.Comment: 4 pages LaTeX, including 1 figure and 1 table, uses emulateapj.sty.
Accepted for publication by Astrophysical Journal Letter
DDO 88: A Galaxy-Sized Hole in the Interstellar Medium
We present an HI and optical study of the gas-rich dwarf irregular galaxy DDO
88. Although DDO 88's global optical and HI parameters are normal for its
morphological type, it hosts a large (3 kpc diameter) and unusually complete
ring of enhanced HI emission. The gas ring is located at approximately
one-third of the total HI radius and one-half the optically-defined Holmberg
radius, and contains 30% of the total HI of the galaxy. The ring surrounds a
central depression in the HI distribution, so it may be a shell formed by a
starburst episode. However, the UBV colors in the HI hole are not bluer than
the rest of the galaxy as would be expected if an unusual star-forming event
had taken place there recently, but there is an old (~1-3 Gyr), red cluster
near the center of the hole that is massive enough to have produced the hole in
the HI. An age estimate for the ring, however, is uncertain because it is not
observed to be expanding. An expansion model produces a lower estimate of 0.5
Gyr, but the presence of faint star formation regions associated with the ring
indicate a much younger age. We also estimate that the ring could have
dispersed by now if it is older than 0.5 Gyr. This implies that the ring is
younger than 0.5 Gyr. A younger age would indicate that the red cluster did not
produce the hole and ring. If this ring and the depression in the gas which it
surrounds were not formed by stellar winds and supernovae, this would indicate
that some other, currently unidentified, mechanism is operating.Comment: 44 pages; 16 figures. To appear in AJ, January 2005. Available from
ftp.lowell.edu, cd pub/dah/papers/d88 and http://www.fiu.edu/~simpsonc/d8
A Uniform Analysis of the Ly-alpha Forest at z=0 - 5: V. The extragalactic ionizing background at low redshift
In Paper III of our series "A Uniform Analysis of the Ly-alpha forest at z=0
- 5", we presented a set of 270 quasar spectra from the archives of the Faint
Object Spectrograph on the Hubble Space Telescope. A total of 151 of these
spectra, yielding 906 lines, are suitable for using the proximity effect
signature to measure J(\nu_0), the mean intensity of the hydrogen-ionizing
background radiation field, at low redshift. Using a maximum likelihood
technique and the best estimates possible for each QSO's Lyman limit flux and
systemic redshift, we find J(\nu_0)= 7.6^+9.4_-3.0 x 10^-23 ergs s^-1 cm^-2
Hz^-1 sr^-1 at at 0.03 < z < 1.67. This is in good agreement with the mean
intensity expected from models of the background which incorporate only the
known quasar population. When the sample is divided into two subsamples,
consisting of lines with z 1, the values of J(\nu_0) found are
6.5^+38._-1.6 x 10^-23 ergs s^-1 cm^-2 Hz^-1 sr^-1, and 1.0^+3.8_-0.2 x 10^-22
ergs s^-1 cm^-2 Hz^-1 sr^-1, respectively, indicating that the mean intensity
of the background is evolving over the redshift range of this data set.
Relaxing the assumption that the spectral shapes of the sample spectra and the
background are identical, the best fit HI photoionization rates are found to be
6.7 x 10^-13 s^-1 for all redshifts, and 1.9 x 10^-13 s^-1 and 1.3 x 10^-12
s^-1 for z 1, respectively. This work confirms that the evolution
of the number density of Ly-alpha lines is driven by a decrease in the ionizing
background from z ~ 2 to z ~ 0 as well as by the formation of structure in the
intergalactic medium. (Abridged)Comment: 71 LaTeX pages, 20 encapsulated Postscript figures, Accepted for
publication in ApJ, Figure 4 available at
http://lithops.as.arizona.edu/~jill/QuasarSpectra/ or
http://hea-www.harvard.edu/QEDT/QuasarSpectra
HST WFPC2 Imaging of Three Low Surface Brightness Dwarf Elliptical Galaxies in the Virgo Cluster
[annotated] HST WFPC2 images were taken of three LSB dwarf elliptical
galaxies in the Virgo cluster. The intent of the observations was to determine
the small scale structure in these enigmatic galaxies, and to attempt to learn
something about the nature of their giant branch through the detection of
luminosity fluctuations. In two of the three studied galaxies, V7L3 and V1L4,
the luminosity fluctuations were unambiguously detected, yielding a density of
2 - 10 red giants/pixel. Using the observed B-V and V-I colors as a constraint,
we could find no model that would reproduce the observed fluctuation signal and
blue colors if there was a significant population of M-giants in these systems.
The third system, V2L8, did not have a detectable fluctuation signal which
possibly implies it is not in the Virgo cluster. Interestingly, this system is
highly nucleated. Our observations have resolved this nucleus and if V2L8 is in
Virgo, then we have discovered what is likely the smallest bulge measured to
date, having an effective radius of only 50 pc. This bulge is quite red (as red
as giant ellipticals) and its entirely possible that this nucleated dE galaxy,
in fact, is a very large galaxy located in the background. As such, it is
highly reminiscent of the manner in which Malin-1 was discovered. Finally, we
find no evidence for small scale clumping of stars in any of the studied
systems at this much improved spatial resolution. This implies these systems
are dynamically well-relaxed and that the physical cause of their observed low
surface brightnesses is their low density. When imaged at the high spatial
resolution of the WFPC2 (~6 pc per pixel), the galaxies are easy to look right
through without evening knowing they are present in the very middle of the
WFPC2 frame. They appear only as elevated ``sky noise''.Comment: 39 pages, 3 figures, 3 tables. Accepted for publication in AJ The
paper, full-sized figures, etc. can also be obtained via
http://guernsey.uoregon.edu/~kare
The disruption of nearby galaxies by the Milky Way
Interactions between galaxies are common and are an important factor in
determining their physical properties such as position along the Hubble
sequence and star-formation rate. There are many possible galaxy interaction
mechanisms, including merging, ram-pressure stripping, gas compression,
gravitational interaction and cluster tides. The relative importance of these
mechanisms is often not clear, as their strength depends on poorly known
parameters such as the density, extent and nature of the massive dark halos
that surround galaxies. A nearby example of a galaxy interaction where the
mechanism is controversial is that between our own Galaxy and two of its
neighbours -- the Large and Small Magellanic Clouds. Here we present the first
results of a new HI survey which provides a spectacular view of this
interaction. In addition to the previously known Magellanic Stream, which
trails 100 degrees behind the Clouds, the new data reveal a counter-stream
which lies in the opposite direction and leads the motion of the Clouds. This
result supports the gravitational model in which leading and trailing streams
are tidally torn from the body of the Magellanic Clouds.Comment: 17 pages with 5 figures in gif format, scheduled for publication in
the August 20th, 1998 issue of Natur
Molecular Hydrogen in the Ring Nebula: Clumpy Photodissociation Regions
Article DOI: 10.1086/345911
Article Stable URL: http://www.jstor.org/stable/10.1086/345911We present a 0 .65 resolution H2 1-0 S(1) 2.122 mm image of the Ring Nebula (NGC 6720),
which was taken with the Near Infrared Imager at the WIYN 3.5 m telescope on Kitt Peak. The high resolution of the H2 observation is sufficient to reveal the finer structure of the molecular material in this nebula. The
morphology of the molecular emission is compared to that of the ionized emission from the Ring Nebula as seen by the Hubble Space Telescope (HST; He ii, [O iii], and [N ii]), and it is clear that the dark clumps seen by
HST match the locations of clumpy H2 emission, suggesting that these clumps are similar to the cometary knots seen in the Helix Nebula. As with the Helix, the clumpy H2 emission from the main ring of the Ring Nebula is contained within the optically bright ionized nebula, implying that the molecular gas is shielded inside dense condensations. Comparison of the observed H2 average surface brightnesses for the Ring Nebula [(1.5 ergs cm 2 s 1 sr 1] with time-dependent models of the expected H2 0.5)#10 4 emission from planetary nebulae
(PNe) shows that it is consistent with H2 excitation in photodissociation regions (PDRs), confirming previous suggestions. Comparison of the Ring Nebula H2 emission with a younger PN, NGC 2346, and an older PN, the
Helix Nebula, suggests an evolution in H2 surface brightness consistent with the time-dependent PDR models. Moreover, the knots of molecular gas appear to become more isolated as the PN evolves, consistent with optical studies of knots in PNe.A. K. S. was supported by NASA JPL 961504 and NASA STI 7898.02-96A. A. K. S. and M. M. were supported by NSF CAREER award AST 97-33697
Evolutionary Status of Dwarf ``Transition'' Galaxies
We present deep B, R and Halpha imaging of 3 dwarf galaxies: NGC3377A,
NGC4286, and IC3475. Based on previous broadband imaging and HI studies, these
mixed-morphology galaxies were proposed by Sandage & Hoffman (1991) to be,
respectively, a gas-rich low surface brightness Im dwarf, a nucleated dwarf
that has lost most of its gas and is in transition from Im to dS0,N, and the
prototypical example of a gas-poor ``huge low surface brightness'' early-type
galaxy. From the combination of our broadband and Halpha imaging with the
published information on the neutral gas content of these three galaxies, we
find that (1) NGC3377A is a dwarf spiral; (2) NGC3377A and NGC4286 have
comparable amounts of ongoing star formation, as indicated by their Halpha
emission, while IC3475 has no detected HII regions to a very low limit; (3) the
global star formation rates are at least a factor of 20 below that of 30
Doradus for NGC3377A and NGC4286; (4) while the amount of star formation is
comparable, the distribution of star forming regions is very different between
NGC3377A and NGC4286; (5) given their current star formation rates and gas
contents, both NGC3377A and NGC4286 can continue to form stars for more than a
Hubble time; (6) both NGC3377A and NGC4286 have integrated total B-R colors
that are redder than the integrated total B-R color for IC3475, and thus it is
unlikely that either galaxy will ever evolve into an IC3475 counterpart; and
(7) IC3475 is too blue to be a dE. We thus conclude that we have not identified
potential precursors to galaxies such as IC3475, and unless signifcant changes
occur in the star formation rates, neither NGC3377A nor NGC4286 will evolve
into a dwarf elliptical or dwarf spheroidal within a Hubble time.Comment: 34 pages, 6 jpg figures, 3 postscript figures, and 4 tables, uses
AASTeX, ApJ, in pres
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
- …