289 research outputs found
Building a CCD Spectrograph for Educational or Amateur Astronomy
We discuss the design of an inexpensive, high-throughput CCD spectrograph for
a small telescope. By using optical fibers to carry the light from the
telescope focus to a table-top spectrograph, one can minimize the weight
carried by the telescope and simplify the spectrograph design. We recently
employed this approach in the construction of IntroSpec, an instrument built
for the 16-inch Knowles Telescope on the Harvard College campus.Comment: 17 pages including 7 figures, PASP, accepted (higher resolution
figures at http://cfa-www.harvard.edu/~sheila/introspec.ps.gz
The Nearby Field Galaxy Survey: a spectrophotometric and photometric study of nearby galaxies
We report on our observing program to obtain integrated spectrophotometry,
intermediate and high resolution major axis spectra, and U,B,R surface photo-
metry of a representative sample of ~200 galaxies in the nearby field. The main
goal of this program is to provide a comparison sample for high redshift
studies and to study the variation in star formation rates (SFR), star forma-
tion history (SFH), excitation, metallicity, and internal kinematics over a
large range in galaxy luminosity and morphological type. In particular, we
extend the work of Kennicutt (1992) to lower luminosity systems.
We present the main results of our analysis sofar. In these proceedings, we
condense the presented two atlases of (1) images and radial surface brightness
profiles and color profiles, and (2) of images and integrated spectra into
several example images, profiles and spectra, showing the general trends
observed. For the original atlasses we refer to the electronic version,
available at http://www.astro.rug.nl/~nfgs/ .Comment: 9 pages, LateX, 6 figures, to appear in the forthcoming issue of
Astrophysics and Space Science: "The Evolution of Galaxies on Cosmological
Timescales
A Broad Search for Counterrotating Gas and Stars: Evidence for Mergers and Accretion
We measure the frequency of bulk gas-stellar counterrotation in a sample of
67 galaxies drawn from the Nearby Field Galaxy Survey, a broadly representative
survey of the local galaxy population down to M_B-15. We detect 4
counterrotators among 17 E/S0's with extended gas emission (24% +8 -6). In
contrast, we find no clear examples of bulk counterrotation among 38 Sa-Sbc
spirals, although one Sa does show peculiar gas kinematics. This result implies
that, at 95% confidence, no more than 8% of Sa-Sbc spirals are bulk
counterrotators. Among types Sc and later, we identify only one possible
counterrotator, a Magellanic irregular. We use these results together with the
physical properties of the counterrotators to constrain possible origins for
this phenomenon.Comment: 19 pages, 4 figures, AJ, accepte
Optical and JWST Mid-IR Emission Line Diagnostics for Simultaneous IMBH and Stellar Excitation in z~0 Dwarf Galaxies
Current observational facilities have yet to conclusively detect intermediate mass black holes (IMBHs) that fill in the evolutionary
gap between early universe seed black holes and supermassive black
holes. Dwarf galaxies present an opportunity to reveal active IMBHs amidst
persistent star formation. We introduce photoionization simulations tailored to
address key physical uncertainties: coincident vs. non-coincident mixing of
IMBH and starlight excitation, open vs. closed surrounding gas cloud
geometries, and different AGN SED shapes. We examine possible AGN emission line
diagnostics in the optical and mid-IR, and find that the diagnostics are often
degenerate with respect to the investigated physical uncertainties. In spite of
these setbacks, and in contrast to recent work, we are able to show that [O
III]/H typically remains bright for dwarf AGN powered by IMBHs down to
. Dwarf AGN are predicted to have inconsistent star-forming and
Seyfert/LINER classifications using the most common optical diagnostics. In the
mid-IR, [O IV] 25.9m and [Ar II] 6.98m are less sensitive to physical
uncertainties than are optical diagnostics. Based on these emission lines, we
provide several mid-IR emission line diagnostic diagrams with demarcations for
separating starbursts and AGN with varying levels of activity. The diagrams are
valid over a wide range of ionization parameters and metallicities out to
, so will prove useful for future JWST observations of local dwarf
AGN in the search for IMBHs. We make our photoionization simulation suite
freely available.Comment: 24 pages, 13 figures, accepted to Ap
Dynamical effects of interactions and the Tully-Fisher relation for Hickson compact groups
We investigate the properties of the B-band Tully-Fisher (T-F) relation for
25 compact group galaxies, using Vmax derived from 2-D velocity maps. Our main
result is that the majority of the Hickson Compact Group galaxies lie on the
T-F relation. However, about 20% of the galaxies, including the lowest-mass
systems, have higher B luminosities for a given mass, or alternatively, a mass
which is too low for their luminosities. We favour a scenario in which outliers
have been brightened due to either enhanced star formation or merging.
Alternatively, the T-F outliers may have undergone truncation of their dark
halo due to interactions. It is possible that in some cases, both effects
contribute. The fact that the B-band T-F relation is similar for compact group
and field galaxies tells us that these galaxies show common mass-to-size
relations and that the halos of compact group galaxies have not been
significantly stripped inside R25. We find that 75% of the compact group
galaxies studied (22 out of 29) have highly peculiar velocity fields.
Nevertheless, a careful choice of inclination, position angle and center,
obtained from the velocity field, and an average of the velocities over a large
sector of the galaxy enabled the determination of fairly well-behaved rotation
curves for the galaxies. However, two of the compact group galaxies which are
the most massive members in M51--like pairs, HCG 91a and HCG 96a, have very
asymmetric rotation curves, with one arm rising and the other one falling,
indicating, most probably, a recent perturbation by the small close companions.Comment: 15 pages, 4 figures, accepted for publication in the Astronomical
Journa
Forming Young Bulges within Existing Disks: Statistical Evidence for External Drivers
Contrary to traditional models of galaxy formation, recent observations
suggest that some bulges form within preexisting disk galaxies. Such late-epoch
bulge formation within disks seems to be linked to disk gas inflow and central
star formation, caused by either internal secular processes or galaxy mergers
and interactions. We identify a population of galaxies likely to be
experiencing active bulge growth within disks, using the criterion that the
color within the half-light radius is bluer than the outer disk color. Such
blue-centered galaxies make up >10% of star-forming disk galaxies within the
Nearby Field Galaxy Survey, a broad survey designed to represent the natural
diversity of the low-z galaxy population over a wide range of luminosities and
environments. Blue-centered galaxies correlate at 99% confidence with
morphological peculiarities suggestive of minor mergers and interactions. From
this and other evidence, we argue that external drivers rather than internal
secular processes probably account for the majority of blue-centered galaxies.
We go on to discuss quantitative plausibility arguments indicating that
blue-centered evolutionary phases may represent an important mode of bulge
growth for most disk galaxies, leading to significant changes in bulge-to-disk
ratio without destroying disks. If this view is correct, bulge growth within
disks may be a natural consequence of the repeated galaxy mergers and
interactions inherent in hierarchical galaxy formation.Comment: 18 pages including 12 figures, AJ, accepte
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