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 $10^3 - 10^4 M_{\odot}$ intermediate mass black holes (IMBHs) that fill in the evolutionary gap between early universe seed black holes and $z \sim 0$ 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$\beta$ typically remains bright for dwarf AGN powered by IMBHs down to $10^3 M_{\odot}$. 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] 25.9$\mu$m and [Ar II] 6.98$\mu$m 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 $z\sim0.1$, 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