M. Trewhella

High-resolution rotation curves of low surface brightness galaxies

High-resolution Halpha rotation curves are presented for five low surface brightness galaxies. These Halpha rotation curves have shapes different from those previously derived from H i observations, probably because of the higher spatial resolution of the Halpha observations. The Halpha rotation curves rise more steeply in the inner parts than the H i rotation curves, and they reach a flat part beyond about two disk scale lengths. With radii expressed in optical disk scale lengths, the rotation curves of the low surface brightness galaxies presented here and those of the high surface brightness galaxies have almost identical shapes. Mass modeling shows that the contribution of the stellar component to the rotation curves may be scaled to explain most of the inner parts of the rotation curves, albeit with high stellar mass-to-light ratios. On the other hand, well-fitting mass models can also be obtained with lower contributions of the stellar disk. These observations suggest that the luminous mass density and the total mass density are coupled in the inner parts of these galaxies.

DEEP SUBMILLIMETER IMAGES OF NGC 891—COLD DUST AT LARGER GALACTIC RADII

We present images in the 450 and 850 μm continuum of the edge-on galaxy NGC 891. These measurements, carried out with the recently commissioned Submillimeter Common-User Bolometer Array, provide the deepest images yet of a nearby galaxy in the submillimeter wave band. We detect dust emission from 2/3 of the optical disk and confirm the presence of dust chimneys escaping from the main absorption layer up to z-heights of nearly 2 kpc. A comparison between the submillimeter surface brightness along the major axis with that corresponding to the IRAS 60 and 100 μm filters implies that large amounts of cold dust (~15 K) are present in the disk (in fact, an order of magnitude more grain material than the warm dust detected by IRAS). These cold grains predominate at larger galactic radii.

ISO Long Wavelength Spectrograph Observations of Cold Dust in Galaxies

We describe observations of five nearby galaxies obtained using the Long Wavelength Spectrograph (LWS) on the Infrared Space Observatory (ISO). We observed five galaxies, using spectrograph apertures positioned at the galactic center and in the outskirts of the disk, to compare the spectral energy distribution of the emitting dust at different positions. The central spectra are typical of those inferred previously from IRAS data; peaking at about 100 μm with estimated dust temperatures of T(d) = 30-35 K. However, there is a rapid change in the spectral energy distribution with increasing galactocentric distance. In the outer regions the spectra are flat, or still rising, out to 197μm, indicating a predominantly cold dust component with T(d) < 20 K. In the central regions, the inferred cold dust component is 2-10 times more massive than the warm dust, and it increasingly dominates the mass and spectral energy distribution in the outer regions. We discuss reasons for believing that emission beyond ≈150 μm in disk galaxies is associated with a separate component of dust with an extended distribution that may be associated with a possible molecular halo.

SCUBA imaging of the NGC 7331 dust ring

We present observations of the spiral galaxy NGC 7331 using the Submillimetre Common User Bolometer Array (SCUBA) on the James Clark Maxwell Telescope. We have detected a dust ring of 45 arcsec radius (3.3 kpc) at wavelengths of 450 and 850 μm. The dust ring is in good correspondence with other observations of the ring in the mid-infrared (MIR), CO and radio continuum, suggesting that the observed dust is associated with molecular gas and star formation. A B − K colour map shows an analogous ring structure with an asymmetry about the major axis, consistent with the extinction being produced by a dust ring. The derived temperature of the dust lies between 16 and 31 K and the gas-to-dust ratio lies between 150 and 570, depending on the assumed dust emission efficiency index (β = 1.5 or 2).

Quantitative Morphology of Galaxies Observed in the Ultraviolet

We present a quantitative study of the far-ultraviolet (FUV) and optical morphology in 32 nearby galaxies and estimate the ii morphological k-correction ˇˇ expected if these objects were observed unevolved at high redshift. Using the common indices of central concentration (C) and rotational asymmetry (A) to quantify morphology, we consider independently two phenomena that give rise to this kcorrection. Bandshifting, the decrease in the rest-frame wavelength of light observed through optical —lters, is explored by measuring these indices in several passbands for each galaxy, and it is found to be the primary driver of changes in C and A. In general, the optical trend found for decreasing C and increasing A when going to shorter wavelengths extends to the FUV. However, the patchy nature of recent star formation in late-type galaxies, which is accentuated in the FUV, results in poor quantitative correspondence between morphologies determined in the optical and FUV. We then arti—cially redshift our FUV images into the Hubble Deep Field (HDF) —lters to simulate various cosmological distance eUects, such as surface brightness dimming and loss of spatial resolution. Hubble types of many galaxies in our sample are not readily identi—able at redshifts beyond z D 1, and the galaxies themselves are difficult to detect beyond z D 3. Because only features of the highest surface brightness remain visible at cosmological distances, the change in C and A between simulated high-z galaxies and their unredshifted counterparts depends on whether their irregular features are primarily bright or faint. Our simulations suggest that k-corrections alone are indeed capable of producing the peculiar morphologies observed at high redshift; for example, several spiral galaxies have C and A indices typical of irregular or peculiar HDF objects viewed at z ” 2. We brie—y discuss some elements of a scheme to classify rest-frame UV images, mergers, protogalaxies, and other objects for which classical Hubble types do not adequately encompass the existing morphology.

ISO observations of cold dust in NGC 6946

A new technique for mapping the internal extinction of disc galaxies has been developed and the results of applying this technique to NGC 6946 are given in another paper in these Proceedings. The study required new and difficult observations covering a wide range of wavelengths. As is often the case when data are assembled in a new way, there have been a number of unexpected discoveries as a by-product of the main study. We present evidence for a new component of dust that is colder and considerably more massive than that detected by the IRAS satellite. This cold component is also more extended than the stars in the radial direction, contrary to the “standard picture” of dust distribution [9].