Measuring the Slope of the Dust Extinction Law and the Power Spectrum of Dust Clouds Using Differentially-Reddened Globular Clusters

We present 3 methods for measuring the Galactic dust extinction law slope R_V and a method for quantifying fine structure in dust clouds in the direction of differentially-reddened globular clusters. We apply these methods to the low- latitude globular cluster NGC4833 which displays variable extinction/reddening about a mean ~ 1. A set of Monte Carlo simulations is used to characterize the efficacy of the methods. The essence of the first 2 methods is to determine, for an assumed value of R_V, the relative visual extinction delta(A_V) of each cluster horizontal branch (HB) star with respect to an empirical HB locus; the locus is derived from the CMD of a subset of stars near the cluster center in which differential reddening is small. A star-by-star comparison of delta(A_V) from the (B-V,V) CMD with that from the (V-I,V) CMD is used to find the optimal R_V. In the third method, R_V is determined by minimizing the scatter in the HB in the (B-V, V) CMD after correcting the photometry for extinction and reddening using the Schlegel et al. (1998) dust maps. The weighted average of the results from 3 methods gives R_V = 3.0 +/- 0.4 for the dust towards NGC4833. The dust fine structure is quantified via the difference, Delta(A_V)_ij = [delta(A_V)]_i - [delta(A_V)]_j, between pairs of cluster HB stars (i,j) as a function of their angular separation r_ij. The variance of Delta(A_V)_ij is found to have a power- law dependence on angular scale: var(r) \propto r^(+0.9 +/- 0.1). This translates into an angular power spectrum P(kappa) \propto kappa^(-1.9 +/- 0.1) for r ~ 1' - 5', where kappa = 1/r. The dust angular power spectrum on small scales (from optical data) matches smoothly onto the larger-scale power spectrum derived from Schlegel et al.'s far-infrared map of the dust thermal emission.Comment: 36 pages, 15 figures, 1 table. Accepted for publication in the Astronomical Journal (July 2004

Keck Spectroscopy of Dwarf Elliptical Galaxies in the Virgo Cluster

Keck spectroscopy is presented for four dwarf elliptical galaxies in the Virgo Cluster. At this distance, the mean velocity and velocity dispersion are well resolved as a function of radius between 100 to 1000 pc, allowing a clear separation between nuclear and surrounding galaxy light. We find a variety of dispersion profiles for the inner regions of these objects, and show that none of these galaxies is rotationally flattened.Comment: 4 pages, 2 figures, to appear in the proceedings of the Yale Cosmology Workshop "The Shapes of Galaxies and their Halos", (ed. P. Natarjan

Internal Kinematics of Distant Field Galaxies: I. Emission Line Widths for a Complete Sample of Faint Blue Galaxies at <z>=0.25

We present measurements of the OII(3727) emission line width for a complete sample of 24 blue field galaxies (21.25=0.25, obtained with the AUTOFIB fibre spectrograph on the Anglo-Australian Telescope. Most emission lines are spectrally resolved, yet all have dispersions sigma<100km/s. Five of the 24 sample members have OII doublet line flux ratios which imply gas densities in excess of 100 cm^-3. The line emission in these galaxies may be dominated by an active nucleus and the galaxies have been eliminated from the subsequent analysis. The remaining 19 linewidths are too large by a factor of two (7sigma significance) to be attributed to turbulent motions within an individual star forming region, and therefore most likely reflect the orbital motion of ionized gas in the galaxy. We use Fabry--Perot observations of nearby galaxies to construct simulated datasets that mimic our observational setup at z=0.25; these allow us to compute the expected distribution of (observable) linewidths sigma_v for a galaxy of a given ``true'' (optical) rotation speed v_c. These simulations include the effects of random viewing angles, clumpy line emission, finite fibre aperture, and internal dust extinction on the emission line profile. We assume a linewidth--luminosity--colour relation: ln[ v_c(M_B,B-R) ] = ln[v_c(-19,1)] - eta*(M_B+10) + zeta*[(B-R)-1] and determine the range of parameters consistent with our data. We find a mean rotation speed of v_c(-19,1)=66+-8km/s (68% confidence limits) for the distant galaxies with M_B=-19 and B-R=1, with a magnitude dependence for v_c of eta=0.07+-0.08, and a colour dependence of zeta =0.28+-0.25. Through comparison with several local samples we show that this value of v_c(-19,1) is significantly lower than the optical rotation speed of present-day galaxies with the same absolute magnitudeComment: TeX Text and Tables, no Figures. Compressed and uuencoded PS file of the complete paper (43 pages including 9 figures) available at http://zwicky.as.arizona.edu/~rix/; submitted to MNRA

BVRI Surface Photometry of Isolated Spiral Galaxies

A release of multicolor broad band (BVRI) photometry for a subsample of 44 isolated spirals drawn from the Catalogue of Isolated Galaxies (CIG) is presented. Total magnitudes and colors at various circular apertures, as well as some global structural/morphological parameters are estimated. Morphology is reevaluated through optical and sharp/filtered R band images, (B-I) color index maps, and archive near-IR JHK images from the Two-Micron Survey. The CAS structural parameters (Concentration, Asymmetry, and Clumpiness) were calculated from the images in each one of the bands. The fraction of galaxies with well identified optical/near-IR bars (SB) is 63%, while a 17% more shows evidence of weak or suspected bars (SAB). The sample average value of the maximum bar ellipticity is 0.4. Half of the galaxies in the sample shows rings. We identify two candidates for isolated galaxies with disturbed morphology. The structural CAS parameters change with the observed band, and the tendencies they follow with the morphological type and global color are more evident in the redder bands. In any band, the major difference between our isolated spirals and a sample of interacting spirals is revealed in the A-S plane. A deep and uniformly observed sample of isolated galaxies is intended for various purposes including (i) comparative studies of environmental effects, (ii) confronting model predictions of galaxy evolution and (iii) evaluating the change of galaxy properties with redshift.Comment: 44 pages, 9 figures and 7 tables included. To appear in The Astronomical Journal. For the 43 appendix figures 4.1-4.43 see http://www.astroscu.unam.mx/~avila/Figs4.1_4.43.tar.gz (7.2 Mb tar.gz file

Intergalactic HI in the NGC5018 group

The cold interstellar and intergalactic medium is in the small group of galaxies whose brightest member is the elliptical galaxy NGC5018. Researchers' attention was first drawn to this galaxy as possibly containing cold interstellar gas by the detection by the Infrared Astronomy Satellite (IRAS) of emission at lambda 60 microns and lambda 100 microns at an intensity of about 1 Jy (Knapp et al. 1989), which is relatively strong for an elliptical (Jura et al. 1987). These data showed that the temperature of the infrared emission is less than 30K and that its likely source is therefore interstellar dust. A preliminary search for neutral hydrogen (HI) emission from this galaxy using the Very Large Array (VLA) showed that there appears to be HI flowing between NGC5018 and the nearby Sc galaxy NGC5022 (Kim et al. 1988). Since NGC5018 has a well-developed system of optical shells (cf. Malin and Carter 1983; Schweizer 1987) this observation suggests that NGC5018 may be in the process of forming its shell system by the merger of a cold stellar system with the elliptical, as suggested by Quinn (1984). Researchers describe follow-up HI observations of improved sensitivity and spatial resolution, and confirm that HI is flowing between NCG5022 and NGC5018, and around NGC5018. The data show, however, that the HI bridge actually connects NGC5022 and another spiral in the group, MCG03-34-013, both spatially and in radial velocity, and that in doing so it flows through and around NGC5018, which lies between the spiral galaxies. This is shown by the total HI map, with the optical positions of the above three galaxies labelled