Marshall L. McCall

H II regions, extinction, and IC 342 - A new view of the galactic neighborhood

The galactic extinction of a heavily reddened spiral galaxy can be gauged from observations of the Balmer decrement of constituent giant H II regions and measurements of annular-averaged H I column densities. Application of the technique to IC 342 reveals that A sub B = 3.05 + or - 0.25. Thus, this galaxy is nearly as luminous as the Milky Way and only 1.9 times farther away from M31. The gravitational acceleration of M31 by IC 342 is between 15 and 25 percent of that due to the Milky Way. It appears that the Milky Way should be regarded dynamically as a member of a quartet of giant galaxies lying roughly in a line in the supergalactic plane. 63 refs.

The IC 342/Maffei Group Revealed

Deep wide-field CCD images in the optical and near-infrared have been acquired for 14 of the 16 known or suspected members of the IC 342/Maffei Group of galaxies, one of the closest groups to the Milky Way, and probably the closest group to M31. Because of their low Galactic latitude, all galaxies are heavily extinguished, and myriads of foreground stars are superimposed. A sophisticated algorithm built around DAOPHOT has been developed which successfully removes the foreground stars, making possible comprehensive morphological and photometric studies. The cleaned near-infrared images reveal the true morphology and extent of many of the galaxies for the first time, three of which are among the largest in the northern sky. Besides surface brightness profiles, precise total magnitudes and colors have been measured. Many of the results represent substantial revisions to previous estimates. The data will make possible new determinations of the distances and masses of the galaxies, which are crucial for evaluating the impact the group may have had upon the dynamical evolution of the Local Group.

The origin of broad emission lines in the extragalactic giant H II region NGC 2363

High signal-to-noise long-slit spectra have been obtained of the giant H II region NGC 2363 located in the dwarf SBm galaxy NGC 2366. A discovery of low-intensity broad spectral components (FWHM is approximately equal to 40 A or 2400 km/s) in the bright nebular lines H-alpha, H-beta, and forbidden O III is reported. The broad spectral components are detected over a large spatial extent (not less than 500 pc) centered on the nebula. Several mechanisms for broadening nebular lines are explored: stellar winds, Thomson scattering by hot gas, supernova remnants, and superbubble blowout. All mechanisms have problems. Superbubble blowout, which is the only known mechanism capable of accelerating interstellar gas over such a volume of space, does not appear consistent with the physical properties of the H II region NGC 2363 or with the nature of the host galaxy. It is concluded that the broad nebular lines are probably due to very high velocity gas whose origin is, at present, unknown.

Uncovering Additional Clues to Galaxy Evolution. II. The Environmental Impact of the Virgo Cluster on the Evolution of Dwarf Irregular Galaxies

The impact of the cluster environment on the evolution of dwarf galaxies is investigated by comparing the properties of a sample of dwarf irregular galaxies (dI’s) in the Virgo Cluster with a control sample of nearby (‘‘ field ’’) dI’s having oxygen abundances derived from [O iii] � 4363 measurements and measured distances from resolved stellar constituents. Spectroscopic data are obtained for H ii regions in 11 Virgo dI’s distributed in the central and outer regions of the cluster. To ensure that oxygen abundances are derived in a homogeneous manner, oxygen abundances for field and Virgo dI’s are computed using the bright-line method and compared with abundances directly obtained from [O iii] � 4363, where available. They are found to agree to within about 0.2 dex, with no systematic offset. At a given optical luminosity, there is no systematic difference in oxygen abundance between the sample of Virgo dI’s and the sample of nearby dI’s. However, five of the 11 Virgo dI’s exhibit much lower baryonic gas fractions than field dI’s at comparable oxygen abundances. Using field dI’s as a reference, a gas-deficiency index for dI’s is constructed, making it possible quantitatively to identify which galaxies have lost gas. For the Virgo sample, some of the dwarfs are gas-deficient by a factor of 30. The gas deficiency correlates roughly with the X-ray surface brightness of the intracluster gas. Ram pressure stripping can best explain the observed gas-poor dI’s in the cluster sample. Together with the lack of significant fading and reddening of the gas-poor dI’s compared with gas-normal dI’s, these observations suggest that the gas-poor dI’s in Virgo have recently encountered the intracluster medium for the first time. Faded remnants of gas-poor dI’s in Virgo will resemble bright dwarf elliptical galaxies currently seen in the cluster core.

IC 10: More evidence that it is a blue compact dwarf

We present optical spectroscopy of Hii regions in the Local Group galaxy IC 10 and UBVR photometry of foreground stars in three fields towards this galaxy. From these data, we find that the foreground reddening due to the Milky Way is

The Extinction and Distance of Maffei 2 and a New View of the IC 342/Maffei Group

E(B-V)=0.77\pm 0.07 \mathrm{mag}

A revised history of the local group and a generalized method of timing

. We find that IC 10 contains considerable internal dust, which qualitatively explains the variety of reddening values found by studies of its different stellar populations. Based upon our foreground reddening, IC 10 has intrinsic photometric properties like those of a blue compact dwarf galaxy, and not those of a dwarf irregular. This result is consistent with much evidence that IC 10 is in the throes of a starburst that began at least

Bright Planetary Nebulae and their Progenitors in Galaxies Without Star Formation

10 \mathrm{Myr}

Chemical Properties of Star-Forming Dwarf Galaxies

ago. We also report the discovery of a new WR star in the Hii region HL111c.

The Progenitors of Planetary Nebulae in Dwarf Irregular Galaxies

We have obtained spectra of H II regions in the heavily obscured spiral galaxy Maffei 2. The observations have allowed for a determination of the Galactic extinction of this galaxy using a correlation between extinction and hydrogen column density observed among spiral galaxies. The technique reveals that the optical depth of Galactic dust at 1 μm obscuring Maffei 2 is τ1 = 2.017 ± 0.211, which implies that AV = 5.58 ± 0.58 mag, significantly higher than observed for the giant elliptical Maffei 1 despite its similar latitude. For comparison, we apply the same technique to IC 342, a neighboring spiral to Maffei 2 but with more moderate obscuration by Galactic dust, owing to its higher Galactic latitude. For this galaxy, we obtain τ1 = 0.692 ± 0.066, which agrees within errors with the value of 0.639 ± 0.102 derived from the reddening estimate of Schlegel et al. (1998). We therefore adopt the weighted mean of τ1 = 0.677 ± 0.056 for the extinction of IC 342, which implies that AV = 1.92 ± 0.16 mag. A new distance estimate for Maffei 2 of 3.34 ± 0.56 Mpc is obtained from a self-consistent Tully-Fisher relation in I adjusted to the NGC 4258 maser zero point. With our new measurement of MI, Maffei 2 joins Maffei 1 and IC 342 as one of three giant members of the nearby IC 342/Maffei Group of galaxies. We present the revised properties of all three galaxies based on the most accurate extinction and distance estimates to date, accounting for shifts in the effective wavelengths of broadband filters as this effect can be significant for highly reddened galaxies. The revised distances are consistent with what would be suspected for the Hubble Flow, making it highly unlikely that the galaxies interacted with the Local Group since the big bang.