K. Y. Lo

The Neutral Interstellar Medium in Nearby Dwarf Galaxies. III. Sagittarius DIG, LGS 3, and Phoenix

We present new VLA observations of the H I medium of the Local Group dwarf galaxies Sag DIG, LGS 3, and Phoenix. Sag DIG is a gas-rich, blue dwarf irregular with some known recent star formation, whereas LGS 3 and Phoenix are gas-poor, red galaxies of intermediate irregular/spheroidal type with little recent star formation. These galaxies complete a small sample of Local Group and near-Local Group irregular and elliptical galaxies that have been mapped in H I and, where possible, in CO. We compare the properties and kinematics of the ISM in these different galaxy types in order to gain some insights into the relationship between galaxy properties, star formation, and the ISM. Both Sag DIG and LGS 3 have larger H I extents and higher H I fluxes than previously known, and in both cases the H I extends significantly farther than the stellar component. Neither one shows convincing signs of rotation; both seem to derive a significant amount of their support against gravity from random motions in the gas. The dwarf galaxies of the sample support the idea that there are large variations in the dark/luminous mass ratio at a given luminosity. The high sensitivity and high spectral and spatial resolution of these observations also make it possible to study the physical properties of the H I medium. The H I in Sag DIG is decomposed into broad (σ = 10 km s-1) and narrow (σ = 5 km s-1) components, with the broad component distributed throughout the galaxy and the narrow component concentrated into a small number of prominent clumps of about 8 × 105 M☉. It is argued that these H I components are in fact cold and warm phases of the H I medium, as in Galactic H I and in the dwarf irregular Leo A. LGS 3, on the other hand, shows little sign of such a two-phase H I structure. This new information on the phase structure of the ISM in dwarf galaxies is consistent with theoretical models of the H I medium if the H I line width is greater than purely thermal widths. The lack of a cold H I phase may be a reason for the lack of recent star formation in LGS 3; we suggest that the presence of a cold H I phase serves as a better indicator of conditions appropriate for star formation than measures of total H I content. The Phoenix dwarf and LGS 3 have been interpreted as two dwarf spheroidal galaxies which are unusual in that they contain H I. The presence of H I in LGS 3 is interesting, then, in the context of models that remove the gas from dwarf spheroidals by a burst of star formation. Either LGS 3 has not had a burst of star formation sufficient to remove its gas, or gas removal was not complete. It is not clear whether the Phoenix dwarf has H I. The current observations show emission in the vicinity of the galaxy at +55 km s-1 and -23 km s-1 (heliocentric). However, none of this emission is coincident with the optical galaxy. Until the stellar velocities in Phoenix are known, we cannot distinguish whether any of the detected H I is actually associated with the galaxy or is perhaps associated with the Magellanic Stream.

The Neutral Interstellar Medium in Nearby Dwarf Galaxies. II. NGC 185, NGC 205, and NGC 147

We present new, high-resolution observations of the interstellar medium (ISM) in the two dwarf elliptical galaxies NGC 185 and NGC 205, as well as a new upper limit on the H I content of the dwarf elliptical galaxy NGC 147. The data consist of VLA observations of H I emission at ~80 pc resolution (a factor of 3.5 improvement over older data), CO spectra at ~80 pc resolution in NGC 185 and NGC 205, an image of CO emission in NGC 205 at 40 ? 20 pc resolution from the BIMA array, and H? images of NGC 185 and NGC 205. These observations represent the intersection of two separate fields of inquiry: (1) the distribution, kinematics, and origin of neutral gas in giant ellipticals, which are structurally similar to the dwarf ellipticals but cannot currently be studied at such high linear resolution; and (2) the physical properties of the ISM in the Galaxy and its neighbors, which can be studied at very high linear resolution but which might have very different interstellar media than elliptical galaxies. The new H I images show that the neutral gas and stars of NGC 205 are two distinct dynamical systems, whereas in NGC 185 the gas and stars may be parts of the same dynamical system. The H I distributions in these galaxies are less extended than the optical emission and do not appear to be stable rotating disks, a result that is very different from the pattern established for giant ellipticals. An internal gas origin (mass loss from evolved stars) may be plausible for NGC 185. No H I emission is detected in NGC 147 with an upper limit of 3 ? 103 M? for an unresolved source with a line width of 8 km s-1. Furthermore, the H I distributions in NGC 185 and 205 are extremely clumpy on size and mass scales less than 200 pc and ~104 M?, with H I velocity dispersions ranging from 3 to 15 km s-1. We present evidence that the molecular gas in these galaxies is associated with individual clumps or clouds of atomic gas, similar to many Galactic giant molecular clouds. However, the H I column densities in the dwarf ellipticals are factors of 5-10 less than the 1021 cm-2 thought to be necessary to shield Galactic giant molecular clouds against photodissociation. Images of NGC 185 in H? + [N II] show an extended region of emission 50 pc in diameter near the center of the galaxy and the H I column density peak.

Infrared Space Observatory Measurements of [C II] Line Variations in Galaxies

We report measurements of the [C II] fine-structure line at 157.714 ?m in 30 normal star-forming galaxies with the Long Wavelength Spectrometer (LWS) on the Infrared Space Observatory (ISO). The ratio of the line to total far-infrared (FIR) luminosity, LC II/LFIR, measures the ratio of the cooling of gas to that of dust, and thus the efficiency of the grain photoelectric heating process. This ratio varies by more than a factor of 40 in the current sample. About two-thirds of the galaxies have LC II/LFIR ratios in the narrow range of (2-7) ? 10 -->?3. The other one-third show trends of decreasing LC II/LFIR with increasing dust temperature, as measured by the flux ratio of infrared emission at 60 and 100 ?m, F?(60 ?m)/F?(100 ?m), and with increasing star formation activity, measured by the ratio of FIR and blue-band luminosity, LFIR/L -->B. We also find three FIR-bright galaxies that are deficient in the [C II] line, which is undetected with 3 ? upper limits of LC II/LFIR ?4. The trend in the LC II/LFIR ratio with the temperature of dust and with star formation activity may be due to decreased efficiency of photoelectric heating of gas at high UV radiation intensity as dust grains become positively charged, decreasing the yield and the energy of the photoelectrons. The three galaxies with no observed photodissociation region lines have among the highest LFIR/L -->B and F?(60 ?m)/F?(100 ?m) ratios. Their lack of [C II] lines may be due to a continuing trend of decreasing LC II/LFIR with increasing star formation activity and dust temperature seen in one-third of the sample with warm IRAS colors. In that case, the upper limits on LC II/LFIR imply a ratio of UV flux to gas density of G -->0/n>10 cm -->3 (where G -->0 is in units of the local average interstellar field). The low LC II/LFIR ratio could also be due to either weak [C II], owing to self-absorption, or a strong FIR continuum from regions weak in [C II], such as dense H II regions or plasma ionized by hard radiation of active galactic nuclei. The mid-infrared and radio images of these galaxies show that most of the emission comes from a compact nucleus. CO and H I are detected in these galaxies, with H I seen in absorption toward the nucleus.

Water Maser Emission and the Parsec-Scale Jet in NGC 3079

We have conducted VLBI observations at subparsec resolution of water maser and radio continuum emission in the nucleus of the nearby active galaxy NGC 3079. The 22 GHz maser emission arises in compact (~0.01 pc at a distance of 16 Mpc) clumps, distributed over ~2 pc along an axis that is approximately aligned with the major axis of the galactic disk. The Doppler velocities of the water maser clumps are consistent with their lying in the inner parsec of a molecular disk with a binding mass ~106 M☉, rotating in the same sense as the edge-on kiloparsec-scale molecular disk observed in CO emission. However, the velocity field has a significant nonrotational component, which may indicate supersonic turbulence in the disk. This distribution is markedly different from that of water masers in NGC 4258, which trace a nearly perfectly Keplerian rotating disk with a binding mass of 3.5 × 107 M☉. The 22 GHz radio continuum emission in NGC 3079 is dominated by a compact (<0.1 pc) source that is offset 0.5 pc to the west of the brightest maser feature. No bright maser emission is coincident with a detected compact continuum source. This suggests that the large apparent luminosity of the maser is not caused by beamed amplification of high brightness temperature continuum emission. At 8 and 5 GHz, we confirm the presence of two compact continuum sources with a projected separation of 1.5 pc. Both have inverted spectra between 5 and 8 GHz and steep spectra between 8 and 22 GHz. NGC 3079 may be a nearby, low-luminosity example of the class of compact symmetric gigahertz-peaked spectrum radio sources. We detected a third continuum component that lies along the same axis as the other two, strongly suggesting that this galaxy possesses a nuclear jet. Faint maser emission was detected near this axis, which may indicate a second population of masers associated with the jet.

Zeeman measurements of the magnetic fields at the Galactic center

There is mounting evidence that suggests magnetic fields are important in a range of phenomena observed in the Galactic center. We report here Zeeman measurements of the magnetic field in the dense molecular gas in the circumnuclear disk (CND; inner radius ∼1.7 pc) using the 1667 MHz OH absorption against the continuum radio source at the Galactic center (Sgr A). These observations with a resolution of 3″×4″ (∼0.15 pc at 8 kpc) were made with the Very Large Array

FIRST DETECTION OF 492 GHz (C I) EMISSION FROM THE LARGE MAGELLANIC CLOUD

The 3 P1 3 3 P0 fine-structure transition of neutral atomic carbon [C I] was observed with the Antarctic Submillimeter Telescope and Remote Observatory (ASTyRO) toward two star-forming regions in the Large Magellanic Cloud: N159 and 30 Doradus. The [C I] line is weak in the vicinity of 30 Dor, a region with a uniquely hard and intense UV field. The I[C I]yICO ratio in N159 is enhanced by a factor?2 compared to the Milky Way Galaxy, a result attributable to the lower metallicity of the Large Magellanic Cloud. Subject headings: galaxies: ISM—ISM: atoms—ISM: individual (N159, 30 Doradus)—Magellanic Clouds— radio lines: ISM

The optical jet of the galaxy NGC 4258: interaction with the interstellar medium

Monochromatic H-alpha and red continuum images, as well as high-resolution aperture synthesis C-12 maps, were obtained in order to study the optical jet of the spiral galaxy NGC 4258. The jet morphology shows strong interaction with the ambient interstellar medium. The optical emission of the jet is due to line emission arising possibly from gas of the ambient interstellar medium which has been entrained and shocked by the radio-emitting gas. The jet near the center is resolved and has a width of about 200 pc. The CO observations show two clouds on each side of the nucleus; these clouds outline a channel, and the H-alpha jet falls right into this channel. These observations are consistent with the jet being in or making a small angle with the galaxy plane. It is suggested that the channel represents a tunnel bored by energetic material and that jet activity in NGC 4258 has been intermittent in the recent past. The interstellar medium may play an important role in making jets detectable optically and in shaping their forms. 28 refs.

Mid-Infrared Images of Luminous Infrared Galaxies in a Merging Sequence

We report mid-infrared observations of several luminous infrared galaxies (LIGs) carried out with the Infrared Space Observatory. Our sample was chosen to represent different phases of a merger sequence of galaxy-galaxy interaction with special emphasis on early/intermediate stages of merging. The mid-infrared emission of these LIGs shows extended structures for the early and intermediate mergers, indicating that most of the mid-infrared luminosities are not from a central active galactic nucleus. Both the infrared hardness (indicated by the IRAS 12, 25, and 60 μm flux density ratios) and the peak-to-total flux density ratios of these LIGs increase as projected separation of these interacting galaxies become smaller, consistent with increasing star formation activities that are concentrated to a smaller area as the merging process advances. These observations provide among the first observational constraint of largely theoretically based scenarios.

MOLECULAR CLOUDS IN THE DWARF ELLIPTICAL GALAXY NGC 205

We present observations of CO emission in NGC 205 (a dwarf elliptical companion of M31) obtained with the Berkeley-Illinois-Maryland Association (BIMA) array and the IRAM 30 m telescope. We compare the CO results to VLA observations of the H I emission at the same spatial resolution. On scales of 90 pc, the atomic gas and dust have very similar distributions, and the atomic and molecular gas have similar kinematics. A12CO (2-1)/(1-0) brightness temperature ratio of 0.9 is derived for one molecular cloud in NGC 205. This cloud is resolved by the 40 pc × 20 pc beam of the BIMA array; the cloud is elongated and shows a velocity gradient along its major axis. CO line widths are comparable to those in the Galaxy and the Small Magellanic Cloud (SMC) for clouds of the same size, which suggests that the molecular clouds in NGC 205 have similar virial masses to those in the Galaxy and the SMC.

BIMA observations of gamma-ray bursters GRB 970111 and GRB 970228

The Satellite per Astronomia X (BeppoSAX) is successfully producing small error boxes to gamma-ray bursts, permitting rapid follow-up multiwavelength searches for fading and quiescent counterparts. For GRB 970228 this led to the discovery of the first convincing fading counterpart. However, no counterpart was found for GRB 970111. In this Letter, we present our observations approximately 1 week after GRB 970228 and 1 month after GRB 970111 using the Berkeley-Illinois-Maryland Association (BIMA) array at 3.5 mm. No fading millimeter-wave sources were detected, but we demonstrate that BIMA is well suited for performing future searches.