Pc In Diameter Research Articles

Dense Shell around a Young Intermediate-Mass Star: NGC 2264 IRS 1

Abstract The results of $\mathrm{H}{{13\atop} \mathrm{CO}}^{+}$$J = 1 \hbox{--} 0$ line and 93-GHz continuum observations of NGC 2264 IRS 1, a luminous infrared source known as Allen$^\prime$s source, are reported. IRS 1 is a young intermediate-mass star with a surrounding YSO cluster, and is associated with a molecular outflow. High angular resolution interferometric observations were performed using the Nobeyama Millimeter Array, and mapping observations were conducted using a 45-m telescope. The continuum map reveals four sources, three of which correspond to a submillimeter source detected by Ward-Thompson et al. (2000, AAA 73.137.062). Four compact clumps are also observed in the $\mathrm{H}{{13\atop} \mathrm{CO}}^{+}$ map, two of which are associated with these millimeter / submillimeter continuum sources without bipolar outflow. These $\mathrm{H}{{13\atop} \mathrm{CO}}^{+}$ clumps as a whole form an incomplete dense shell of 0.12 pc in diameter around IRS 1, and the outer part of the shell is clearly traced on the single-dish map. The shell, or cavity, structure suggests that the outflow from IRS 1 has evacuated part of the surrounding envelope in $\sim 0.1 \,\mathrm{Myr}$. The kinematic properties can be interpreted as being two clumps of entrained or compressed material derived from steady outflow. It is concluded that the activity of IRS 1 may have triggered the formation of cluster members in the dense surrounding shell.

Feedback of massive stars on the ISM: a XMM-Newton view of the LMC superbubble N 51D

N 51D (= DEM L 192) appears at first glance as a nearly circular, 120 pc diameter bubble of ionized gas around the LMC OB association LH 54. A deeper look reveals a complex web of filaments and deviations from radial expansion. Using a deep XMM-Newton pointing centered on N 51D we find that diffuse soft X-ray emitting gas fills the whole superbubble as delineated by the Hα filaments. Contrary to recent findings for galactic winds, the correlation between Hα and X-ray surface brightness is not good. The X-ray spectrum of this diffuse gas cannot be fitted with the LMC abundance pattern, but implies an overabundance of at least oxygen and neon, consistent with recent enrichment from supernovae Type II. Some indications for enhanced mixing at the brightest region of the Hα shell and for a beginning outflow of the hot gas were also detected.

A 60 pc counter-rotating core in NGC 4621

We present adaptive optics assisted OASIS integral field spectrography of the S0 galaxy NGC 4621. Two-dimensional stellar kinematical maps (mean velocity and dispersion) reveal the presence of a ~60 pc diameter counter-rotating core (CRC), the smallest observed to date. The OASIS data also suggests that the kinematic center of the CRC is slightly offset from the center of the outer isophotes. This seems to be confirmed by archival HST/STIS data. We also present the HST/WFPC2 colour map, which exhibits a central elongated red structure, also slightly off-centered in the same direction as the kinematic centre. We then construct an axisymmetric model of NGC 4621: the two-integral distribution function is derived using the Multi-Gaussian Expansion and the Hunter & Qian ([CITE]) formalisms. Although the stellar velocities are reasonably fitted, including the region of the counter-rotating core, significant discrepancies between the model and the observations demonstrate the need for a more general model (e.g. a three-integral model).

6-m telescope spectroscopic observations of the bubble complex in NGC 6946

We describe the results of a long-slit spectroscopic study of an unusual star complex in the nearby spiral galaxy NGC 6946 using the SAO 6-m telescope and the Keck 10-m telescope. The complex resembles a circular bubble 600 pc in diameter with a young super star cluster (SSC) near the center. The kinematics of ionized gas is studied through Ha emission with several slit positions. Position-velocity diagrams show two distinct features with high speed motions. One is an irregularly shaped region to the east of the SSC, 270 pc in size, in which most of the Ha emission is blue shifted by 120 km/s, and another is a 350 pc shell centered on the SSC with positive and negative velocity shifts of 60 km/s. Balmer and HeI absorption lines in the SSC give an age of 12-13 Myr, which is consistent with the photometric age but significantly older than the kinematic ages of the high speed regions. The energetics of the SSC and its interaction with the environment are considered. The expansion energies exceed 10^52 ergs, but the power outputs from winds and supernova in the SSC are large enough to account for this. The intensities of Balmer, [NII], and [SII] emission lines within and around the complex indicate that shock excitation makes a significant contribution to the emission from the most energetic region.

VLA High-Resolution 1.4 and 8.4 GH[CLC]z[/CLC] Mapping of the Barred Galaxy NGC 3367

We report new radio continuum observations with an angular resolution of 21 at 1.4 GHz (20 cm) and 028 at 8.4 GHz (3.6 cm) of the barred galaxy NGC 3367. In the map at 1.4 GHz, the central nuclear region connects to the southwest lobe, with a projected structure at a position angle (P.A.) of ∼230°, forming a jetlike structure. The map at 8.4 GHz shows a compact unresolved source (smaller than 65 pc in diameter) associated with emission from the nucleus and several compact sources located within a radius of about 300 pc, forming a circumnuclear structure. The compact core, jet, and lobes form a small, low-power counterpart to radio galaxies, with a flow axis that is out of the plane of the galaxy. The flow axis (P.A. ∼ 230°) coincides with the P.A. of the major axis of the galaxy and is thus inclined to the rotation axis of the disk. In addition, the flow axis differs by about 20° from the major axis of the stellar bar. Assuming that the stellar bar rotates counterclockwise (i.e., assuming trailing spiral arms), this difference in angle is taken as an argument in favor of having the jetlike structure out of the plane of the disk and not associated with the stellar bar.

S[CLC]i[/CLC]O Chimneys and Supershells in M82

In this Letter we present the first images of the emission of SiO and H13CO+ in the nucleus of the starburst galaxy M82. Contrary to other molecular species that mainly trace the distribution of the star-forming molecular gas within the disk, the SiO emission extends noticeably out of the galaxy plane. The bulk of the SiO emission is restricted to two major features. The first feature, referred to as the SiO supershell, is an open shell of 150 pc diameter, located 120 pc west from the galaxy center. The SiO supershell represents the inner front of a molecular shell expanding at 40 km/s, produced by mass ejection around a supercluster of young stars containing supernova remnant SNR 41.95+57.5. The second feature is a vertical filament, referred to as the SiO chimney, emanating from the disk at 200 pc east from the galaxy center. The SiO chimney reaches a 500 pc vertical height, and it is associated with the most prominent chimney identified in radio continuum maps. The kinematics, morphology, and fractional abundances of the SiO gas features in M82 can be explained in the framework of shocked chemistry driven by local episodes of gas ejection from the starburst disk.

Supernova Remnants in the Southwestern Part of the Small Magellanic Cloud

We have compared radio images from the Australia Telescope Compact Array and X-ray images from ROSAT of an area about 560 pc in diameter around the very extended emission nebula N19 on the southwestern edge of the bright ridge of the Small Magellanic Cloud. These data have allowed us to identify tentatively seven supernova remnants in this region. This number is large enough to suggest that the area has recently undergone some starburst activity. N19 itself appears to contain two overlapping supernova remnants within the extended H II region.

G28.17+0.05: An Unusual Giant HiCloud in the Inner Galaxy

New 21 cm H I observations have revealed a giant H I cloud in the Galactic plane that has unusual properties. It is quite well defined, about 150 pc in diameter at a distance of 5 kpc, and contains as much as 105 M☉ of atomic hydrogen. The outer parts of the cloud appear in H I emission above the H I background, while the central regions show H I self-absorption. Models that reproduce the observations have a core with a temperature 40 K and an outer envelope as much as an order of magnitude hotter. The cold core is elongated along the Galactic plane, whereas the overall outline of the cloud is approximately spherical. The warm and cold parts of the H I cloud have similar and relatively large line widths, ~7 km s-1. The cloud core is a source of weak, anomalously excited 1720 MHz OH emission, also with a relatively large line width, which delineates the region of H I self-absorption but is slightly blueshifted in velocity. The intensity of the 1720 MHz OH emission is correlated with NH derived from models of the cold core. There is 12CO emission associated with the cloud core. Most of the cloud mass is in molecules, and the total mass is greater than 2 × 105 M☉. In the cold core the H I mass fraction may be ~10%. The cloud has only a few sites of current star formation. There may be ~100 more objects like this in the inner Galaxy; every line of sight through the Galactic plane within 50° of the Galactic center probably intersects at least one. We suggest that G28.17+0.05 is a cloud being observed as it enters a spiral arm and that it is in the transition from the atomic to the molecular state.

Infrared Emission from the Radio Supernebula in NGC 5253: A Proto–Globular Cluster?

Hidden from optical view in the starburst region of the dwarf galaxy NGC 5253 lies an intense radio source with an unusual spectrum, which could be interpreted variously as nebular gas ionized by a young stellar cluster or nonthermal emission from a radio supernova or an active galactic nucleus. We have obtained 11.7 and 18.7 μm images of this region at the Keck telescope and find that it is an extremely strong mid-infrared emitter. The infrared-to-radio flux ratio rules out a supernova and is consistent with an H II region excited by a dense cluster of young stars. This supernebula provides at least 15% of the total bolometric luminosity of the galaxy. Its excitation requires 105-106 stars, giving it the total mass and size (1-2 pc diameter) of a globular cluster. However, its high obscuration, small size, and high gas density all argue that it is very young, no more than a few hundred thousand years old. This may be the youngest globular cluster yet observed.

A Submillimeter View of Star Formation near the HiiRegion KR 140

We present the results of 450 and 850 μm continuum mapping of the H II region KR 140 using the Submillimeter Common-User Bolometer Array (SCUBA) instrument on the James Clerk Maxwell Telescope (JCMT). KR 140 is a small (5.7 pc diameter) H II region at a distance of 2.3 ± 0.3 kpc. Five of the six IRAS point sources near KR 140 were mapped in this study. Our analysis shows that two of these IRAS sources are embedded late B-type stars lying well outside the H II region, two are a part of the dust shell surrounding the H II region, and one is the combined emission from an ensemble of smaller sources unresolved by IRAS. We have discovered a number of relatively cold submillimeter sources not visible in the IRAS data, ranging in size from 0.2 to 0.7 pc and in mass from 0.5 to 130 M☉. The distribution of masses for all sources is well characterized by a power law N(> M) ∝ M-α with α = 0.5 ± 0.04, in agreement with the typical mass function for clumped structures of this scale in molecular clouds. Several of the submillimeter sources are found at the H II molecular gas interface and have probably been formed as the result of the expansion of the H II region. Many of the submillimeter sources we detect are gravitationally bound and most of these follow a mass-size relationship expected for objects in virial equilibrium with nonthermal pressure support. Upon the loss of nonthermal support, they could be sites of star formation. Along with the two B stars that we have identified as possible cluster members along with VES 735, we argue that five nearby highly reddened stars are in a pre-main-sequence stage of evolution.

NGC 1614: A Laboratory for Starburst Evolution

The modest extinction and reasonably face-on viewing geometry make the luminous infrared galaxy NGC 1614 an ideal laboratory for study of a powerful starburst. HST/NICMOS observations show: (1) deep CO stellar absorption, tracing a starburst nucleus about 45 pc in diameter; (2) surrounded by an approx. 600 pc diameter ring of supergiant H II regions revealed in Pa-alpha line emission; (3) lying within a molecular ring indicated by its extinction shadow in H - K; and (4) all at the center of a disturbed spiral galaxy. The luminosities of the giant H II regions in the ring axe extremely high, an order of magnitude brighter than 30 Doradus; very luminous H II regions, comparable with 30 Dor, are also found in the spiral arms of the galaxy. Luminous stellar clusters surround the nucleus and lie in the spiral arms, similar to clusters observed in other infrared luminous and ultraluminous galaxies. The star forming activity may have been initiated by a merger between a disk galaxy and a companion satellite, whose nucleus appears in projection about 300 pc to the NE of the nucleus of the primary galaxy. The relation of deep stellar CO bands to surrounding ionized gas ring to molecular gas indicates that the luminous starburst started in the nucleus and is propagating outward into the surrounding molecular ring. This hypothesis is supported by evolutionary starburst modeling that shows that the properties of NGC 1614 can be fitted with two short-lived bursts of star formation separated by 5 Myr (and by inference by a variety of models with a similar duration of star formation). The total dynamical mass of the starburst region of 1.3 x 10(exp 9) solar masses is mostly accounted for by the old pre-starburst stellar population. Although our starburst models use a modified Salpeter initial mass function (turning over near one solar mass), the tight mass budget suggests that the IMF may contain relatively more 10 - 30 solar masses stars and fewer low mass stars than the Salpeter function. The dynamical mass is nearly four times smaller than the mass of molecular gas estimated from the standard ratio of (C-12)O (1 - 0) to H2. A number of arguments place the mass of gas in the starburst region at approx. 25% of the dynamical mass, nominally about 1/15 and with an upper limit of 1/10 of the amount estimated from (C-12)O and the standard ratio.

An Imaging and Spectroscopic Study of the Very Metal-deficient Blue Compact Dwarf Galaxy Tol 1214−277

We present a spectrophotometric study based on VLT/FORS I observations of one of the most metal-deficient blue compact dwarf (BCD) galaxies known, Tol 1214-277 (Z ∼ Z⊙/25). The data show that roughly half of the total luminosity of the BCD originates from a bright and compact starburst region located at the northeastern tip of a faint dwarf galaxy with cometary appearance. The starburst has ignited less than 4 Myr ago and its emission is powered by several thousand O7V stars and ∼170 late-type nitrogen Wolf-Rayet stars located within a compact region with ≲500 pc in diameter. For the first time in a BCD, a relatively strong [Fe V] λ4227 emission line is seen which together with intense He II λ4686 emission indicates the presence of a very hard radiation field in Tol 1214-277. We argue that this extraordinarily hard radiation originates from both Wolf-Rayet stars and radiative shocks in the starburst region. The structural properties of the low surface brightness (LSB) component underlying the starburst have been investigated by means of surface photometry down to 28 B mag arcsec-2. We find that, for a surface brightness level fainter than ∼24.5 B mag arcsec-2, an exponential fitting law provides an adequate approximation to its radial intensity distribution. The broadband colors in the outskirts of the LSB component of Tol 1214-277 are nearly constant and are consistent with an age below one Gyr. This conclusion is supported by the comparison of the observed spectral energy distribution (SED) of the LSB host with theoretical SEDs.

The Location of the Dense and Ionized Gas in the NGC 2023 Photon‐dominated Region

The VLA and the BIMA array were used to obtain high-resolution (10''-20'') observations of C+, traced by the C91α recombination line at 8.6 GHz, and the dense molecular gas, traced by HCN and HCO+(1-0), of the photon-dominated region (PDR) associated with the reflection nebula NGC 2023. Using the VLA, continuum emission is detected at 8.6 GHz from a faint H II region associated with HD 37903. The C91α emission originates from a 0.4 pc long filament, extending from the east to the south of the exciting star HD 37903. Within the filament three C91α clumps can be distinguished, each associated with filamentary vibrationally excited H2 emission in the direction toward HD 37903. The HCO+ emission has a clumpy appearance superimposed on a more extended component. C91α is, in general, closer to the exciting star than HCO+ emission as expected from PDR models. The morphologies of HCO+ and HCN are quite similar. Based on the C91α line width toward one of the clumps, a limit of 170 K on the kinetic temperature in the ionized carbon layer can be derived. This value is consistent with PDR models with H2 densities of about 105 cm-3. However, this result suggests surprisingly low limits on the turbulence in the PDR. We detected a compact 3 mm continuum source in the PDR, which appears to be a cold "core" of density 107 cm-3, 0.03 pc diameter, and 6 M☉. We conclude that it may have formed within the PDR. In an appendix, observations of the C91α recombination line toward five additional PDRs using the Effelsberg 100 m telescope are described.

Submillimeter Observations of [ITAL]Midcourse Space Experiment[/ITAL] Galactic Infrared-Dark Clouds

We present 850 and 450 μm continuum images of infrared-dark clouds (IRDCs) taken with the Submillimeter Common-User Bolometer Array (SCUBA) submillimeter camera at the James Clerk Maxwell Telescope. The IRDCs are large (1-10 pc diameter) molecular cores with gas densities ~106 cm-3 and temperatures ≈15 K. We detected strong submillimeter sources with peak flux densities of ≈1 Jy beam-1 at 850 μm in all eight clouds that were observed. The submillimeter emission generally lies within the envelope of the mid-infrared extinction where dense gas has been detected using H2CO as a tracer. The dust temperatures in the bright, compact sources are calculated to lie in the range 10-25 K. The masses of these sources are estimated to be in the range of several tens up to about a thousand solar masses. The corresponding gas column densities range over an order of magnitude, up to about 1023 cm-2. Several of the sources are detected in emission at both 850 and 8 μm. Two of the sources have HCO+ line profiles characteristic of molecular infall. It is likely that the bright, compact sources seen in the SCUBA images are in various early stages of star formation, from preprotostellar cores to class I objects.

Resolving SNR 0540-6944 from LMC X-1 with Chandra.

We examine the supernova remnant SNR 0540-697 in the Large Magellanic Cloud (LMC) using data from the Chandra Advanced CCD Imaging Spectrometer. The X-ray emission from this supernova remnant (SNR) had previously been hidden in the bright emission of the nearby X-ray binary LMC X-1; however, new observations with Chandra can finally reveal the SNR's structure and spectrum. We find the SNR to be a thick-shelled structure about 19 pc in diameter, with a brightened northeast region. Spectral results suggest a temperature of 0.31 keV and an X-ray luminosity (0.3-3.0 keV) of 8.4x1033 ergs s(-1). We estimate an age of 12,000-20,000 yr for this SNR, but note that this estimate does not take into account the possibility of cavity expansion or other environmental effects.

Cloud Collision–induced Star Formation in Sagittarius B2. I. Large‐Scale Kinematics

We present maps of a ~ 14 pc × 20 pc region covering the Sgr B2 molecular cloud complex in the 13CO (1-0) and CS (1-0) lines with high angular resolution. A more restricted central area was also mapped in the C18O (1-0) line. The 13CO cloud consists of two components: a compact core of ~ 3 pc in diameter roughly centered on Sgr B2(M) and an extended plateau of ~ 17 pc × 7.5 pc area containing the maser sources and most of the H II regions. Compact, massive (~ 3 × 105 M☉) C18O cores are associated with Sgr B2(M) and Sgr B2(N). The obtained intensity distributions are compared with those of molecular masers and compact H II regions reported in the literature. Low-velocity (40-65 km s-1) masers, with positions roughly aligned in the north-south line connecting the three major compact H II region complexes, are located near the eastern margin of a large hole in the low-velocity molecular cloud. In contrast, most of the high-velocity (65-80 km s-1) masers are distributed near the center of a compact molecular cloud at higher velocity, where a shock seems to have occurred. These results provide further evidence supporting the cloud-cloud collision hypothesis based on limited 13CO (1-0) data. In addition, this hypothesis appears to be consistent with a wide range of new observations that have appeared in the literature.

Two Large H [CSC]i[/CSC] Shells in the Outer Galaxy near [ITAL][CLC]l[/CLC][/ITAL] = 279°

As part of a survey of H I λ21 cm emission in the southern Milky Way, we have detected two large shells in the interstellar neutral hydrogen near l = 279°. The center velocities are +36 and +59 km s-1, which puts the shells at kinematic distances of 7 and 10 kpc. The larger shell is about 610 pc in diameter and very empty, with density contrast of at least 15 between the middle and the shell walls. It has expansion velocity of about 20 km s-1 and swept-up mass of several million solar masses. The energy indicated by the expansion may be as high as 2.4 × 1053 ergs. We estimate its age to be 15 to 20 million years. The smaller shell has diameter of about 400 pc, expansion velocity about 10 km s-1, and swept-up mass of about 106 M⊙. Morphologically both regions appear to be shells, with high-density regions mostly surrounding the voids, although the first appears to have channels of low density that connect with the halo above and below the H I layer. They lie on the edge of the Carina arm, which suggests that they may be expanding horizontally into the interarm region, as well as vertically out of the disk. If this interpretation is correct, this is the first detection of an H I chimney which has blown out of both sides of the disk.

ITAL]Chandra[/ITAL] X-Ray Observations of the Hydra A Cluster: An Interaction between the Radio Source and the X-Ray–emitting Gas

We present Chandra X-ray observations of the Hydra A cluster of galaxies, and we report the discovery of structure in the central 80 kpc of the cluster's X-ray-emitting gas. The most remarkable structures are depressions in the X-ray surface brightness, approximately 25-35 kpc in diameter, that are coincident with Hydra A's radio lobes. The depressions are nearly devoid of X-ray-emitting gas, and there is no evidence for shock-heated gas surrounding the radio lobes. We suggest that the gas within the surface brightness depressions was displaced as the radio lobes expanded subsonically, leaving cavities in the hot atmosphere. The gas temperature declines from 4 keV at 70 kpc to 3 keV in the inner 20 kpc of the brightest cluster galaxy (BCG), and the cooling time of the gas is approximately 600 Myr in the inner 10 kpc. These properties are consistent with the presence of an approximately 34 M middle dot in circle yr-1 cooling flow within a 70 kpc radius. Bright X-ray emission is present in the BCG surrounding a recently accreted disk of nebular emission and young stars. The star formation rate is commensurate with the cooling rate of the hot gas within the volume of the disk, although the sink for the material that may be cooling at larger radii remains elusive. A bright, unresolved X-ray source is present in the BCG's nucleus, coincident with the radio core. Its X-ray spectrum is consistent with a power law absorbed by a foreground NH approximately 4x1022 cm-2 column of hydrogen. This column is roughly consistent with the hydrogen column seen in absorption toward the less, similar24 pc diameter VLBA radio source. Apart from the point source, no evidence for excess X-ray absorption above the Galactic column is found.

The Complex Environment of the High‐Excitation Planetary Nebula NGC 3242

Spatially resolved profiles of the Hα, [N II] λ6584, and [O III] λ5007 nebular emission lines, obtained with the Manchester echelle spectrometer combined with the 2.1 m San Pedro Martir telescope, have revealed the velocity structure of the nebular core and of one of the three (A, B, and C) inner of the high-excitation planetary nebula NGC 3242. The core is shown to have a cylindrical structure expanding at 25 km s-1. The bright, diffuse, line-emitting, inner spherical halo A surrounding the intensely bright elliptical core is shown to be limb brightened, but its expansion velocity is unclear. The surrounding diffuse, 2.5 pc diameter halo B is modeled by a thick shell expanding at 20 km s-1, although the contribution of scattered [O III] λ5007 emission is unknown at present. The origin of the broad profiles from the fainter, patchy, 0.44 pc diameter halo C is again somewhat uncertain and may originate in scattered core light. Far-infrared (FIR) observations with IRAS and the Infrared Space Observatory (ISO) reveal the presence of warm dust throughout the core and inner halos on NGC 3242. Alternatively, the split profiles could imply an expansion velocity of around 20 km s-1 for halo C if scattering is discounted. There is a large filamentary structure of line-emitting gas 1.7 pc to the west of the nebular core. This is shown to be the western boundary of extended FIR emission from warm dust. A patchy component associated with this filamentary nebulosity emits the [O III] λ5007 line with anomalously high intensity yet is shown here to be kinematically relatively inert. Likewise, [N II] λ6584 profiles from the filamentary edge are very narrow with turbulent motions of 8 km s-1. Certainly, the arc is being photoionized by leakage radiation from NGC 3242, but the radial velocities of the line profiles are inconsistent with its origin as a simple shell expanding radially from this planetary nebula. An asymmetric lobe remains a possibility, in which case the arc could be an ancient halo of NGC 3242. The more mundane possibility that the arc is simply photoionized ambient gas is not completely ruled out by the present observations. Strange, faint, broad [O III] λ5007 line profiles are found over the whole 15' diameter area being considered here. The extended FIR emission region implies the presence of hot dust and suggests therefore that these [O III] λ5007 line profiles could have a scattered origin.

Detection and Mapping of Decoupled Stellar and Ionized Gas Structures in the Ultraluminous Infrared Galaxy IRAS 12112+0305.

Integral field optical spectroscopy with the INTEGRAL fiber-fed system and Hubble Space Telescope optical imaging are used to map the complex stellar and warm ionized gas structure in the ultraluminous infrared galaxy IRAS 12112+0305. Images reconstructed from wavelength-delimited extractions of the integral field spectra reveal that the observed ionized gas distribution is decoupled from the stellar main body of the galaxy, with the dominant continuum and emission-line regions separated by projected distances of up to 7.5 kpc. The two optical nuclei are detected as apparently faint emission-line regions, and their optical properties are consistent with being dust-enshrouded weak [O i] LINERs. The brightest emission-line region is associated with a faint (mI=20.4), giant H ii region of 600 pc diameter, in which a young ( approximately 5 Myr) massive cluster of about 2x107 M middle dot in circle dominates the ionization. Internal reddening toward the line-emitting regions and the optical nuclei ranges from 1 to 8 mag in the visual. Taking the reddening into account, the overall star formation in IRAS 12112+0305 is dominated by starbursts associated with the two nuclei and corresponds to a star formation rate of 80 M middle dot in circle yr-1.