Ice Mission Research Articles

Magnetic fields in the nearby spiral galaxy IC 342: A multi-frequency radio polarization study

The total and polarized radio continuum emission of IC 342 was observed in four wavelength bands with the Effelsberg and VLA telescopes. The frequency dependence of the radial scalelength of synchrotron emission indicates energy-dependent propagation of the cosmic-ray electrons, probably via the streaming instability. The equipartition strength of the total magnetic field is typically 15 muG, that of the ordered field 5 muG. Faraday rotation of the polarization angles reveals an underlying regular field of only about 0.5 muG strength with a large-scale axisymmetric spiral pattern, signature of a mean-field dynamo, and an about 10x stronger field that fluctuates on scales of a few 100 pc. The magnetic field around the bar in the central region of IC 342 resembles that of large barred galaxies; its regular spiral field is directed outwards, opposite to that in the disk. The polarized emission in the disk is concentrated in: (1) a narrow arm of about 300 pc width, displaced inwards with respect to the eastern arm by about 200 pc, indicating magnetic fields compressed by a density wave; (2) a broad arm of 300-500 pc width around the northern arm with systematic variations in polarized emission, polarization angles, and Faraday rotation measures on a scale of about 2 kpc, indicative of a helically twisted flux tube generated by the Parker instability; (3) a rudimentary "magnetic arm" in an interarm region in the north-west; (4) several broad spiral arms in the outer galaxy, related to spiral arms in the total neutral gas; (5) short features in the outer south-western galaxy, probably distorted by tidal interaction. - The generation and development of "magnetic arms" by a mean-field dynamo probably need a spiral pattern that is stable over a few galactic rotation periods. The dynamo in IC 342 is slow and weak, probably disturbed by the bar, tidal interaction, or a transient spiral pattern.

XMM-NewtonandChandraX-ray follow-up observations of the VHE gamma-ray source HESS J1507-622

Context. The discovery of the unique source HESS J1507-622 in the very high energy (VHE) range (100 GeV-100 TeV) opened new possibilities to study the parent population of ultra-relativistic particles found in astrophysical sources and underlined the possibility of new scenarios/mechanisms crucial for understanding the underlying astrophysical processes in nonthermal sources. Aims. The follow-up X-ray (0.2 - 10 keV) observations on HESS J1507-622 are reported, and possibilities regarding the nature of the VHE source and that of the newly discovered X-ray sources are investigated. Methods.We obtained bservations with the X-ray satellites XMM-Newton and Chandra. Background corrections were applied to the data to search for extended diffuse emission. Since HESS J1507-622 covers a large part of the field of view of these instruments, blank-sky background fields were used. Results. The discovery of several new X-ray sources and a new, faint, extended X-ray source with a flux of ~6e-14 erg cm^-2 s^-1 is reported. Interestingly, a new, variable point-like X-ray source with a flux of ~8e-14 erg cm^-2 s^-1 appeared in the 2011 observation, which was not detected in the previous X-ray observations. Conclusions. The X-ray observations revealed a faint, extended X-ray source that may be a possible counterpart for HESS J1507-622. This source could be an X-ray pulsar wind nebula (PWN) remnant of the larger gamma-ray PWN, which is still bright in IC emission. Several interpretations are proposed to explain the newly detected variable X-ray source.

Does Flood Pulsing Act as a Switch to Store or Release Sediment-Bound Carbon in Seasonal Floodplain Lakes? Case Study from the Colombian Orinoco-Llanos and the Brazilian Pantanal

Wetlands have been identified as key elements of global carbon budgets. Today, due to increasing anthropogen- ic carbon emissions, the carbon storage capacity of these ecosystems has become of global interest. The focus of this research was to determine the storage capacity and mecha- nisms ofcarbonsequestration ofNeotropical seasonalshallow lakes (SSLs), assessing their flood pulse, biomass, geomor- phology and sediment composition. We applied a standard- ized and extensive sampling to five SSLs from the Colombian Orinoco-Llanos and five from the Brazilian Pantanal, during both flooded and non-flooded seasons. We found that sedi- ment organic carbon (SOC) storage capacity in SSLs is sig- nificantly driven by the number of flooded days (p-value= 0.0057) ratherthanbytheir biomassproduction.TheOrinoco- Llanos and Pantanal lakes differ in morphology, and thus in their inundation patterns. SSLs of the Orinoco-Llanos are more concave, having a significantly higher number of flooded days/year than SSLs from Pantanal. These hydrologic differences affect SOC storage, resulting in a significantly higher SOC storage in SSLs from Orinoco-Llanos than in Pantanal. A synopsis of data from both regions suggests the existence of a critical threshold between "non-flooded season losses"and "non-flooded seasonstorage"ofSOC inSSLsina range of 225-275 flooded days/year.

Imaging on PAPER: Centaurus A at 148 MHz

We present observations taken with the Precision Array for Probing the Epoch of Reionization (PAPER) of the Centaurus A field in the frequency range 114 to 188 MHz. The resulting image has a 25' resolution, a dynamic range of 3500 and an r.m.s. of 0.5 Jy\beam (for a beam size of 25' x 23'). A spectral index map of Cen A is produced across the full band. The spectral index distribution is qualitatively consistent with electron reacceleration in regions of excess turbulence in the radio lobes, as previously identified morphologically. Hence, there appears to be an association of 'severe weather' in radio lobes with energy input into the relativistic electron population. We compare the PAPER large scale radio image with the X-ray image from the ROSAT All Sky Survey. There is a tentative correlation between radio and X-ray features at the end of the southern lobe, some 200 kpc from the nucleus, as might be expected from inverse Compton scattering of the CMB by the relativistic electrons also responsible for the radio synchrotron emission. The magnetic fields derived from the (possible) IC and radio emission are of similar magnitude to fields derived under the minimum pressure assumptions, ~ 1 {\mu}G. However, the X-ray field is complex, with large scale gradients and features possibly unrelated to Cen A. If these X-ray features are unrelated to Cen A, then these fields are lower limits.

Radio Loud AGN Unification: Connecting Jets and Accretion

While only a fraction of Active Galactic Nuclei are observed to host a powerful relativistic jet, a cohesive picture is emerging that radio-loud AGN may represent an important phase in the evolution of galaxies and the growth of the central super-massive black hole. I will review my own recent observational work in radio-loud AGN unification in the context of understanding how and why jets form and their the connection to different kinds of accretion and growing the black hole, along with a brief discussion of possible connections to recent modeling work in jet formation. Starting from the significant observational advances in our understanding of jetted AGN as a population over the last decade thanks to new, more sensitive instruments such as Fermi and Swift as well as all-sky surveys at all frequencies, I will lay out the case for a dichotomy in the jetted AGN population connected to accretion mode onto the black hole. In recent work, we have identified two sub-populations of radio-loud AGN which appear to be distinguished by jet structure, where low-effi ciency accreting systems produce 'weak' jets which decelerate more rapidly than the 'strong' jets of black holes accreting near the Eddington limit. The two classes are comprised of: (1)The weak jet sources, corresponding to the less collimated, edge-darkened FR Is, with a decelerating or spine-sheath jet with velocity gradients, and (2) The strong jet sources, having fast, collimated jets, and typically displaying strong emission lines. The dichotomy in the vp-Lp plane can be understood as a broken power sequence in which jets exist on one branch or the other based on the particular accretion mode (Georganopolous 2011).We suggest that the intrinsic kinetic power (as measured by low-frequency, isotropic radio emission), the orientation, and the accretion rate of the SMBH system are the the fundamental axes needed for unification of radio-loud AGN by studying a well-characterized sample of several hundred Fermi-detected jets. Finally, we present very recent findings that the most powerful strong jets produce gamma-rays by external Compton rather than SSC emission, placing the origin of the IC emission in these strong jets at a radius inside the BLR and/or molecular torus (Meyer 2012).

Jets and gamma-ray emission from isolated accreting black holes

The large number of isolated black holes (IBHs) in the Galaxy, estimated to be 10^8, implies a very high density of 10^-4 pc^-3 and an average distance between IBHs of 10 pc. Our study shows that the magnetic flux, accumulated on the horizon of an IBH because of accretion of interstellar matter, allows the Blandford-Znajeck mechanism to be activated. Thus, electron-positron jets can be launched. We have performed 2D numerical modelling which allowed the jet power to be estimated. Their inferred properties make such jets a feasible electron accelerator which, in molecular clouds, allows electron energy to be boosted up to 1 PeV. For the conditions expected in molecular clouds the radiative cooling time should be comparable to the escape time. Thus these sources can contribute both to the population of unidentified point-like sources and to the local cosmic ray (CR) electron spectrum. The impact of the generated electron CRs depends on the diffusion rate inside molecular clouds (MCs). If the diffusion regime in a MC is similar to Galactic diffusion, the produced electrons should rapidly escape the cloud and contribute to the Galactic CR population at very high energies >100 TeV. However, due to the modest jet luminosity (at the level of 10^35 erg s^-1) and low filling factor of MC, these sources cannot make a significant contribution to the spectrum of cosmic ray electrons at lower energies. On the other hand, if the diffusion within MCs operates at a rate close to the Bohm limit, the CR electrons escaping from the source should be confined in the cloud, significantly contributing to the local density of CRs. The IC emission of these locally-generated CRs may explain the variety of gamma ray spectra detected from nearby MCs.

Predicting module level RF emissions from IC emissions measurements using a 1 GHz TEM or GTEM cell — A review of related published technical papers

This paper reviews some of the many published technical papers relating to the measurement of IC emissions using a 1 GHz TEM cell or GTEM cell modified to accept an IC test board and provides a basis for correlation between these IC level measurements and module level RF emissions. We will follow the development of this methodology from the early investigations through the later applications to show the viability of the technique for IC qualification. Developments over time have shown that this method of IC emission measurement is repeatable, correlatable to other methods and provides, within limits, useful prediction of module level RF emissions performance.

GIANT GAMMA-RAY BUBBLES FROMFERMI-LAT: ACTIVE GALACTIC NUCLEUS ACTIVITY OR BIPOLAR GALACTIC WIND?

Data from the Fermi-LAT reveal two large gamma-ray bubbles, extending 50 degrees above and below the Galactic center, with a width of about 40 degrees in longitude. The gamma-ray emission associated with these bubbles has a significantly harder spectrum (dN/dE ~ E^-2) than the IC emission from electrons in the Galactic disk, or the gamma-rays produced by decay of pions from proton-ISM collisions. There is no significant spatial variation in the spectrum or gamma-ray intensity within the bubbles, or between the north and south bubbles. The bubbles are spatially correlated with the hard-spectrum microwave excess known as the WMAP haze; the edges of the bubbles also line up with features in the ROSAT X-ray maps at 1.5-2 keV. We argue that these Galactic gamma-ray bubbles were most likely created by some large episode of energy injection in the Galactic center, such as past accretion events onto the central massive black hole, or a nuclear starburst in the last ~10 Myr. Dark matter annihilation/decay seems unlikely to generate all the features of the bubbles and the associated signals in WMAP and ROSAT; the bubbles must be understood in order to use measurements of the diffuse gamma-ray emission in the inner Galaxy as a probe of dark matter physics. Study of the origin and evolution of the bubbles also has the potential to improve our understanding of recent energetic events in the inner Galaxy and the high-latitude cosmic ray population.

Estimation of tropical forest height and biomass dynamics using lidar remote sensing at La Selva, Costa Rica

In this paper we present the results of an experiment to measure forest structure and biomass dynamics over the tropical forests of La Selva Biological Station in Costa Rica using a medium resolution lidar. Our main objective was to observe changes in forest canopy height, related height metrics, and biomass, and from these map sources and sinks of carbon across the landscape. The Laser Vegetation Imaging Sensor (LVIS) measured canopy structure over La Selva in 1998 and again in 2005. Changes in waveform metrics were related to field‐derived changes in estimated aboveground biomass from a series of old growth and secondary forest plots. Pairwise comparisons of nearly coincident lidar footprints between years showed canopy top height changes that coincided with expected changes based on land cover types. Old growth forests had a net loss in height of −0.33 m, while secondary forests had net gain of 2.08 m. Multiple linear regression was used to relate lidar metrics with biomass changes for combined old growth and secondary forest plots, giving an r2 of 0.65 and an RSE of 10.5 Mg/ha, but both parametric and bootstrapped confidence intervals were wide, suggesting weaker model performance. The plot level relationships were then used to map biomass changes across La Selva using LVIS at a 1 ha scale. The spatial patterns of biomass changes matched expected patterns given the distribution of land cover types at La Selva, with secondary forests showing a gain of 25 Mg/ha and old growth forests showing little change (2 Mg/ha). Prediction intervals were calculated to assess uncertainty for each 1 ha cell to ascertain whether the data and methods used could confidently estimate the sign (source or sink) of the biomass changes. The resulting map showed most of the old growth areas as neutral (no net biomass change), with widely scattered and isolated sources and sinks. Secondary forests in contrast were mostly sinks or neutral, but were never sources. By quantifying both the magnitude of biomass changes and the sensitivity of lidar to detect them, this work will help inform the formulation of future space missions focused on biomass dynamics, such as NASA's Deformation Ecosystem Structure and Dynamics of Ice mission.

Characterization of the Evolution of IC Emissions After Accelerated Aging

With the evolving technological development of integrated circuits, ensuring electromagnetic compatibility (EMC) is becoming a serious challenge for electronic circuit and system manufacturers. Although electronic components must pass a set of EMC tests to ensure safe operations, the evolution of EMC over time is not characterized and cannot be accurately forecast. This paper presents an original study about the consequences of the aging of circuits on electromagnetic emissions. Different types of standard applicative and accelerated life tests are applied on a mixed power circuit dedicated to automotive applications. Its conducted emissions are measured before and after these tests, showing variations in EMC performance. Comparisons between each type of aging procedure show that the emission level of the circuit under test is differently affected.

Approximation of the radiation power of electrons due to the inverse-Compton process in the black-body photon field

An approximation for the inverse-Compton radiation power of electrons in the isotropic black-body photon field is presented. The approximation allows one to calculate inverse-Compton emissivity as integral over the energies of incident electrons rather than over the field photon energies. Such an approach allows for accurate modeling of IC emission of electrons with energy spectra being different from power-law, in situation where the CPU resources are limited. High accuracy of this approximation allows one to use it in a wide range of conditions, from Thomson to extreme Klein-Nishina limits. The approach adopted results also in some new analytic expressions representing known results in the Thomson limit.

Prompt high-energy emission from gamma-ray bursts in the internal shock model

The prompt GRB emission is thought to arise from electrons accelerated in internal shocks propagating within a highly relativistic outflow. The launch of Fermi offers the prospect of observations with unprecedented sensitivity in high-energy (>100 MeV) gamma-rays. The aim is to explore the predictions for HE emission from internal shocks, taking into account both dynamical and radiative aspects, and to deduce how HE observations constrain the properties of the relativistic outflow. The emission is modeled by combining a time-dependent radiative code with a dynamical code giving the evolution of the physical conditions in the shocked regions.Synthetic lightcurves and spectra are compared to observations. The HE emission deviates significantly from analytical estimates, which tend to overpredict the IC component, when the time dependence and full cross-sections are included. The exploration of the parameter space favors the case where the dominant process in the BATSE range is synchrotron emission. The HE component becomes stronger for weaker magnetic fields. The HE lightcurve can display a prolonged pulse duration due to IC emission, or even a delayed peak compared to the BATSE range.Alternatively, having dominant IC emission in the BATSE range requires most electrons to be accelerated into a steep power-law distribution and implies strong 2nd order IC scattering. In this case, the BATSE and HE lightcurves are very similar. The combined dynamical and radiative approach allows a firm appraisal of GRB HE prompt emission. A diagnostic procedure is presented to identify from observations the dominant emission process and derive constrains on the bulk Lorentz factor, particle density and magnetic field of the outflow.

Non-thermal emission in the core of Perseus: results from a longXMM-Newtonobservation

We employ a long XMM-Newton observation of the core of the Perseus cluster to validate claims of a non-thermal component discovered with Chandra. From a meticulous analysis of our dataset, which includes a detailed treatment of systematic errors, we find the 2-10 keV surface brightness of the non-thermal component to be smaller than about 5x10^-16 erg cm^-2s^-1arcsec^-2. The most likely explanation for the discrepancy between the XMM-Newton and Chandra estimates is a problem in the effective area calibration of the latter. Our EPIC based magnetic field lower limits are not in disagreement with Faraday rotation measure estimates on a few cool cores and with a minimum energy estimate on Perseus. In the not too distant future Simbol-X may allow detection of non-thermal components with intensities more than 10 times smaller than those that can be measured with EPIC; nonetheless even the exquisite sensitivity within reach for Simbol-X might be insufficient to detect the IC emission from Perseus.

Mapping Warm Molecular Hydrogen with theSpitzerInfrared Array Camera (IRAC)

Photometric maps, obtained with the Spitzer Infrared Array Camera (IRAC), can provide a valuable probe of warm molecular hydrogen within the interstellar medium. IRAC maps of the supernova remnant IC 443, extracted from the Spitzer archive, are strikingly similar to spectral line maps of the H2 pure rotational transitions that we obtained with the Infrared Spectrograph (IRS) instrument on Spitzer. IRS spectroscopy indicates that IRAC bands 3 and 4 are indeed dominated by the H2 v = 0-0 S(5) and S(7) transitions, respectively. Modeling of the H2 excitation suggests that bands 1 and 2 are dominated by H2 v = 1–0 O(5) and v = 0-0 S(9). Large maps of the H2 emission in IC 443, obtained with IRAC, show band ratios that are inconsistent with the presence of gas at a single temperature. The relative strengths of IRAC bands 2, 3, and 4 are consistent with pure H2 emission from shocked material with a power-law distribution of gas temperatures. CO vibrational emissions do not contribute significantly to the observed band 2 intensity. Assuming that the column density of H2 at temperatures T to T + dT is proportional to T−b for temperatures up to 4000 K, we obtained a typical estimate of 4.5 for b. The power-law index, b, shows variations over the range ~3-6 within the set of different sight lines probed by the maps, with the majority of sight lines showing b in the range 4-5. The observed power-law index is consistent with the predictions of simple models for paraboloidal bow shocks.

Isolated X‐Ray–Infrared Sources in the Region of Interaction of the Supernova Remnant IC 443 with a Molecular Cloud

The nature of the extended hard X-ray source XMMU J061804.3+222732 and its surroundings is investigated using XMM-Newton, Chandra, and Spitzer observations. This source is located in an interaction region of the IC 443 supernova remnant with a neighboring molecular cloud. The X-ray emission consists of a number of bright clumps embedded in an extended structured non-thermal X-ray nebula larger than 30 in size. Some clumps show evidence for line emission at ~1.9 keV and ~3.7 keV at the 99% confidence level. Large-scale diffuse radio emission of IC 443 passes over the source region, with an enhancement near the source. An IR source of about 14 x 7 size is prominent in the 24 um, 70 um, and 2.2 um bands, adjacent to a putative Si K-shell X-ray line emission region. The observed IR/X-ray morphology and spectra are consistent with those expected for J/C-type shocks of different velocities driven by fragmented supernova ejecta colliding with the dense medium of a molecular cloud. The IR emission of the source detected by Spitzer can be attributed to both continuum emission from an HII region created by the ejecta fragment and line emission excited by shocks. This source region in IC 443 may be an example of a rather numerous population of hard X-ray/IR sources created by supernova explosions in the dense environment of star-forming regions. Alternative Galactic and extragalactic interpretations of the observed source are also discussed.

H I IMAGING OBSERVATIONS OF SUPERTHIN GALAXIES. II. IC 2233 AND THE BLUE COMPACT DWARF NGC 2537

We have used the Very Large Array to image the H I 21 cm line emission in the edge-on Sd galaxy IC 2233 and the blue compact dwarf NGC 2537. We also present new optical B, R, and Hα imaging of IC 2233 obtained with the WIYN telescope. Despite evidence of localized massive star formation in the form of prominent H II regions and shells, supergiant stars, and a blue integrated color, IC 2233 is a low surface brightness system with a very low global star formation rate (0.05 M☉ yr−1), and we detect no significant 21 cm radio continuum emission from the galaxy. The H I and ionized gas disks of IC 2233 are clumpy and vertically distended, with scale heights comparable to that of the young stellar disk. Both the stellar and H I disks of IC 2233 appear flared, and we also find a vertically extended, rotationally anomalous component of H I extending to ~ 2.4d10 kpc from the midplane. The H I disk exhibits a mild lopsidedness as well as a global corrugation pattern with a period of ~7d10 kpc and an amplitude of ~150d10 pc. To our knowledge, this is the first time corrugations of the gas disk have been reported in an external galaxy; these undulations may be linked to bending instabilities or to underlying spiral structure and suggest that the disk is largely self-gravitating. Lying at a projected distance of from IC 2233, NGC 2537 has an H I disk with a bright, tilted inner ring and a flocculent, dynamically cold outer region that extends to ~3.5 times the extent of the stellar light (D25). Although NGC 2537 is rotationally-dominated, we measure H I velocity dispersions as high as km s−1 near its center, indicative of significant turbulent motions. The inner rotation curve rises steeply, implying a strong central mass concentration. Our data indicate that IC 2233 and NGC 2537 do not constitute a bound pair and most likely lie at different distances. We also find no compelling evidence of a recent minor merger in either IC 2233 or NGC 2537, suggesting that both are examples of small disk galaxies evolving in relative isolation.

The Active Nucleus of IC 4970: A Nearby Example of Merger‐Induced Cold‐Gas Accretion

We present results from Chandra X-ray and Spitzer mid-infrared observations of the interacting galaxy pair NGC 6872/IC4970inthePavogalaxygroupandshowthat thesmaller companiongalaxyIC4970hostsahighlyobscured activegalacticnucleus(AGN).The0.5Y10keVX-rayluminosityof thenucleusisvariable,increasingbyafactor2.9 to a luminosity of 1:7 ;10 42 erg s � 1 (bright state) on � 100 ks timescales. The X-ray spectrum of the bright state is heavily absorbed (NH ¼ 3 ;10 23 cm � 2 for power-law models with � ¼ 1:5Y2:0) and shows a clear 6.4 keV Fe K� line with equivalent width of 144Y195 eV. Limits on the diffuse emission in IC 4970 from Chandra X-ray data suggest that the available power from Bondi accretion of IC 4970’s hot interstellar gas may be an order of magnitude toosmalltopowertheAGN.Spitzerimagesshowthat8 � mnonstellaremissionisconcentratedinthecentral1kpcof IC 4970, consistent with high obscuration in this region. The mid-infrared colors of the nucleus are consistent with thoseexpectedfor ahighlyobscuredAGN.Takentogetherthesedatasuggest thatthenucleusof IC4970isaSeyfert2, triggered and fueled by cold material supplied to the central supermassive black hole as a result of the off-axis collision of IC 4970 with the cold-gas-rich spiral galaxy NGC 6872. Subject headingg galaxies: active — galaxies: clusters: general — galaxies: individual (IC 4970) — galaxies: interactions — infrared: galaxies — X-rays: galaxies

Testing the cosmic ray content in galaxy clusters

The effective content of cosmic rays (CR) in galaxy clusters remains elusive. This paper aims to estimate a maximum production of both secondary relativistic electrons, SRE, and gamma rays, GR, from the relativistic protons, RP, that have supposedly accumulated throughout the entire history of a cluster. The production rate is normalized by adopting a reference value of 0.3 for the ratio of RP to thermal pressure. The SRE content which obtains, when constrained to reproduce the observed radio brightness profile, yields univocally B(r), if the presence of primary RE were negligible. This procedure is applied to four radio halo clusters (Coma, A2163, A2255, A2319). In these objects, the central value B_0 required is consistent with typical, albeit rather uncertain, values derived from FR. On the other hand, B(r) typically increases beyond the thermal core, a hardly acceptable condition. This problem is alleviated by assuming a mix of SRE and of primary RE, with the latter becoming the dominant component beyond the thermal core. These results suggest that in clusters without a radio halo detected so far a diffuse radio emission should also be observable due to SRE alone, and therefore more centrally condensed. To encourage deeper radio observations of such clusters, some examples were selected that seem rather promising. Efforts in this direction, if accompanied by FR measurements, could provide highly significant constraints on the CR content in clusters, even before the GLAST mission will have accomplished the hard task of detecting the GR. A complementary result concerns the excess far UV in the Coma cluster, that some authors have attributed to IC emission from SRE. It is shown that this hypothesis can be excluded, because it requires a RP energy content in excess of the thermal one.

Hot Molecular Gas in the Nuclear Region of IC 342

We present the first interferometric detection of extragalactic NH3 (6, 6) emission in the nearby galaxy IC 342 made using the VLA. The data have a resolution of 78 × 50 and trace hot (T ~ 412 K) and dense (>104 cm-3) molecular gas. We have covered a 170'' × 300'' area and detect two very strong line emission peaks, likely associated with the two strongest star formation regions of the central part of the galaxy. We compare these emission peaks to CO (1-0) and (2-1) emission data, which are the most abundant CO transitions and trace spatially extended emission. The NH3 (6, 6) emission is also compared to emission data from three high-density, nitrogen-bearing tracers: HNC (1-0), HC3N (10-9), and N2H+ (1-0). Our results suggest that the molecular mass in the nuclear region of IC 342 has at least two different components, a dense and cold component and a less dense and hotter component.

Molecular Gas in the Low‐Metallicity, Star‐forming Dwarf IC 10

We present a complete survey of CO (1 → 0) emission in the Local Group dwarf irregular IC 10. The survey, conducted with the BIMA interferometer, covers the stellar disk and a large fraction of the extended H I envelope with the sensitivity and resolution necessary to detect individual giant molecular clouds (GMCs) at the distance of IC 10 (950 kpc). We find 16 clouds with a total CO luminosity of 1 × 106 K km s-1 pc2, equivalent to 4 × 106 M☉ of molecular gas using the Galactic CO-to-H2 conversion factor. Observations with the ARO 12 m find that BIMA may resolve out as much as 50% of the CO emission, and we estimate the total CO luminosity as ~2.2 × 106 K km s-1 pc2. We measure the properties of 14 GMCs from high-resolution OVRO data. These clouds are very similar to Galactic GMCs in their sizes, line widths, luminosities, and CO-to-H2 conversion factors, despite the low metallicity of IC 10 (Z ≈ 1/5 Z☉). Comparing the BIMA survey to the atomic gas and stellar content of IC 10, we find that most of the CO emission is coincident with high surface density H I. IC 10 displays a much higher star formation rate per unit molecular (H2) or total (H I+H2) gas than most galaxies. This could be a real difference or may be an evolutionary effect—the star formation rate may have been higher in the recent past.