First Interferometric Observations of Molecular Gas in a Polar Ring: The Helix Galaxy NGC 2685

We have detected four Giant Molecular cloud Associations (GMAs) (sizes < 6.6'' ~ 430 pc) in the western and eastern region of the polar ring in NGC2685 (the Helix galaxy) using the Owens Valley Radio Observatory (OVRO) millimeter interferometer. Emission from molecular gas is found close to the brightest Halpha and HI peaks in the polar ring and is confirmed by new IRAM 30m single dish observations. The CO and HI line velocities are very similar, providing additional kinematic confirmation that the CO emission emerges from the polar ring. For the first time, the total molecular mass within a polar ring is determined (M_H2~(8-11)x10^6 M_sol, using the standard Galactic conversion factor). We detect about M_H2~4.4x10^6 M_sol in the nuclear region with the single dish. Our upper limit derived from the interferometric data is lower (M_H2<0.7x10^6 M_sol) suggesting that the molecular gas is distributed in an extended (< 1.3 kpc) diffuse disk. These new values are an order of magnitude lower than in previous reports. The total amount of molecular gas and the atomic gas content of the polar ring are consistent with formation due to accretion of a small gas-rich object, such as a dwarf irregular. The properties of the NGC2685 system suggest that the polar ring and the host galaxy have been in a stable configuration for a considerable time (few Gyr). The second (outer) HI ring within the disk of NGC2685 is very likely at the outer Lindblad resonance (OLR) of the ~ 11 kpc long stellar bar.Comment: 8 pages, 4 figures, accepted by ApJ Letter

Nanostructured exchange coupled hard / soft composites: from the local magnetization profile to an extended 3D simple model

In nanocomposite magnetic materials the exchange coupling between phases plays a central role in the determination of the extrinsic magnetic properties of the material: coercive field, remanence magnetization. Exchange coupling is therefore of crucial importance in composite systems made of magnetically hard and soft grains or in partially crystallized media including nanosized crystallites in a soft matrix. It has been shown also to be a key point in the control of stratified hard / soft media coercive field in the research for optimized recording media. A signature of the exchange coupling due to the nanostructure is generally obtained on the magnetization curve $M(H)$ with a plateau characteristic of the domain wall compression at the hard/soft interface ending at the depinning of the wall inside the hard phase. This compression / depinning behavior is clearly evidenced through one dimensional description of the interface, which is rigorously possible only in stratified media. Starting from a local description of the hard/soft interface in a model for nanocomposite system we show that one can extend this kind of behavior for system of hard crystallites embedded in a soft matrix.Comment: 18 pages, 8 figures. To be published in the Journal of Magnetism and Magnetic Materials. (To be found at http://www.sciencedirect.com/science/journal/03048853

Bar-driven Transport of Molecular Gas to Galactic Centers and Its Consequences

We study the characteristics of molecular gas in the central regions of spiral galaxies on the basis of our CO(J=1-0) imaging survey of 20 nearby spiral galaxies using the NRO and OVRO millimeter arrays. Condensations of molecular gas at galactic centers with sizescales < 1 kpc and CO-derived masses M_gas(R<500pc) = 10^8 - 10^9 M_sun are found to be prevalent in the gas-rich L^* galaxies. Moreover, the degree of gas concentration to the central kpc is found to be higher in barred systems than in unbarred galaxies. This is the first statistical evidence for the higher central concentration of molecular gas in barred galaxies, and it strongly supports the theory of bar-driven gas transport. It is most likely that more than half of molecular gas within the central kpc of a barred galaxy was transported there from outside by the bar. The supply of gas has exceeded the consumption of gas by star formation in the central kpc, resulting in the excess gas in the centers of barred systems. The mean rate of gas inflow is statistically estimated to be larger than 0.1 - 1 M_sun/yr. The correlation between gas properties in the central kpc and the type of nuclear spectrum (HII, LINER, or Seyfert) is investigated. A correlation is found in which galaxies with larger gas-to-dynamical mass ratios tend to have HII nuclear spectra, while galaxies with smaller ratios show spectra indicating AGN. Also, the theoretical prediction of bar-dissolution by condensation of gas to galactic centers is observationally tested. It is suggested that the timescale for bar dissolution is larger than 10^8 - 10^10 yr, or a bar in a L^* galaxy is not destroyed by a condensation of 10^8 - 10^9 M_sun gas in the central kpc.Comment: AASTeX, 20 pages, 8 eps figs, ApJ in press (10 Nov. 1999 issue

Two massive star-forming regions at early evolutionary stages

We report sensitive ATCA radio-continuum observations toward IRAS 15596-5301 and 16272-4837, two luminous objects (> 2x10^4 Lsun) thought to represent massive star-forming regions in early stages of evolution (due to previously undetected radio emission at the 1-sigma level of 2 mJy per beam). Also reported are 1.2-millimeter continuum and a series of molecular-line observations made with the SEST telescope. For IRAS 15596-5301, the observations reveal the presence of three distinct compact radio-continuum sources associated with a dense molecular core. We suggest that this core contains a cluster of B stars which are exciting compact HII regions that are in pressure equilibrium with the dense molecular surroundings. No radio continuum emission was detected from IRAS 16272-4837 (3-sigma limit of 0.2 mJy). However, a dense molecular core has been detected. The high luminosity and lack of radio emission from this massive core suggests that it hosts an embedded young massive protostar that is still undergoing an intense accretion phase. This scenario is supported by the observed characteristics of the line profiles and the presence of a bipolar outflow detected from observations of the SiO emission. We suggest that IRAS 16272-4837 is a bona fide massive star- forming region in a very early evolutionary stage, being the precursor of an ultra compact HII region.Comment: 25 pages, 9 figures, accepted for publication in The Astrophysical Journa

Galaxies with unusually high abundances of molecular hydrogen

A sample of 66 galaxies from the catalog of Bettoni et al. (CISM) with anomalously high molecular-to-atomic hydrogen mass ratios (M_{mol}/M_{HI}>2) is considered. The sample galaxies do not differ systematically from other galaxies in the catalog with the same morphological types, in terms of their photometric parameters, rotational velocities, dust contents, or the total mass of gas in comparison with galaxies of similar linear sizes and disk angular momentum. This suggests that the overabundance of $H_2$ is due to transition of HI to H_2. Galaxies with bars and active nuclei are found more frequently among galaxies which have M_{mol} estimates in CISM. In a small fraction of galaxies, high M_{mol}/M_{HI} ratios are caused by the overestimation of M_{mol} due to a low conversion factor for the translation of CO-line intensities into the number of H_2 molecules along the line of sight. It is argued that the "molecularization" of the bulk of the gas mass could be due 1) to the concentration of gas in the inner regions of the galactic disks, resulting to a high gas pressure and 2) to relatively low star-formation rate per unit mass of molecular gas which indeed takes place in galaxies with high M_{mol}/M_{HI} ratios.Comment: 11 pages,7 figures, published in Astronomy Report

Cold and warm dust along a merging galaxy sequence

We investigate the cold and warm dust properties during galaxy interactions using a merging galaxy sample ordered into a chronological sequence from pre- to post-mergers. Our sample comprises a total of 29 merging systems selected to have far-infrared and sub-millimeter observations. The sub-millimeter data are mainly culled from the literature while for 5 galaxies (NGC 3597, NGC 3690, NGC 6090, NGC 6670 and NGC 7252) the sub-millimeter observations are presented here for the first time. We use the 100-to-850 micron flux density ratio, f_{100}/f_{850}, as a proxy to the mass fraction of the warm and the cold dust in these systems. We find evidence for an increase in f_{100}/f_{850} along the merging sequence from early to advanced mergers and interpret this trend as an increase of the warm relative to the cold dust mass. We argue that the two key parameters affecting the f_{100}/f_{850} flux ratio is the star-formation rate and the dust content of individual systems relative to the stars. Using a sophisticated model for the absorption and re-emission of the stellar UV radiation by dust we show that these parameters can indeed explain both the increase and the observed scatter in the f_{100}/f_{850} along the merging galaxy sequence. We also discuss our results under the hypothesis that elliptical galaxies are formed via disc galaxy mergers.Comment: Accepted for publication in MNRA

Dust enshrouded star-forming activity in Arp 299

We present mid-infrared spectro-imaging (5 - 16 microns) observations of the infrared luminous interacting system Arp 299 (=Mrk171 =IC694+NGC3690) obtained with the ISOCAM instrument aboard ISO. Our observations show that nearly 40% of the total emission at 7 and 15 microns is diffuse, originating from the interacting disks of the galaxies. Moreover, they indicate the presence of large amounts of hot dust in the main infrared sources of the system and large extinctions toward the nuclei. While the observed spectra have an overall similar shape, mainly composed of Unidentified Infrared Bands (UIB) in the short wavelength domain, a strong continuum at ~ 13 microns and a deep silicate absorption band at 10 microns, their differences reveal the varying physical conditions of each component. For each source, the spectral energy distribution (SED) can be reproduced by a linear combination of a UIB "canonical" spectral template and a hot dust continuum due to a 230-300 K black body, after independently applying an extinction correction to both of them. We find that the UIB extinction does not vary much throughout the system (A_V ~ 5 mag) suggesting that most UIBs originate from less enshrouded regions. IC694 appears to dominate the infrared emission of the system and our observations support the interpretation of a deeply embedded nuclear starburst located behind an absorption of about 40 mag. The central region of NGC3690 displays a hard radiation field characterized by a [NeIII]/[NeII] ratio > 1.8. It also hosts a strong continuum from 5 to 16 microns which can be explained as thermal emission from a deeply embedded (A_V ~ 60 mag) compact source, consistent with the mid-infrared signature of an active galactic nucleus (AGN), and in agreement with recent X-ray findings.Comment: to be published in Astronomy and Astrophysics - 12 page

The Gas Content in Galactic Disks: Correlation with Kinematics

We consider the relationship between the total HI mass in late-type galaxies and the kinematic properties of their disks. The mass $M_HI$ for galaxies with a wide variety of properties, from dwarf dIrr galaxies with active star formation to giant low-brightness galaxies, is shown to correlate with the product $V_c R_0$ ($V_c$ is the rotational velocity, and $R_0$ is the radial photometric disks scale length), which characterizes the specific angular momentum of the disk. This relationship, along with the anticorrelation between the relative mass of HI in a galaxy and $V_c$, can be explained in terms of the previously made assumption that the gas density in the disks of most galaxies is maintained at a level close to the threshold (marginal) stability of a gaseous layer to local gravitational perturbations. In this case, the regulation mechanism of the star formation rate associated with the growth of local gravitational instability in the gaseous layer must play a crucial role in the evolution of the gas content in the galactic disk.Comment: revised version to appear in Astronomy Letters, 8 pages, 5 EPS figure

Kinetic analysis of Human T-cell Leukemia Virus type 2 expression in chronically-infected cells and patient PBMCs

Introduction: The elucidation of the viral gene expression profile provides useful information in assessing the function of specific viral genes in the process of infection and cellular transformation. HTLV-2 pattern of mRNAs expression produces three major classes of mRNAs: unspliced genomic mRNA for Gag, protease and Pol proteins; singly spliced mRNAs encoding Env and the accessory proteins p28, p22/p20-1 and -2; and a doubly spliced mRNA for the regulatory proteins Tax, Rex and for the p10/p11 and p? accessory ones (Ref.1 and Fig. 1). To date, very little information has been obtained on the temporal regulation of different HTLV-2 transcripts expression in infected cells. Aim of this study was to investigate the kinetics of gene expression from HTLV-2 infected cell lines and from PBMCs of HTLV-2B infected subjects. The expression profile and kinetics of the different transcripts were analysed by real time RT-PCR using splice-junction-specific primers. Results: This approach was used to first determine the steady-state levels of expression for the different viral transcripts in three different cell lines in log phase of growth . Experiments performed indicated that gag/pol is the most abundant transcript. The expression level of env was comparable in the two T-cell lines, Mo-T and C344, infected by the 2A subtype, and was considerably higher than in the B-cells infected with HTLV-2B subtype, where p10/p11 and p? transcripts were below the limit of detection. We next investigated the kinetics of viral transcripts expression in infected BJAB-Gu cells. As in the previous experiment, the absolute copy number of gag/pol was the highest over the time period analysed . Among the accessory transcripts, p28,p22/p20-2 was the most abundant while other regulatory and accessory genes were lower. The analysis of fold variation, reported in g. 4B, indicated that tax/rex and p28, p22/p20-1 showed a biphasic profile with an early peak at 24 hours and a second one at 72 hours, whereas the transcripts gag/pol, env and p28,p22/p20-2 were expressed later.The kinetics of gene expression also was analysed from ex-vivo PBMCs of HTLV-2B infected subjects. Fig. 5 shows a typical pattern of expression. Also in this case, among the mRNAs species, gag/pol was consistently the most abundant transcript, p28, p22/p20-2 was approximately 15 fold lower than gag/pol, followed by tax/rex and p? that were present at approximately 25 fold lower than the unspliced mRNA coding for gag/pol . Very low levels of expression were found for p28, p22/p20-1, while env and p10/p11 transcripts were below the limit of detection. In Fig. 5B the fold variation analysis showed that the first mRNAs expressed were tax/rex and p28,p22/p20-1 with a peak at 4 hours followed by all the other transcripts that showed a later peak at 24 hours.These results indicate that tax/rex is the earliest transcript expressed, while the other genes, coding for accessory and structural proteins, are expressed in a later phase of the viral cycle. Conclusions: The expression of different HTLV-2 genes follows a distinct timing both in infected cell lines and PBMCs isolated from infected patients. The transcript tax/rex is the first to be expressed, thus indicating that it is necessary at the beginning of the infection cycle to transactivate and regulate viral and cellular transcripts. These results also suggest that the control of viral gene expression is highly regulated both in its kinetics and expression level