Tidally-Triggered Star Formation in Close Pairs of Galaxies

We analyze new optical spectra of a sample of 502 galaxies in close pairs and n-tuples, separated by <= 50/h kpc. We extracted the sample objectively from the CfA2 redshift survey, without regard to the surroundings of the tight systems. We probe the relationship between star formation and the dynamics of the systems of galaxies. The equivalent widths of H\alpha (EW(H\alpha) and other emission lines anti-correlate strongly with pair spatial separation (\Delta D) and velocity separation. We use the measured EW(H\alpha) and the starburst models of Leitherer et al. to estimate the time since the most recent burst of star for- mation began for each galaxy. In the absence of a large contribution from an old stellar population to the continuum around H\alpha, the observed \Delta D -- EW(H\alpha) correlation signifies that starbursts with larger separations on the sky are, on average, older. By matching the dynamical timescale to the burst timescale, we show that the data support a simple picture in which a close pass initiates a starburst; EW(H\alpha) decreases with time as the pair separation increases, accounting for the anti-correlation. This picture leads to a method for measuring the duration and the initial mass function of interaction-induced starbursts: our data are compatible with the starburst and orbit models in many respects, as long as the starburst lasts longer than \sim10^8 years and the delay between the close pass and the initiation of the starburst is less than a few \times 10^7 years. If there is no large contribution from an old stellar population to the continuum around H\alpha the Miller-Scalo and cutoff (M <= 30 M_\sun) Salpeter initial mass functions fit the data much better than a standard Salpeter IMF. (Abridged.)Comment: 43 pages, 22 figures, to appear in the ApJ; we correct an error which had minor effects on numerical values in the pape

The Central Engines of 19 LINERs as Viewed by Chandra

Using archival Chandra observations of 19 LINERs we explore the X-ray properties of their inner kiloparsec to determine the origin of their nuclear X-ray emission, to investigate the presence of an AGN, and to identify the power source of the optical emission lines. The relative numbers of LINER types in our sample are similar to those in optical spectroscopic surveys. We find that diffuse, thermal emission is very common and is concentrated within the central few hundred parsec. The average spectra of the hot gas in spirals and ellipticals are very similar to those of normal galaxies. They can be fitted with a thermal plasma (kT~0.5 keV) plus a power law (photon index of 1.3-1.5) model. There are on average 3 detected point sources in their inner kiloparsec with L(0.5-10 keV)~10^37-10^40 erg/s. The average cumulative luminosity functions for sources in spirals and ellipticals are identical to those of normal galaxies. In the innermost circle of 2.5" radius in each galaxy we find an AGN in 12 of the 19 galaxies. The AGNs contribute a median of 60% of the 0.5-10 keV luminosity of the central 2.5" region, they have luminosities of 10^37-10^39 erg/s (Eddington ratios 10^-8 to 10^-5). The ionizing luminosity of the AGNs is not enough to power the observed optical emission lines in this particular sample. Thus, we suggest that the lines are powered either by the mechanical interaction of an AGN jet (or wind) with the circumnuclear gas, or by stellar processes, e.g. photoionization by post-AGB stars or young stars.Comment: Accepted by Ap.J. 23 pages, 8 figures, emulatepj format, images of fig 1 not included, for complete PDF preprint see http://www.astro.psu.edu/users/mce/preprints

Star Formation in the Interacting Pair NGC7733/34

The problem of star formation within the interacting pair NGC7733/34 has been studied, based on the UBVRI photometry of the source. The distribution of the colors of selected regions within the galaxies is used to infer an estimate for the age distribution of the star forming regions. The results seem to indicate the presence of numerous extended young star-forming regions in the disk of one of the two galaxies, NGC 7733, with ages in the range of $10^6$--$10^8$ yr. However, there exist no evidence for any violent star formation activity, in the past $10^8$ yr, in the nuclei of the two galaxies. The pair seems to be a merger bound system with the brightest, youngest, site of star forming activity lying at the disk interface.Comment: Accepted for publication in Astronomical Journa

Mapping IR Enhancements in Closely Interacting Spiral-Spiral Pairs. I. ISO~CAM and ISO~SWS Observations

Mid-infrared (MIR) imaging and spectroscopic observations are presented for a well defined sample of eight closely interacting (CLO) pairs of spiral galaxies that have overlapping disks and show enhanced far-infrared (FIR) emission. The goal is to study the star formation distribution in CLO pairs, with special emphasis on the role of 'overlap starbursts'. Observations were made with the Infrared Space Observatory (ISO) using the CAM and SWS instruments. The ISO~CAM maps, tracing the MIR emission of warm dust heated by young massive stars, are compared to new ground based H$\alpha$ and R-band images. We identify three possible subgroups in the sample, classified according to the star formation morphology: (1) advanced mergers (Arp~157, Arp~244 and Arp~299), (2) severely disturbed systems (Arp~81 and Arp~278), and (3) less disturbed systems (Arp~276, KPG 347 and KPG 426). Localized starbursts are detected in the overlap regions in all five pairs of subgroups (1) and (2), suggesting that they are a common property in colliding systems. Except for Arp~244, the 'overlap starburst' is usually fainter than the major nuclear starburst in CLO pairs. Star formation in 'less disturbed systems' is often distributed throughout the disks of both galaxies with no 'overlap starburst' detected in any of them. These systems also show less enhanced FIR emission, suggesting that they are in an earlier interaction stage than pairs of the other two subgroups where the direct disk collisions have probably not yet occurred.Comment: 27 pages text, 4 JPEG figures, 3 PS figures. To be accepted by ApJ. High quality figures (included in a PS file of the paper) can be found in http://spider.ipac.caltech.edu/staff/cxu/papers/ss_iso.ps.g

Spitzer and JCMT Observations of the Active Galactic Nucleus in the Sombrero Galaxy (NGC 4594)

We present Spitzer 3.6-160 micron images, Spitzer mid-infrared spectra, and JCMT SCUBA 850 micron images of the Sombrero Galaxy (NGC 4594), an Sa galaxy with a 10^9 M_solar low luminosity active galactic nucleus (AGN). The brightest infrared sources in the galaxy are the nucleus and the dust ring. The spectral energy distribution of the AGN demonstrates that, while the environment around the AGN is a prominent source of mid-infrared emission, it is a relatively weak source of far-infrared emission, as had been inferred for AGN in previous research. The weak nuclear 160 micron emission and the negligible polycyclic aromatic hydrocarbon emission from the nucleus also implies that the nucleus is a site of only weak star formation activity and the nucleus contains relatively little cool interstellar gas needed to fuel such activity. We propose that this galaxy may be representative of a subset of low ionization nuclear emission region galaxies that are in a quiescent AGN phase because of the lack of gas needed to fuel circumnuclear star formation and Seyfert-like AGN activity. Surprisingly, the AGN is the predominant source of 850 micron emission. We examine the possible emission mechanisms that could give rise to the 850 micron emission and find that neither thermal dust emission, CO line emission, bremsstrahlung emission, nor the synchrotron emission observed at radio wavelengths can adequately explain the measured 850 micron flux density by themselves. The remaining possibilities for the source of the 850 micron emission include a combination of known emission mechanisms, synchrotron emission that is self-absorbed at wavelengths longer than 850 microns, or unidentified spectral lines in the 850 micron band.Comment: Accepted to ApJ, 200

Kinematics and physical properties of Southern interacting galaxies: the minor merger AM 2306-721

We present an observational study about the effects of the interactions in the kinematics, stellar population and abundances of the components of the galaxy pair AM2306-721. Rotation curves for the main and companion galaxies were obtained, showing a deprojected velocity amplitude of 175 km/s and 185 km/s, respectively. The interaction between the main and companion galaxies was modeled using numerical N-body/hydrodynamical simulations, with the result indicating that the current stage of the merger would be about 250 Myr after perigalacticum. The spatial variation in the distribution of the stellar population components in both galaxies was analysed by fitting combinations of stellar population models of different age groups. The central region of main galaxy is dominated by an old (5-10 Gyr) population, while significant contributions from a young (200 Myr) and intermediate (1 Gyr) components are found in the disk, being enhanced in the direction of the tidal features. The stellar population of the companion galaxy is overall much younger, being dominated by components with 1 Gyr or less, quite widely spread over the whole disk. Spatial profiles of the oxygen abundance were obtained from the a grid of photoionization models using the R23 line ratio. The disk of the main galaxy shows a clear radial gradient, while the companion galaxy presents an oxygen abundance relatively homogeneous across the disk. The absence of an abundance gradient in the secondary galaxy is interpreted in terms of mixing by gas flows from the outer parts to the center of the galaxy due to the gravitational interaction with the more massive primary.Comment: 13 pages, 12 figures, accepted for publication on MNRA

Reorganizing the Intrinsic Functional Architecture of the Human Primary Motor Cortex during Rest with Non-Invasive Cortical Stimulation

The primary motor cortex (M1) is the main effector structure implicated in the generation of voluntary movements and is directly involved in motor learning. The intrinsic horizontal neuronal connections of M1 exhibit short-term and long-term plasticity, which is a strong substrate for learning-related map reorganization. Transcranial direct current stimulation (tDCS) applied for few minutes over M1 has been shown to induce relatively long-lasting plastic alterations and to modulate motor performance. Here we test the hypothesis that the relatively long-lasting synaptic modification induced by tDCS over M1 results in the alteration of associations among populations of M1 neurons which may be reflected in changes of its functional architecture. fMRI resting-state datasets were acquired immediately before and after 10 minutes of tDCS during rest, with the anode/cathode placed over the left M1. For each functional dataset, grey-matter voxels belonging to Brodmann area 4 (BA4) were labelled and afterwards BA4 voxel-based synchronization matrices were calculated and thresholded to construct undirected graphs. Nodal network parameters which characterize the architecture of functional networks (connectivity degree, clustering coefficient and characteristic path-length) were computed, transformed to volume maps and compared before and after stimulation. At the dorsolateral-BA4 region cathodal tDCS boosted local connectedness, while anodal-tDCS enhanced long distance functional communication within M1. Additionally, the more efficient the functional architecture of M1 was at baseline, the more efficient the tDCS-induced functional modulations were. In summary, we show here that it is possible to non-invasively reorganize the intrinsic functional architecture of M1, and to image such alterations

Glucose Depletion in the Airway Surface Liquid Is Essential for Sterility of the Airways

Diabetes mellitus predisposes the host to bacterial infections. Moreover, hyperglycemia has been shown to be an independent risk factor for respiratory infections. The luminal surface of airway epithelia is covered by a thin layer of airway surface liquid (ASL) and is normally sterile despite constant exposure to bacteria. The balance between bacterial growth and killing in the airway determines the outcome of exposure to inhaled or aspirated bacteria: infection or sterility. We hypothesized that restriction of carbon sources –including glucose– in the ASL is required for sterility of the lungs. We found that airway epithelia deplete glucose from the ASL via a novel mechanism involving polarized expression of GLUT-1 and GLUT-10, intracellular glucose phosphorylation, and low relative paracellular glucose permeability in well-differentiated cultures of human airway epithelia and in segments of airway epithelia excised from human tracheas. Moreover, we found that increased glucose concentration in the ASL augments growth of P. aeruginosa in vitro and in the lungs of hyperglycemic ob/ob and db/db mice in vivo. In contrast, hyperglycemia had no effect on intrapulmonary bacterial growth of a P. aeruginosa mutant that is unable to utilize glucose as a carbon source. Our data suggest that depletion of glucose in the airway epithelial surface is a novel mechanism for innate immunity. This mechanism is important for sterility of the airways and has implications in hyperglycemia and conditions that result in disruption of the epithelial barrier in the lung