Radio-Source Problems

Context Radio continuum surveys of the Galactic plane can find and characterize H II regions, supernova remnants (SNRs), planetary nebulae (PNe), and extragalactic sources. A number of surveys at high angular resolution (≤25″) at different wavelengths exist to study the interstellar medium (ISM), but no comparable high-resolution and high-sensitivity survey exists at long radio wavelengths around 21 cm. Aims. Our goal is to investigate the 21 cm radio continuum emission in the northern Galactic plane at < 25″ resolution. Methods We observed a large percentage of the Galactic plane in the first quadrant of the Milky Way (l = 14.0−67.4° and |b| ≤ 1.25°) with the Karl G. Jansky Very Large Array (VLA) in the C-configuration covering six continuum spectral windows (SPW). These data provide a detailed view on the compact as well as extended radio emission of our Galaxy and thousands of extragalactic background sources. Results We used the BLOBCAT software and extracted 10 916 sources. After removing spurious source detections caused by the side lobes of the synthesized beam, we classified 10 387 sources as reliable detections. We smoothed the images to a common resolution of 25″ and extracted the peak flux density of each source in each SPW to determine the spectral indices α (assuming I(ν) ∝ να). By cross-matching with catalogs of H II regions, SNRs, PNe, and pulsars, we found radio counterparts for 840 H II regions, 52 SNRs, 164 PNe, and 38 pulsars. We found 79 continuum sources that are associated with X-ray sources. We identified 699 ultra-steep spectral sources (α < −1.3) that could be high-redshift galaxies. About 9000 of the sources we extracted are not classified specifically, but based on their spatial and spectral distribution, a large percentage of these are likely to be extragalactic background sources. More than 7750 sources do not have counterparts in the SIMBAD database and more than 3760 sources do not have counterparts in the NED database. Conclusions Studying the long wavelengths centimeter continuum emission and the associated spectral indices allows us to characaterize a large percentage of Galactic and extragalactic radio sources in the area of the northern inner Milky Way. This database will be extremely useful for future studies of a diverse set of astrophysical objects.

Clusters of galaxies represent the largest laboratories in the universe for testing the incredibly chaotic physics governing the collapse of baryons into the stars, galaxies, groups, and diffuse clouds that we see today. Within the cluster environment, there are a wide variety of physical processes that may be acting to transform galaxies. In this thesis, we combine extensive Keck spectroscopy with wide-field HST imaging to perform a detailed case study of two intermediate redshift galaxy clusters, Cl 0024+1654 (z=0.395) and MS 0451-03 (z=0.540). Leveraging a comprehensive multiwavelength data set that spans the X-ray to infrared, and with spectral-line measurements serving as the key to revealing both the recent star-formation histories and kinematics of infalling galaxies, we aim to shed light on the environmental processes that could be acting to transform galaxies in clusters. We adopt a strategy to make maximal use of our HST-based morphologies by splitting our sample of cluster galaxies according to morphological type, characterizing signs of recent evolution in and early types separately. This approach proves to be powerful in identifying galaxies that are currently being altered by an environmental interaction: early-type galaxies that have either been newly transformed or prodded back into an active phase, and spiral galaxies where star formation is being suppressed or enhanced all stand out in our sample. We begin by using variations in the early-type galaxy population as indicators of recent activity. Because ellipticals and S0s form such a homogeneous class in the local universe, we are sensitive to even very subtle signatures of recent and current environmental interactions. This study has yielded two key results: By constructing the Fundamental Plane (FP) of Cl 0024, we observe that elliptical and S0 galaxies exhibit a high scatter in their FP residuals, which occurs only among galaxies in the cluster core, suggesting a turbulent assembly history for Cl 0024 early types. Near the Virial radius of Cl 0024, we observe a number of compact, intermediate-mass ellipticals undergoing a burst of star formation or weak AGN activity, indicated by strong [O II] emission; their locations may mark the minimum radius at which merging is effective in each cluster. While E+S0 galaxies do prove to be sensitive indicators of environmental interaction, it is the spiral galaxies that, of course, host the bulk of star formation within and around these clusters. We therefore probe for kinematic disturbances in spiral disks by measuring resolved rotation curves from optical emission lines, and constructing the Tully-Fisher relation for across Cl 0024 and MS 0451. We find that the cluster Tully-Fisher relation exhibits significantly higher scatter than the field relation. In probing for the origin of this difference, we find that the central mass densities of star-forming exhibit a sharp break near the cluster Virial radius, with in the cluster outskirts exhibiting significantly lower densities. We argue that these results considered together demonstrate that cluster are kinematically disturbed by their environment, likely due to galaxy-galaxy interactions (harassment). We then discuss our most powerful method of tracking galaxy evolution across Cl 0024 and MS 0451: identifying and studying transition galaxies-galaxies whose stellar populations or dynamical states indicate a recent or ongoing change in morphology or star formation rate. Such galaxies are often revealed by star formation histories that seem to be at odds with the galaxy morphologies: for example, spiral galaxies with no signs of star formation, or elliptical galaxies that do show signs of star formation. We identify and study one such class of objects, the spirals in Cl 0024. These objects exhibit no emission lines in their spectra, suggesting a lack of star formation, yet are surprisingly detected in the UV, revealing the presence of young stars. By modeling the different temporal sensitivities of UV and spectroscopic data to recent activity, we show that star formation in Cl 0024 passive has decayed on timescales of less than 1 Gyr, consistent with the action of gas starvation. We then build on and link together our previous indications of galaxy evolution at work, aiming to piece together a more comprehensive picture of how cluster galaxies are affected by their environment at intermediate redshift. To accomplish this, we document what we believe to be the first direct evidence for the transformation of into S0s: through an analysis of their stellar populations and recent star formation rates, we link the passive spiral galaxies in both clusters to their eventual end states as newly generated cluster S0 galaxies. Differences between the two clusters in both the timescales and spatial location of this conversion process allow us to evaluate the relative importance of several proposed physical mechanisms that could be responsible for the transformation. Combined with other diagnostics that are sensitive to either ICM-driven galaxy evolution or galaxy-galaxy interactions, we describe a self-consistent picture of galaxy evolution in clusters. We find that spiral galaxies within infalling groups have already begun a slow process of conversion into S0s primarily via gentle galaxy-galaxy interactions that act to quench star formation. The fates of upon reaching the core of the cluster depend heavily on the cluster ICM, with rapid conversion of all remaining into S0s via ram-pressure stripping in clusters where the ICM is dense. In the presence of a less-dense ICM, the conversion continues at a slower pace, with galaxy-galaxy interactions continuing to play a role along with starvation by the ICM. We conclude that the buildup of the local S0 population through the transformation of spiral galaxies is a heterogeneous process that nevertheless proceeds robustly across a variety of different environments from cluster outskirts to cores.

view Abstract Citations (82) References (47) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Optical properties of spiral galaxies with strong nuclear radio sources : active nuclei, starbursts, and composite objects. Keel, W. C. Abstract New optical spectrophotometry, accurate positions, and narrow-band emission-line images have been obtained for the nuclei of spiral and irregular galaxies containing strong radio sources. The optical data support a general division of these objects into those powered by star formation and those dominated by nonthermal processes. In several cases, however, nuclei are classified differently from radio and optical properties, suggesting a composite nature for some nuclei. Further work on this group could indicate whether the two processes are causally related and in which direction. All these nuclei show emission lines, systematically stronger than in a sample of optically selected nuclei. The emission-line spatial distributions, in most cases, are much like those in normal (optically selected) nuclei. Correlations between emission-line and 6 cm luminosities are explored for each class of nucleus. A correlation with forbidden N II luminosity for (optically) nonstellar-powered nuclei suggests a common energy source for the emission-line region and the radio emission, while uncertainties in optical reddening obscure any relation for nuclear H II regions. Publication: The Astrophysical Journal Pub Date: July 1984 DOI: 10.1086/162177 Bibcode: 1984ApJ...282...75K Keywords: Astronomical Spectroscopy; Galactic Nuclei; Galactic Radiation; Radio Galaxies; Spiral Galaxies; Emission Spectra; H Alpha Line; H Ii Regions; Luminosity; Spectral Line Width; Spectrophotometry; Stellar Evolution; Astrophysics full text sources ADS | data products SIMBAD (34) NED (29)

Clusters of galaxies

Synthesis mapping of extragalactic systems at frequencies below 1.4 GHz has revealed important new clues about what we might discover at much lower frequencies in intergalactic space. State-of-the-art images at frequencies between 1420 MHz and 300 MHz can be considered important precursors to even lower frequency images such as might be generated by spaceor ground-based antenna arrays. In this talk I shall discuss how recent images at 327 and 408 MHz provide us with new information and some interesting clues about the physical phenomena which could be explored if we had sensitive, even lower frequency images at comparable or better resolution. The recent outfitting of both the NRAO Very Large Array and the Westerbork Synthesis Telescope (WSRT) with 327 MHz receivers has permitted us to generate images which combine the virtues of high dynamic range, good resolution and high sensitivity at a relatively low frequency. The DRAO Synthesis Telescope at 408 MHz, and the Cambridge 151 MHz array (also described at this meeting) though less sensitive, have wide field capability which is also important for some types of low frequency imaging which we would like to attempt at lower frequencies. In the following, I shall describe results of detailed mapping of intergalactic radio emission at 1.4GHz, 408MHz, and 326 MHz in the region of the Coma cluster of galaxies. The results provide some new information on intergalactic magnetic fields, and reveal large scale synchrotron emission at 327 MHz on intercluster scales for the first time. They point to potentially interesting discoveries that could be made in future at lower frequencies, where the diffuse intergalactic synchrotron emission will be even stronger. The Coma cluster, due both to its richness and relative proximity, is the most accessible "passive laboratory" for studying the intracluster medium of a cluster of galaxies. However, even for the Coma cluster the properties of its halo have been known in less than adequate detail. This is partly because most radio telescopes tuned to relatively low radio frequencies (at which radio halos are most prominent) have inadequate resolution, poorly known beam characteristics, or are insensitive to the faint, large scale radio emission.

We review the principal radiation mechanisms which can be responsible for the production of high energy emission of Galactic and extragalactic sources. The spectral properties of Comptonization processes are studied. We present a rigorous treatment of the problem in terms of the Boltzmann kinetic equation formalism. We also overview the different exact and approximate analytical and numerical techniques which decouple the photon transport in six dimensions to that in the configuration space and in energy space separately. CGRO, RXTE, and SAX observations of Galactic black hole candidates, neutron star systems, and Seyfert nuclei show that their spectra can be fitted by thermal Comptonization models in the hard state, and two components in the soft state. In the latter case, these components show a blackbody like spectrum with the color temperature which is a fraction of keV and tens of eV for Galactic and extragalactic sources, respectively, and an extended power-law with spectral index between 1 and 2 (in the neutron star systems in their soft state this power-law component is not detected). We give arguments for an explanation of these two types of spectral states in the framework of the thermal and bulk motion Comptonization processes.

Aims: Following the study already presented in our previous paper, based on the Nordic Optical Telescope (NOT) sample, which consists of five clusters of galaxies within the redshift range 0.18 $\leq$ z $\leq$ 0.25, imaged in the central 0.5-2 Mpc in very good seeing conditions, we have studied the quantitative morphology of their bright galaxy population Methods: We have analyzed the surface brightness profiles of the galaxy population in those clusters. Previously, we have performed simulations in order to check the reliability of the fits. We have also derived a quantitative morphological classification. Results: The structural parameters derived from these analysis have been analyzed. We have obtained that the structural parameters of E/S0 galaxies are similar to those showed by galaxies in low redshift clusters. However, the disc scales are different. In particular, the scales of the discs of galaxies at medium redshift clusters are statistically different than those located in similar galaxies in the Coma cluster. But, the scales of the discs of galaxies in medium redshift clusters are similar to nearby field galaxies. Conclusions: The results suggest that the evolution of the disc component of galaxies in clusters is faster than in field ones. Mechanisms like galaxy harassment showing timescales of $\sim 1$Gyr could be the responsible of this disc scale evolution. This indicates that spiral galaxies in clusters have suffered a strong evolution in the last 2.5 Gyr or that Coma is in some way anomalous.

The jets in several extragalactic radio sources appear to open with a more or less constant angle out to some critical distance from the nucleus, and then broaden much less rapidly or even become more narrow. This ''reconfinement shoulder'' seems to be a fairly typical property of jets observed with sufficiently high resolution to map the structure transverse to the jet axis. This may be due to the fact that the internal pressure in a free jet decreases very rapidly with distance from the nucleus, and therefore the jet would be expected to come into pressure equilibrium with an ambient medium. But the flow in free jets must be highly supersonic; thus, the process of reconfinement is certain to be accompanied by shocks. In the present paper, the structure of inviscid steady state jets in the presence of a realistic ambient medium is calculated by means of the method of characteristics. It is shown that reconfinement is accompanied by conical shocks which heat the jet causing it to reexpand as free jet. The typical jet shape can be reproduced by this process. If particle acceleration results from internal shocks, multiple ''attempts'' at reconfinement could produce symmetric knots in a two-sidedmore » jet symmetrically placed with respect to the ambient medium.« less

We found an unusual complex of narrow blue filaments, bright blue knots, and Hα-emitting filaments and clouds, which morphologically resembled a complex of fireballs, extending up to 80 kpc south from an E+A galaxy RB 199 in the Coma Cluster. The galaxy has a highly disturbed morphology indicative of a galaxy-galaxy merger remnant. The narrow blue filaments extend in straight shapes toward the south from the galaxy, and several bright blue knots are located at the southern ends of the filaments. The RC band absolute magnitudes, half-light radii, and estimated masses of the bright knots are ~–12 to –13 mag, ~200-300 pc, and ~106-107 M☉, respectively. Long, narrow Hα-emitting filaments are connected at the south edge of the knots. The average color of the fireballs is B − RC ≈ 0.5, which is bluer than RB 199 (B − R = 0.99), suggesting that most of the stars in the fireballs were formed within several times 108 yr. The narrow blue filaments exhibit almost no Hα emission. Strong Hα and UV emission appear in the bright knots. These characteristics indicate that star formation recently ceased in the blue filaments and now continues in the bright knots. The gas stripped by some mechanism from the disk of RB 199 may be traveling in intergalactic space, forming stars left along its trajectory. The most plausible fireball formation mechanism is ram pressure stripping by high-speed collision between the galaxy and the hot intracluster medium. The fireballs may be a snapshot of diffuse intracluster population formation, or halo star population formation in a cluster galaxy.

The non-thermal radiation–cluster merger connection

The field of Very High Energy (VHE) gamma ray astronomy using the Atmospheric Cerenkov Technique has entered an interesting phase with detection of various galactic and extragalactic sources. Among galactic sources, only the Crab nebula has been established as a standard candle. Most observations on pulsars are in agreement as to the necessity for the GeV spectra to steepen at 1.5 TeV which could be the second component of the Outer Gap pulsar models.

view Abstract Citations (71) References (141) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS High-Energy and Ultra--High-Energy Cosmic Rays Fichtel, C. E. ; Linsley, J. Abstract The nature of the source and history of Galactic cosmic rays is examined in the light of new measurements on cosmic rays and related astronomical measurements. The possibility of both a Galactic center source and sources of one type distributed throughout the Galaxy is addressed. The concept of escape first from a Galactic plane trapping region and then from the larger region of the Galaxy as a whole is addressed. Two types of sources are considered, along with the reasons that at least two types appear to be necessary. Among the combinations considered are two distributed Galactic sources, one distributed source type and a Galactic center source, and one distributed source type and an extragalactic source. The constraints are seen to be fairly restrictive, leading to the conclusion that there must be two Galactic source types with quite different characteristics. Publication: The Astrophysical Journal Pub Date: January 1986 DOI: 10.1086/163825 Bibcode: 1986ApJ...300..474F Keywords: Galactic Cosmic Rays; High Energy Interactions; Anisotropy; Black Holes (Astronomy); Galactic Nuclei; Neutron Stars; Particle Energy; X Ray Sources; Space Radiation; COSMIC RAYS: GENERAL; PARTICLE ACCELERATION full text sources ADS |

We present 6 and 4 cm radio images of the inner disks of the nearby merger pair NGC 4038 and NGC 4039 at resolutions between ~ 1'' and 26, or ~ 100 and 260 pc. We detect numerous compact radio sources embedded in more diffuse radio emission; the compact radio sources produce ~ 12% and ~ 25% of the 6 and 4 cm radio emission in the system, respectively. The strongest radio continuum emission occurs between the galaxies, at an optically unremarkable location near but not coincident with an extremely red cluster detected by Whitmore & Schweizer. The radio peak is at a location of intense star formation (as detected in the mid-IR) and of dense molecular gas. We identify 115 individual compact 6 cm radio sources and 63 individual 4 cm sources in the system, to a limiting luminosity of ~ 2 × 1018 W Hz-1, or ~ 4 times the luminosity of Cas A; 90% of the 6 cm sources and 67% of the 4 cm sources are unresolved in these observations. Of the strongest sources, for which the flux densities are large enough to measure spectral indices with reasonable errors, one-third have nominally flat radio spectra, indicating that they are dominated by thermal radio emission from H II regions, and two-thirds have nominally steep spectra dominated by nonthermal emission from supernova remnants. For the compact sources dominated by thermal emission, we derive typical masses of 104 to 105 M⊙ in massive young stars. Using reasonable assumptions about source sizes, we estimate associated ionized gas masses of 103 to 104 M⊙. A comparison with the ~ (2–5) × 103 M⊙ typical optical clusters detected recently by Whitmore et al. suggests that the thermal radio sources detected here may be composed of tens of these typical young star clusters or else that they are more like the individual massive 105–106 M⊙ Whitmore et al. clusters. The strongest thermal radio source is associated with none of the Whitmore et al. clusters but is coincident with strong CO and 15 μm emission, implying that massive star clusters are still enshrouded by dust in this region. The second strongest thermal source is near several optical clusters, suggesting that these clusters are just now emerging from their birth cloud and are clearing away the surrounding material. Estimates of stellar ages and sound speeds indicate that we do not see star formation propagating in this system; rather, the star formation sequence is probably related to more global properties of the interaction. From the steep-spectrum sources, we derive a total system supernova rate of 0.2–0.3 yr-1, about an order of magnitude larger than that expected from the currently observed O star population. This suggests that a large number of O stars may have formed in the system in a short-lived burst, less than 106 yr in duration and about 3–4 Myr ago.

VLA observations of radio emissions at 1465 and 4885 MHz, of Sco X-1 confirm the existence of a colinear triple structure. Evidence that the three components of Sco X-1 are physically associated is presented, including the morphology, spectrum, variability, volume emissivity and magnetic field strength. The possibility of a physical phenomenon occurring in Sco X-1 similar to that occurring in extragalactic radio sources is discussed, and two galactic sources are found having extended emission similar to that in extragalactic objects. The extended structure of Sco X-1 is also observed to be similar to that of the hot spots in luminous extragalactic sources, and a radio source 20 arcmin from Sco X-1 is found to lie nearly along the radio axis formed by the components of Sco X-1.

All galaxies without a radio-loud AGN follow a tight correlation between\ntheir global FIR and radio synchrotron luminosities, which is believed to be\nultimately the result of the formation of massive stars. Two colliding pairs of\ngalaxies, UGC12914/5 and UGC 813/6 deviate from this correlation and show an\nexcess of radio emission which in both cases originates to a large extent in a\ngas bridge connecting the two galactic disks. We are aiming to clarify the\norigin of the radio continuum emission from the bridge. The radio synchrotron\nemission expected from the bridge regions is calculated, assuming that the\nkinetic energy liberated in the predominantly gas dynamic interaction of the\nrespective interstellar media (ISM) has produced shock waves that efficiently\naccelerate nuclei and electrons to relativistic energies. We present a model\nfor this acceleration and calculate the resulting radio emission, its spectral\nindex and the expected high-energy gamma-ray emission. It is found that the\nnonthermal energy produced in the collision is large enough to explain the\nradio emission from the bridge between the two galaxies. The calculated\nspectral index at the present time also agrees with the observed value. The\ndeviation of these two interacting galaxy systems from the standard FIR-radio\ncorrelation is consistent with the acceleration of an additional population of\nelectrons. This process is not related to star formation and therefore it is\nexpected that the systems do not follow the FIR-radio correlation. The\nacceleration of relativistic electrons in shocks caused by an ISM collision, in\nthe same way as described here, is likely to take place in other systems as\nwell, as in galaxy clusters and groups or high-redshift systems.\n