Observational Constraints on the Diffusion of Cosmic-Ray Electrons in Spiral Galaxies
We have performed some quantitative tests of the smearing model for the radio emission in spiral galaxies.
infrared galaxies photometry radio continuum smear↗
Engineering topics
Helou, G.
Publications and source records attributed to Helou, G..
ISM Parameters in the Normal Galaxy NGC 5713
We report ISO Long Wavelength Spectrometer (LWS) observations fo the Sbc(s) pec galaxy NGC 5713. We have obtained strong detections of the fine-structure forbidden transitions [C(sub ii)] 158(micro)m, [O(sub i)]63(micro)m, and [O(sub iii)] 88(micro)m, and significant upper limits for[N(sub ii)]122(micro)m, [O(sub iii)] 52(micro)m, and [N(sub iii)] 57(micro)m. We also detect the galaxy's dust continuum emission between 43 and 197 microns.
galaxies NGC5713 ISM infrared spectroscopy↗
An ISOPHOT Study of the Disk of Galaxy NGC6946: 60(micro)m Infrared and Radio Continuum Correlation
We combine the ISOPHOT 60(micro)m image of Tuffs, et al. (1996) and VLA radio continuum images at 6 and 20cm from the literature to study the 60(micro)m light distribution and its correlation with radio continuum within the disk of nearby spiral galaxy NGC6946, at an effective resolution of + - 52.
galaxies NGC6946 infrared radio continuum↗
The Mid-Infrared Color of NGC 6946
We analyze the new mid-infrared maps of NGC 6946 for variations in the color ratio fo the 7-to-15(micro)m emission. Our preliminary findings are that this mid-infrared color is remarkably constant between arms and inter-arm regions, and as a function of radius in the disk, excluding the nuclear region.
Infrared galaxies ISM ngc 6946↗
ISOCAM* Observations of NGC 6946: Mid-IR Structure
The nearby spiral galaxy NGC 6946 was observed with ISO-CAM in the mid-infrared, achieving 7inch resolution and sub-MJy sr(sup -1) sensitivity. Images taken with CAM filters LW2 (7(micro)m) and LW3 (15(micro)m) are analyzed to determine the morphology of this galaxy and understand beter the emission mechanisms.
Infrared galaxies morphology individual galaxies N↗
The spatial variation of the infrared-to-radio ratio in spiral galaxies
We have produced two-dimensional maps of the intensity ratio, Q(sub 60), of 60 micron infrared to 20 cm radio continuum emission, for a set of 25 nearby galaxies, mostly spirals. The ratio maps were obtained from infrared images made using IRAS data with the maximum correlation method, and radio images made using VLA data. Before taking the ratio, the radio images were processed so as to have the same resolution properties as the infrared images; the final spatial resolution in all cases is approximately 1 min, corresponding to 1 - 2 kpc for most galaxies. This resolution represents a significant improvement over previous studies. Our new high-resolution maps confirm the slow decrease of Q(sub 60) with increasing radial distance from the nucleus, but show additional structure which is probably associated with separate sites of active star formation in the spiral arms. The maps show Q(sub 60) to be more closely related to infrared surface brightness than to the radial distance r in the galaxy disk. We note also that the Q(sub 60) gradients are absent (or at least reduced) for the edge-on galaxies, a property which can be attributed to the dilution of contrast due to the averaging of the additional structure along the line of sight. The results are all in qualitative agreement with the suggestion that the radio image represents a smeared version of the infrared image, as would be expected on the basis of current models in which the infrared-radio correlation is driven by the formation of massive stars, and the intensity distribution of radio emission is smeared as a result of the propagation of energetic electrons accelerated during the supernova phase.
Marsh, K. A.↗
NED and SIMBAD Conventions for Bibliographic Reference Coding
The primary purpose of the 'reference code' is to provide a unique and traceable representation of a bibliographic reference within the structure of each database. The code is used frequently in the interfaces as a succinct abbreviation of a full bibliographic reference. Since its inception, it has become a standard code not only for NED and SIMBAD, but also for other bibliographic services.
NED SIMBAD bibliographic reference coding astronom↗
High Resolution IRAS Maps and IR Emission of M31 -- II. Diffuse Component and Interstellar Dust
Large-scale dust heating and cooling in the diffuse medium of M31 is studied using the high resolution (HiRes) IRAS maps in conjunction with UV, optical (UBV), and the HI maps. A dust heating/cooling model is developed based on a radiative transfer model which assumes a 'Sandwich' configuration of dust and stars takes account of the effect of dust grain scattering.
infrared astronomy galaxies interstellar grains ph↗
Revised sizes and positions for the Mailyan dwarf galaxy catalog
New positions (good to +/- 15 arcsec), revised sizes, and sample cross-identifications are presented for dwarf galaxies discovered and published by Mailyan (1973, Astrofizika, 9, 33). Of the 104 originally cataloged galaxies we were able to recover only 100, despite an extensive search of the red and blue Palomar Observatory Sky Survey (POSS) prints. Over half of the recovered objects were found to correspond to previously cataloged galaxies; few if any of these are dwarf spheriodal galaxies, but rather mostly dwarf irregulars of low surface brightness.
Madore, B. F.↗
The Spatial Variation of the Infrared-to-Radio Ratio in Spiral Galaxies
We have produced two-dimensional maps of the intensity ratio, Q60, of 60 µm infrared to 20 cm radio continuum emission, for a set of 25 nearby galaxies, mostly spirals.
galaxies↗
A physical model of the infrared-to-radio correlation in galaxies
We explore the implications of the IR-radio correlation in star-forming galaxies, using a simple physical model constrained by the constant global ratio q of IR to radio emission and by the radial falloff of this ratio in disks of galaxies. The modeling takes into account the diffusion, radiative decay, and escape of cosmic-ray electrons responsible for the synchrotron emission, and the full range of optical depths to dust-heating photons. We introduce two assumptions: that dust-heating photons and radio-emitting cosmic-ray electrons are created in constant proportion to each other as part of the star formation activity, and that gas and magnetic field are well coupled locally, expressed as B proportional to n exp beta, with beta between 1/3 and 2/3. We conclude that disk galaxies would maintain the observed constant ratio q under these assumptions if the disk scale height h(0) and the escape scale length l(esc) for cosmic-ray electrons followed a relation of the form l(esc) proportional to h(0) exp 1/2; the IR-to-radio ratio will then depend very weakly on interstellar density, and, therefore, on magnetic field strength or mean optical depth.
Helou, G.↗
Brackett- Line Emission Maps of Four Galaxies
Observatons of the Br hydrogen recombination line have been made in a sample of four galaxies. The data were acquired with the new Palomar Prime-Focus Infrared Camers (PFIRCAM) mounted at the prime focus (f/3.3) of the Hale 5-m telescope.
Galaxies Massive Star Formation H II Regions↗
The 'Taffy' galaxies UGC 12914/5
The interacting spiral galaxy pair UGC 12914/5 was mapped with the 'VLA' for a 1.49 GHz atlas of the IRAS Bright Galaxy Sample. Half of the flux from UGC 12914/5 arises from the gap between UGC 12914 and UGC 12915, across which the radio contours are drawn like filaments of taffy. The bright radio continuum connecting UGC 12914 with UGC 12915 is a synchrotron source which therefore probably contains both cosmic rays and magnetic fields. It is suggested that the filaments were stripped from UGC 12914 and UGC 12915 as their stellar disks interpenetrated during a recent encounter; this implies that the radio bridge is a relic whose properties contain information about direct collisions of galaxies with gaseous disks. New maps have been made of the radio continuum and H I line emission from UGC 12914/5 in order to verify the stripping hypothesis.
Condon, J. J.↗
Revised Sizes and Positions for the Mailyan Dwarf Galaxy Cataolgue
This paper presents new positions, revised sizes, and sample cross-identifications for dwarf galaxies discovered and published by Mailyan in 1973.
galaxies Mailyan Dwarf Galaxies↗
IRAS Colors Within M31; Evidence for Deficiency of Very Small Grains?
Significant differences are found in the IRAS color-color diagrams of small regions within the disk of M31 compared to Galactic cirrus, most noticeably demonstrated by a trend of low surface brightness ratio.
individual↗
Very small grains and the infrared colors of galaxies
The contribution of very small grains with fluctuating temperature to the IR emission of disk galaxies is evaluated in light of IRAS data, and the abundance variation of these small grains among galaxies, relative to the 'classical', thermally stable grains, is estimated. It is suggested that the low dispersion in the 'cold' galaxies is representative of the true dispersion in the small-to-large grain abundance ratio among galaxies, while the larger dispersion among warmer galaxies results from increasing optical depth-heating dust effects due to AGNs, in conjunction with the destruction of small dust grains at high radiative energy densities.
Helou, G.↗
Correlations between the far-infrared, radio, and blue luminosities of spiral galaxies
FIR-radio luminosity correlations are presently derived for two statistically complete samples spanning a wide luminosity range. While observed correlations are tight, they are not linear; the observed scattering is mostly intrinsic. An empirical correction to the radio luminosities, based on observed blue luminosities, makes the corrected FIR-radio correlation linear and eliminates most of the residual variance. A two-component model that is consistent with the improved correlation is proposed for the heating luminosity within galaxies. The first, radio-loud component contains young massive stars that heat the dust, while contributing to the cool cirrus dust-heating interstellar radiation field. The second, radio-quiet component is made up of less massive older stars.
Condon, J. J.↗
What COBE might see - The far-infrared cosmological background
The redshifted emission from infrared luminous galaxies should produce a diffuse background around 300 microns detectable by the Cosmic Background Explorer (COBE). The detection of such emission would provide evidence for evolution in the density or luminosity of galaxies in the infrared.
Beichman, C. A.↗