arXiv subjects

George Helou

Publications and source records attributed to George Helou.

134 records · Page 8Linked to original sources

arXiv2000

The Infrared Spectral Energy Distribution of Normal Star-Forming Galaxies

We present a new phenomenological model for the spectral energy distribution of normal star-forming galaxies between 3 and 1100 microns. A sequence of realistic galaxy spectra are constructed from a family of dust emission curves assuming a power law distribution of dust mass over a wide range of interstellar radiation fields. For each interstellar radiation field heating intensity we combine emission curves for large and very small grains and aromatic feature carriers. The model is constrained by IRAS and ISOCAM broadband photometric and ISOPHOT spectrophotometric observations for our sample of 69 normal galaxies; the model reproduces well the empirical spectra and infrared color trends. These model spectra allow us to determine the infrared energy budget for normal galaxies, and in particular to translate far-infrared fluxes into total (bolometric) infrared fluxes. The 20 to 42 micron range appears to show the most significant growth in relative terms as the activity level increases, suggesting that the 20-42 micron continuum may be the best dust emission tracer of current star formation in galaxies. The redshift dependence of infrared color-color diagrams and the far-infrared to radio correlation for galaxies are also explored.

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arXiv2000

Normal Galaxies in the Infrared

This review addresses what can be learned from infrared observations about galaxies powered predominantly by star formation. Infrared techniques mostly probe the interstellar medium of galaxies, yielding physical and chemical information on the medium out of which stars form, which is in turn affected by those stars. Methods traditionally used in the study of such normal galaxies at wavelengths longer than 3 microns are described, and major questions currently pursued in the field are outlined. The most prominent results from the IRAS survey are reviewed. Contributions by ISO in the field of broad-band photometry are then presented, followed by ISO results in spectrospcopy. Normal galaxy studies not directly concerned with the ISM are quickly reviewed. The outlook and challenges in pursuing the interpretation of infrared data on the ISM are discussed.

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arXiv2000

Mid-Infrared Observations of Normal Star-Forming Galaxies: The Infrared Space Observatory Key Project Sample

We present mid-infrared maps and preliminary analysis for 61 galaxies observed with the ISOCAM instrument aboard the Infrared Space Observatory. Many of the general features of galaxies observed at optical wavelengths---spiral arms, disks, rings, and bright knots of emission---are also seen in the mid-infrared, except the prominent optical bulges are absent at 6.75 and 15 microns. In addition, the maps are quite similar at 6.75 and 15 microns, except for a few cases where a central starburst leads to lower 6.75/15 ratios in the inner region. We also present infrared flux densities and mid-infrared sizes for these galaxies. The mid-infrared color 6.75/15 shows a distinct trend with the far-infrared color 60/100. The quiescent galaxies in our sample (60/100 < 0.6) show 6.75/15 near unity, whereas this ratio drops significantly for galaxies with higher global heating intensity levels. Azimuthally-averaged surface brightness profiles indicate the extent to which the mid-infrared flux is centrally concentrated, and provide information on the radial dependence of mid-infrared colors. The galaxies are mostly well resolved in these maps: almost half of them have < 10% of their flux in the central resolution element. A comparison of optical and mid-infrared isophotal profiles indicates that the optical flux at 4400 Angstroms near the optical outskirts of the galaxies is approximately eight (seven) times that at 6.75 microns (15 microns), comparable with observations of the diffuse quiescent regions of the Milky Way.

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arXiv1999

Towards an Understanding of the Mid-Infrared Surface Brightness of Normal Galaxies

We report a mid-infrared color and surface brightness analysis of IC 10, NGC 1313, and NGC 6946, three of the nearby galaxies studied under the Infrared Space Observatory Key Project on Normal Galaxies. Images with < 9 arcsecond (170 pc) resolution of these nearly face-on, late-type galaxies were obtained using the LW2 (6.75 mu) and LW3 (15 mu) ISOCAM filters. Though their global I_nu(6.75 mu)/I_nu(15 mu) flux ratios are similar and typical of normal galaxies, they show distinct trends of this color ratio with mid-infrared surface brightness. We find that I_nu(6.75 mu)/I_nu(15 mu) ~< 1 only occurs for regions of intense heating activity where the continuum rises at 15 micron and where PAH destruction can play an important role. The shape of the color-surface brightness trend also appears to depend, to the second-order, on the hardness of the ionizing radiation. We discuss these findings in the context of a two-component model for the phases of the interstellar medium and suggest that star formation intensity is largely responsible for the mid-infrared surface brightness and colors within normal galaxies, whereas differences in dust column density are the primary drivers of variations in the mid-infrared surface brightness between the disks of normal galaxies.

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arXiv1999

ISOCAM 15um Search for Distant Infrared Galaxies Lensed by Clusters

In a search for lensed infrared galaxies, ISOCAM images have been obtained toward the rich clusters Abell 2218 and Abell 2219 at 15um. Nine galaxies (four in Abell 2218 and five in Abell 2219) were detected with flux levels in the range 530-1100 microJy. Three of the galaxies detected in Abell 2218 have previously known redshifts; of these one is a foreground galaxy and the other two are lensed background galaxies at z=0.474 and z=1.032. One of the objects detected in the field of Abell 2219 is a faint, optically red, extreme infrared-dominated galaxy with a probable redshift of 1.048.

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arXiv1998

Emission Features and Source Counts of Galaxies in Mid-Infrared

In this work we incorporate the newest ISO results on the mid-infrared spectral-energy-distributions (MIR SEDs) of galaxies into models for the number counts and redshift distributions of MIR surveys. A three-component model, with empirically determined MIR SED templates of (1) a cirrus/PDR component (2) a starburst component and (3) an AGN component, is developed for infrared (3--120\micron) SEDs of galaxies. The model includes a complete IRAS 25\micron selected sample of 1406 local galaxies ($z \leq 0.1$; Shupe et al. 1998a). Results based on these 1406 spectra show that the MIR emission features cause significant effects on the redshift dependence of the K-corrections for fluxes in the WIRE 25\micron band and ISOCAM 15\micron band. This in turn will affect deep counts and redshift distributions in these two bands, as shown by the predictions of two evolution models (a luminosity evolution model with $L\propto (1+z)^3$ and a density evolution model with $ρ\propto (1+z)^4$). The dips-and-bumps on curves of MIR number counts, caused by the emission features, should be useful indicators of evolution mode. The strong emission features at $\sim 6$--8\micron will help the detections of relatively high redshift ($z\sim 2$) galaxies in MIR surveys. On the other hand, determinations of the evolutionary rate based on the slope of source counts, and studies on the large scale structures using the redshift distribution of MIR sources, will have to treat the effects of the MIR emission features carefully. We have also estimated a 15\micron local luminosity function from the predicted 15\micron fluxes of the 1406 galaxies using the bivariate (15\micron vs. 25\micron luminosities) method. This luminosity function will improve our understanding of the ISOCAM 15\micron surveys.

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arXiv1995

High Resolution IRAS Maps and IR Emission of M31 --- I. Morphology and Sources

The morphology of the IR emission and the properties of discrete FIR sources in M31 disk are studied using the HiRes maps from IRAS. Very thin and bright FIR arm segments are shown in these maps, which have similar structure as the HI gas but with much enhanced arm/inter-arm contrast, typically a factor of 5 on the 60$μ$m image. We identify 39 unconfused sources (excluding the nucleus) and 14 confused sources (7 pairs) at 60 $μ$m by direct Gaussian fittings to the image. IRAS colors of 10 bright isolated sources are studied, which are presumably representative of the discrete sources in general. The colors follow the well known anticorrelation between the 60micron/100micron flux ratio and the 12micron/25micron flux ratio. All sources coincide with optical HII regions. A comparison with H$α$ observations shows that the total luminosity (in the wavelength range 8 --- 1000$μ$m) associated with HII regions is $7.2\pm 2.9 \; 10^8$L_sun, namely $30\pm 14$\% of the total IR emission of M31. This luminosity translates into a present-day star formation rate of 0.36$\pm 0.14$ M_sun/yr, about an order of magnitude lower than that of the Milky Way Galaxy.

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arXiv1995

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 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 and takes fully into account the effect of scattering of dust grains. The model is applied to a complete sample of 'cells' (small areas of size $2'\times 2'$), generated from the above maps. The sample covers the M31 disk in the galactocentric radius range 2 --- 14 kpc, and includes only the cells for which the contribution of the discrete sources to the 60$μm$ surface brightness is negligible ($< 20$\%). This effectively excludes most of the bright arm regions from our analysis. We find that: (1) The mean optical depth (viewed from the inclination angle of 77$^\circ$) increases with radius from $τ_V\sim 0.7$ at r=2kpc outwards, reaches a peak of ~1.6 near 10kpc, and stays quite flat out to 14kpc, where the signal falls below the 5sigma level. (2) A correlation between $τ_V$ and HI surface density is suggested by the similarity between their radial profiles. Significant differences are found between the radial profiles of the H_2 gas (estimated from CO) and of the dust (from $τ_V$), which are most probably due to the large uncertainty in the CO-to-H_2 conversion factor, and to the under-representation of H_2-rich regions in the sample of cells of diffuse regions. (3) The $τ_V / N(HI)$ ratio decreases with increasing radius in the disk of M31, with an exponential law fit yielding an e-folding scale length of $9.6\pm 0.4$ kpc. (4) The optical depth adjusted for this gradient, $τ_{V,c}$, is strongly and linearly correlated with N(HI) over one and a half order of magnitude of column density, indicating that at a given

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