arXiv subjects

G. Helou

Publications and source records attributed to G. Helou.

179 records · Page 10Linked to original sources

arXiv2003

SINGS: The SIRTF Nearby Galaxies Survey

The SIRTF Nearby Galaxy Survey is a comprehensive infrared imaging and spectroscopic survey of 75 nearby galaxies. Its primary goal is to characterize the infrared emission of galaxies and their principal infrared-emitting components, across a broad range of galaxy properties and star formation environments. SINGS will provide new insights into the physical processes connecting star formation to the interstellar medium properties of galaxies, and provide a vital foundation for understanding infrared observations of the distant universe and ultraluminous and active galaxies. The galaxy sample and observing strategy have been designed to maximize the scientific and archival value of the data set for the SIRTF user community at large. The SIRTF images and spectra will be supplemented by a comprehensive multi-wavelength library of ancillary and complementary observations, including radio continuum, HI, CO, submillimeter, BVRIJHK, H-alpha, Paschen-alpha, ultraviolet, and X-ray data. This paper describes the main astrophysical issues to be addressed by SINGS, the galaxy sample and the observing strategy, and the SIRTF and other ancillary data products.

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arXiv2003

Nascent starbursts in synchrotron-deficient galaxies with hot dust

Three nearby galaxies which have abnormally high infrared to radio continuum ratios, NGC 1377, IC 1953 and NGC 4491, are investigated with a view to understanding the physical origin of their peculiarity. We review the existing data and present new radio continuum measurements along with near-infrared integral-field spectroscopy and molecular gas observations. The three galaxies have low luminosities but starburst-like infrared colors; in NGC 1377, no synchrotron emission is detected at any wavelength; in IC 1953, the observed synchrotron component is attributable to the spiral disk alone, and is lacking in the central regions; the radio spectrum of NGC 4491 is unusually flat. We also compare and contrast them with NGC 4418, a heavily extinguished galaxy which shares some attributes with them. After examining various scenarios, we conclude that these galaxies are most likely observed within a few Myr of the onset of an intense star formation episode after being quiescent for at least ~ 100 Myr. This starburst, while heating the dust, has not produced optical signatures nor a normal amount of cosmic rays yet. We briefly discuss the statistics of such galaxies and what they imply for star formation surveys.

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arXiv2003

The bi-variate luminosity-color distribution of IRAS galaxies, and implications for the high redshift Universe

We present a characterization of the local luminosity-color, bi-variate distribution of IRAS galaxies from the 1.2Jy sample, selected at 60 micron. The S60/S100 infrared color is used as the best single parameter description of the IR spectral energy distribution of galaxies. We derive an analytical form of the distribution and use it to constrain the effect of the IR color distribution on evolution models for high redshift, farIR luminous galaxies. Our adopted evolution retains the locally observed correlation between luminosity and color, such that the larger characteristic luminosities at higher redshift have a warmer characteristic color. The width of the color distribution at a given luminosity remains constant for all redshifts. We demonstrate that there is the potential for both hotter and colder sources to be missed in cosmological surveys. An evolving bi-variate luminosity function coupled with the cold source bias of sub-mm selected surveys suggests the existence of a large population of cold sources appearing in such surveys. Likewise, a hot source bias for most SIRTF wavebands together with a bi-variate model suggests an excess of hot sources being selected. We test the evolutionary form against available data for higher redshift, farIR galaxies. The data do not reveal evidence for any strong evolution in the characteristic luminosity-color distribution as a function of redshift over 0<z<1. However, there is marginal evidence for a broadening of the color distribution at higher redshifts, consistent with our locally characterized trend of a broadening in the IR color distribution at the highest luminosities.

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arXiv2002

Hot dust in normal star-forming galaxies: JHKL' photometry of the ISO Key Project sample

We present JHK and 3.8-micron photometry of 26 galaxies in the Infrared Space Observatory (ISO) Normal Galaxy Key Project (KP) sample and of seven normal ellipticals with the aim of investigating the origin of the 4-micron emission. The majority of the KP galaxies, and all the ellipticals, have K-L<~1.0, consistent with stellar photospheres plus moderate dust extinction. Ten of the 26 KP galaxies have K-L>~1.0, corresponding to a flat or rising 4-micron continuum, consistent with significant emission from hot dust at 600-1000K. K-L is anticorrelated with ISO flux ratio F_{6.75}/F_{15}, weakly correlated with line ratio [OI]/[CII], but not with [CII]/FIR or IRAS ratio F_{60}/F_{100}. Photodissociation-region models for these galaxies show that the hot dust responsible for red K-L resides in regions of high pressure and intense far-ultraviolet radiation field. Taken together, these results suggest that star formation in normal star-forming galaxies can assume two basic forms: an ``active'', relatively rare, mode characterized by hot dust, suppressed Aromatic Features in Emission (AFEs), high pressure, and intense radiation field; and the more common ``passive'' mode that occurs under more quiescent physical conditions, with AFEs, and without hot dust. The occurrence of these modes appears to only weakly depend on the star-formation rate per unit area. Passive star formation over large scales makes up the bulk of star-forming activity locally, while the ``active'' regime may dominate at high redshifts.

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arXiv2002

ISO--LWS observations of the two nearby spiral galaxies: NGC6946 and NGC1313

(Abridged) We present the analysis of the main FIR fine structure lines emission in NGC1313 and NGC6946. We calculate that a component probably associated with the diffuse disks contributes <~40% in N6946 and ~30 % in N1313 to the total [CII] emission. The main PDR physical parameters responsible for the neutral atomic gas emission in N1313 and N6946 do not significantly differ from what Malhotra etal (2001) found by modelling the integrated emission of a sample of 60 normal galaxies,although there are evidences for a beam averaged contribution of a less active component inside NGC6946 higher than its contribution in the integrated emission of normal galaxies. CO and [CII] in N6946 are well correlated with a mean [CII]/CO ratio similar to that of the normal galaxies sample. In N1313 the [CII]/CO seems to systematically increase from the North to the south, along the S-shaped spiral arm, indicating much more inhomogeneous conditions than in N6946. HI and [CII] in N6946 are completely de-correlated, probably because they arise from different gas components. In N1313 we successfully detect two distinct gas components: a cirrus-like component where HI and [CII] are weakly correlated as observed in our Galaxy, and a component associated with dense PDRs completely de-correlated from HI as observed in N6946.Finally, we find that the HI residing in dense PDRs and presumably recently photo-dissociated, constitutes a few % of the total HI. In turn, this dense gas component produces most of the [CII] emission emitted by the atomic neutral medium.

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arXiv2002

A best fit evolution scenario for the SCUBA galaxies

We explore the evolution of the SCUBA sub-mm galaxy population using Monte Carlo simulations to generate synthetic sub-mm/radio color magnitude diagrams. To represent the local distribution of observed dust properties, we use a local far-infrared luminosity function derived from the IRAS 1.2 Jy catalog, bivariate in FIR luminosity and 60micron/100micron color. We assume a peak luminosity evolution scenario and a fixed cosmology, to fit a single parameter model to the existing sub-mm/radio data. Our best fitting model has a peak in the luminosity evolution at z=2.6.

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arXiv2002

Optically faint counterparts to the ISO-FIRBACK 170 micron population: the discovery of cold, luminous galaxies at high redshift

We present Keck spectroscopy and UKIRT near-IR imaging observations of two 170micron-selected sources from the ISO-FIRBACK survey which have faint counterparts in the optical, and r-K~5. Both sources were expected to lie at z>1 based on their far-infrared, submillimeter and radio fluxes, assuming a similar spectral energy distribution to the local ultra-luminous infrared galaxy (ULIRG) Arp220. However, our spectroscopy indicates that the redshifts of these galaxies are z<1: z=0.91 for FN1-64 and z=0.45 for FN1-40. While the bolometric luminosities of both galaxies are similar to Arp220, it appears that the dust emission in these systems has a characteristic temperature of 30K much cooler than the ~50K seen in Arp220. Neither optical spectrum shows evidence of AGN activity. If these galaxies are characteristic of the optically faint FIRBACK population, then evolutionary models of the far-infrared background must include a substantial population of cold, luminous galaxies. These galaxies provide an important intermediate comparison between the local luminous IR galaxies, and the high redshift submillimeter-selected galaxies, for which there is very little information available.

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arXiv2001

Far Infrared Spectroscopy of Normal Galaxies: Physical Conditions in the Interstellar Medium

The most important cooling lines of the neutral interstellar medium (ISM) lie in the far-infrared (FIR). We present measurements by the Infrared Space Observatory Long Wavelength Spectrometer of seven lines from neutral and ionized ISM of 60 normal, star-forming galaxies. The galaxy sample spans a range in properties such as morphology, FIR colors (indicating dust temperature), and FIR/Blue ratios (indicating star-formation activity and optical depth). In two-thirds of the galaxies in this sample, the [CII] line is proportional to FIR dust continuum. The other one-third show a smooth decline in [CII]/FIR with increasing F60/F100 and FIR/B, spanning a range of a factor of more than 50. Two galaxies, at the warm and active extreme of the range have [CII]/FIR < 2 \times 10^{-4} (3-sigma upper limit). This is due to increased positive grain charge in the warmer and more active galaxies, which leads to less efficient heating by photoelectrons from dust grains. The ratio of the two principal photodissociation region (PDR) cooling lines [CII]/[OI] shows a tight correlation with F60/F100, indicating that both gas and dust temperatures increase together. We derive a theoretical scaling between [NII] and [CII] from ionized gas and use it to separate [CII] emission from neutral PDRs and ionized gas. Comparison of PDR models of Kaufman et al. (1999) with observed ratios of (a) [OI]/[CII] and ([CII]+[OI])/FIR and (b) [OI]/FIR and F60/F100 yields far-UV flux G0 and gas density n. The derived G0 scales as n to the power 1.4. We interpret this correlation as arising from Stromgren sphere scalings if much of the line and continuum luminosity arises near star-forming regions. The differences in G0 and n may be due to differences in the physical properties of the star-forming clouds.(Short abstract)

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arXiv2000

The Interstellar Medium of Star-forming Irregular Galaxies: The View with ISO

We present mid-infrared imaging and far-infrared (FIR) spectroscopy of 5 IBm galaxies observed by ISO as part of our larger study of the inter- stellar medium of galaxies. Most of the irregulars in our sample are very actively forming stars. The mid-infrared imaging was in a band centered at 6.75 microns that is dominated by polycyclic aromatic hydrocarbons (PAHs) and in a band centered at 15 microns that is dominated by small dust grains. The spectroscopy of 3 of the galaxies includes [CII]158 microns and [OI]63 microns, important coolants of photodissociation regions (PDRs), and [OIII]88 microns and [NII]122 microns, which come from ionized gas. [OI]145 microns and [OIII]52 microns were measured in one galaxy as well. These data are combined with PDR and HII region models to deduce properties of the interstellar medium of these galaxies.

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arXiv2000

Probing the Interstellar Medium in Early type galaxies with ISO observations

Four IRAS-detected early type galaxies were observed with ISO. With the exception of the 15 micron image of NGC1052, the mid-IR emission from NGC1052, NGC1155, NGC5866 and NGC6958 at 4.5, 7 and 15 microns show extended emission. Mid-IR emission from NGC1052, NGC1155, and NGC6958 follows a de Vaucouleurs profile. The ratio of 15/7 micron flux decreases with radius in these galaxies, approaching the values empirically observed for purely stellar systems. In NGC5866, the 7 and 15 micron emission is concentrated in the edge-on dust lane. All the galaxies are detected in the [CII] line, and the S0s NGC1155 and NGC5866 are detected in the [OI] line as well. The ISO-LWS observations of the [CII] line are more sensitive measures of cool, neutral ISM than HI and CO by about a factor of 10-100. Three of four early type galaxies, namely NGC1052, NGC6958 and NGC5866, have low ratio FIR/Blue and show a lower [CII]/FIR, which is due to a softer radiation field from old stellar populations. The low [CII]/CO ratio in NGC5866 ([CII]/CO(1-0) < 570) confirms this scenario. We estimate the UV radiation expected from the old stellar populations in these galaxies and compare it to that needed to heat the gas to account for the cooling observed [CII] and [OI] lines. In three out of four galaxies, NGC1052, NGC5866 and NGC6958, the predicted UV radiation falls short by a factor of 2-3. In view of the observed intrinsic scatter in the "UV-upturn" in elliptical galaxies and its great sensitivity to age and metallicity effects, this is not significant. However, the much larger difference (about a factor of 20) between the UV radiation from old stars and that needed to produce the FIR lines for NGC 1155 is strong evidence for the presence of young stars, in NGC1155.

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arXiv2000

Near/Mid Infrared-Wavelength Study of the Interacting Galaxy Pair UGC 12914/5 -- "Taffy"

We report on an infrared 1 to 17 micron study of the nearby (cz = 4600 km/s) interacting spiral galaxy system, UGC 12914/12915, using the ground-based Palomar 200" telescope and PFIRCAM near-infrared detector and space-based mid-infrared imaging and spectral observations using ISOCAM and Phot-S on the Infrared Space Observatory (ISO). Strong line emission from aromatic band features at 6.2, 7.7 and 11.3 microns are seen in both the mid-IR imaging and Phot-S spectro-photometry centered on the nuclei. In the connecting bridge, the mid-infrared imaging reveals dust grains intermixed with the atomic hydrogen gas. The heating mechanism for the hot dust is likely to be UV photons diffusing out from the galactic disks and the H II complex located along the extreme northeastern portion of the bridge: The dust emission, or mid-infrared intensity per atomic hydrogen column density ratio, is consistent with heating from the local (bridge) interstellar radiation field.

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arXiv2000

The Mid-Infrared Spectra of Normal Galaxies

The mid-infrared spectra (2.5 to 5 and 5.7 to 11.6 mu) obtained by ISO-PHOT reveal the interstellar medium emission from galaxies powered by star formation to be strongly dominated by the aromatic features at 6.2, 7.7, 8.6 and 11.3 mu. Additional emission appears in-between the features, and an underlying continuum is clearly evident at 3-5 mu. This continuum would contribute about a third of the luminosity in the 3 to 13 mu range. The features together carry 5 to 30% of the 40-to-120 mu `FIR' luminosity. The relative fluxes in individual features depend very weakly on galaxy parameters such as the far-infrared colors, direct evidence that the emitting particles are not in thermal equilibrium. The dip at 10 mu is unlikely to result from silicate absorption, since its shape is invariant among galaxies. The continuum component has a f_nu \~ nu^{0.65} shape between 3 and 5 mu and carries 1 to 4% of the FIR luminosity; its extrapolation to longer wavelengths falls well below the spectrum in the 6 to 12 mu range. This continuum component is almost certainly of non-stellar origin, and is probably due to fluctuating grains without aromatic features. The spectra reported here typify the integrated emission from the interstellar medium of the majority of star-forming galaxies, and could thus be used to obtain redshifts of highly extincted galaxies up to z=3 with SIRTF.

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arXiv1997

ISO measurements of [CII] line variations in galaxies

We report measurements of the [CII] fine structure line at 157.714 micron in 30 normal star-forming galaxies with the Long Wavelength Spectrometer (LWS) on the Infrared Space Observatory (ISO). The ratio of the line to total far-infrared luminosity, [CII]/FIR, measures the ratio of the cooling of gas to that of dust; and thus the efficiency of the grain photoelectric heating process. This ratio varies by a factor of 40 in the current sample. About two-thirds of the galaxies have [CII]/FIR = 2-7 x 10^{-3}. The other one-third show trends of decreasing [CII]/FIR with increasing dust temperature, indicated by the ratio of infrared emission at 60 and 100 microns; and with increasing star-formation activity, measured by the ratio of far-infrared and blue band luminosity. We find three FIR bright galaxies with undetected [CII] line at 3-sigma upper limits of [CII]/FIR < 0.5-2 x 10^{-4}. The trend in the [CII]/FIR ratio with the temperature of dust and with star-formation activity may be due to decreased efficiency of photoelectric heating of gas at high UV radiation intensity as dust grains become positively charged, decreasing the yield and the energy of the photoelectrons. The three galaxies with no observed PDR lines have among the highest ratios of FIR to Blue luminosity and the ratio of 60 and 100 micron emission. Their lack of [CII] lines may be due to a continuing trend of decreasing [CII]/FIR with increasing star-formation activity and dust temperature seen in one-third of the sample with warm IRAS colors. In that case the upper limits on [CII]/FIR imply a ratio of UV flux to gas density G_0/n > 10 cm$^3. The low [CII]/FIR could also be due to either weak [CII] because of self-absorption or strong FIR continuum from regions weak in [CII], such as dense HII regions or plasma ionized by hard radiation of AGNs.

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arXiv1996

ISOCAM Observations of NGC 6946: Mid-IR Structure

The nearby spiral galaxy NGC 6946 was observed with ISO-CAM in the mid-infrared, achieving 7 arcsec resolution and sub-MJy/steradian sensitivity. Images taken with CAM filters LW2 (7 microns) and LW3 (15 microns) are analysed to determine the morphology of this galaxy and understand better the emission mechanisms. The mid-infrared emission follows an exponential disk with a scale length 75 arcsec. This is 60% of the scale length in the optical R-band and radio continuum. The nuclear starburst region is too bright for reliable measurement in these images. Its surface brightness exceeds the inner disk by at least a factor of 12. The arms and interarm regions are clearly detected, with each of these components contributing about equally to the disk emission. The arm-interarm contrast is 2-4 in the mid-IR, close to that measured in the visible R band light and lower than the contrast in $Hα$, suggesting that non-ionizing radiation contributes significantly to dust heating.

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arXiv1996

The Mid-Infrared Color of NGC 6946

We analyze the new mid-infrared maps of NGC 6946 for variations in the color ratio of the 7-to-15 micron 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. As surface brightness ranges by more than an order of magnitude and the radius runs from about 0.5 to 3kpc, the color ratio remains constant to about +/-20%. Our interpretation is that (1) hard UV radiation from OB stars does not dominate the heating of the grains radiating in the mid-infrared; and (2) that surface brightness variations are driven primarily by surface-filling fraction in the disk, and by radiation intensity increases in starburst environments, such as the nucleus of NGC 6946.

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arXiv1996

An ISOPHOT of the disk of galaxy NGC 6946: 60 micron infrared and radio continuum correlation

We use the ISOPHOT 60 micron image of Tuffs et al. (1996) to study the 60 micron light distribution and its correlation with radio continuum within the disk of spiral galaxy NGC6946, and to resolve the long-standing IRAS controversy as to whether the infrared-to-radio continuum ratio, Q, decreases radially out in galaxy disks. Our main results are: (1) The disk at 60 microns also appears to follow an exponential fall-off. Its e-folding scale-length lies between 1'.3 and an upper limit of 1'.9, obtained from the mean radial profile between 1'.5 and 4' in radius. This scale length is smaller than that derived from the radio continuum data at either 20cm or 6cm, with or without the thermal component removed, at a significance level of >~ 2 sigma. From 1'.5 to 4' in radius, the 60 micron -to-radio continuum surface brightness ratio, Q, decreases by a factor of 1.5 to 1.7 on average. These quantitative results agree with that of Marsh & Helou (1995) using IRAS HiRes data, but not with that of Fitt et al. (1992) based on IRAS CPC data. Our results also fit into the picture of the radio disk as a ``smeared'' version of the infrared disk, but are not consistent with the picture in which the variation of Q is a result of the thermal and non-thermal radio components having two distinct, constant values of the infrared-to-radio ratio.

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arXiv1996

Arecibo HI Mapping of a Large Sample of Dwarf Irregular Galaxies

Neutral hydrogen mapping of 70 dwarf irregular (Sm, Im and BCD) galaxies is reported, with position-velocity contour maps presented for those resolved by the Arecibo beam. The galaxies were selected either from the Virgo Cluster Catalog, from similarly identified field galaxies, or from a distance-limited sample within the Arecibo declination range. We do not find any isolated dwarfs with a larger HI to optical radius ratio than DDO 154; the ``protogalaxy'' HI 1225+01 (Giovanelli \& Haynes 1989) continues to be a unique object among dwarfs that have been mapped in HI. For all dwarfs with significant rotation we are able to determine the sense of the spin. For a number of better-resolved dwarfs we are able to determine rotation curves, in most cases extending well beyond the last measured point in available synthesis array maps. Correlations among the several measures of galaxy size and mass are studied; in Paper II (Salpeter \& Hoffman 1996) we combine these data with those for the set of all available mapped dwarf irregular galaxies and for mapped spirals spanning a similar range of redshifts to investigate variations in Tully-Fisher relations and in surface densities as functions of galaxy size and luminosity or mass.

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