T. J. Sumner

Observations of the Hubble Deep Field with the Infrared Space Observatory V. Spectral energy distributions starburst models and star formation history

We have modelled the spectral energy distributions of the 13 HDF galaxies reliably detected by ISO. For 2 galaxies the emission detected by ISO is consistent with being starlight or the infrared ’cirrus’ in the galaxies. For the remaining 11 galaxies there is a clear mid-infrared excess, which we interpret as emission from dust associated with a strong starburst. 10 of these galaxies are spirals or interacting pairs, while the remaining one is an elliptical with a prominent nucleus and broad emission lines. We give a new discussion of how the star formation rate can be deduced from the far infrared luminosity and derive star formation rates for these galaxies of 8-1000 φM⊙ per yr, where φ takes account of the uncertainty in the initial mass function. The HDF galaxies detected by ISO are clearly forming stars at a prodigious rate compared with nearby normal galaxies. We discuss the implications of our detections for the history of star and heavy element formation in the universe. Although uncertainties in the calibration, reliability of source detection, associations, and starburst models remain, it is clear that dust plays an important role in star formation out to redshift 1 at least.

The European Large Area ISO Survey - I. Goals, definition and observations

We describe the European Large Area ISO Survey (ELAIS). ELAIS was the largest single Open Time project conducted by ISO, mapping an area of 12deg^2 at 15μm with ISOCAM and at 90μm with ISOPHOT. Secondary surveys in other ISO bands were undertaken by the ELAIS team within the fields of the primary survey, with 6deg^2 being covered at 6.7μm and 1deg^2 at 175μm. This paper discusses the goals of the project and the techniques employed in its construction, as well as presenting details of the observations carried out, the data from which are now in the public domain. We outline the ELAIS `preliminary analysis which led to the detection of over 1000 sources from the 15 and 90-μm surveys (the majority selected at 15μm with a flux limit of ~3mJy), to be fed into a ground-based follow-up campaign, as well as a programme of photometric observations of detected sources using both ISOCAM and ISOPHOT. We detail how the ELAIS survey complements other ISO surveys in terms of depth and areal coverage, and show that the extensive multi-wavelength coverage of the ELAIS fields resulting from our concerted and on-going follow-up programme has made these regions amongst the best studied areas of their size in the entire sky, and, therefore, natural targets for future surveys. This paper accompanies the release of extremely reliable subsets of the `preliminary analysis products. Subsequent papers in this series will give further details of our data reduction techniques, reliability and completeness estimates and present the 15- and 90-μm number counts from the `preliminary analysis, while a further series of papers will discuss in detail the results from the ELAIS `final analysis, as well as from the follow-up programme.

The European Large-Area ISO Survey (ELAIS): the final band-merged catalogue

We present the final band-merged European Large-Area ISO Survey (ELAIS) Catalogue at 6.7, 15, 90 and 175 mum, and the associated data at U, g, r, i, Z, J, H, K and 20 cm. The origin of the survey, infrared and radio observations, data-reduction and optical identifications are briefly reviewed, and a summary of the area covered and the completeness limit for each infrared band is given. A detailed discussion of the band-merging and optical association strategy is given. The total Catalogue consists of 3762 sources. 23 per cent of the 15-mum sources and 75 per cent of the 6.7-mum sources are stars. For extragalactic sources observed in three or more infrared bands, colour-colour diagrams are presented and discussed in terms of the contributing infrared populations. Spectral energy distributions (SEDs) are shown for selected sources and compared with cirrus, M82 and Arp220 starburst, and active galactic nuclei (AGN) dust torus models. Spectroscopic redshifts are tabulated, where available. For the N1 and N2 areas, the Isaac Newton Telescope ugriz Wide Field Survey permits photometric redshifts to be estimated for galaxies and quasars. These agree well with the spectroscopic redshifts, within the uncertainty of the photometric method [similar to10 per cent in (1 + z) for galaxies]. The redshift distribution is given for selected ELAIS bands and colour-redshift diagrams are discussed. There is a high proportion of ultraluminous infrared galaxies (log(10) of 1-1000 mum luminosity L-ir > 12.22) in the ELAIS Catalogue (14 per cent of 15-mum galaxies with known z), many with Arp220-like SEDs. 10 per cent of the 15-mum sources are genuine optically blank fields to r = 24: these must have very high infrared-to-optical ratios and probably have z > 0.6, so are high-luminosity dusty starbursts or Type 2 AGN. Nine hyperluminous infrared galaxies (L-ir > 13.22) and nine extremely red objects (EROs) (r - K > 6) are found in the survey. The latter are interpreted as ultraluminous dusty infrared galaxies at z similar to 1. The large numbers of ultraluminous galaxies imply very strong evolution in the star formation rate between z = 0 and 1. There is also a surprisingly large population of luminous (L-ir > 11.5), cool (cirrus-type SEDs) galaxies, with L-ir L-opt > 0, implying A(V) > 1.

The European Large Area ISO Survey – III. 90-μm extragalactic source counts

We present results and source counts at 90micron extracted from the Preliminary Analysis of the European Large Area ISO Survey (ELAIS). The survey covered about 11.6 square degrees of the sky in four main areas and was carried out with the PHOT instrument onboard the Infrared Space Observatory (ISO). The survey is at least an order of magnitude deeper than the IRAS 100micron survey and is expected to provide constraints on the formation and evolution of galaxies. The majority of the detected sources are associated with galaxies on optical images. In some cases the optical associations are interacting pairs or small groups of galaxies suggesting the sample may include a significant fraction of luminous infrared galaxies. The source counts extracted from a reliable subset of the detected sources are in agreement with strongly evolving models of the starburst galaxy population.

Observations of the Hubble Deep Field with the Infrared Space Observatory - IV. Association of sources with Hubble Deep Field galaxies

We discuss the identification of sources detected by ISO at 6.7 and 15μm in the Hubble Deep Field (HDF) region. We conservatively associate ISO sources with objects in existing optical and near-infrared HDF catalogues using the likelihood ratio method, confirming these results (and, in one case, clarifying them) with independent visual searches. We find fifteen ISO sources to be reliably associated with bright [I814(AB) < 23] galaxies in the HDF, and one with an I814(AB) = 19.9 star, while a further eleven are associated with objects in the Hubble Flanking Fields (ten galaxies and one star). Amongst optically bright HDF galaxies, ISO tends to detect luminous, star-forming galaxies at fairly high redshift and with disturbed morphologies, in preference to nearby ellipticals.

The European Large Area ISO Survey — IV. The preliminary 90-μm luminosity function

We present the luminosity function of 90-mum-selected galaxies from the European Large Area ISO Survey (ELAIS), extending to z=0.3. Their luminosities are in the range 109 =100mJy data base, we have found optical, 15-mum or 1.4-GHz identifications for 24 (65 per cent). We have obtained 2dF and UK Schmidt FLAIR spectroscopy of 89 per cent of identifications to rigid multivariate flux limits. We construct a luminosity function assuming that (i) our spectroscopic subset is an unbiased sparse sample, and (ii) there are no galaxies that would not be represented in our spectroscopic sample at any redshift. We argue that we can be confident of both assumptions. We find that the luminosity function is well described by the local 100-mum luminosity function of Rowan-Robinson, Helou & Walker. Assuming this local normalization, we derive luminosity evolution of (1+z)2.45+/-0.85 (95 per cent confidence). We argue that star formation dominates the bolometric luminosities of these galaxies, and we derive comoving star formation rates in broad agreement with the Flores et al. and Rowan-Robinson et al. mid-infrared-based estimates.

Near- and mid-infrared colours of star-forming galaxies in European Large Area ISO Survey fields

We present J- and K-band near-infrared (near-IR) photometry of a sample of mid-infrared (mid-IR) sources detected by the Infrared Space Observatory (ISO) as part of the European Large Area ISO Survey (ELAIS) and study their classification and star-forming properties. We have used the Preliminary ELAIS Catalogue for the 6.7-mum (LW2) and 15-mum (LW3) fluxes. All of the high-reliability LW2 sources and 80 per cent of the LW3 sources are identified in the near-IR survey reaching K~ 17.5 mag. The near-IR/mid-IR flux ratios can effectively be used to separate stars from galaxies in mid-IR surveys. The stars detected in our survey region are used to derive a new accurate calibration for the ELAIS ISOCAM data in both the LW2 and LW3 filters. We show that near- to mid-IR colour-colour diagrams can be used to classify galaxies further, as well as to study star formation. The ELAIS ISOCAM survey is found mostly to detect strongly star-forming late-type galaxies, possibly starburst-powered galaxies, and it also picks out obscured active galactic nuclei. The ELAIS galaxies yield an average mid-IR flux ratio LW2/LW3 = 0.67 +/- 0.27. We discuss the fnu(6.7 mu m)/fnu(15 mu m) ratio as a star formation tracer using ISO and IRAS data of a local comparison sample. We find that the fnu(2.2 mu m)/fnu(15 mu m) ratio is also a good indicator of activity level in galaxies and conclude that the drop in the fnu(6.7 mu m)/fnu(15 mu m) ratio seen in strongly star-forming galaxies is a result of both an increase of 15-mum emission and an apparent depletion of 6.7-mum emission. Near-IR together with the mid-IR data make it possible to estimate the relative amount of interstellar matter in the galaxies.

Starburst activity in a ROSAT narrow emission-line galaxy

We present multiwaveband photometric and optical spectropolarimetric observations of the R = 15.9 narrow emission line galaxy R117 A which lies on the edge of the error circle of the ROSAT X-ray source R117 (from M c Hardy et al. 1998). The overall spectral energy distribution of the galaxy is well modelled by a combination of a normal spiral galaxy and a moderate-strength burst of star formation. The far infrared and radio emission is extended along the major axis of the galaxy, indicating an extended starburst. On positional grounds, the galaxy is a good candidate for the identification of R117 and the observed X-ray flux is very close to what would be expected from a starburst of the observed far infra-red and radio fluxes. Although an obscured high redshift QSO cannot be entirely ruled out as contributing some fraction of the X-ray flux, we find no candidates to K = 20.8 within the X-ray errorbox and so conclude that R117 A is responsible for a large fraction, if not all, of the X-ray emission from R117. Searches for indicators of an obscured AGN in R117 A have so far proven negative; deep spectropolarimetric observations show no signs of broad lines to a limit of one per cent and, for the observed far infra-red and radio emission, we would expect a ten times greater X-ray flux if the overall emission were powered by an AGN. We therefore conclude that the X-ray emission from R117 is dominated by starburst emission from the galaxy R117 A.

50–500 MeV γ‐ray emission in the early phase of SN1987A

SN1987A was observed on 19th April 1987 with a combined high energy γ‐ray and hard x‐ray payload, flown on a stratospheric balloon from Alice Springs, Australia. The γ‐ray detector, sensitive in the energy range 50–500 MeV, was an optical spark chamber with 400cm2 area, a field of view of 60° FWHM and a time resolution of 10 μs. The counting rate profile at ∼2.2 mb float altitude has lead to a 3σ upper limit to the steady γ‐ray flux of 7×10−4 ph cm−2 s−1 in the 50–500 MeV range. This upper limit is compared to our predictions for the time profile of γ‐ray emission from SN1987A resulting from pulsar acceleration of particles to cosmic ray energies.

A ROSAT EUV SURVEY OF CORONAL ACTIVITY IN NEARBY STARS AND COSMIC RAY INJECTION INTO THE INTERSTELLAR MEDIUM.

It has been argued by Shapiro (Proc 21st ICRC Moscow 2, 260, 1987 and references therein), that the dominant population of red dwarf stars provides the seed particles for a subsequent, interstellar medium based cosmic ray acceleration process. Computation of the energy budget for the injection process has depended to date on sample estimates of the flaring activity of dMe and dKe stars. The ROSAT Wide Field Camera is providing the first systematic survey of general coronal activity of the nearby sky. Using results of the all sky survey, we are able to estimate the value for the x-ray input into the galaxy from coronally active stars to be 1036 ergs s-1 kpc∼3. Results of the analysis are related to the possible ISM proton input rate through the rough equality of particle and x-ray energy release demonstrated for the solar case.