II. Comparison of ISO and IRAS galaxy counts

Transcription

1 New Astronomy 6 (2001) locate/ newast ISOCAM observations of the deep IRAS 60 micron sample in the NEP region q II. Comparison of ISO and IRAS galaxy counts a a,b c a,d a,e P. Mazzei, H. Aussel, C. Xu, M. Salvo, G. De Zotti, A. Franceschini* a Osservatorio Astronomico, Vicolo dell Osservatorio 5, I Padova, Italy b Institute For Astronomy, 2680 Woodlawn Drive, Honolulu, HI 96822, USA c IPAC, Jet Propulsion Laboratory, Caltech , Pasadena, CA 91125, USA d RSAA, Australian National University, Mt.Stromlo Observatory, Cotter Rd., 2611 Weston ACT, Australia e Dipartimento di Astronomia, Vicolo dell Osservatorio 5, I Padova, Italy Received 31 October 2000; received in revised form 13 March 2001; accepted 13 March 2001 Communicated by J.I. Silk Abstract We have determined IRAS flux densities at positions of sources detected by our ISOCAM LW3 observations of the IRAS deep survey sample in the north ecliptic polar region [ApJS 63 (1987) 311]. Exploiting the higher angular resolution of ISOCAM to deal with confusion effects and properly correcting the fluxes for observational biases, we show that 60 mm counts can be reliably determined only above 80 mjy. We have also derived the distributions of to 60 mm and of 100 to 60 mm flux density ratios for the sample and used them to extrapolate the 60 mm counts of galaxies to the ISOCAM LW3 mm and to the ISOPHOT C mm bands. Our results cover a flux density range intermediate between those of the European Large Area ISO Survey and the IRAS Faint Source Survey, hence allowing to establish a direct link between ISO and IRAS surveys. The present estimate has the further advantage of being automatically rid of the stellar contamination to the galaxy counts and provides a crucial boundary to constrain evolutionary properties of faint mid- and far-ir sources Published by Elsevier Science B.V. PACS: Gn; g; Jg; s Keywords: Galaxies: starburst; Galaxies: photometry; Infrared; Surveys q Based on observations with the Infrared Space Observatory (ISO). ISO is an ESA project with instruments funded by ESA Member States (especially by the PI countries: France, Germany, The Netherlands and the United Kingdom) and with participation of ISAS and NASA. *Corresponding author. addresses: mazzei@pd.astro.it (P. Mazzei), aussel@carp.ifa.hawaii.edu (H. Aussel), cxu@ipac.caltech.edu (C. Xu), salvo@pd.astro.it (M. Salvo), dezotti@pd.astro.it (G. De Zotti), afrances@mostro.pd.astro.it (A. Franceschini) / 01/ $ see front matter 2001 Published by Elsevier Science B.V. PII: S (01) Introduction Thanks to recent deep explorations by the Infrared Space Observatory (ISO) and the SCUBA bolometer camera on JCMT, faint IR sources are starting to reveal remarkable properties of cosmological evolution. A particularly pronounced effect is shown by the mid-ir ( mm) galaxy counts at flux density levels of 0.1 to 5 mjy (Elbaz et al., 1999), where the

2 266 P. Mazzei et al. / New Astronomy 6 (2001) ISO mission has produced statistically rich and 1998), and exploiting the ISO data to test the reliable samples and where it has been possible to reliability of sources and the effect of confusion identify the evolving sources as a (mostly starburst) (multiple sources within a single IRAS beam, see galaxy population at typical z 1 (e.g. Aussel et al., Section 3). We also determine the distributions of the 1999). However, assessing and interpreting the evo- to 60 mm and of the 100 to 60 mm flux density lution properties requires to tie in faint samples with ratios, and exploit them to estimate the contribution brighter ones, representative of the low-redshift of IRAS sources to counts in the ISOCAM LW3 source population. Unfortunately, the closest IRAS filter and in the ISOPHOT C mm filter. These surveys at 12 mm severely suffered from a very poor results are discussed in Section 4. spatial sampling, implying a modest limiting sensitivity ( 300 mjy, Rush et al., 1993), an uncertain flux scale and substantial contamination by local large-scale structure (Xu et al., 1998; Fang et al., 2. IRAS data 1998). To add such crucial information at moderately The new determinations of IRAS flux densities at deep mm fluxes, and to add constraints on the still the positions of the ISOCAM sources have been uncertain far-ir (90 mm) galaxy counts, we have obtained from the IRAS survey database using the exploited data from the 60 mm IRAS north ecliptic IPAC software SCANPI. The co-addition method polar region (NEPR) sample (Hacking, 1987; Hack- forming at each point the statistical median of all the ing and Houck, 1987; henceforth HH87), the deepest data scans co-added has been used (SCAN number far-ir sample before the advent of ISO surveys, still 1002), since it is believed to be the less vulnerable to providing crucial information on the far-ir evolution the non-gaussianity of the noise (see the description of galaxies, in a waveband non-optimally covered by of SCANPI processing in the Web-page ISO and where only SIRTF in the next several years / ipac/ iras/ scanpi ] over.html). could allow significant improvements. The flux density of the best-fitting point source In a previous paper (Aussel et al., 2000) we template was adopted, since it is recommended as presented ISO observations with the CAM LW3 filter the best flux estimator for weak point sources (see (range mm, leff514.3 mm; as usual (e.g. the above Web-page). No source showed evidence of Genzel and Cesarsky, 2000), we will refer to them as being resolved by IRAS. For some sources unde- mm observations) of 94 out of the 98 galaxies tected by SCANPI but for which data from pointed comprising the sample. Altogether, mm observations are available in addition to survey data, sources (plus the 25 mm source 2 16) were detected SUPERSCANPI was also run. with a signal to noise ratio $ 3 (68 of which with Xu (2000) exploited the new determinations only S/N $ 5) in the fields centered on the of 60 mm fluxes, obtained within the present pronominal positions of each observed IRAS source. gram, to derive a new estimate of the mm local For 11 additional sources, signals at the 2 3s level luminosity function through the bivariate ( mm vs. were found either close to the IRAS position (six 60 mm) function. Here we present the complete cases) or associated to optical objects. Sixty-five results of our reanalysis of the data in all 4 IRAS $ 3s detections (49 of which at $ 5s) are likely bands. Appropriate statistical corrections for the identifications of IRAS sources. Ten additional IRAS observational bias affecting weak sources have been sources have possible $ 3s ISOCAM counterparts. applied, and the better angular resolution of ISO- In this paper we present in Section 2 a new CAM data has been exploited to deal as far as determination of IRAS flux densities at the positions possible with confusion effects. Neither of these of the ISOCAM sources in the fields of 60 mm points was addressed by Xu (2000). NEPR sources. This new analysis is used to re-assess Whenever there are more than one ISOCAM the faint 60 mm counts, updating the flux densities to source within the ISOCAM fields we the new IRAS standard (Moshir et al., 1992), by adopted the following criteria to estimate the posapplying appropriate corrections for the bias affect- sible contamination of the target flux density by ing low signal-to-noise fluxes (Hogg and Turner, nearby sources:

3 P. Mazzei et al. / New Astronomy 6 (2001) (i) contamination by companion sources lying at where r is the signal to noise ratio. Clearly, SML can 2 least 19 away was assumed to be negligible; be evaluated only if r. 4b 1 4. (ii) stars detected by ISOCAM in the LW3 band In Table 2 we give the value of the maximum were assumed not to appreciably contaminate the 60 likelihood flux density, if it exists, computed assumand 100 mm flux densities of near galaxies; ing b 5 1.5, or the 3s upper limit. (iii) in the case of ISOCAM-detected galaxies It may be noted that, due to its statistical nature, closer than 19 from each other we have tried to infer the best applications of above correction to flux the relative intensities from a detailed analysis of estimates concern statistical rather than individual flux densities derived from IRAS scans in different properties of sources. In particular, this correction directions, taking advantage of the much better removes the Eddington bias (Eddington, 1913) on positional accuracy in the in-scan direction in com- source counts. parison with the cross-scan direction (Beichman et al., 1985). The results are presented in Table 1. Column 1 lists the source names according to Table 5 of HH Data analysis The other columns contain the flux densities and Fig. 1 shows the distribution of 60 mm maximum their errors in mjy, as well as the signal-to-noise likelihood flux densities. These can be evaluated for ratio (S/N) yielded by SCANPI, defined as the ratio only a minor fraction of sources fainter than about 80 of the peak flux density to the rms deviation (s) of mjy. Above this limit, our data allow a reassessment the residual after the baseline subtraction. Since the of source counts, with updated flux density estimates, peak flux density is, in general, somewhat different properly allowing for source confusion effects and from the flux of the best-fitting point source temcorrecting flux densities for the bias affecting weak plate, S, also given in Table 1, S/N generally differs sources. As shown by Fig. 1, the effect of the Hogg from S/s. It may also be noted that the values of S and Turner (1998) correction is quite significant for yielded by SCANPI are rounded to tens of mjy. This number counts in the bin centered at 100 mjy. implies that the rounding error for 12 and 25 mm flux We have assumed an effective area of 6 square densities is comparable to s. For the cases labelled degrees, slightly lower than the area of the map confused no reliable estimate of the flux could be obtained. constructed by HH87, on account of the fact that As pointed out by Xu (2000), the new 60 mm flux ISOCAM observations covered 94 out of the 98 densities are about 20% higher than those originally galaxies in the NEPR sample. Note that the effect of reported by HH87, but in good agreement with those non-uniform coverage of the area, due to the complilisted in the Faint Source Catalog and in the Faint cated geometry of scans used (see HH87), is negli- Source Reject File in the IRAS database. The gible above 80 mjy (cf. Fig. 13 of HH87). difference with HH87 is probably due to subsequent In estimating the 60 mm counts (Fig. 2) we must revisions of the IRAS data processing pipeline. allow for sources detected by ISOCAM in crowded As stressed by Hogg and Turner (1998), flux fields, whose 60 mm flux densities cannot be derived estimates for faint sources are systematically biased from IRAS data because of confusion effects. There high (in a statistical sense) because in any given are three such sources (3-26c, 3-36a, and 3-36b) observed flux interval there are more sources brigcentered at 100 mjy if their /60 mm flux whose 60 mm flux densities would fall in the bin htened than dimmed by measurement errors, density simply due to the fact that faint sources are more numerous than bright ones. If b is the slope of integral source counts, the maximum likelihood true flux density SML is related to the observed flux density So by (Hogg and Turner, 1998): S S 1 1 4b 1 4 1/2 ML ]] 5] 1] 1 2 ]], (1) S 2 2 r 2 o D ratio corresponds to either the average or the median value of log(s /S 60) (see Table 3), and one source (3-37b) that would fall in the bin centered at logs All other sources of this kind would fall in lower flux density bins. Moreover, so far we have implicitly assumed that the 9 NEPR sources lacking an (even doubtful) ISOCAM/ LW3 counterpart are not real. This is plausible for at least some of them, since they are

4 268 P. Mazzei et al. / New Astronomy 6 (2001) Table 1 Uncorrected ISOCAM/ LW3 and IRAS flux densities (mjy) Name LW3 IRAS 12 mm IRAS 25 mm IRAS 60 mm IRAS 100 mm flux err flux err S/N flux err S/N flux err S/N flux err S/N a b a b confused 7 confused confused 3-19c confused 11 confused confused 3-20a b confused a confused b confused c confused 5 confused confused 3-26d confused confused 3-27a b confused confused a b confused a b a b a confused confused 3-38b a b

5 P. Mazzei et al. / New Astronomy 6 (2001) Table 1. Continued Name LW3 IRAS 12 mm IRAS 25 mm IRAS 60 mm IRAS 100 mm flux err flux err S/N flux err S/N flux err S/N flux err S/N a b a b a b a b c confused confused 3-79a confused b confused confused c a confused confused b confused confused a b confused confused a b a confused confused 3-92b c a b

6 270 P. Mazzei et al. / New Astronomy 6 (2001) Table 2 Maximum likelihood ISOCAM/ LW3 and IRAS flux densities or upper limits (mjy) Name LW3 12 mm 25 mm 60 mm 100 mm Name LW3 12 mm 25 mm 60 mm , 21, 81, , , , , 27, , ,, , 39, a 3.1, 30, b 1.8, 24, 21, , 27, , 2.7, , 36, , 2.1, 24, , 33, 27, 75, , 18, , , , 24 7, , , , , 21, 30, 75, , 18, , 36, , , 33, , 33, , 30, , 33, 30 84, a a 2.7, 30, 30, 48, b , 30, b 3.0, 33, 36 48, , 30, 33, 75, , 27, a 3.7, 33, , 207, b 2.0 conf, 21 conf conf 3-67a , c, 2.1 conf, 33 conf conf 3-67b 5.2, 18, 30, a 2.4, 27, , , 24, b 2.0, 27, 27 conf, , 21, 18, , 27, , , 33, , , 27, 24 83, , 36, 21, , 27, a 4.1 conf, , 27, b 8.0 conf 17 84, , 42, 27, c 5.6 conf, conf conf , 39, d, 2.1, 21, conf conf , 27, a 4.5, 24, , a 3.3, 24, 24, b 7.8, 27, 27 conf conf 3-78b 2.5, 24, , 21, c, 1.8, 27, 24 conf , a.0, 30 conf , 3.0, 24, 33 59, b 31.8 conf conf, , c 6.1, 24,, a 6.0, 30, 30 71, a 23.3 conf conf, b, 2.4, 27, 18 conf, b 22.1 conf conf a , 18 64, a 5.7, 30, b 2.7, 21, 21, b 2.2, 30, 36 conf , , , , 3.0, 21 24, , 24, a 5.2, 27, 36, 213, , b 5.2, 27, 39, 216, , 24, , 36, a 3.3, 18, 18 conf conf 3-88a 2.9, 24, b , b 2.4, 24, 30, , , , 24, a 7.5, 36, 39 86, , b, 2.1, 36, 36, 87, , 21, a 3.0, 24, 27 conf , 30, 24 45, b 3.5, 27, 27, c, 2.1, 30, , , 27, , 33, a.2, 21, , 2.1, b 3.5, 33, 30, , 30, ,

7 P. Mazzei et al. / New Astronomy 6 (2001) Fig. 1. Distribution of maximum likelihood 60 mm flux densities Fig. 2. Normalized 60 mm source counts. The filled squares show (solid line). Also shown, for comparison, are the distributions of the present estimates, compared with earlier results by Gregorich HH87 fluxes (dashed) and of uncorrected SCANPI/ SUPER- et al. (1995; open circles), Bertin et al. (1997; open squares), SCANPI fluxes (dotted); the latter are, on the average, 20% higher Hacking and Houck (1987; filled circles at S 60, 0.4 Jy), Lonsdale than those reported by HH87 (see text). et al. (1990; filled triangles), Rowan-Robinson et al. (1991; filled circles at S Jy). The solid lines encompass most data sets. The results by Gregorich et al. (1995), which are substantially higher than both those of HH87 and Bertin et al. (1997), may be contaminated by cirrus and by source confusion effects. relatively faint (S 60(HH87), 90 mjy), and, according to HH87, the reliability of sources having flux densities close to the survey limit is about 80%. On the other hand, the observed distributions of / of the ISOPHOT C100 detector array; only two 60 mm flux density ratios both for the NEPR sample sources were detected, brighter than 236 mjy. and for the sample of Dale et al. (2000) have a tail In Figs. 3 and 4 we show the distributions of to extending to, 0.01 (see Fig. 3). Sources with S. 60 mm and of 100 to 60 mm flux density ratios, 2 mjy may have been missed by our ISOCAM/ LW3 respectively, for those sources for which S ML (60 observations having a median (1s) error of 0.9 mjy. mm) can be evaluated. The Kaplan and Meier (1958) Based on the observed distribution of / 60 mm flux estimator was used to properly take into account density ratios, we estimate that we may have missed upper limits at and 100 mm. Calculations were. 10% of sources in the bin centered at S60 5 carried out using the ASURV package Rev 1.2 (Isobe 100 mjy and we have applied the corresponding and Feigelson, 1990; LaValley et al., 1992), which correction to the source counts in Fig. 2. The effect implements methods presented in Feigelson and is negligible for higher flux density bins. Nelson (1985) and Isobe et al. (1986). Our estimated 60 mm counts (Fig. 2) are in good The Kaplan Meier estimator is a nonparametric, agreement with those by Bertin et al. (1997) and maximum-likelihood-type estimator of the true tend to be slightly higher than those by HH87. distribution function (i.e. with all quantities properly So far analyses of ISO data have provided very measured, no upper limits). The survivor function, little additional information on 60 mm counts. A giving the estimated proportion of objects with upper 2 survey of a small area (0.4 deg ) has been carried out limits falling in each bin, does not produce, in by Linden-Vørnle et al. (2000) using the C-60 filter general, integer numbers, but it is normalized to the

8 272 P. Mazzei et al. / New Astronomy 6 (2001) Table 3 Coefficients and parameters describing the distribution functions 2 Case y0 a1 a2 m1 m2 Mode Mean s Skew Kurt x n NEPR S /S NEPR S /S Dale S /S Dale S 100 /S total number. This is why non-integer numbers of quality of the fit is quantified by the value of x per 2 objects appear in the histograms of Figs. 3 and 4. degree of freedom (x n), given in the last column, The distributions per unit interval of the logarithm and computed adopting Poisson errors, as tabulated of the flux density ratios are well described by Type by Gehrels (1986). I Pearson s curves (Pearson, 1924;, 1969): Also shown in Figs. 3 and 4 are the analytical representations of the flux density ratio distributions m1 m2 y 5 y 0(1 1 x/a 1) (12 x/a 2) of the nearby bright galaxy sample of Dale et al. 2 a # x # a, (2) (2000), plotted in Figs. 5 and 6. The latter sample is 1 2 a collection of galaxies covering a broad range of with origin at the position of the peak of the morphologies, luminosities and IRAS colours seen in distribution (mode). normal galaxies, and is by no means complete. The values of the parameters are given in Table 3, Hence, the comparison with the NEPR sample together with the mean, the standard deviation s, the should be dealt with caution. The distributions are skewness ( m 3/m 2) and the kurtosis ( m 4/m22 3), significantly broader for the NEPR sample, possibly where m is the ith moment about the mean. The reflecting its broader redshift distribution, and the i 2 Fig. 3. Distribution of mm to60mm flux density ratios for the Fig. 4. Distribution of 100 mm to60mm flux density ratios for NEPR sample. The solid line is the fitting Type I Pearson s curve the NEPR sample and the fitting Type I Pearson s curve (solid [Eq. (2) with coefficients given in Table 3]. The dotted line is the line). The dotted line is the Pearson s curve for the sample of Dale Pearson s curve for the sample of Dale et al. (2000), scaled to the et al. (2000), scaled to the same total number of sources (the scale same total number of sources (the scale factor is 82/54). factor is 82/56).

9 P. Mazzei et al. / New Astronomy 6 (2001) Fig. 5. Distribution of mm to60mm flux density ratios for the Dale et al. (2000) sample and the fitting Type I Pearson s curve (solid line). peak of the distribution of log S /S60 is shifted to a slightly higher value. With the distributions of flux density ratios derived above, we may extrapolate the IRAS 60 mm counts to the ISOCAM LW3 band and to the 90 mm band of the ISOPHOT C100 detector array. In fact, the IRAS 100 mm and the ISOPHOT C-90 filters have fairly similar spectral responses so that the fluxes in the two bands can be assumed to be approximately equal (see Linden-Vørnle et al., 2000). The mean value of the 100 mm to60mm flux density ratio for the NEPR sample (. 2.1) is close to the flux factor between the IRAS 100 mm and 60 mm point source counts at equal number densities, determined to be 2.4 (Beichman et al., 1985; Linden- Vørnle et al., 2000). This indicates that there is no significant change in the S 100 /S60 ratio between sources in the IRAS Point Source Catalog and those in our deeper sample. Therefore, IRAS 60 mm counts can be straightforwardly extrapolated to the ISO- PHOT C100 band using a constant flux density ratio from Jy levels down to S 90 mm. 200 mjy. The minimum flux density of our extrapolated counts is determined by the condition that contributions from sources fainter than 80 mjy at 60 mm must be well within the assumed uncertainty, under reasonable assumptions. Particular care must be taken at mm, since we are close to the flux density region where counts show a strong upturn. Thanks to the fast decline of the tails of the distributions of flux density ratios, such minimum flux density corresponds approximately to 80 mjy times the mean flux density ratio plus 1 standard deviation. As illustrated by Figs. 7 and 8, our estimates reach flux densities partially covered by the European Large Area ISO Survey (ELAIS) in both bands, and link directly the ISO to the IRAS surveys, allowing to extend the counts by about two orders of magnitude in flux density. 4. Discussion and conclusions Fig. 6. Distribution of 100 mm to60mm flux density ratios for the Dale et al. (2000) sample and the fitting Type I Pearson s curve (solid line). This paper presents IRAS flux densities at the positions of sources detected by our ISOCAM LW3 ( mm ) observations of the IRAS deep survey sample in the North Ecliptic Polar (NEP) region

10 274 P. Mazzei et al. / New Astronomy 6 (2001) Fig. 7. Comparison of extrapolated IRAS 60 mm (Euclidean Fig. 8. Comparison of extrapolated IRAS 60 mm differential normalized) differential counts with results of ISOCAM mm counts with counts obtained with the C-90 filter of the C100 surveys. The shaded region is the estrapolation, exploiting the ISOPHOT detector array [filled squares: Efstathiou et al. (2000); distribution of to 60 mm flux density ratios for the NEPR open circle: Linden-Vørnle et al. (2000); filled circles: Juvela et al. sample shown in Fig. 3, of the region bounded by the solid lines (2000)]. Units are as in Fig. 7. The shaded region corresponds to in Fig. 2. The open squares at S( mm).4 mjy show the the region bounded by the two solid lines in Fig. 2, extrapolated preliminary ELAIS results (Serjeant et al., 2000). The other data using the distribution of 100 to 60 mm flux density ratios for the points correspond to the normalized counts from a variety of NEPR sample shown in Fig. 4. The lines correspond to the surveys in the ISO Guaranteed and Open Time, as compiled by evolutionary model mentioned in the caption to Fig. 7, and Elbaz et al. (1999): A2390 (the two filled triangles on the left), best-fitting multi-frequency data on faint IR galaxies. ISOHDF-North (open circles), ISOHDF-South (filled squares), Marano FIRBACK (MFB) Ultra-Deep (the three filled triangles with smaller error bars), Marano Ultra-Deep (open triangles), sample, possibly reflecting its broader redshift dis- MFB Deep (open squares at S( mm),4 mjy), Lockman Deep tribution, and the peak of the distribution of log and Shallow (filled circles). The lines show the predictions of a S /S60 is shifted to a slightly higher value. recent model, whereby counts of bright sources are dominated by The flux density ratio distributions were exploited a non-evolving population (comprising normal spiral and irregular to extrapolate the 60 mm counts of galaxies to the galaxies, the dotted line) while the sub-mjy peak is mostly due to strongly evolving starburst galaxies (dashed line); the contribution ISOCAM LW3 and ISOPHOT C100 bands. Our of Active Galactic Nuclei corresponds to the long-dash line; for results cover a flux density interval partly overlap- more details, see Franceschini et al. (2001) or Franceschini ping that of the European Large Area ISO Survey (2000). The continuous line sums up all various contributions. and of the survey by Juvela et al. (2000), and allow to set reliably the bright-flux limiting boundary to the (HH87). This establishes a well defined link between faint IR galaxy counts. ISO and IRAS counts. Note that our 60 mm primary selection has the We have derived the distributions of to 60 mm important advantage of almost completely avoiding and of 100 to 60 mm flux density ratios and have contamination by stars, which is a rather serious obtained accurate analytic formulae fitting them. A problem for direct mid-ir surveys at such bright flux comparison with the corresponding distributions for limits. the nearby bright galaxy sample of Dale et al. At mm (Fig. 7) IRAS data allow to extend the (2000), for which also accurate fitting formulae are flux coverage by about two orders of magnitude on provided, shows relatively minor differences: the the bright side, thus strongly tightening the condistributions are significantly broader for the NEPR straints on the evolutionary history of galaxies. Our

11 P. Mazzei et al. / New Astronomy 6 (2001) estimate is a factor. 2 lower than the source counts (Hogg and Turner, 1998). We find that 60 mm counts from the preliminary analysis of ELAIS data (Ser- can be reliably determined only above 80 mjy. Our jeant et al., 2000), but consistent with those from the estimates are slightly above those by HH87 and in shallow Lockman Hole ISOCAM survey reported by good agreement with those by Bertin et al. (1997). Elbaz et al. (1999). According to Serjeant et al. (2000), the discrepancy between the ELAIS and the Lockman Hole counts is entirely attributable to Acknowledgements differences in the assumed flux calibration. A more recent analysis (Lari et al., 2001, and private com- We are indebted to the Referee for pointing out an munication), however, does not confirm this interpre- error in the original version of the paper and for tation. A proper understanding of the origin of the several constructive comments. Work supported in discrepancy must await for a more refined analysis of part by ASI, MURST and the TMR Network the data, currently in progress. Galaxy Formation under contract n. ERBFMRX- Putting together our results with those of deep ISO CT This research has made use of the surveys (see Elbaz et al., 1999), which have de- NASA/ IPAC Extragalactic Database (NED) which termined the mm counts down to flux densities of is operated by the Jet Propulsion Laboratory, Califorabout 0.1 mjy, a dramatic evolution effect becomes nia Institute of Technology, under contract with the apparent at S, 10 mjy, corresponding to a factor National Aeronautics and Space Administration. 5 excess in the Euclidean-normalized differential counts at S 0.4 mjy with respect to the plain 22.5 Euclidean (N ~ S ) extrapolation from our esti- References mated bright end. At 90 mm our results (Fig. 8) are in good Aussel, H., Cesarsky, C., Elbaz, D., Starck, J.L., A&A 342, agreement with the preliminary analysis of ELAIS 313. data (Efstathiou et al., 2000) and consistent with the Aussel, H., Coia, D., Mazzei, P., De Zotti, G., Franceschini, A., A&AS 141, 257. counts by Linden-Vørnle et al. (2000) and Juvela et Beichman, C.A., Neugebauer, G., Habing, H.J., Clegg, P.E., al. (2000), and provide independent evidence that the Chester, T.J. (Eds.), Infrared Astronomical Satellite extragalactic source counts at this wavelength are (IRAS) Catalogs and Atlases, Explanatory Supplement. consistently established down to a flux density limit Bertin, E., Dennefeld, M., Moshir, M., A&A 323, 685. of S 5 0 mjy. The ISOPHOT counts by Ef- Dale, D.A. et al., AJ 120, Eddington, A.S., MNRAS 73, 359. stathiou et al. (2000) and Juvela et al. (2000) Efstathiou, A. et al., MNRAS 319, indicate an excess in the counts fainter than this Elbaz, D., Cesarsky, C.J., Fadda, D. et al., A&A 351, L37. limit, which could be evidence for evolution. How- Elderton, W.P., Johnson, N.L., Systems of Frequency ever, more refined analyses of the ISOPHOT 90 mm Curves. Cambridge University Press. data, or deeper surveys by SIRTF, will be needed to Fang, F., Shupe, D.L., Xu, C., Hacking, P.B., ApJ 500, 693. Feigelson, E.D., Nelson, P.I., ApJ 293, 192. assess this result. Franceschini A., In: Sanchez, F. et al. (Eds.), Proc. XI On the other hand, our ISOCAM observations are Canary Islands Winter School of Astrophysics on Galaxies at also beneficial for the assessment of the deepest 60 High Redshift, Cambridge University Press, in press (astroph/ mm IRAS counts in the NEP region, since they test ). the reliability of barely detected sources and the Franceschini, A., Aussel, H., Cesarsky, C., Elbaz, D., Fadda D., A&A, submitted. higher angular resolution of ISOCAM allows to deal Gehrels, N., ApJ 303, 336. with source confusion effects. Two further factors Genzel, R., Cesarsky, C.J., ARA&A 38, 761. are relevant in updating the 60 mm source count Gregorich, D.T., Neugebauer, G., Soifer, B.T., Gunn, J.E., Herter, estimate: first, the new SCANPI processing of IRAS T.L., AJ 110, 259. data yields fluxes about 20% higher than those Hacking, P., Ph.D. Thesis, Cornell University. Hacking, P., Houck, J.R., ApJS 63, 311, (HH87). reported by HH87, and, second, significant correc- Hogg, D.W., Turner, E.L., PASP 110, 727. tions are needed for the bias affecting flux measure- Isobe, T., Feigelson, E.D., BAAS 22, 917. ments of these relatively low signal-to-noise sources Isobe, T., Feigelson, E.D., Nelson, P.I., ApJ 306, 490.

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