Canada-France Redshift Survey Research Articles

3D spectroscopy with VLT/GIRAFFE

Luminous Compact Galaxies (LCGs) are enigmatic sources in many aspects. They can reach the luminosity of the Milky Way within a radius of only a few kpc. They also represent one of the most rapidly evolving populations of galaxies since they represent up to 1/5 of the luminous galaxies at redshift , while being almost absent in the local Universe. The measurement of their dynamics is crucial to our understanding of LCGs since this has the potential of telling us which physical process(es) drive(s) them and ultimately link them to the existing present-day galaxies. Here, we derive the 3-dimensional velocity fields and velocity dispersion (σ) maps of 17 LCGs selected from the Canada France Redshift Survey and the Hubble Deep Field South with redshifts ranging from to . We find that only 18% of them show rotational velocity fields typical of rotating disks while the others show more complex kinematics. Assuming that LCGs are not too far from equilibrium, about half of LCGs then appear to be either non-relaxed objects, or objects that are not supported by velocity dispersion alone. This supports the view that an important fraction of LCGs are probably mergers. It brings additional support to the “spiral rebuilding scenario” in which LCGs correspond to a previous or post-merger phase before the disk re-building.

Spectrophotometric properties of galaxies at intermediate redshifts (z ~ 0.2–1.0)

We present the spectrophotometric properties of a sample of 141 emission-line galaxies at redshifts in the range $0.2<z<1.0$ with a peak around $z\in[0.2,0.4]$. The analysis is based on medium resolution ($R\_{\mathrm{s}}=500-600$), optical spectra obtained at VLT and Keck. The targets are mostly {} "Canada-France Redshift Survey" emission-line galaxies, with the addition of field galaxies randomly selected behind lensing clusters. We complement this sample with galaxy spectra from the {} "Gemini Deep Deep Survey" public data release. We have computed absolute magnitudes of the galaxies and measured the line fluxes and equivalent widths of the main emission/absorption lines. The last two have been measured after careful subtraction of the fitted stellar continuum using the \texttt{platefit} software originally developed for the SDSS and adapted to our data. We present a careful comparison of this software with the results of manual measurements. The pipeline has also been tested on lower resolution spectra, typical of the {} "VIMOS/VLT Deep Survey" ($R\_{\mathrm{s}}=250$), by resampling our medium resolution spectra. We show that we can successfully deblend the most important strong emission lines. These data are primarily used to perform a spectral classification of the galaxies in order to distinguish star-forming galaxies from AGNs. Among the initial sample of 141 emission-line galaxies, we find 7 Seyfert 2 (narrow-line AGN), 115 star-forming galaxies and 16 {} "candidate" star-forming galaxies. Scientific analysis of these data, in terms of chemical abundances, stellar populations, etc, will be presented in subsequent papers of this serie.

The Gemini Deep Deep Survey. VII. The Redshift Evolution of the Mass‐Metallicity Relation

We have investigated the mass-metallicity (M-Z) relation using galaxies at 0.4 < z < 1.0 from the Gemini Deep Deep Survey (GDDS) and Canada-France Redshift Survey (CFRS). Deep K- and z'-band photometry allowed us to measure stellar masses for 69 galaxies. From a subsample of 56 galaxies, for which metallicity of the interstellar medium is also measured, we identified a strong correlation between mass and metallicity for the first time in the distant universe. This was possible because of the larger baseline spanned by the sample in terms of metallicity (a factor of 7) and mass (a factor of 400) than in previous works. This correlation is much stronger and tighter than the luminosity-metallicity relation, confirming that stellar mass is a more meaningful physical parameter than luminosity. We find clear evidence for temporal evolution in the M-Z relation in the sense that at a given mass, a galaxy at z ~ 0.7 tends to have lower metallicity than a local galaxy of similar mass. We use the z ~ 0.1 Sloan Digital Sky Survey M-Z relation and a small sample of z ~ 2.3 Lyman break galaxies with known mass and metallicity to propose an empirical redshift-dependent M-Z relation. According to this relation the stellar mass and metallicity in small galaxies evolve for a longer time than they do in massive galaxies. This relation predicts that the generally metal-poor damped Lyα galaxies have stellar masses of the order of 108.8 M☉ (with a dispersion of 0.7 dex) all the way from z ~ 0.2 to 4. The observed redshift evolution of the M-Z relation can be reproduced remarkably well by a simple closed-box model in which the key assumption is an e-folding time for star formation that is higher or, in other words, a period of star formation that lasts longer in less massive galaxies than in more massive galaxies. Such a picture supports the downsizing scenario for galaxy formation.

XMM-Newtonsurveys of the Canada-France Redshift Survey Fields - III. The environments of X-ray selected active galactic nuclei at 0.4 &lt;z&lt; 0.6

The environmental properties of a sample of 31 hard X-ray selected active galactic nuclei (AGN) are investigated, from scales of 500 kpc down to 30 kpc, and are compared to a control sample of inactive galaxies. All the AGN lie in the redshift range 0.4 < z < 0.6. The accretion luminosity density of the Universe peaks close to this redshift range, and the AGN in the sample have X-ray luminosities close to the knee in the hard X-ray luminosity function, making them representative of the population that dominated this important phase of energy conversion. Using both the spatial clustering amplitude and near-neighbour counts, it is found that the AGN have environments that are indistinguishable from normal, inactive galaxies over the same redshift range and with similar optical properties. Typically, the environments are of subcluster richness, in contrast to similar studies of high-z quasars, which are often found in clusters with comparable richness to the Abell R≥ 0 clusters. It is suggested that minor mergers with low-mass companions are a likely candidate for the mechanism by which these modest luminosity AGN are fuelled.

Did most present-day spirals form during the last 8 Gyr?

Studies of distant galaxies have shown that ellipticals and large spirals (Schade et al. [CITE], ApJ, 525, 31; Lilly et al. [CITE], ApJ, 500, 75) were already in place 8 Gyr ago, leading to a very modest recent star formation (Brinchmann & Ellis [CITE], ApJ, 536, L77) in intermediate mass galaxies ( ). This is challenged by a recent analysis (Heavens et al. [CITE], Nature, 428, 625) of the fossil record of the stellar populations of ~105 nearby galaxies, which shows that intermediate mass galaxies formed or assembled the bulk of their stars 4 to 8 Gyr ago. Here we present direct observational evidence supporting this findings from a long term, multi-wavelength study of 195 intermediate mass galaxies, mostly selected from the Canada France Redshift Survey (CFRS). We show that recent and efficient star formation is revealed at IR wavelengths since ~15% of intermediate mass galaxies at are indeed luminous IR galaxies (LIRGs), a phenomenon far more common than in the local Universe. The star formation in LIRGs is sufficient in itself to produce 38% of the total stellar mass of intermediate mass galaxies and then to account for most of the reported stellar mass formation since . Observations of distant galaxies have also the potential to resolve their star formation and mass assembly histories. The high occurrence of LIRGs is easily understood only if they correspond to episodic peaks of star formation, during which galaxies are reddened through short IREs (infrared episodes). We estimate that each galaxy should experience 4 to IREs from to , being the characteristic timescale. An efficient and episodic star formation is further supported by the luminosity-metallicity relation of emission line galaxies, which we find to be on average metal deficient by a factor of ~2 when compared to those of local spirals. We then examine how galaxy IREs can be related to the emergence at high redshift of the abundant population of galaxies with small size (but not with small mass), blue core and many irregularities. We show that recent merging and gas infall naturally explain both morphological changes and episodic star formation history in a hierarchical galaxy formation frame. We propose a simple scenario in which % of intermediate mass spirals have recently experienced their last major merger event, leading to a drastic reshaping of their bulges and disks during the last 8 Gyr. It links in a simple manner distant and local galaxies, and gives account of the simultaneous decreases during that period, of the cosmic star formation density, of the merger rate, and of the number densities of LIRGs, compact and irregular galaxies, while the densities of ellipticals and large spirals are essentially unaffected. It predicts that 42, 22 and 36% of the IR (episodic) star formation density is related to major mergers, minor mergers and gas infall, respectively.

XMM-Newtonsurveys of the Canada-France Redshift Survey fields - II. The X-ray catalogues, the properties of the host galaxies and the redshift distribution

We present the X-ray source catalogues for the XMM surveys of the 3-h and 14-h Canada–France Redshift Survey fields (0.5–10 keV flux range ∼2 × 10−15–10−13 erg cm−2 s−1). We use a subset of the XMM sources, which have Chandra positions, to determine the best method of obtaining optical identifications of sources with only XMM positions. We find optical identifications for 79 per cent of the XMM sources for which there are deep optical images. The sources without optical identifications are likely to be optically fainter and have higher redshifts than the sources with identifications. We have estimated ‘photometric redshifts’ for the identified sources, calibrating our method using ∼200 galaxies in the fields with spectroscopic redshifts. We find that the redshift distribution has a strong peak at z∼ 0.7. The host galaxies of active galactic nuclei (AGN) identified in this work cover a wide range of optical properties, with every galaxy type being represented and no obvious preference for one type over another. Redder types tend to be more luminous than blue types, particularly at lower redshifts. The host galaxies also span a wide range of optical luminosity, in contrast to the narrow range found for the starburst galaxies detected in μJy radio surveys. We find a strong correlation between optical and X-ray luminosity similar to the Magorrian relation, although selection effects cannot be ruled out.

Oii] as a Star Formation Rate Indicator

We investigate the [O II] emission line as a star formation rate (SFR) indicator using integrated spectra of 97 galaxies from the Nearby Field Galaxies Survey (NFGS). The sample includes all Hubble types and contains SFRs ranging from 0.01 to 100 M⊙ yr-1. We compare the Kennicutt [O II] and Hα SFR calibrations and show that there are two significant effects that produce disagreement between SFR([O II]) and SFR(Hα): reddening and metallicity. Differences in the ionization state of the interstellar medium do not contribute significantly to the observed difference between SFR([O II]) and SFR(Hα) for the NFGS galaxies with metallicities log (O/H) + 12 ≳ 8.5. The Kennicutt [O II]-SFR relation assumes a typical reddening for nearby galaxies; in practice, the reddening differs significantly from sample to sample. We derive a new SFR([O II]) calibration that does not contain a reddening assumption. Our new SFR([O II]) calibration also provides an optional correction for metallicity. Our SFRs derived from [O II] agree with those derived from Hα to within 0.03–0.05 dex. We show that the reddening, E(B-V), increases with intrinsic (i.e., reddening-corrected) [O II] luminosity for the NFGS sample. We apply our SFR([O II]) calibration with metallicity correction to two samples: high-redshift 0.8 < z < 1.6 galaxies from the NICMOS Hα survey and 0.5 < z < 1.1 galaxies from the Canada-France Redshift Survey. The SFR([O II]) and SFR(Hα) for these samples agree to within the scatter observed for the NFGS sample, indicating that our SFR([O II]) relation can be applied to both local and high-z galaxies. Finally, we apply our SFR([O II]) to estimates of the cosmic star formation history. After reddening and metallicity corrections, the star formation rate densities derived from [O II] and Hα agree to within ∼30%.

Misleading results from low-resolution spectroscopy: From galaxy interstellar medium chemistry to cosmic star formation density

Low resolution spectroscopy (R=150) from the Canada-France-Redshift Survey (CFRS) had revealed intriguing properties for low redshift galaxies (z 600), reveals that the CFRS H\alpha-single galaxies and most of the star forming spirals have high extinctions (A_V>2), high stellar masses and over-solar oxygen abundances. The present study shows that it is hard to derive the detailed properties of galaxies (gas chemical abundances, interstellar extinction, stellar population, star formation rates and history) using spectra with resolution below 600. One major drawback is indeed that the underlying Balmer absorption cannot be estimated properly, which could cause the SFRs be either underestimated or overestimated by factors reaching 10 (average 3.1) for the sample galaxies. These effects are prominent for a large fraction of evolved massive galaxies especially those experiencing successive bursts (A and F stars dominating their absorption spectra). Further estimates of the cosmic star formation density at all redshifts mandatorily requires moderate resolution spectroscopy to avoid severe biases.

The Metallicities of Star‐forming Galaxies at Intermediate Redshifts 0.47 &lt;z&lt; 0.92

Estimates of the [O/H] metallicity of the star-forming gas in a sample of 66 Canada-France Redshift Survey galaxies in the range 0.47 75%) have the [O/H] ~ 8.9 metallicities that are seen locally in galaxies of similar luminosities. However, a minority (<25%) appear to have significantly lower metallicities [O/H] < 8.6, as indicated by high values of the Pagel R23 parameter. The fraction of the sample at these lower [O/H] values would be reduced if the reddening in these objects were lower than the EB-V ~ 0.3 assumed. The high metallicities of the majority of the galaxies suggest that they do not fade to be low-metallicity galaxies today. Only one of the 66 galaxies has an optical emission-line spectrum similar to the few Lyman break galaxies recently observed at z ~ 3, emphasizing the differences from that population. The inferred emission-line gas [O/H] metallicity broadly correlates with luminosity in both the rest B and J bands but with considerable scatter introduced, especially at lower luminosities, by the range in [O/H] observed. The metallicity does not appear to correlate well with galaxy size, Hβ strength, or, with the very limited data available, kinematics. The metallicity does correlate well with the continuum optical and optical to infrared colors in a way that could be explained as a combination of photospheric effects, differing ages of the stellar population, and/or different amounts of reddening. None of these should produce large variations in the M/L ratio. These results support a downsizing picture of galaxy evolution, in which the manifestations of star-forming evolutionary activity appear in progressively more massive galaxies at earlier epochs, rather than a fading dwarf picture in which the luminous active galaxies at high redshift are highly brightened galaxies. The overall change in metallicity of star-forming galaxies over the last half of the age of the universe appears to have been modest, Δ[O/H] = 0.08 ± 0.06. This is consistent with the age-metallicity relation in the Galactic disk and is broadly consistent with models for the chemical evolution of the universe, especially those that consider different environments.

XMM-Newton surveys of the Canada-France Redshift Survey fields -- I. The submillimetre/X-ray relation

First results from XMM–Newton observations of the Canada–France Redshift Survey (CFRS) 3-h, 10-h and 14-h fields are presented. Limited regions of two of the XMM surveys (3 and 14 h) are compared with the Canada–UK Deep Submillimetre Surveys (CUDSS) undertaken with SCUBA. None of the 27 SCUBA sources in the 3-h field is detected by XMM, while one of the 23 SCUBA sources in the 14-h field is found to coincide with an X-ray source. The SCUBA population as a whole is not significantly detected in either the 0.5–2 keV or the 2–10 keV X-ray bands, even after coadding the X-ray flux at the SCUBA positions, in both fields. The 18 X-ray sources within the CUDSS 3-h map yield a mean submillimetre (submm) flux of 0.48 ± 0.27 mJy after coadding the submm flux at the X-ray positions. Using this result we place an upper limit on the contribution of AGN to the submm background at 850 ?m of ?7 per cent. Conversely we estimate the contribution of submm sources to the 0.5–2 keV X-ray background to be 2.3. The ratio of the X-ray to submm flux for the X-ray sources, however, implies that even when a galaxy does contain an AGN, most of the energy heating the dust is from young stars and not from the active nucleus.

The Hαluminosity function and star formation rate up toz ∼ 1

We describe ISAAC/ESO-VLT observations of the Ha(6563) Balmer line of 33 field galaxies from the Canada-France Redshift Survey (CFRS) with redshifts selected between 0.5 and 1.1. We detect Ha in emission in 30 galaxies and compare the properties of this sample with the low-redshift sample of CFRS galaxies at z~0.2 (Tresse & Maddox 1998). We find that the Ha luminosity, L(Ha), is tightly correlated to M(B(AB)) in the same way for both the low- and high-redshift samples. L(Ha) is also correlated to L([OII]3727), and again the relation appears to be similar at low and high redshifts. The ratio L([OII])/L(Ha) decreases for brighter galaxies by as much as a factor 2 on average. Derived from the Ha luminosity function, the comoving Ha luminosity density increases by a factor 12 from =0.2 to =1.3. Our results confirm a strong rise of the star formation rate (SFR) at z<1.3, proportional to (1+z)^{4.1+/-0.3} (with H_0=50 km/s/Mpc, q_0=0.5). We find an average SFR(2800 Ang)/SFR(Ha) ratio of 3.2 using the Kennicutt (1998) SFR transformations. This corresponds to the dust correction that is required to make the near UV data consistent with the reddening-corrected Ha data within the self-contained, I-selected CFRS sample.

The Growth of Galaxies in Cosmological Simulations of Structure Formation

We use hydrodynamic simulations to examine how the baryonic components of galaxies are assembled, focusing on the relative importance of mergers and smooth accretion in the formation of ~L* systems. In our primary simulation, which models a (50 h-1 Mpc)3 comoving volume of a Λ-dominated cold dark matter universe, the space density of objects at our (64 particle) baryon mass resolution threshold Mc = 5.4 × 1010 M☉ corresponds to that of observed galaxies with L ~ L*/4. Galaxies above this threshold gain most of their mass by accretion rather than by mergers. At the redshift of peak mass growth, z ≈ 2, accretion dominates over merging by about 4 : 1. The mean accretion rate per galaxy declines from ~40 M☉ yr-1 at z = 2 to ~10 M☉ yr-1 at z = 0, while the merging rate peaks later (z ≈ 1) and declines more slowly, so by z = 0 the ratio is about 2 : 1. We cannot distinguish truly smooth accretion from merging with objects below our mass resolution threshold, but extrapolating our measured mass spectrum of merging objects, dP/dM ∝ M-α with α ~ 1, implies that subresolution mergers would add relatively little mass. The global star formation history in these simulations tracks the mass accretion rate rather than the merger rate. At low redshift, destruction of galaxies by mergers is approximately balanced by the growth of new systems, so the comoving space density of resolved galaxies stays nearly constant despite significant mass evolution at the galaxy-by-galaxy level. The predicted merger rate at z 1 agrees with recent estimates from close pairs in the Canada-France Redshift Survey and Canadian Network for Observational Cosmology Redshift Survey.

The Gravitational Lens CFRS 03.1077

An exquisite gravitational arc with a radius of 21 has been discovered around the z = 0.938 field elliptical galaxy CFRS 03.1077 during HST observations of Canada-France Redshift Survey (CFRS) fields. Spectroscopic observations of the arc show that the redshift of the resolved lensed galaxy is z = 2.941. This gravitational lens source system is well fitted using the position angle and ellipticity derived from the visible matter distribution and an isothermal mass profile with a mass corresponding to σ = 387 ± 5 km s-1. Surprisingly, given the evidence for passive evolution of elliptical galaxies, this is in good agreement with an estimate based on the fundamental plane for z = 0 ellipticals. This, perhaps, indicates that this galaxy has not shared in the significant evolution observed for average elliptical galaxies at z ~ 1. A second elliptical galaxy with similar luminosity from the CFRS survey, CFRS 14.1311 at z = 0.807, is also a lens, but in this case the lens model gives a much smaller mass-to-light ratio; i.e., it appears to confirm the expected evolution. This suggests that this pair of field elliptical galaxies may have very different evolutionary histories, which would be a significant result if confirmed. Clearly, CFRS 03.1077 demonstrates that these Einstein rings are powerful probes of high-redshift galaxies.

The Evolution of Galaxy Morphology for Galaxies in the Canada-France Redshift Survey

The images of 229 galaxies in the Canada France Redshift Survey have been classified on the DDO system. These observations were combined with previous classifications of galaxies with known redshifts in the Hubble Deep Field. The combined sample provides homogeneous morphological classifications for 425 galaxies of known redshift. The fraction of all galaxies that are of types E + S0 + E/S0 appears to remain approximately constant at ~17% over the redshift range 0.25 < z < 1.2. Over the same range the fraction of irregular (Ir) galaxies increases from ~5% to ~12%. Part of this increase may be due to mild luminosity evolution of Ir galaxies. The frequency of mergers is found to rise by a factor of two or three over the redshift range covered by the present survey.

The Metallicity of 0.5 &lt; [CLC][ITAL]z[/ITAL][/CLC] &lt; 1 Field Galaxies

We have measured the emission-line ratios in a sample of 34 Canada-France Redshift Survey star-forming galaxies with redshifts in the range of 0.5 < z < 1.0, and we have computed their metallicities by means of the empirically calibrated R23-metallicity estimator introduced by Pagel et al. The current analysis concentrates on the 15 galaxies with LHβ > 1.2 × 1041 ergs s-1. Although our results can only be regarded as preliminary until near-IR spectroscopy of Hα and [N II] λ6583 are available, the metallicities of these galaxies appear to be remarkably similar to those of local galaxies selected in the same way, and there appears to have been little change in the relationship between metallicity and line and continuum luminosity from z ~ 1 to today. At this stage, our results do not support the idea that these galaxies, known to be generally small and with late-type morphologies, are dwarf galaxies brightened by large bursts of star formation, as had been suggested from previous studies. Rather, our findings are more consistent with a picture in which these systems are the progenitors of today's massive metal-rich galaxies.

Wide‐Field Imaging of the Hubble Deep Field–South Region. II. The Evolution of Galaxy Clustering atz&lt; 1

We present the galaxy-galaxy angular correlations as a function of photometric redshift in a deep, wide galaxy survey centered on the Hubble Deep Field-South (HDF-S). Images were obtained with the Big Throughput Camera on the Blanco 4 m telescope at CTIO, of square degree in broadband uBVRI, reaching ~24 mag. Approximately 40,000 galaxies are detected in the survey. We determine photometric redshifts using galaxy template fitting to the photometry. Monte Carlo simulations show that redshifts from these data should be reliable out to z ~ 1, where the 4000 A break shifts into the I band. The inferred redshift distribution, n(z), shows good agreement with the distribution of galaxies measured in the Hubble Deep Field-North (HDF-N) and the Canada-France Redshift Survey. After assigning galaxies to redshift bins with width Δz = 0.33, we determine the two-point angular correlation function in each bin. We find that the amplitude of the correlation, Aw, drops across the three bins to redshift z ~ 1. Simple models of clustering evolution fit this result, with the best agreement for = 0. Hierarchical cold dark matter models best fit in a low-density, Λ-dominated universe.

The Evolution Of The Star Formation Rate In Galaxies

The aim of this work is to provide clues for the understanding of the star formation history of the universe. We study the process of star formation in galactic objects in hierarchical clustering scenarios using hydrodynamical cosmological simulations of standard Cold Dark Matter Universe. We estimate the specific star formation rate (sSFR) of galactic haloes, which is defined as the SFR per unit luminosity; their luminosities are obtained from the evolutionary synthesis population models of Charlot and Bruzual (1991). To confront the results of the simulations with observations, we consider the sample of field galaxies at high redshift up to z = 1.3 of the Canada-France Redshift Survey (CFRS, Hammer et al.,1997). The sSFR for these galaxies were estimated from their spectral properties. From the confrontation of models and observations, we see that galactic objects formed in hierarchical clustering scenarios seem to reproduce the specific SFR observed at intermediate (0.35 < z < 0.7) and high (0.7 < z < 1.3) redshifts in the CFRS and also their dependence with mass and redshift. Our results suggest that in both simulations and observations, the mechanisms controlling the transformation of gas into stars is more effective at high redshift and in smaller objects producing larger sSFR in low lµ-luminosity galaxies. We find that the sSFR depends on the redshift (Cora et al.,2000, in prep.). Starbursts tidally induced during mergers might be responsible for this trend.

Hubble Space Telescope imaging survey of sub-mJy star-forming galaxies — I. Morphologies at z ∼ 0.2

We present the first results of our Hubble Space TelescopeHST WFPC2 F814W snapshot imaging survey, targeting virtually all sub-mJy decimetric radio-selected star-forming galaxies. The radio selection at ∼1 GHz is free from extinction effects and the radio luminosities are largely unaffected by AGN contamination, making these galaxies ideal tracers of the cosmic star formation history. A subsample of four targets is presented here, selected at 1.4 GHz from the spectroscopically homogenous and complete samples of Benn et al. and Hopkins et al. The redshifts are confined to a narrow range around z∼0.2, to avoid differential evolution, with a radio luminosity close to L* where the galaxies dominate the comoving volume-averaged star formation rate. We find clearly disturbed morphologies resembling those of ultraluminous infrared galaxies, indicating that galaxy interactions may be the dominant mechanism for triggering star formation at these epochs. The morphologies are also clearly different from those of coeval quasars and radio galaxies, as found in star-forming galaxies selected at other wavelengths. This may prove challenging for models that propose direct causal links between AGN evolution and the cosmic star formation history at these epochs. The asymmetries are typically much larger than seen in the Canada—France Redshift Survey at similar redshifts, optical luminosities and Hα -derived star formation rates, indicating the possible existence of an obscuration-related morphological bias in such samples.

Spectral analysis of the Stromlo-APM Survey -- I. Spectral properties of galaxies

We analyse spectral properties of 1671 galaxies from the Stromlo—APM Survey, selected to have 15 ≤ bJ ≤ 17.15 and having a mean redshift z = 0.05. This is a representative local sample of field galaxies, so the global properties of the galaxy population provide a comparative point for analysis of more distant surveys. We measure Hα, [O ii] λ3727, [S ii] λλ6716, 6731, [N ii] λ6583 and [O i] λ6300 equivalent widths and the D4000 break index. The 5-Å-resolution spectra use an 8-arcsec slit, which typically covers 40-50 per cent of the galaxy area. We find no evidence for systematic trends depending on the fraction of galaxy covered by the slit, and further analysis suggests that our spectra are representative of integrated galaxy spectra. We classify spectra according to their Hα emission, which is closely related to massive star formation. Overall we find that 61 per cent of galaxies are Hα emitters with rest-frame equivalent widths EW(Hα) ≳2 Å. The emission-line galaxy (ELG) fraction is smaller than seen in the Canada—France Redshift Survey (CFRS) at z = 0.2, and is consistent with a rapid evolution of Hα luminosity density. The ELG fraction and EW(Hα) increase at fainter absolute magnitudes, smaller projected area and smaller D4000. In the local Universe, faint, small galaxies are dominated by star formation activity, while bright, large galaxies are more quiescent. This picture of the local Universe is quite different from that of the distant one; bright galaxies appear to show rapidly increasing activity as one moves further back in time. We find that the ratio [N ii] λ6583/Hα is anticorrelated with EW(Hα), and that the value of 0.5 commonly used to remove the [N ii] contribution from blended Hα+[N ii] λλ6548, 6583 applies only for samples with an EW distribution similar to that seen at low redshift. We show that the [O ii], [N ii], [S ii] and Hα EWs are correlated, but with large dispersions (∼50 per cent) owing to the diversity of galaxy contents sampled. Our [O ii]-Hα relation is similar to the one derived by Kennicutt, but is 10 per cent higher at 1σ significance. We show that this relation is not valid for distant, strong [O ii] emitters with blue colours, which are more numerous than locally. This relation would overestimate the individual star formation rate by ∼50 per cent for these kinds of galaxies. We find that, on average, luminous blue ELGs are likely to be enhanced in nitrogen abundance. This suggests that in faint, low-mass, late-type ELGs nitrogen is a primary element, whereas in brighter, more massive galaxies nitrogen comes from a secondary source. We also find that 4 per cent of early-type galaxies show star formation activity; this fraction seems to increase at higher redshifts.

Spectral energy distribution of submillimeter sources

The submillimeter universe has now been explored with the Submillimeter Common User Bolometer Array (SCUBA) camera on the James Clerk Maxwell Telescope, and claims have been made to the presence of a new population of optically unidentified starforming dusty galaxies at high redshifts. An alternative interpretation of the submillimeter (submm) emission from these sources, however, is that their far infrared emission is due to the presence of an active galactic nucleus (AGN) enshrouded by dust. Using examples of submm sources, selected primarily due to the availability of multiwavelength observations, we investigate whether the spectral energy distribution (SED) selected sources is consistent with either the dusty starforming or dusty AGN scenarios. Among these sources, we find that the SED of HDF850.4, one of the sources detected in the Hubble Deep Field (HDF), and CFRS14F, one of the sources in the Canada France Redshift Survey Fields, are fully consistent with the dusty starformation hypothesis, while the SED of APM 08279+5255 is consistent with a dusty AGN. For some submm sources, it is suggested that observations at rest wavelengths between ∼1 μm and 100 μm may be needed to differentiate between the dusty starformation and dusty AGN scenarios. We discuss implications of our results for the interpretation of submm sources and suggest that dusty AGNs can contribute up to ∼30% of the submm source counts. However, an exact observational determination on the ratio of dusty star-forming to dusty AGN contribution to submm counts require detailed followup observations of a flux limited source catalog, beyond what is currently available.