Gopal-Krishna1 - C. Ledoux2 - J. Melnick2 - E. Giraud3 - V. Kulkarni1 - B. Altieri4
1 - National Centre for Radio Astrophysics, TIFR, Post Bag 3, Ganesh Khind, Pune 411 007, India
2 - European Southern Observatory, Alonso de Córdova 3107, Casilla 19001, Vitacura, Santiago, Chile
3 - GAM, Univ. Montpellier II, Place E. Bataillon, 34095 Montpellier Cedex, France
4 - European Space Agency, Villafranca del Castillo, Apartado 50727, 28080 Madrid, Spain
Received 12 January 2005 / Accepted 22 February 2005
We present the results of radio (VLA) and optical (ESO/La Silla) imaging of a sample of 52 radio sources having an ultra-steep radio spectrum with mostly steeper than -1.1 at decimetre wavelengths (median ). Radio-optical overlays are presented to an astrometric accuracy of . For 41 of the sources, radio spectral indices are newly determined using unpublished observations made with the 100-m Effelsberg radio telescope. For 14 of the sources identified with relatively brighter optical counterparts, spectroscopic observations were also carried out at La Silla and their redshifts are found to lie in the range 0.4 to 2.6. These observations have revealed three distant clusters of galaxies with redshifts of 0.55, 0.75 and 0.79, and we suggest that, together with an ultra-steep radio spectrum and relaxed radio morphology, the presence of a LINER spectrum in the optical can be used as a powerful indicator of rich clusters of galaxies. Additional candidates of this type in our sample are pointed out. Also, sources exhibiting particularly interesting radio-optical morphological relationships are highlighted. We further note the presence of six sources in our sample for which the optical counterpart (either detected or undetected) is fainter than and the radio extent is small (< ). These ultra-steep spectrum radio sources are good signposts for discovering massive galaxies out to very large redshifts.
Key words: cosmology: observations - galaxies: active - cooling flows - galaxies: clusters: general - quasars: general - radio continuum: galaxies
The discovery of a correlation between the output of radio galaxies in optical emission lines and the radio band (e.g., McCarthy 1993; Rawlings & Saunders 1991) led to the recognition of using radio source samples as a means to detect galaxies located at very large distances. Indeed, radio surveys backed up with optical spectroscopy provided the first detections of objects beyond a redshift of two (e.g., Spinrad 1986). A major technical breakthrough came with the demonstration of a statistical correlation between radio spectral index and distance (McCarthy et al. 1987; Chambers et al. 1987), as already hinted in some earlier studies (e.g., Blumenthal & Miley 1979; Gopal-Krishna & Steppe 1981; Gopal-Krishna et al. 1980; Tielens et al. 1979). This correlation is now understood to be largely the result of a radio K-correction operating on the integrated spectrum which usually has a downward curvature arising from radiative losses (Laing & Peacock 1980). Consequently, the radio spectrum in a given frequency range appears increasingly steeper with redshift. The most distant radio galaxy found so far using the clue of ultra-steep radio spectrum has a redshift z=5.2 (Venemans et al. 2004).
The spectacular success of using ultra-steep spectrum radio sources (hereafter USSRS) to discover high-z galaxies, from mid-80s to early 90s, later faced a stiff competition from a purely optical technique which involved multi-color photometry (Steidel et al. 1996). Nonetheless, the radio technique continues to offer at least two unique advantages. Firstly, the high-z galaxies thus found belong to the most massive stellar systems existing at their redshifts; this is particularly relevant for cosmological theories of structure formation (e.g., De Breuck et al. 2002; Lilly & Longair 1984). Secondly, an eventual discovery of radio sources beyond z=6 would provide a direct probe of the "re-ionization era'', via H I spectroscopy (e.g., Djorgovski et al. 2001). In this context, finding USSRS whose optical counterparts remain undetected down to fairly deep magnitudes ("empty fields'') is particularly important. Moreover, from the astrophysical perspective, it is also of great interest to detect optical emission associated with the lobes and hot spots of radio galaxies at high redshifts where inverse Compton losses against the intense cosmic microwave background would severely limit the range of the electrons capable of optical synchrotron radiation (e.g., Gopal-Krishna et al. 2001; Brunetti et al. 2003).
In this paper, we present radio and optical imaging data for a sample of 52 USSRS (Sects. 2 and 3). Redshift measurements for the brighter subset of 14 sources are also presented. The radio maps are based on our Very Large Array (VLA) observations while the optical data were obtained using the telescopes of the European Southern Observatory (ESO) at La Silla.
The present sample of 52 USSRS with (but mostly <-1.1; see Table 1) is an assortment derived from the following four data-sets:
|Figure 1: Radio-optical overlays. The insets show the optical counterparts without radio contour. The radio contours are based on VLA snapshots taken at 4.8 GHz.|
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For all sources in our sample, except those few for which VLA maps at 4.8 GHz (A-array) were available in Bennett et al. (1986), we took five to ten minutes snapshots using the VLA at 4.8 GHz. The majority of these observations were made in the BnC hybrid configuration while the DnC array was used in the remaining cases. The data restoration was done using the AIPS package (details will be given in Kulkarni et al. 2005, in prep.). The derived radio images are shown as overlays in Fig. 1 while the main structural parameters are given in Table 1.
The optical observations were taken during several observing runs between 1989 and 1996 using telescopes at La Silla. The bulk of the data was obtained in the period 1989-1991 using EFOSC 1 at the La Silla 3.6 m telescope. Standard Bessel filters were used at all observing runs, and both imaging and spectroscopy were performed using EFOSC-type instruments. The characteristics of these instruments is summarized in Tables 2 and 3, that condense the relevant information from successive generations of User's Manuals. The data reduction was performed using MIDAS, the data reduction and analysis software developed at ESO. Most observations were taken under reasonably good seeing conditions (at least by the standards of those days) ranging between one and , with little or no moon illumination. However, the observations of the sources 0001+058, 2232-062, 2236-039 and 2245-037 were made on a bright moonlit night with rather poor seeing conditions ( ). Consequently, the detection limit attained for these fields is only around . The optical and radio images are shown in Fig. 1 when not already published elsewhere.
We emphasize that, by itself, the astrometric error class can be treated as the principal reliability indicator of optical identification only provided the radio source is compact (smaller than ), or it contains a compact central radio component. For extended radio sources larger than and lacking a central component, offsets of up to 25% of the size of the radio source, between the optical identification and the radio centroid, can be accepted.
Direct imaging observations were not always obtained under photometric conditions, although, due to the faintness of the sources, rather good transparency was always required. Not all observations, therefore, were photometrically calibrated. On photometric nights, calibrations were done using several stars from the Landolt (1992) CCD fields. Given the excellent photometric stability of the EFOSC instruments, we were able to determine the magnitudes of the objects observed under non-photometric conditions using the standard zero points given in the User's Manuals. Due to the unknown atmospheric extinction, however, these magnitudes carry rather large observational errors (in some cases even up to 0.5 mag).
To determine the magnitudes of the optical counterparts of the radio sources, we followed two procedures. For most objects, we carried out circular aperture photometry with adequate aperture size (using the MIDAS software) whereas for objects located in crowded/confused regions we additionally employed the SExtractor algorithm (Bertin & Arnouts 1996) to deblend the different close-by objects. Each of these procedures gives a magnitude estimate with an associated error. These values are reflected in the estimates given in Table 1. It should be stressed that additional uncertainties can be expected due to systematic effects, mainly non-photometric conditions (see above). For blank fields, a lower limit to the magnitude was determined by circular aperture photometry taking the radius to be equal to the mean FWHM of several stars on the same CCD frame.
Whenever optical identification could be established and weather conditions allowed, we used the available telescope time to obtain spectra of the sources using the identification images to place the slit on the objects. Otherwise, we performed the source identifications off-line, and carried out spectroscopy subsequently on any available nights. In spite of considerable efforts, however, the number of sources for which we were able to obtain redshifts remains rather small and for several sources the spectroscopy did not yield redshift (these sources are marked with an asterisk in the last column of Table 1). Plausibly, this is due to the fact that, due to the high read-out noise and low blue sensitivity of the CCDs of those days, our ability to detect Ly emission in the range 1.8<z<2.3 was rather limited, while powerful radio sources tend to cluster at redshifts between one and two (see, e.g., Condon 2003).
The optical identification is a fuzzy object close to the detection limit situated between the two radio lobes. An object of R=22.7 0.3 is located about to the SW of the western lobe. Another close-by, much fainter object (R=23.2 0.4) is seen about North of the eastern lobe.
It cannot be entirely excluded that the optical identification of this radio source is not located on a bad CCD column. For this reason, the source remains undetected down to R=23.
This is a clear empty field since no optical object is visible within a radius of from the radio source.
The optical field without radio contours is shown in the inset of the overlay in Fig. 1. A faint object with R=21.9 0.1 seen at the centre of the inset, which coincides with the mid-point of the two radio lobes, is a likely identification. Another candidate is the brighter object with R=20.8 0.1 situated NW of the radio centre.
This radio source is located in a clear empty field.
The R-band overlay image shown in Fig. 1 has been taken with EFOSC 2 at the NTT which has a higher resolution than the B-band image used for the photometry (see Table 1). The optical identification, more clearly seen in the inset of the overlay, is a QSO with redshift z=2.53based on prominent broad Ly and C IV lines.
The hint of a wide-angle-tail radio morphology signifies the presence of a cluster. Interestingly, the optical counterpart is a QSO with redshift z=1.12based on several emission lines visible in our spectra. About to the SW of the quasar, a foreground elliptical galaxy is seen for which we find a redshift z=0.27. The R-band image shown in Fig. 1 has been taken with EFOSC 2 at the NTT which has a higher resolution than the R-band image used for the photometry.
The R-band overlay image shown in Fig. 1 resulted from a 30 min exposure taken with EFOSC 2 at the NTT which has a higher resolution than the R-band image used for the photometry (see Table 1). The optical ID lies closer to the eastern lobe of the radio source. It has a fuzzy appearance, roughly extended along the radio axis. It has a northward extension of , which is also seen on the EFOSC 1 image used for the photometry. It could possibly be a chain of fainter galaxies, or even a tidal tail resulting from a merger event.
Our results for this source are published in Giraud et al. (1996a). It is identified with the dominant galaxy of a z=0.79 cluster. A remarkable optical extended emission-line region (EELR) of low excitation and size, 100 kpc, was found associated with the radio galaxy. The EELR bears a close morphological relationship to the radio lobes which exhibit Z-shaped symmetry. The EELR appears to consists of three cones associated with the radio galaxy and a neighbouring galaxy also located at the same redshift and connected to the former by a long stellar filament.
The optical counterpart is clearly extended along the two radio lobes (see inset in Fig. 1). Our low-dispersion spectra yielded a redshift z=1.12 based on three (narrow?) emission lines (C III] 1909, Mg II 2800, [O II] 3727).
An almost point-like object lies at the position of this barely resolved radio source.
Only a pointing optical image (3 min exposure) is available for this small diameter radio source which remains unidentified down to R=22.7. A deeper image is needed.
The optical counterpart is a diffuse object with two peaks separated by along the radio axis.
The optical counterpart of this marginally resolved radio source is not visible in our 3 min pointing image. A deeper image is needed for this source.
A galaxy of R=22.3 0.2 which is seen at the centre of the overlay is situated roughly midway between the two widely separated radio "lobes''. This is a possible identification. However, since the southern lobe is itself resolved into two components, it is possible that the two lobes are in fact independent radio sources. In that event, their optical counterparts are undetected down to R=23.7.
This barely resolved radio source is a clear empty field located towards the Galactic anticentre ( ).
Our results for this source are published in Gopal-Krishna et al. (1995). A giant cloud of Ly emission at z=2.468 extended over 100 kpc is associated with the southern radio lobe. The equivalent width of Ly is exceptionally large (1000 Å in the rest-frame) and the line profile indicates expansion at a velocity of 550 km s-1, probably driven by the radio lobe from within. The Ly emission shows a sharp cut-off near the radio galaxy indicative of a dusty disc around the galaxy, oriented roughly perpendicular to the radio axis.
The only optical object detected between the two radio lobes is a galaxy at , (J 2000).
The optical counterpart of this small diameter ( ) radio source is below the detection limit in our image, R>23.3. There is a bright object about West of the radio position.
Since our VLA map shows the size of this source to be , its flux density given in the Molonglo Reference Catalogue at 408 MHz (MRC does not give integrated flux) might be significantly underestimated. Hence we have computed the spectral index using the flux densities estimated from the NVSS map (353 mJy at 1.4 GHz) and the Parkes survey (87 mJy at 4.85 GHz; Griffith et al. 1995). We further note that our VLA map shows a prominent radio core in this triple source, which contributes 9 mJy at 4.85 GHz.
The optical identification is a bright, narrow emission-line radio galaxy (NELG) at a redshift z=0.618. A close inspection of the image shows that the optical counterpart could be a tight chain of three objects. The southern radio hot spot coincides with a point-like optical object with R=19.7 0.1.
The optical identification lies closer to the northern radio lobe and is seen more clearly near the centre of the inset. After 4 h of integration, the optical spectrum shows two narrow faint emission lines that we tentatively identify with Mg II 2800 and [O II] 3727 at z=0.71. Note also that a faint optical object is coincident with the outer edge of the southern radio lobe.
The optical ID is a galaxy that coincides with the central radio component and is elongated along the radio axis. A relatively bright star is located to the SE of the galaxy.
The optical ID of this radio source is a point-like object with R=22.0 0.1.
The best candidate for optical identification is a R=22.6 0.1galaxy situated at , (J 2000). It could be the dominant member of a distant cluster. Our 2 h spectrum of this galaxy shows a strong [O II] 3727 emission line and a rich Balmer absorption spectrum, reminiscent of other cluster sources (Melnick et al. 1997). Unfortunately, our B 300 spectra cuts off just where we would expect to see the [O III] emission lines at z=0.42, so we cannot be certain of the LINER nature of this source.
The identified optical structure consists of a R=23.5 0.3 point-like object and a faint, long wisp to SW coincident with the southern radio lobe and elongated along the radio axis. The entire structure has .
The radio structure is poorly resolved in our low-resolution VLA map. The only object within of the radio peak is a fuzzy object situated at , (J 2000).
The likely optical counterpart is a point-like object located NW of a bright star.
The optical counterpart of this slightly resolved ( ) radio source is a R=22.8 0.4 object. A fainter object is also seen NE of it.
The central radio component lies within of a pair of optical objects which are themselves separated by and have an integrated R=22.7 0.3. One of these objects is the likely optical ID.
An object of R=24.4 0.2 is seen NE of the central component of this extended radio source. The astrometry has been independently checked using a 10 min exposure taken with EFOSC 1 at the ESO 3.6 m telescope. This image is less deep but includes five reference stars and confirms the astrometry as shown in the overlay.
The likely optical ID is close to the north-eastern radio lobe and appears to be extended.
An optical object is seen about offset from the radio peak towards NE. Within the astrometric uncertainties for this case, this object is the likely identification. Faint and narrow emission lines of [C II] 2326 and Mg II 2800 are visible in our 2 h spectrum of this object indicating that the radio source is probably a NELG at a redshift z=0.98.
Our results for this source are published in Gopal-Krishna et al. (1992). An EELR of size 100 kpc and z=0.477 is associated with this double radio source. Its [O II] 3727 emission line has a very large rest-frame equivalent width (350 Å), consistent with the high radio luminosity. Intriguingly, we found that its optical spectrum is ultra-soft, with [O II] 3727/[O III] 5007 = 10, nearly 30 times the typical value for distant 3CR radio galaxies (van Breugel & McCarthy 1990). Together with Hydra A, this source thus provides an outstanding example of extremely low-ionization optical emission-line spectra being associated with powerful radio galaxies (Melnick et al. 1997). Unfortunately, the object is seen very close to a bright star, so it is difficult to say whether it is a cluster source like Hydra A.
Deep multi-color optical observations of this source have been reported by us (Giraud et al. 1996b). The source was identified with a z=0.753 elliptical galaxy in the process of formation by accreting material from neighbouring galaxies, all members of a group. The radio source has a central core and two relatively relaxed lobes consistent with its being in a cluster.
The peak of this unresolved radio source lies just from a pair of bright stars. No optical identification is visible above a detection limit of . The radio flux measurement at 2.7 GHz (Effelsberg) contains a significant contribution from a double radio source located to the NW of 1509-158. Due to this, the true spectral index of this source is probably even steeper than the -1.24 value given in Table 1. The parameters of the confusing source, estimated from our VLA observations, are , (J 2000), with a flux density equal to 36 mJy at 5 GHz (Kulkarni et al. 2005, in prep.).
The radio map of this triple source is taken from the VLA 5 GHz observations by Bennett et al. (1986). The redshift of the optical counterpart, z=0.93, was derived from a 3 h low-dispersion (B 1000) spectrum taken with EFOSC 1. The spectrum shows prominent, probably narrow, Mg II and [O II] emission lines. The presence of H+K in absorption and the lack of strong [O III] emission lines indicate that this object may be a LINER.
The optical counterpart of this unresolved radio source is quite bright and point-like. The blue spectrum of this object obtained from 2 h integration with EFOSC 1 is typical of an elliptical galaxy at z=0.31. The lack of emission lines and the compactness of the object, however, are rather puzzling.
Although this object has a large radio size, its optical identification remains uncertain. The nearest object to the radio centre is a R=21.6 0.2point-like object, which we suggest as the possible identification. However, it is offset by to the South of the faint radio peak seen between the two radio lobes. In addition to the R-band photometry, we also took a 15 min V-band exposure using EFOSC 1 from which we get V=21.2 0.1 and V-R=-0.4assuming that the object did not vary over the 11 months time interval between the two observations. If a higher resolution radio map confirms the faint, central peak to be the radio nucleus then its separation from the suggested optical ID would be four times the rms astrometric error. Any other optical ID would have to be below the detection limit of the image (for a point source).
The optical counterpart of this radio source is a point-like object.
The source lies in a crowded field close to the Galactic plane (latitude ). The only detected object within is a R=25.5 0.4 object located about North of the radio peak. This is the likely optical ID for this unresolved radio source.
This compact steep spectrum radio source lies in a clear empty field.
This extended radio source lies in a crowded field close to the Galactic plane (latitude ). The inset shows five bright objects near the mid-point between the two radio lobes. Of these, the western-most object, which is slightly extended, is the most likely optical identification.
The optical counterpart of this compact steep spectrum radio source is undetected down to a very deep level (R=25.6).
There is a hint of a central radio component. The NS extended optical object is offset from this component by to the North, which is within the astrometric error, and is therefore the likely optical counterpart.
The suggested optical ID partially overlaps with the image of a 1.6 mag brighter elliptical galaxy located about to the South. A 2 h spectrum obtained with EFOSC 2 at the ESO/MPI 2.2 m telescope shows a rich emission-line spectrum at z=0.92. The adjacent elliptical galaxy is at z=0.6.
The best candidate appears to be the slightly extended object located about West of the radio position. A very faint fuzz is coincident with the radio peak.
The spectral index of this radio source has a large uncertainty of 0.2(see Sect. 2.1). No object is seen within the radio contours above the detection limit of the image ().
A tight clustering of bright stars is seen within West of this radio source. This considerably degrades the quality of detection of the faint ( ) optical counterpart observed between the two radio lobes.
This compact steep-spectrum radio source coincides with a barely detected fuzz with situated in a crowded field.
The results of our detailed optical observations of this source have been reported by Melnick et al. (1993). The distorted double radio source is identified with a z=0.552 cD galaxy which is member of a cluster (Kulkarni et al. 2005, in prep.). Between this galaxy and another similarly bright galaxy to the NE, which is a member of the same cluster, an unusually bright and straight arc-like feature was found by Melnick et al. (1993). We interpreted it as the image of a pair of merging galaxies at z=1.116, highly magnified due to gravitational lensing by the foreground cluster (see also Kneib et al. 1994).
On the assumption that the two radio components of this source are physically associated, the most likely optical identification is the bright (R=20.8 0.1) object roughly located midway between them. On the other hand, it is quite possible that the two radio components are independent radio sources (particularly since the western component is itself a double). In that case, the optical features detected towards each of them would be their likely optical IDs. The magnitudes of these objects are R=22.7 0.2 for the western lobe and R=20.8 0.1 for the source close the eastern lobe.
More sensitive radio imaging is needed to confirm if this source is a wide-angle tail. The likely optical identification is coincident with the radio peak.
We have presented a catalogue of 52 powerful radio sources with very steep spectra at decimetre wavelengths (median ). For 15% of the sources, the observing conditions were poor or the optical fields are confused, so even if the optical counterparts may not be very faint, they could not be identified with the available material. Out of the 41 (i.e., 80%) identified sources, in spite of considerable efforts, we were also able to obtain redshifts for only about a third of them. This is most likely due to the fact that the CCDs of those days had rather high read-out noise and poor blue sensitivity. We are confident that using modern detectors on the same telescopes we should be able to do a lot better.
Our sample is found to contain six USSRS that are not only optically very faint () but also have small radio sizes (< ). These sources, namely, 0001+058, 0634-196, 1509-158, 1631-222, 2011-169 and 2105-119, are prime candidates to be at very large redshifts, and indeed, we plan to make a new round of optical identifications to complete the catalogue.
Finally, we note that, even though optical spectroscopy has been completed for only about a quarter of the sample (14 sources), already this small subset is found to contain two powerful ultra-steep spectrum radio sources having LINER-type ultra-soft emission-line spectra (0410-198 and 1411-192 at, respectively, z=0.79 and 0.48) plus at least one good candidate (1523-017 at z=0.93). Another three candidates, namely, 0423-199 (z=1.12), 1146+052 (z=0.42) and 2057-179 (z=0.92), show strong [O II] emission but since their existing spectra do not extend to the region of the [O III] line, spectral softness based on the strengths of these two emission lines remains to be confirmed. Our observations indicate that the former two sources are associated with distant () galaxy clusters having a dense intra-cluster medium, as is the case for the nearby massive cooling flow cluster Hydra A (Melnick et al. 1997; also, McNamara 1995). Thus, together with an ultra-steep radio spectrum and relaxed radio morphology, the presence of a LINER spectrum in the optical can be used as a powerful indicator of high-z clusters which are not merely concentrations of galaxies, but have acquired an intra-cluster medium dense enough to sustain cooling flow activity.
G.K. thanks ESO for the hospitality in Chile during a visit when this project was started. JM thanks NCRA, Pune, for their hospitality when this project was finished. We also thank Samir Dhurde and Alok Gupta for their invaluable help with data handling. This work made use of the MIDAS and AIPS data reduction and analysis packages as well as the NASA/IPAC extragalactic database (NED). VLA is operated by Associated Universities, Inc., under contract with the National Science Foundation.
Table 1: Overall properties of the sample galaxies.
Table 2: Optical imaging observations.
Table 3: Optical spectroscopy observations.
|Figure 1: Radio-optical overlays. The insets show the optical counterparts without radio contour. The radio contours are based on VLA snapshots taken at 4.8 GHz.|
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