Volume 566, June 2014
|Number of page(s)||21|
|Section||Cosmology (including clusters of galaxies)|
|Published online||05 June 2014|
The catalogue presented in this paper contains 14 EW values that are among the most prominent emission and absorption lines in the optical range (see Table 1), as measured in the rest-frame observed spectra. Errorbars on the measures, taking into account both the spectral S/N and the measurement method, are also provided with the details of their calculation being explained in Sect. 3.1. These values, inparticular those of the [Oii] and Hδ, are used to derive a rough spectral classification, which is included in the catalogue in the form of a numerical flag. Emission-line galaxies are labelled with 1, 2, and 3 for e(a), e(b) and e(c), respectively, while the “non-starforming” types are flagged with 4, 5, and 6 (k, k+a, and a+k respectively). Where no classification was possible, we flag the spectrum with a 0. In Fig. A.1, we report the number of spectra in which each line was successfully measured, for the entire sample and distinguish those that were measured in emission (red histograms).
Non-detections and noise-dominated lines are flagged with a 99.00, while the value 999.00 is used to identify those lines that lie outside the observed range. We also measure two classical, widely-used, indices: D(4000) (Bruzual A. 1983) and Dn(4000) (Balogh et al. 1999). As already mentioned, whenever a [Oii] and [Oiii] EW has a value, which is higher than −2 Å, we treat this as a non-reliable detection, which is flagged by a 0.000 value. In the catalogue we are also reporting the values of the magnitude and radial completeness (C(m) and C(r)). Table A.1 shows an example of how the catalogue looks like. Column 1 reports the wings name, based on the galaxy’s coordinate; Cols. 2 to 15 report the equivalent width values expressed in Å; Cols. 16 to 29 report the uncertainties, following the same order as the EW values; Cols. 30 and 31 contain the spectral indexes D4000 and Dn4000, respectively; in Col. 32, we give the spectral class flag and in Cols. 33 and 34, we provide the photometric and geometrical completeness, respectively.
Since the measurements of lines shortwards of ~4300 Å is not reliable in most of them for [Oii], Hδ, and the two calcium lines (Cak and Cah+Hϵ), for the spectra of the north sample we
give the values that were manually measured, while values for the other lines that are longwards of 4300 Å are taken from the automatic measurement.
The presence of sky lines, which are not always properly subtracted from observed spectra, might also influence both the detection and the lines’ measurement. If we consider the most prominent of such lines, or those at 5577, 5894, 6300, and 7246 Å, we find that only the Mg and the Na lines might fall close to these sky features, as far as wings cluster members are concerned especially in the higher redshift clusters. None of the other lines are instead affected, but we strongly suggest a visual check before using the values for these two lines. Similarly, a visual check should be performed in the spectra of higher redshift, foreground galaxies in our samples. For example, spectra of galaxies at z ~ 0.108, have both Hα and the [Oiii] lines critically close to such sky lines.
Statistics on the line measurement. The histograms show the number of spectra for which a given line was properly measured. Red, dashed-line histograms refer to lines in emission (EW < 0).
|Open with DEXTER|
Example of the catalogue as it is made publicly available.
The parent catalogue from which spectroscopic targets were selected has been generated by adopting the following selection criteria:
Vtot < 20
Vfib < 21.5
(B − V)5 kpc ≲ 1.4
where Vfib is the V-band magnitude in an aperture matching that of the spectroscopic fiber, Vtot is the total V magnitude, and (B − V)5 kpc is the colour computed from a 5 kpc aperture.
The exact cut in the colour–magnitude diagram varied slightly from cluster to cluster due to the small differences in cluster redshift and to minimise the level of contamination from the background. In a few cases, the cut has purposely included with a secondary red sequence, such as for Abell 151, to be able to study also background clusters. To optimize the observational setup in a few cases, galaxies at fainter magnitudes or larger colours have been observed. These loose selection limits were applied to avoid any bias in the observed galaxy type, as is the case of a selection based on the colour-magnitude relation only (which selects only red, early type galaxies).
We computed magnitude and geometrical completeness from which we define a specific weight for each galaxy in the catalogue defined as (B.1)where C(m)i and C(r)i are the magnitude and geometrical completeness in the opportune radial and V-band magnitude bin.
The completeness as a function of magnitude is defined as (B.2)where Nz is the number of galaxies with measured redshifts and Nph is the number of galaxies in the parent photometric catalogue. Completeness is usually a decreasing function of the magnitude because priority was given to brighter objects in observations.
The success rate, which is the fraction of galaxies with redshift determination with respect to the total number of observed galaxies, is similarly defined as (B.3)where Nz is defined as in Eq. (B.2) and Ntg is the number of target galaxies we actually observed. Besides that, we also computed the radial completeness for our sample. It is known that fiber collision problems can lead to a variable density of observed sources at different radii, given that it is more difficult to allocate many fibers near the crowded cluster centre. On the other hand, central parts of the clusters are usually privileged due
to the higher density of galaxies, and observers tend to allocate as many fibers as possible there. Having several fibre configurations for a given cluster, as in our case, helps mitigating the fibre collision problem even further. The net result in our case is pretty flat behaviour of the defined radial completeness function defined, which is analogous to the magnitude completeness: (B.4)with Nz and Nph defined as in Eq. (B.2) but for radial bins.
Due to the absence of calibration lines in the UV, several spectra taken at WHT (the “north sample”) suffer from a wavelength calibration issue affecting the wavelength range shortwards of ~4300 Å. Hence, it is often not possible to automatically recognize and properly measure a spectral line in this wavelength range due to a displacement, which can in few extreme cases reach some tens of Å in the case of the [Oii] line (which is the bluest line that we measure). Since this displacement is not only wavelength dependent but it may also vary from spectrum to spectrum, it is not possible to automatically correct for this effect in a straightforward way. It is not always possible to properly measure some of the lines that typically characterize the stellar populations. This makes the automatic spectral fitting, that was the main reason for having a reliable EW measure (Fritz et al. 2007, 2011), meaningless in some cases.
To recover as much information as possible and use as many spectra as we could from the north sample, we proceeded as follows. After running our fitting procedure over the entire north sample, we measured the EW of the four most prominent UV lines, namely [Oii], the two calcium lines (h and k; 3969 and 3934 Å, respectively), and Hδ. This was done manually for each of these four lines and for each spectrum displaying the calibration problem. Due to these issues, care must be taken when judging the quality of a fit by means of the χ2 value only. We recalculate χ2 by only considering spectral features (i.e. lines and continuum emission) longwards of 4300 Å and rejected all the spectra with a . On the remaining spectra, we checked that the values, which we have manually measured were compatible with those of the best fit model. When this was the case, the fit with the derived physical quantities (e.g. stellar mass, star formation rate, etc.) was considered to be acceptable, and the spectrum considered in the analysis.
After the selection procedure described above, some of the clusters from the north sample were very poorly sampled. They only had a few galaxies that were spectroscopically confirmed members (Cava et al. 2009). We decided not to include those clusters with a number of confirmed members less than 20 in all of the statistical studies that will follow in the paper. This leaves us with only seven clusters of the north sample, which still contains more than 50% of the usable spectra from this catalogue.
© ESO, 2014
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