Spectral index analysis

The star-formation histories of individual cluster members can be constrained through a spectral index analysis (e.g. Balogh et al. (1999)). VIMOS spectra withS/N >4 allowed us to robustly determine the strength of the 4000˚A break (D4000), and the equivalent widths of the Hδphotospheric absorption line (W0(Hδ) and the [OII] neb- ular emission line (W0([OII])). In particular, D4000 is particularly sensitive to the

presence of old (i.e., ≥ 1–2 Gyr), passively evolving stellar populations; W0(Hδ) is sensitive to episodes of star formation in the last Gyr by measuring the presence of A-type stars; W0([OII]) gives an estimate of the present SFR (i.e., on timescales

of ≤ 0.01 Gyr). The combined use of the diagnostic diagrams W0(Hδ)–W0([OII])

andW0(Hδ)–D4000 limits misinterpretations arising from the absence of corrections for intrinsic extinction by dust or additional line emission affecting W0([OII]) and

W0(Hδ), respectively.

Figure 3.10 reproduces the behaviour of each spectral index as a function of the observed B−R colour (uncorrected for internal extinction) for the spectroscopic cluster member galaxies. In this section, all plots will have the same colour-shape coding, chosen to underline different types of galaxies in each cluster and to allow direct comparison between the two clusters and the different diagrams. Thus, blue triangles will be used to mark faint galaxies, red dots for the bright ones.

A clear trend with colour can be clearly seen in both clusters for theW0([OII]) and

D4000 spectral indices. Values ofEW(OII) decrease with increasing values of (B-R). This is consistent with a change from actively star-forming (blue) galaxies to passively evolving spectral types. For (B-R)<1.5, EW(OII) is almost stable beyond 0 with a clear increase towards large values for very blue colours ((B-R)<1.2), confirming star-formation activity; on the other hand, at (B-R) ∼ 2 its mean value is of order 0, typical of passive systems. Conversely, D4000 intensity steadily increases from blue to red colours, as expected if moving from star-formation systems to old stellar populations. For (B-R)∼2,D4000 is significantly beyond 1.5 and increasing rapidly towards redder colours. Although with a somewhat larger scatter, a feeble trend can also be detected inW0(Hδ) from higher values (i.e., more absorbed) for bluer objects to lower or negative values for redder objects. This could be due to contamination from nebular emission, which we did not take into account in the calculation of our spectral indices.

We empirically determine threshold values for the three spectral indices, which will be used to discriminate different star-formation regimes, by combining colour and spectroscopy information. Specifically, mean values for EW(OII), D4000 and W0(Hδ) are calculated from red-sequence objects, as representative of passively- evolving systems; objects are then defined as passively evolving if they lie within 3σ from the mean. We thus define objects as passively evolving if they have [OII] equivalent widths lower than 4.85 in the [EW(OII),W0(Hδ)] plane, or if they have

Figure 3.10: Distribution of spectral indices against galaxy (B-R) colour for RXCJ0014.3-3022 (left column) and for RXCJ2308.3-0211 (right column). The notations are explained in Sect. 5.2. The values ofEW(OII) and D4000 are found to be well correlated with galaxy colours, showing a steady increase of D4000 for redder systems and a decrease inEW(OII). A less evident trend of W0(Hδ) can

still be noticed.

D4000 in excess of 1.4 in the [D4000,W0(Hδ)] plane. This approach is similar to the definition found in Barger et al. ((1996)) and Balogh et al. ((1999)), but adapted to our specific case to better describe galaxies based on the definition of red sequence, i.e. the locus of passively evolving systems at the clusters redshift. The values found are comparable to those from Balogh et al. ((1999)), or Dressler et al. ((1999)), who similarly tailored the thresholds to their data-sets. For theW0(Hδ) index we assume instead the same definitions as previously applied: a lower limit of 0 ˚Aand an upper limit of 5 ˚A to discriminate between different regimes in star-forming galaxies. It is

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Table 3.7: Definitions in the [W0(OII),W0(Hδ)] plane

Definition W0(OII) W0(Hδ) PEV (passive) <4.85 ≤5 SF (star-forming) ≥4.85 ∈[0, 5] SSB (short starburst) ≥4.85 <0

Table 3.8: Definitions in the [D4000,W0(Hδ)] plane

Definition D4000 W0(Hδ) PEV (passive) ≥1.4 ≤5 SF (star-forming) <1.4 ∈[0, 5] SSB (short starburst) <1.4 <0

worth noting that our thresholds are in good agreement with ones in the literature, i.e. W0([OII]) = 5˚A and D4000 = 1.45˚A. We will use our definitions for these two

indices, as better tuned to our data.

These definitions allow to flag galaxies as a function of their star-formation regime, as defined from their position in the [W0([OII]), W0(Hδ)] and [D4000, W0(Hδ)] planes. Since we do not detect extreme values ofW0(Hδ) in our sample, we will only use the following definitions (see also tables 3.7 and 3.8), consistent with previous works (for an extended explanation, see Balogh et al. ((1999))).

Passive galaxies (PEV): galaxies not currently undergoing significant star for- mation activity, mainly ellipticals and S0. They are defined by W0(Hδ) < 5 and W0([OII])≤4.85, orD4000≥1.4.

Star-forming galaxies (SF): galaxies undergoing significant star formation activity since at least several hundred million years, mainly late-type spirals and starburst galaxies. These are identified by 0< W0(Hδ)<5 andW0([OII])>4.85, orD4000<

1.4.

Short starburst galaxies (SSB): galaxies currently undergoing a short-lived, intense episode of star formation (i.e., ≤200 Myr), with strong nebular emission. These galaxies haveW0(Hδ)< 0 andW0([OII])>4.85, orD4000< 1.4.

As shown in Fig. 3.11, RXCJ0014.3-3022 and RXCJ2308.3-0211 host galaxy pop- ulations with rather different distributions in the diagnostic plotsW0(Hδ)–W0([OII])

and W0(Hδ)–D4000.

In particular, RXCJ0014.3-3022 exhibits a larger fraction of objects with large W0([OII]) and star-forming systems, which confirms the presence of the Butcher-

Oemler effect in this cluster.

To assess if the galaxy populations in the two clusters are significantly different, a diagnostic two-dimensional Kolmogorov-Smirnov test was run on their distribution in both planes. As a result, the probability that the two distributions are drawn from

Figure 3.11: Distribution of spectral indices for the two clusters. Top row shows the members of RXCJ0014.3-3022; bottom row shows the members of RXCJ2308.3- 0211. The dashed lines mark the position of boundary values for the different spectral indices to discriminate different star formation regimes (see Tables 3.7 and 3.8). Average error values are shown in the top left corner of each plot. Note that the error onW0([OII]) is rather small in the diagram scale.

the same parent population is absolutely negligible: 0.02% in the [D4000, W0(Hδ)] plane,0.41% in the [EW(OII),W0(Hδ)]. We want to recall that the two spectroscopic

samples were built with the same selection function (cfr. Sect. 3.2.1). We attribute this significant difference to the largely different dynamical state and morphology of the two clusters (see Sect. 3.5.1 and Braglia et al. 2008b, in prep.). As an independent test, we consider the galaxy distribution in the (FUV-V) vs. D4000 plane (cfr. Moran et al. ((2007)). For our spectrosocpic sample (FUV-V) colours were calculated from our total magnitudes and archival GALEX data. In absence of

3.5 Results

GALEX FUV detections, we assumed 3σ lower limits on the UV magnitude, based on the completeness magnitude of the two fields.

Fig. 3.12 shows the (FUV-V) vs. D4000 plots for both clusters, where galaxies are flagged as star-forming or passive according to their [OII] emission. RXCJ0014.3- 3022 has a larger number of UV-detected objects than RXCJ2308.3-0211 (12 objects in the first cluster, 7 in the second). As our spectroscopic catalogue for RXCJ0014.3- 3022 contains 101 objects, against 269 in RXCJ2308.3-0211, this suggests that an intrinsically larger fraction of objects are actively forming stars in the first cluster. Most of the FUV-detected galaxies in RXCJ0014.3-3022 lie close to the cluster core and in the region of the southern filament, where our density maps reveal the presence of blue clumps of galaxies.

A more complete picture is obtained when lower limits on the (FUV-V) colour are considered. Fig. 3.12 shows that in RXCJ2308.3-0211 the bulk of spectroscopic mem- bers avoid the region with (FUV-V)<2, whatever the value of D4000. Conversely, RXCJ0014.3-3022 hosts a non-negligible population of galaxies with D4000 < 1.6 and (FUV-V)<2 mag.

It is now interesting to compare the behaviour of spectral indices of individual galaxies in the two clusters as a function of cluster-centric radius, as shown in Fig. 3.13.

In RXCJ0014.3-3022, the radial trend of W0([OII]) shows a strong and localized

increase of the star formation activity at R200. This recalls our results in BPB07,

where we found a sharp peak in the blue-to-red galaxies ratio along the two filaments connected to the cluster. Here this is confirmed by spectroscopy (see also the next section), suggesting that the filaments really begin to interact with the cluster at this distance through an intense increase of the star formation. The brightest cluster members always show small or negligible W0([OII]). The distribution of D4000

shows again a sharp change acrossR200. The number of galaxies with D4000>1.4

increases towards the center, whereas beyond R200 no galaxy shows a strong break,

its value suddenly dropping.

The distribution of spectral indices for RXCJ2308-0211 shows different features. The radial behaviour of W0([OII]) shows that the largest part of cluster members

is no more forming stars. All but one bright galaxy lie below the threshold value for this index, and the few objects still showing signs of star formation are scattered both below and beyondR200. The radial distribution ofD4000 shows this cluster as

largely dominated by passively evolving galaxies. Although slightly larger values are found towards the center, high values ofD4000 are found also beyondR200, both for

bright and faint galaxies.

The presence of bright passive galaxies outside R200 for RXCJ2308-0211 suggests

that in this region massive galaxies could be moving towards a passive sequence al- ready beyond the cluster structure. The separation between bright and faint galaxies is evident in the radial plot of D4000, where the two populations are largely segre-

Figure 3.12: Distribution in the (FUV-V) vs. D4000 plane for the two clusters. In addition to the usual coding for this section, red and blue bars mark lower limits for bright and red galaxies, respectively. Green circles mark galaxies with significant (i.e., > 4.85) [OII] emission. The dashed and dotted lines mark respectively a starvation and a truncation evolutionary track, similarly to Moran et al. ((2007)).

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Figure 3.13: Radial dependence of spectral indices for the two clusters. The left column shows the radial behavior of the three measured indices for RXCJ0014.3- 3022; the right one shows the same for RXCJ2308.3-0211. The horizontal dashed lines mark the characteristic values for the spectral indices, while the vertical dotted line marks the scaled position ofR200for each cluster.

gated. This is in agreement with previous works (Kodama et al. (2001) and (2003); Balogh et al. (1997)). In particular, Balogh et al. ((1997)) found that star formation is generally suppressed out to 2R200.

3.5.2.1 Environmental dependence of spectral indices

To better check and confirm our already published findings of an increase in the star- formation activity along filaments in RXCJ0014.3-3022, equivalent widths are plotted against cluster-centric radius for the cluster core, the filaments and the comparison

Figure 3.14: Radial dependence of spectral indices in the different regions of RXCJ0014.3-3022. Cluster-centric distance is expressed in arcminutes, to directly compare with the definitions given in Section 3.5.1. Notations are as in Fig. 3.13.

field (cfr. Fig. 3.14).

The cluster core shows a population rather typical of a cluster: galaxies have large values of D4000 and negligible [OII] emission. Still, at least one object is intensely forming stars; this is found to be a small, compact galaxy just outside the largest density peak in the center. Analysis on the two planes of the spectral indices shows that all but two objects are indeed star forming galaxies, with the most prominent being at the boundary between normal star formation and short starburst. This suggests that in the core of this cluster, short-lived and young episodes of star formation can still happen. This is likely associated with strong gravitational stresses (see Couch et al. (1998)).

3.5 Results

The southern filament is mostly outside the region targeted by VIMOS (see BPB07 and Sect. 3.5.1). Thus, we cannot spectroscopically confirm the star-formation mode of the “flaming giants” discovered by BPB07, which are mostly located along this filament but beyondR200 (i.e., ∼9.4′). According to our density maps, up to R200

the cluster shows mostly a population of passively evolving galaxies. This is also confirmed by the spectral index analysis. A slight but systematic decrease in the values ofD4000 suggests the presence of a population somewhat younger than in the core. In the northern filament, several galaxies are found to be actively star forming across the clusterR200, while beyond and below this distance the population is again

more typical of a cluster in the distribution of W0([OII]). However, the values of

D4000 are sensibly lower, suggesting again younger systems. The position of these galaxies with respect to the spectral indices confirms them as star-forming galaxies, with a small fraction of short-starbursts.

The comparison field is rather quiet, with a single bright and passive galaxy and several low-luminosity star-forming galaxies. Analysis of the spectral indices shows that the field is made of a mix of passive and mildly-to-high star-forming systems. No clear trends with radius is found, confirming this region as a fair sample of the field unaffected by the cluster. Also no short-starbursts are found, suggesting that the star formation in the field is quietly evolving without noticeable interaction with structures. New observations are currently under completion to spectroscopically sample 1200-1400 additional galaxies in this region to robustly map the star formation activity across the cluster core and filaments and beyond R200.

Although no specific regions of interest are detected in RXCJ2308.3-0211 which are also covered by spectroscopy, it is still possible to better examine the radial behaviour of spectral indices referring to the regions defined and discussed in Sect. 3.5.1. Thus, equivalent widths are plotted against cluster-centric radius in the regions of the cluster core, inner outskirts and outer outskirts, as well as for the comparison field.

From Fig. 3.15 it is possible to notice that active star-forming systems are almost exclusively represented by faint galaxies. Their presence in the core is practically negligible, while for increasing cluster-centric radii it is possible to see a slight increase of their fraction. Also, the mean value of W0([OII]) is slightly increasing from the

core to the outskirts. This is in good agreement with previous findings (i.e. Balogh et al. (1999)) and suggests the presence of age sequences in the radial distribution of galaxies in evolved clusters. Accordingly, typical values of D4000 show a small decrease with increasing cluster-centric radius, confirming that in the core are found on average older galaxies. Although the field coverage is scarce, it is still possible to detect the signature of the cluster in its galaxy population, suggesting that the whole region surrounding RXCJ2308.3-0211 is globally evolved. In particular, the mean values of D4000 show that massive galaxies outside the clusterR200 are already old

Figure 3.15: Radial dependence of spectral indices in the different regions of RXCJ2308.3-0211. Cluster-centric distance is expressed in arcminutes, to directly compare with the definitions given in Section 3.5.1. Notations are as in Fig. 3.13.

although with no significant star-formation activity.