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Astron. Astrophys. 355, 99-112 (2000)

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6. Stellar population synthesis

Stellar population synthesis is better constrained when one considers a wavelength range as wide as possible, as emphasised already by e.g., Bica (1988) and Alloin & Bica (1990). As shown in Paper II for spiral galaxies, synthesis in the UV range is well constrained because of the marked variety of spectral features and continuum distribution occurring in the star cluster components in the UV range, which are predominantly age dependent (Paper I). For this reason, the simplest approach is to perform an independent population synthesis in the UV range alone, and apply the same method to all groups.

Following Bica (1988) and Schmitt et al. (1996), we have adapted the population synthesis algorithm to the UV range in Paper II. Basically this algorithm uses EWs of the most prominent absorption features and selected continuum points observed in a given spectrum, and compares them to those of a model computed from a base of simple stellar population elements. The algorithm is not a minimisation procedure, instead it generates all possible combinations of the base elements according to a given flux contribution step and compares the resulting EWs and continuum points to the input ones. The code successively de-reddens galaxy input continuum points and tests them against a given model generated from the base elements. The acceptable solutions, within error bars, are averaged out: this average is adopted as the final synthesis solution.

The IUE is well-known to have a low-response in the region [FORMULA] Å, and we do not include this region in the syntheses. Well-known spikes have also been taken into account at observed wavelengths (Paper I). Redshift corrections further ensure the real nature of spectral features in groups spectra, averaging out eventual detector deficiencies (Papers II, III and IV).

The base elements that we use in the present UV synthesis are taken from the star cluster population templates described in Paper I to represent young (including an HII region template) and intermediate age components, and a galaxy template taken to represent an old metal-rich bulge (Paper III). EWs and continuum points of the HII region element were measured on the average spectrum of the M 101, M 33, LMC and SMC groups from Paper I. For young and intermediate age elements, we use LMC star cluster groups with ages [FORMULA] Myr (respectively the groups I, II, III, IVA and V in Paper I). Finally, we use a bulge template (named E2E5 by Bica and collaborators - see the database in Leitherer et al. 1996), which is the average of the far-UV weak elliptical galaxy groups G[FORMULA]N1553 and G[FORMULA]N3115 (Paper III). EWs and continuum points for these base elements are listed in Table 8 of Paper II.

All synthesis runs included the 7 age components described above. Initially, we used a 5% step for testing flux contributions at [FORMULA] Å, generating 230 230 combinations for each assumed [FORMULA]. Typically, for the appropriate [FORMULA], only 5% of these 230 230 element combinations are acceptable solutions within the error windows. Reddenings were tested in the range [FORMULA] with a step of [FORMULA]. Thus, in total, [FORMULA] element combinations are tested for each group, and acceptable solutions amount to less than 1%. Finally, after having probed as above a large space of combinations, we calculate the final solution with finer steps (2% in flux contributions).

The Galactic (Seaton 1979), LMC (Fitzpatrick 1986) and SMC (Prévot et al. 1994) reddening laws have been tested in the synthesis of each group.

It is interesting to point out that a very reddened blue stellar population cannot be confused with a red (predominantly old and/or intermediate age) stellar population because there are spectral features which differ considerably in strength (Paper I), such as those in windows 44 (FeII ) and 45 (MgII ). Compare also the typically blue and red stellar populations respectively of G[FORMULA]Mrk1261 and G[FORMULA]Mrk1267 (Fig. 9) and their EW values in Table 3.

Synthesis results for the present normal stellar population galaxy groups are given in Table 4 where we present the percentage flux contribution of each base element to the group stellar population, at [FORMULA] Å; [FORMULA] values are also given in the table, using the SMC extinction law. It is interesting to point out that in no case Seaton's Galactic reddening law can be applied to correct for the internal reddening of these galaxies, such a correction producing prohibitive flux excesses in the region around [FORMULA] Å. A similar result has been found for the sample of normal nearby spirals in which there are cases where an SMC reddening law applies and cases where an LMC law applies (Paper II). The same conclusion was reached by Kinney et al. (1994). For the enhanced star formation galaxies of Paper IV, only the SMC extinction law applies.

Extragalactic reddening laws for extended objects seem to work differently than interstellar ones deduced from stars (Calzetti et al. 1994), so that the term attenuation is often used now in place of extinction when referring to the loss of light in a galaxy spectrum due to intervening dust mixed with the stars. The latter extragalactic obscuration law (EOL) was compared to the LMC and SMC laws (deduced from stars) in Fig. 1 of Calzetti et al. (1995). The EOL on average follows the LMC curve, except that it presents no [FORMULA] Å bump. In this respect it is similar to the SMC curve, which however is steeper. As a consequence, the application of the EOL curve to the present spectra would produce similar results (i.e. SMC law, absence of bump), but the associated colour excesses [FORMULA] would be somewhat higher.

The synthesis procedure is illustrated in detail in Fig. 6 for the nearby starburst galaxy ESO 338-IG4. This galaxy shows emission-lines superposed on a steep, blue continuum with dominant flux contributions from the HII region and LMC I components (Table 4). These two stellar population elements, along with the sum of the remaining contributions from the other components are displayed in Fig. 6, together with the resulting synthesis spectrum. No reddening was found in this galaxy. The spectra in Fig. 6 are shown in their true proportions (in terms of flux fraction at [FORMULA] Å), according to the synthesis. The case of a synthesis with large reddening is shown in Fig. 7 for G[FORMULA]N5860, a spectral group of distant interacting galaxies with a red continuum (Table 1). The main flux contributors for this group are the intermediate-age LMC V component (age [FORMULA] Gyr) and secondly, the young LMC I component (age [FORMULA] Myr, Table 4). The synthesis for each group is discussed in detail below.

[FIGURE] Fig. 6. Top panel: synthesis of ESO 338-IG4, the nearby starburst galaxy with blue continuum and emission lines. Components are: (a) - HII region; (b) - LMC I; (c) - sum of LMC II, LMC III, LMC IVA, LMC V and E2E5. The main emission-lines are indicated. The spectra are shown in their true proportion according to the synthesis, except that a constant has been added to the top one (observed galaxy spectrum) for clarity purposes. The spectrum shown with bold line is the synthesized one. Bottom panel: resulting pure-emission spectrum of ESO 338-IG4, observed - synthesized.

[FIGURE] Fig. 7. Same as Fig. 6 for G[FORMULA]N5860, the high-reddening group of distant interacting galaxies. Components are: (a) - sum of the HII region and LMC I; (b) - sum of LMC II, LMC II and LMC IVA; (c) - sum of LMC V and E2E5.

The ability of the synthesis algorithm to reproduce the observed EWs of normal stellar population spectra can be evaluated by the residuals in each absorption feature, which are given in Table 5. We remind that, according to Paper I, the base elements with ages between 10 and 500 Myr correspond to LMC clusters, and the HII region template is an average of SMC, LMC, M 33 and M 101 HII regions, thus in general with subsolar metallicity. In this sense, absorption features of solar and above solar-metallicity stellar populations cannot be well reproduced by our synthesis, except when the old (bulge) component is dominant. We remind again that our approach is essentially a synthesis of stellar population ages.


[TABLE]

Table 5. Equivalent Width Residuals
Table notes:
[FORMULA] - Emission line contamination.


In the following we discuss the synthesis results for each group.

6.1. Nearby disturbed galaxy groups

ESO 338-IG4: This is a well studied blue compact galaxy with an irregular and patchy central region in which there are two nuclei separated by [FORMULA] (Iye et al. 1987). ESO 338-IG4 harbours an extremely blue starburst with age [FORMULA] years as well as extended systems of ionised gas (Bergvall 1985).

The UV spectrum of ESO 338-IG4 is steeper than that of G[FORMULA]N2782, the bluest among the luminous starburst groups studied in Paper IV. According to our stellar population synthesis, this difference is explained by a more important flux contribution from the HII region component in the former ([FORMULA]), and more significant contributions from evolved bursts (ages [FORMULA] Myr) in the latter. The intermediate (age [FORMULA] Gyr) and old bulge components together contribute less than 3% of the flux at [FORMULA] Å, while the HII region together with the LMC I components (age [FORMULA] Myr) are responsible for more than 86%, in agreement with previous results (Bergvall 1985). As can be seen in Fig. 6, ESO 338-IG4 presents conspicuous permitted emission lines such as Ly[FORMULA], [FORMULA] and [FORMULA] as well as semi-forbidden lines such as [FORMULA], [FORMULA] and [FORMULA]. These emission lines are isolated and better seen after the subtraction of the stellar population continuum, as shown in Fig. 6, bottom panel. A probable mechanism for the permitted emission lines in this object is photoionisation by hot stars either young, associated with the recent starburst, or evolved post-AGBs (Binette et al. 1994); the semi-forbidden lines may originate from shock-heated gas in supernova remnants and/or winds (Sutherland et al. 1993).

G[FORMULA]N4438: This group contains only the galaxies ESO 383-G35, a very elongated lenticular galaxy in a cluster, and NGC 4438, which is in pair with NGC 4435 in the Virgo cluster, probably being disrupted by M 87 (Rauscher 1995); optical and near-IR integrated spectroscopy suggests that NGC 4438 might contain a low-luminosity LINER nucleus (Bonatto et al. 1989) which is further confirmed by the near-IR photometry by Jungwiert et al (1997).

Although the IUE spectrum of G[FORMULA]N4438 has a low (S/N), the presence of a bulge population can be clearly seen in Fig. 1. In fact, the population synthesis of G[FORMULA]N4438 shows that the old bulge population is the dominant flux contributor ([FORMULA]) to its spectrum, with indications of a series of important bursts distributed in age among the younger populations (Table 4). Despite the low (S/N), a few conspicuous emission lines can be seen in the spectrum of G[FORMULA]N4438: [FORMULA], [FORMULA], [FORMULA] and [FORMULA]. They might be related to the presence of a LINER nucleus. The synthesis of G[FORMULA]N4438 is shown in Fig. 8.

[FIGURE] Fig. 8. Synthesis of the nearby group of disturbed galaxies G[FORMULA]N4438. Although the low (S/N), the presence of the old population is evident. Conspicuous emission lines are indicated.

6.2. Distant isolated galaxy groups

Groups G[FORMULA]Mrk1261 and G[FORMULA]Mrk1267 contain distant isolated, moderate-luminosity galaxies with a variety of morphological classifications such as compact, BCG and barred spirals (Table 1 and Fig. 2, bottom panel), most of which are indicative of recent star formation. For both groups, the average spatial region covered by the IUE slit is large (Table 2) and should encompass the bulge. Indeed, such a bulge contribution is clearly detected on the spectrum of the flat/red continuum group G[FORMULA]Mrk1261 (Fig. 1).

G[FORMULA]Mrk1261: Galaxy members of this flat/red continuum group are IC 1586, a blue elliptical compact with H and K absorption lines and sharp emission lines of HI as well as [FORMULA], and NGC 118, a spherical compact with a blue disc (Zwicky 1971).

According to the stellar population synthesis (Table 4), the flat/red UV spectrum of G[FORMULA]Mrk1261 can be explained essentially by a dominant ([FORMULA]) flux contribution from the old bulge population and a significant contribution from the HII region component ([FORMULA]) together with a noticeable amount of reddening, [FORMULA]. The presence of enhanced star formation in the galaxies forming this group can be traced by the [FORMULA] flux contribution from young components (age [FORMULA] Myr). We emphasise, however, that G[FORMULA]Mrk1261, which contains isolated galaxies, is the group with the smallest flux contribution from young stellar populations among the present sample. In Fig. 9, top panel, we show the spectrum of G[FORMULA]Mrk1261 corrected with the SMC extinction law and [FORMULA], along with the corresponding synthesis.

[FIGURE] Fig. 9. Synthesis for the isolated galaxy groups. Top panel: flat/red continuum group G[FORMULA]Mrk1261, which presents an important contribution from the old bulge population. Bottom panel: blue continuum group G[FORMULA]Mrk1267. Reddening corrections have been applied with an SMC law and with [FORMULA] and 0.01 respectively for G[FORMULA]Mrk1261 and G[FORMULA]Mrk1267.

G[FORMULA]Mrk1267: The blue continuum group G[FORMULA]Mrk1267 includes Mrk 702 which contains three individual giant HII region complexes (Telles & Terlevich 1997) and Mrk 1267 in which Kinney et al. (1993) detected star-formation activity, in particular the presence of many early-supergiant stars.

As expected, the stellar population synthesis shows that the HII region is the dominant flux contributor to the UV spectrum of this group, while intermediate age and old bulge components together contribute up to [FORMULA] only, in marked contrast with the flat/red continuum group G[FORMULA]Mrk1261. In terms of flux fractions, galaxies of this group are characterised by a series of star formation bursts distributed in age among the young populations. The synthesis indicates a very small reddening in the spectrum of G[FORMULA]Mrk1267, [FORMULA], which was subsequently corrected for, using the SMC extinction law. A small contribution from the old population ([FORMULA] in flux at [FORMULA] Å) was found. The resulting synthesis and reddening-corrected spectra are displayed in Fig. 9, bottom panel.

6.3. Distant interacting galaxy groups

These groups contain galaxies in systems (pair, triple, etc) and/or with signs of interactions and even mergers, i.e. phenomena usually associated to enhanced star formation (Table 1). Observationally, their IUE spectra classify as red (G[FORMULA]N5860), blue (G[FORMULA]N4410) and very blue (G[FORMULA]Mrk54 - Fig. 2, top panel). The IUE slit covered similar spatial regions as in the distant isolated groups and, accordingly, it should include similar old bulge fractions.

G[FORMULA]N5860: Galaxy members of the red continuum group G[FORMULA]N5860 are NGC 5860, which is in a merging pair with fading starburst signatures (Mazzarella & Boroson 1993); NGC 2623 which is part of a well-studied triple system included in Arp's atlas of peculiar galaxies (1987) with bright tidal tails suggesting a merger, while its emission is dominated by a compact starburst (Condon et al. 1991); NGC 828 which presents a dust lane and is described as a merger with disturbed morphology (Wang et al. 1991); and Mrk 789 which has been classified as a starburst galaxy by Kukula et al. (1995).

The stellar population synthesis of this group (Table 4) shows that its dominant flux contribution is from the intermediate-age component ([FORMULA]) with a negligible ([FORMULA]) HII region contribution. Combined with the large reddening ([FORMULA]), this produces the red appearance of the G[FORMULA]N5860 spectrum. Despite its red shape, the total flux contribution from the young populations (age [FORMULA] Myr) still amounts to [FORMULA], and is dominated by a somewhat evolved [FORMULA] Myr burst (LMC I component). Interestingly, the young populations are larger flux-contributors to this red continuum group of interacting galaxies than to the very red continuum group of isolated galaxies G[FORMULA]Mrk1261 (Fig. 2). This probably reinforces the existence of the link between interactions and star formation activity. The non-detection of the old bulge population in the spectrum of G[FORMULA]N5860 can be accounted for by dust obscuration, as implied by the large reddening derived in the synthesis (Table 4). The synthesis of G[FORMULA]N5860 (reddening-corrected with [FORMULA], SMC law) is illustrated in Fig. 7. We conclude that the red appearance of these two observed spectra arises primarily from very different combinations in terms of reddening and stellar-population.

G[FORMULA]N4410: The following galaxies, members of the blue continuum group G[FORMULA]N4410 have been previously studied: IC 298 with a ring containing many knots of HII regions with H[FORMULA] emission (Horellou et al. 1995) and NGC 6090 with two compact, blue nuclei (Rakos et al. 1996).

The stellar population synthesis shows that the bluer observed spectrum of G[FORMULA]N4410 with respect to that of G[FORMULA]N5860 is due to a much smaller extinction ([FORMULA]) as well as to a more uniform distribution of flux contributions among the different age components (Table 4). As well, the total contribution from the young components amounts to [FORMULA], a considerably larger figure than that in the red group G[FORMULA]N5860. The small extinction derived in the synthesis allowed the detection of a significant old bulge population contribution ([FORMULA]). The reddening-corrected ([FORMULA], SMC law) spectrum of G[FORMULA]N4410 along with its corresponding synthesis are shown in Fig. 10, top panel. A few emission-lines are also present in the spectrum of G[FORMULA]N4410: in particular Ly[FORMULA], [FORMULA], [FORMULA] and [FORMULA]. These emission lines may arise in shock-heated gas in supernova remnants and/or winds (Sutherland et al. 1993), with some photoionisation from the hot stars associated to recent starburst.

[FIGURE] Fig. 10. Same as Fig. 9 for the groups of distant interacting galaxies with a blue continuum G[FORMULA]N4410 (top panel) and very blue continuum G[FORMULA]Mrk54 (bottom panel). The spectrum of G[FORMULA]N4410 has been reddening-corrected with an SMC law.

It should be noted that the stellar population syntheses of both groups G[FORMULA]N5860 and G[FORMULA]N4410 are similar to that of the nearby starburst group with flat continuum G[FORMULA]N3256 (Paper IV), in the sense that in all three groups there are large contributions from intermediate and young ages. Therefore, starburst activity is inferred in the mergers, and the merging time-scale should be of [FORMULA] Gyr, during which most of the mass has been converted into stars (Sect. 6.4 and Table 7).

G[FORMULA]Mrk54: Among the members of the very blue continuum group G[FORMULA]Mrk54, the following galaxies have been previously studied along the lines of our analysis: Mrk 54, identified as a starburst galaxy by Glass & Brinks (1998); ESO 350-IG38 which exhibits three nuclei with emission lines typical of HII galaxies (Heisler & Vader 1994); Mrk 66 experiencing active star formation (Kinney et al. 1993); Mrk 220S in strong interaction with UGC 7905N, probably triggering the star formation activity in Mrk 220S (Kinney et al. 1993); and the now classical merger/starburst galaxy NGC 6240 which presents a disturbed morphology and a dominant contribution from supergiant stars in the H band (Lançon et al. 1996). It is worth remarking that the observed IUE spectrum of NGC 6240 is blue, as a member of the group G[FORMULA]Mrk54 (Fig. 2), while its [FORMULA] Å spectrum, which has been synthesised by Schmitt et al. (1996) is very red owing to high reddening. This apparent discrepancy can be explained by different optical depths probed by spectroscopy in different wavelengths. Indeed, the population synthesis in Schmitt et al. (1996) indicated a higher reddening in the near-IR range than in the near-UV range. The IUE spectrum of NGC 6240 supports this scenario, and in such case the present blue spectrum would correspond to an outer galaxy shell, probing the external spatial zones of the starburst. Evidence of this effect has also been found in the dust lane of Centaurus A, NGC 5128 (Storchi-Bergmann et al. 1997).

The enhanced and recent star-formation activity among galaxy members of the group G[FORMULA]Mrk54 implied by previous studies is fully confirmed by our stellar population synthesis (Table 4). The UV flux is dominated by the HII component ([FORMULA]) while the intermediate and old bulge components together contribute with only [FORMULA]. The small reddening value derived in the synthesis allowed the detection of a significant old bulge population contribution. The bulge contribution is also found in the synthesis of Schmitt et al. (1996). According to the synthesis, the main differences between the spectrum of G[FORMULA]Mrk54 and that of the blue starburst ESO 338-IG4 are: a larger old population flux contribution (due to the larger spatial area sampled by the IUE slit) in the former, and a very important flux contribution from a 10 Myr burst in the latter. The synthesis of G[FORMULA]Mrk54 is shown in Fig. 10, bottom panel. Ly[FORMULA] and [FORMULA] in emission are clearly visible on the spectrum of G[FORMULA]Mrk54, and are probably resulting from photoionisation by the hot, young stars associated to the strong and recent starburst.

6.4. Mass fractions

Similarly to what has been performed in the case of the normal spiral (Paper II) and irregular galaxy groups (Paper IV), the flux fractions derived from the synthesis have been converted into mass fractions. Previously, the synthesis results in the visible/near-IR ranges (Bica 1988) had been converted into mass fractions by means of mass to light ratios [FORMULA] computed from a stellar evolution model of star clusters (Bica et al., 1988). Applying these results, we recall in Table 6, for each age component used in the present paper, the corresponding age interval and mass to light ratio expressed in luminosity at [FORMULA] Å. Using the [FORMULA] Å stellar population templates from Bica and collaborators (see Leitherer et al. 1996), we measured the ratio [FORMULA], and derived the mass to light ratio [FORMULA], respectively shown in Columns 4 and 5 of Table 6. Note that small old population flux fractions in fact correspond to large mass fractions. Mass fractions for the present galaxy groups can be obtained from the synthesis flux fractions by means of [FORMULA]. The results are shown in Table 7.


[TABLE]

Table 6. Mass to light ratios



[TABLE]

Table 7. Synthesis results in terms of mass fractions


In terms of mass fractions, the old stellar population is dominant in all groups, except G[FORMULA]N5860 in which most of the mass is stored in the intermediate age component (Table 7). The fact that very large mass fractions are stored in the old stellar population still applies for the groups of distant interacting galaxies G[FORMULA]Mrk1267 and G[FORMULA]Mrk54 with a blue/very blue continuum, and the nearby starburst galaxy ESO 338-IG4, despite the fact that for these three groups the stellar population synthesis shows that their UV light is strongly dominated by recent star formation (Table 4).

For the distant galaxy groups, the fact that most of the mass is stored in the old stellar population is a consequence of the large spatial region sampled by the IUE aperture, hence including important fractions of the old population: [FORMULA] flux fraction at [FORMULA] Å for G[FORMULA]Mrk1267 and [FORMULA] for G[FORMULA]Mrk54. Even the small fraction of old stellar population flux in ESO 338-IG4 ([FORMULA]), when converted into mass, corresponds to the dominant fraction of [FORMULA]. However, in the latter case of a nearby starburst galaxy, the small spatial area covered by the IUE aperture allowed the detection of noticeable mass fractions stored in younger populations (Table 7).

The groups containing distant interacting galaxies, with a blue continuum (G[FORMULA]N4410) and particularly that with a red (attenuated) continuum (G[FORMULA]N5860), have significant amounts of mass stored in the intermediate age population. This is probably related to the old disc contribution and/or evolved starbursts associated to early interactions.

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Online publication: March 17, 2000
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