Using infrared template fits (§4.2.3-4.2.5), we are able to estimate bolometric in-
frared luminosity, Lir, which in the case where there is no contribution from an
AGN dust torus, can be a measure of the total star formation rate in a galaxy. One of the things that someone should bear in mind when trying to estimate bolometric infrared luminosities is that this estimate is highly uncertain if far-infrared data are not available [283]. As a result in this section we have included an estimate of bolometric infrared luminosities only from sources which have significant 24 µm
detections (S24> 150µJ y) and for which we were able to estimate their bolometric
tic sources. It is reccomended to use sources that have longer wavelength detections which not only provide more accurate luminosities measurements but also preciser SED fittings.
Figure 4.5 shows the ratio Lir/Lopt versus Lir for a sample of 112 sources with
24µm detections which were fitted with one of the four available infrared templates. The remaining 60 24µm sources are either single band infrared excess or have no excess. Different colours represent sources dominated by cirrus, M82–like starburst, Arp 220–like starburst or AGN dust tori templates. From the 112 sources, 19 (17%) are fitted with a cirrus template, 26 (23%) with an M82 starburst template, 5 (4%) with an Arp220 starburst template or single band infrared excess and 62 (56%) with a dust torus template. Among this sample there are 42 (38%) Luminous Infrared
Galaxies ( LIR > 1011 L ), 25 (22%) of which have LIR > 1012 L most of them
fitted with a dust torus template.
Since the optical bolometric luminosity of a galaxy, corrected for extinction, is a
measure of the stellar mass, the ratio LIR/Loptical can be interpreted as the spe-
cific star formation rate which is basically the inverse of the timescale in years to accumulate the present stellar mass, forming stars at the current rate. For galax-
ies with LIR < 1010 L the ratio LIR/Loptical should be interpreted as the optical
depth of the interstellar dust since most of the infrared emission is reemission of the general starlight absorbed by interstellar dust. 64 sources are found to have LIR/Loptical < 0.2. These should be interpreted as very low star–forming galaxies. There is a sloping cutoff to the right part of Figure 4.5. This could be explained as a limit on the optical bolometric luminosity and stellar mass. As it can be seen to the right of the limit there are AGN and some starbursts. In the case that these AGN
were Type 1 we would expect them to lie on LIR = Loptical but since they appear to
have LIR> Loptical that means that they are probably Type 2.
We now look more closely at each infrared template type in turn. In the case
of cirrus dominated sources infrared emission is due to interstellar dust heated by
the general stellar radiation field. As a result, the ratio LIR/Loptical is a measure of
the dust opacity of the interstellar medium. From the 19 cirrus dominated sources, 4 are fitted with a starburst optical template, 14 with a spiral template and 1 with an elliptical template. We also notice a very interesting population of luminous cirrus
galaxies. 10 (53 %) of which have LIR > 1011 L and one of them LIR > 1012 L
optical bolometric luminosity, suggesting that the optical depth of the interstellar
medium in these galaxies is 1. Cirrus galaxies with LIR < 1011 L have values
of log10(LIR/Loptical) of -0.2. This can be explained if the quiescent phase of star
formation was significantly more luminous in the past and that dust of interstellar medium is much more opaque in galaxies at z∼1.
Although M82-like galaxies lie in the same luminosity range as cirrus, a much higher proportion (70%) lie at the LIRG range. Most of the M82 starbursts are fitted with a spiral optical SED with very high dust opacities. One very interesting finding is the presence of 2 ULIRG elliptical which would result in very high star formation rate.
In the case of Arp 220 dominated sources the higher values of Lir result in higher
values of Lir/Lopt consistent with the idea that Arp 220 are very dusty. Highly ex-
tinguished starbursts like Arp220 look like ellipticals in the optical because the light from the young stars is almost extinguished which explains the large percentage of Arp220 fitted with an elliptical template.
Due to selection effects (see §3.4.2) the 24µm population is dominated by torus
sources. Lir/Lopt in this case can be interpreted as ftorkopt where ftor is the cover-
ing factor of the torus and kopt is the bolometric correction that needs to be applied
to the optical luminosity to account for emission shortward of the Lyman limit [283].
Among the ULIRGs found in our sample there are seven galaxies that would be classified as early-type red galaxies but which are all fitted with starburst SEDs, 4 Arp220 and 3 by a combination of M82 with significant contribution (20-60%) from a torus template. As mentioned before, the 4 sources fitted with an Arp220 tem- plate are most likely highly extinguished starbursts, which are probably the product of a merger event, that look like ellipticals in the optical because the optical light from the young stars is almost completely extinguished. In addition the very high
values of Lir, reaching up to LIR = 1013.3 L , results to such high values of star
formation rates that is not feasible to be generated by a single starburst event but rather from significant accretion of gas through a galaxy merger. The broad band SEDs of the remaining three elliptical-like galaxies with composite infrared SEDs (both an AGN and an M82 starburst) points to the presence of Type-2 QSOs, with the central engine hidden behind large columns of obscuring dust. The presence of a strong star-formation component imply that there is either an obscured starburst or that the star-formation event is causing the obscuration of the central engine. In both cases there seems to be a strong underlying relation between the supermassive
za J1b J2c SCd TCe S1f S2g S3h D1i D2j
0.066 sb Cirrus 0.2 0 -0.45 -0.72 -0.06 SF SF
0.13 sb M82 1 0 -0.68 -0.74 0.20 SF SF
0.063 Sab Cirrus 0.4 0 -0.29 -0.54 0.07 COM SF
0.126 Scd Cirrus 0.4 0 -0.71 -0.97 0.01 SF SF
0.315 Scd Cirrus 0.25 0 -0.55 -0.71 -0.20 SF SF
0.266 E Torus 0.45 0.55 -0.17 0.22 0.18 COM AGN
0.453 Sdm Torus 0.1 0.9 -0.69 -0.44 1.14 AGN AGN
0.263 Scd Cirrus 0.4 0 -0.64 -0.68 -0.30 SF SF
0.127 E M82 0.55 0.45 -0.07 -0.41 -0.04 COM SF
0.268 Sdm M82 0.65 0.35 -0.29 -0.46 0.56 AGN AGN
0.336 QSO M82 0.55 0.45 -0.37 -0.59 1.14 AGN AGN
0.339 Scd M82 1 0 -0.53 -0.85 -0.12 SF SF 0.143 Sbc Cirrus 0.45 0 -0.91 -0.15 -0.13 SF AGN 0.041 sb M82 0.55 0.45 -1.38 -0.35 0.81 COM AGN 0.092 sb M82 0.55 0.45 -1.34 -0.59 1.35 AGN AGN 0.048 Sdm M82 1 0 -0.71 -0.49 0.23 SF SF 0.212 Scd Cirrus 0.2 0 -0.91 -0.97 -0.52 SF SF 0.338 Sdm Cirrus 0.4 0 -0.69 -0.58 0.15 SF SF 0.300 Sdm M82 0.65 0.35 -0.52 -0.43 -0.26 SF SF 0.227 Scd M82 0.6 0.4 -0.31 -0.29 -0.16 COM SF
0.063 Sab Cirrus 0 0 -0.27 -0.81 0.85 AGN AGN
0.069 Sab Cirrus 0 0 -0.11 -0.82 0.60 AGN SF
0.144 E Cirrus 0.25 0 -0.62 -0.51 0.03 SF SF
0.299 Scd M82 0.8 0.2 0.15 0.22 0.09 COM AGN
0.157 Scd M82 0.5 0.5 0.18 -0.38 -0.39 COM SF
0.214 Scd M82 1 0 -0.77 -0.87 0.53 SF SF
0.281 E Torus 0.25 0.75 -1.28 -0.66 0.87 AGN AGN
0.26 Sbc M82 1 0 -0.36 -0.24 0.29 SF AGN
0.19 Scd Cirrus 0.5 0 -0.31 -0.27 0.14 COM AGN
0.176 E M82 0.6 0.4 -0.27 -0.47 -0.37 COM SF
aSpectroscopic Redshift bOptical SED best fit cInfrared SED best fit
dFraction of Starburst contribution at 8 µm eFraction of Torus contribution at 8 µm
flog([NII]/Hα) glog([SII]/Hα) hlog([OIII]/Hβ)
iSpectroscopic Classification according to the[N II]/Hα versus [OIII]/Hβ diagnostic. jSpectroscopic Classification according to the[SII]/Hα versus [OIII]/Hβ diagnostic.
Figure 4.6: Representative Flag 3 spectra of the spectra with available [SII], Hα, [OIII], Hβ, [N II] lines, used to estimate line ratios.
central black hole and the star-formation event either by the presence of gas that both feeds the SMBH and the starburst, as a result of a major merger event or points to the fact that these ULIRG ellipticals belong to the proposed green-valley transition objects between blue star-forming galaxies and red QSO remnants.