Figure 2.10:MH2 vsMHIwith a color code to indicate the evolutive stage of the HCG where the galaxy is. Signs: Black: galaxies in Phase 1 HCGs; Green: galaxies in Phase 2 HCGs; Red: galaxies in Phase 3 HCGs. The squares represent the late-type (T>0) galaxies and the circles the early-type galaxies. The y=x line is plotted for orientation and does not represent a fit to the data.
there is no clear relation with the HI content of the group, but there might be a tentative trend of the Def(MH2) to increase as the group evolves.
2.3.6 SFE and sSFR as a function of the deficiencies of the galaxies
In Fig. 2.11 we display the SFE and the sSFR of the galaxies in our sample as a function of their Def(MHI) and Def(MH2). We see that there is no clear trend of the SFE with the gas defi- ciency of the galaxies, neither atomic nor molecular. On the other hand, the sSFR is related with the gas deficiency, with a higher sSFR in the galaxies showing a excess ofMH2 orMHI, as can be seen in Table 2.8. In particular, the trend withMHIis interesting as it suggest that, although the Def(MHI) of a galaxy has only a weak influence on the absolute SFR, it has a stronger influence on the SFR per stellar mass.
We also note that, in contrast to Tzanavaris et al. (2010), we do not see a gap in the sSFR dis- tribution. They found the sSFR in a sample of 41 galaxies in 12 HCGs, to show a clear bimodality, with a gap in the sSFR distribution between the star-forming galaxies and the quiescent ones.
This trend is not seen in our sample (see Fig. 2.11) but it must be taken into account that the sSFRs in Tzanavaris et al. (2010) are calculated, considering Spitzer infrared 24µm and Swift UV observations, which gives a more precise measurement, since it takes into account both extincted and unextincted SF tracers. Thus, we would need more sensitive data to set a strong conclusion on the sSFR distribution.
2.4 Comparison between HCG and isolated galaxies
There is an intrinsic difference between the AMIGA and the HCG samples that must be taken into account when performing a comparison between both samples: the HCG sample have much more eary-type galaxies (∼45%), whereas 87 % of the AMIGA galaxias are spirals (Fig. 2.12), which can be explained in terms of the morphological type changes due to inter-
Figure 2.11: Upper left: Star formation efficiency vsMHIdeficiency. Upper right: Specific star formation rate vsMHI deficiency.
Lower left: Star formation efficiency vsMH2 deficiency. Lower right: Specific star formation rate vsMH2 deficiency. Signs: Black squares: late-type (T>0) galaxies; Red circles: early-type galaxies.
Table 2.8: Average log(sSFR) and log(SFE) depending on Def(MHI) and Def(MH2) for the full sample and the spiral galaxies.
sSFR
Full Sample nU L/n T>0 nU L/n Def(MHI)<0.75 -10.50±0.11 (8/23) -10.42±0.10 (5/19) Def(MHI)>0.75 -11.67±0.14 (28/41) -10.94±0.13 (6/16) Def(MH2)<-0.25 -10.73±0.10 (18/38) -10.44±0.06 (6/23) Def(MH2)>-0.25 -11.53±0.15 (28/43) -10.93±0.13 (8/21)
SFE
Full Sample nU L/n T>0 nU L/n Def(MHI)<0.75 -8.87±0.07 (8/23) -8.88±0.07 (5/19) Def(MHI)>0.75 -8.86±0.13 (28/41) -8.91±0.12 (6/16) Def(MH2)<-0.25 -8.95±0.06 (18/38) -8.99±0.06 (6/23) Def(MH2)>-0.25 -8.89±0.10 (24/43) -8.82±0.11 (8/21) For each subsample,nis the number of galaxies andnU Lis the number of upper limits.
2.4 Comparison between HCG and isolated galaxies 37
actions between galaxies within the groups. In order not to be affected by this difference, we compare only the spiral galaxies.
Figure 2.12: Morphological types for the AMIGA isolated (black line) and HCGs (red filled bars) galaxies
We have compared the HCG galaxies in our sample with the isolated galaxies in the AMIGA sample. The average values ofMH2,LFIR andLBfor the two samples, listed in Table 2.3, show slightly larger values for the HCGs. The difference between the average values of the HCGs and the AMIGA galaxies is larger forMH2 (∼0.7 for spiral galaxies) than forLFIR andLB (∼0.4 and
∼0.2 respectively, also considering only spiral galaxies). The sample of AMIGA galaxies with availableMH2 data is restricted to a velocity range of 1500<V <5000 km s−1, while the range of the HCGs extends to larger velocities. Thus, the isolated galaxies are, in average, at a lower distance from us (47 Mpc of average distance versus the 68 Mpc of the HCG sample), which can explain their lower averageMH2, since galaxies with a low emission are easier to detect when located at shorter distances. Nevertheless, an intrinsic enhancement of the molecular gas content in HCGs cannot be discarded.
The Def(MH2) of the isolated galaxies versus their Def(LFIR) is displayed in Fig. 2.13. The behavior of the isolated galaxies in this plot do not show a significative difference with the one shown by the HCGs in Fig. 2.3.4.1, with both deficiencies showing a tight correlation. The iso- lated galaxies show values of the Def(MH2) with respect to their Def( LFIR) a bit lower than the ones shown by the HCGs, as can be seen when comparing the distribution of the points of both samples around the Def(MH2)=Def(LFIR) line. This is also reflected in the mean values of Def(MH2) and Def(LFIR) of AMIGA and the HCGs (Table 2.3): whereas the values of Def(LFIR) for spiral galaxies are almost the same for both samples, Def(MH2) in spirals is larger by 0.2 for HCG than for AMIGA. This could show a real excess of molecular gas in HCG galaxies. Alterna- tively, we might have overestimated the molecular gas by our extrapolation (Sec. 2.2.1.3) if the molecular gas extent in HCGs is smaller than in isolated galaxies.
The comparison of the linear regression between LFIR and LB, MH2 and LB orMH2 to LFIR
(Table 2.5) shows no significative differences. In the case of theLFIRversusLBandMH2 versus LB, the slope and intercept of both samples match within the error bar.
The main difference between HCGs and the AMIGA isolated galaxies is on theirMHI, where HCGs presentMHIdeficiencies that are one order of magnitude larger than the ones inMH2 and SFR (Sec. 2.3.4.2).
Figure 2.13: MH2 deficiency vsLFIRdeficiency for the isolated galaxies. Signs: Black squares: late-type (T>0) galaxies; Red circles:
early-type galaxies. The y=x line is plotted for orientation and does not represent a fit to the data.
Table 2.9: Average log(SFE) for different samples and measurements.
Sample log(SFE) (log yr−1)
All T>0
HCGs -8.78±0.07(1)/-9.00±0.06(2) -8.86±0.05(1)/-9.01±0.06(2) CIGs -8.72±0.03(1)/-8.85±0.04(2) -8.74±0.03(1)/-8.87±0.04(2)
THINGS∗ -8.68±0.21
HERACLES -9.02
(1)Values obtained from the galaxies detected in both CO and FIR and galaxies detected in FIR but not detected in CO.(2)Values obtained using galaxies detected in both CO and FIR and galaxies detected in CO but not detected in FIR. (∗) Value for 11 galaxies with T=1-5 calculated with the data in Leroy et al. (2008).
We have calculated the average SFE for the AMIGA sample of isolated galaxies, using theMH2
data from Lisenfeld et al. (in prep.) and theLFIR data from Lisenfeld et al. (2007), to compare this value with the one on HCGs. The average SFE of 11 galaxies with T=1-5 in the THINGS sample (Walter et al. 2008) using IRAS data from Leroy et al. (2008) is also calculated. The SFR we are considering (Eq. 2.7) assumes a Salpeter (1955) Initial Mass Function (IMF), while in Leroy et al. (2008), a Kroupa (2001) IMF is considered. In order to express the SFR as a function of a common IMF, we have converted the THINGS SFE to the Salpeter IMF, applying a correction factor of 1.59, as detailed in the appendix D of Leroy et al. (2008). Finally, we have included also the SFE calculated in Bigiel et al. (2011) for a sample of 30 galaxies in the HERACLES Survey (whose first results can be found in Leroy et al. 2009), calculated from 24µm data from Spitzer and far ultraviolet maps from the GALEX Nearby Galaxy Survey (Gil de Paz et al. 2007). From the value given in the paper, we have substracted the Helium fraction they included as, as well as for the THINGS SFE, we have converted it from a Kroupa to a Salpeter IMF. All the values are shown in Table 2.9.
A comparison between the average values of the SFE shows no significant difference be- tween the SFE of galaxies in HCGs and the one in isolated or field galaxies, showing there is not a significant dependence of theMH2toLFIRrelation as a result of the environment in HCGs. We must note that, as detailed in Sec. 2.3.2, those galaxies not detected inMH2 nor inLFIRhave not