X- RAY Groups of Galaxies in the AEGIS deep and wide fields
4.3 Results
4.3 Results 97
0 0.1 0.2
−4.2 −3.6 −3 −2.4 −1.8 −1.2 −0.6 0 0.6 1.2 1.8
Normalized number
SFRobs−SFRMS 9 < log(M*/MO·) < 9.5
0 0.1 0.2 0.3
−4.2 −3.6 −3 −2.4 −1.8 −1.2 −0.6 0 0.6 1.2 1.8
Normalized number
SFRobs−SFRMS 9.5 < log(M*/MO·) < 10
0 0.1 0.2
−4.2 −3.6 −3 −2.4 −1.8 −1.2 −0.6 0 0.6 1.2 1.8
Normalized number
SFRobs−SFRMS 10 < log(M*/MO·) < 10.2
0 0.1 0.2
−4.2 −3.6 −3 −2.4 −1.8 −1.2 −0.6 0 0.6 1.2 1.8
Normalized number
SFRobs−SFRMS 10.2 < log(M*/MO·) < 10.4
0 0.1 0.2 0.3
−4.2 −3.6 −3 −2.4 −1.8 −1.2 −0.6 0 0.6 1.2 1.8
Normalized number
SFRobs−SFRMS 10.4 < log(M*/MO·) < 10.6
0 0.1 0.2
−4.2 −3.6 −3 −2.4 −1.8 −1.2 −0.6 0 0.6 1.2 1.8
Normalized number
SFRobs−SFRMS 10.6 < log(M*/MO·) < 10.8
0 0.1 0.2
−4.2 −3.6 −3 −2.4 −1.8 −1.2 −0.6 0 0.6 1.2 1.8
Normalized number
SFRobs−SFRMS 10.8 < log(M*/MO·) < 11
0 0.1 0.2
−4.2 −3.6 −3 −2.4 −1.8 −1.2 −0.6 0 0.6 1.2 1.8
Normalized number
SFRobs−SFRMS 11 < log(M*/MO·) < 12
Figure 4.3: ∆SFR= SFRobs-SFRM S for 0.15<z<0.5 in different stellar mass bins. The bright blue histograms show just IR detected galaxies and the dark blue histograms show galaxies with IR and SED SFR simultaneously.
distribution of the residuals around the peak of the MS at any given stellar mass bin. The location of the peak and the dispersion of the relation is highly consistent between the IR detected galaxy subsample and the whole sample. This assures that we do not have to worry about biases due to our estimate of the SFR via SED technique. Thus, to have a better statistics we use the whole galaxy sample for our analysis.
Both low-z and high-z sample clearly show a deviation from 0 (consistent with the location of the linear MS) in the peak of ∆SFR starting since 10.4 <log(M∗/M⊙)<10.6.
We estimate the mean deviation from the MS by isolating the Main sequence galaxies at any mass bin as those with |∆SFR| < 1 (consistent with a 3σ cut according to the values of the dispersion reported in the literature) from a first guess of the peak of the Gaussian distribution. We fit both the peak and the dispersion of the Gaussian iteratively by selecting at any iteration all galaxies with |∆SFR | < 3×σ, where σ is the Gaussian dispersion. This is done until we reach a stable solution. Fig. 4.5 shows the mean deviation from the MS and the dispersion of the Gaussian distribution as a function the galaxy stellar mass. Above 10.4 < log(M∗/M⊙) < 10.6 we observe a deviation of ∼0.5 dex at 1011 M⊙
and ∼0.6-0.8 dex at 1011.5 M⊙ towards the quiescence region. In addition, as shown in the
0 0.1 0.2 0.3
−4.2 −3.6 −3 −2.4 −1.8 −1.2 −0.6 0 0.6 1.2 1.8
Normalized number
SFRobs−SFRMS 9 < log(M*/MO·) < 9.5
0 0.1 0.2 0.3
−4.2 −3.6 −3 −2.4 −1.8 −1.2 −0.6 0 0.6 1.2 1.8
Normalized number
SFRobs−SFRMS 9.5 < log(M*/MO·) < 10
0 0.1 0.2 0.3
−4.2 −3.6 −3 −2.4 −1.8 −1.2 −0.6 0 0.6 1.2 1.8
Normalized number
SFRobs−SFRMS 10 < log(M*/MO·) < 10.2
0 0.1 0.2 0.3
−4.2 −3.6 −3 −2.4 −1.8 −1.2 −0.6 0 0.6 1.2 1.8
Normalized number
SFRobs−SFRMS 10.2 < log(M*/MO·) < 10.4
0 0.1 0.2
−4.2 −3.6 −3 −2.4 −1.8 −1.2 −0.6 0 0.6 1.2 1.8
Normalized number
SFRobs−SFRMS 10.4 < log(M*/MO·) < 10.6
0 0.1 0.2
−4.2 −3.6 −3 −2.4 −1.8 −1.2 −0.6 0 0.6 1.2 1.8
Normalized number
SFRobs−SFRMS 10.6 < log(M*/MO·) < 10.8
0 0.1 0.2 0.3
−4.2 −3.6 −3 −2.4 −1.8 −1.2 −0.6 0 0.6 1.2 1.8
Normalized number
SFRobs−SFRMS 10.8 < log(M*/MO·) < 11
0 0.1 0.2 0.3
−4.2 −3.6 −3 −2.4 −1.8 −1.2 −0.6 0 0.6 1.2 1.8
Normalized number
SFRobs−SFRMS 11 < log(M*/MO·) < 12
Figure 4.4: ∆SFR= SFRobs-SFRM S for 0.5<z<1.1 in different stellar mass bins. The bright red histograms show just IR detected galaxies and the dark red histograms show galaxies with IR and SED SFR simultaneously.
left panel, above the same mass threshold, we observe also an increase of the dispersion around the MS location at high masses. The scatter of the MS is of 0.3-0.4 dex below 10.4<log(M∗/M⊙)<10.6, consistent with the value reported by many of previous studies (Daddi et al. 2007; Elbaz et al. 2007; Peng et al. 2010). It increases to 0.6-0.7 dex at higher masses. In the high mass range the dispersion seems to be larger at low redshift (∼ 0.7 dex) with respect to the high redshift MS (∼ 0.5 dex). The final appearance of the MS estimated as explained above is shown in Fig. 4.6 in the low (left panel) and high (right panel) redshift bins.
We also fit the best relation with two different exponential laws above and below M∗ = 1010.4M⊙. The best fit relation is reported below in the two redshift bins.
0.15< z <0.5 :
log(SF R) = (0.97±0.004)log(M∗)−(9.15±0.04), log(M∗)<10.4 (4.1)
log(SF R) = (0.27±0.02)log(M∗)−1.99(±0.19), log(M∗)≥10.4 (4.2)
4.3 Results 99
0.3 0.4 0.5 0.6 0.7 0.8
9 9.5 10 10.5 11 11.5 12
Dispersion
Stellar Mass [MO• ] 0.15<z<0.5
0.5<z<1.1
-1 -0.8 -0.6 -0.4 -0.2 0 0.2
9 9.5 10 10.5 11 11.5 12
Deviation from MS
Stellar Mass [MO• ] 0.15<z<0.5
0.5<z<1.1
Figure 4.5: Left panel: dispersion around the MS location as a function of the galaxy stellar mass. Right panel: peak of the residual ∆(SF R) as a function of the galaxy stellar mass.
0.5< z <1 :
log(SF R) = (1.1±0.005)log(M∗)−(10.16±0.04), log(M∗)<10.4 (4.3) log(SF R) = (0.24±0.09)log(M∗)−(1.37±0.09), log(M∗)≥10.4 (4.4) In both cases the slope of the relation below M∗ <1010.4M⊙ is consistent with a linear relation consistently with the results in the literature. Above this mass limit the relation is much flatter, which is in agreement with the most recent findings of Rodighiero et al.
(2010) and Whitaker et al. (2012). We use this relation to identify all MS galaxies as those within±3σ(see Fig. 4.7 and 4.8 for the new residual distribution at low and high redshift, respectively) from these best fit relations as a function of the stellar mass. This MS galaxy sample will be used in the next paragraph to study the dependence of the dispersion and location of the MS from the environment.
4.3.2 The role of the environment in the shape and dispersion of the MS
In order to check whether the slope of the MS at higher masses and the increase of its dispersion as a function of the stellar mass is environment dependent, we analyse separately the location of the MS and its dispersion in the three environmental classes defined above:
field galaxies, “filament-like” galaxies and group galaxies. In order to define the MS in these three environment we perform the same exercise described above by looking for the location of the peak of the Gaussian distribution of the residual ∆SFR with respect to the best fit MS defined by the two exponential laws at low and high stellar masses.
-3 -2 -1 0 1 2
7 8 9 10 11 12
SFR [MO• yr-1 ]
Stellar Mass [MO• ] 0.15<z<0.5
IR+SED
-2 -1 0 1 2 3
7 8 9 10 11 12
SFR [MO• yr-1 ]
Stellar Mass [MO• ] 0.5<z<1
IR+SED
Figure 4.6: left panel :SFR vs. M∗ for galaxies in 0.15<z<0.5. The black dots show the peak of distribution of SFR in different mass bins. right panel: Same as left panel for galaxies in 0.5<z<1.
Fig. 4.9 shows the mean MS for field galaxies (grey connected points), “filament-like”
galaxies (green connected points) and group galaxies (violet connected points) in the low redshift bin (left panel) and in the high redshift bin (right panel). These two plots are revealing the following information:
- The flattening of the MS at stellar masses above 1010.4−10.6 M⊙ is clearly visible in all environments. The flattening is in place already at z∼1.
- Below the stellar mass threshold of 1010.4−10.6 M⊙ the MS is the same in all three environments both at high and low redshift.
- Above this mass threshold we observe a different behaviour of MS galaxies in the three different environments. At low redshift, group galaxies show a significant departure from the mean MS and an even flatter MS. These galaxies seem to deviate from the MS at lower masses (∼ 1010 M⊙) with respect to the MS galaxies in the other environments. At higher redshifts, groups member galaxies do not deviate from the mean relation and their MS coincides with the MS of the other two environments.
- Field (isolated) and “filament-like” galaxy MS are perfectly consistent at any redshift.
This shows that the relative vicinity of galaxies as expressed by the density field is not playing an important role in affecting and/or regulating the galaxy SF activity.
This, in addition to the blue galaxies selection, explains why Peng et al. (2010) did not observe any difference between the MS location of galaxies at different densities.
Fig. 4.9 indicates clearly that the evolution of the star formation activity in galaxies does not simply depend on the galaxy stellar mass as suggested by Peng et al. (2010) though galaxy internal process (e.g. AGN feedback) but it must be regulated by the