SDSS active galaxies

We compared the rates of diferent types of activity derived from SDSS data for AMIGA galaxies with the ones found for galaxies in denser environments. In§10.4 we compare the rate of nuclear activity found in other samples and its relation with the environment and in§10.4we compare the ratio of broad line AGN over narrow line AGN in different samples.

Rate of activity

The effect of the environment on the rate of AGN is not clear yet. While the studies ofCarter et al.(2001) andMiller et al.(2003) claim that this rate is constant,Dressler et al.(1985);Kauffmann et al.(2004) detect a decrease of this rate towards higher local densities whileReviglio & Helfand(2006) detect an increase of the fractional abundance of AGN with increasing density if passive galaxies with AGN activity (ra- dio AGN) are taken into account. Constantin & Vogeley(2006) andMontero-Dorta et al.(2009) found that Seyfert galaxies are less clustered and LINERs are more clus- tered in comparison to normal galaxies in the local universe. Alonso et al. (2007) found a slight increase (≈10%) in the rate of AGN in pairs of galaxies in comparison with field galaxies. These studies used optical spectra to determine the presence of an AGN (with the exception ofReviglio & Helfand 2006, which also used radio data) but their samples are defined in different ways and the selection criteria and definition for an “AGN” are usually different from one study to another.Sorrentino et al.(2006) require the detection of all the lines used in the three diagnostics diagrams (see Sec- tion7.6) to classify a galaxy.Dressler et al.(1985);Huchra & Burg(1992);Ho et al.

(1997a);Ivezi´c et al.(2002);Maia et al.(2004) andHao et al.(2005) also require this

10.4. SDSS ACTIVE GALAXIES 127 whileCarter et al.(2001);Miller et al.(2003) andMartinez(2008) allow the use of only the log([Oiii]/Hβ) - log([Nii]/Hα) diagram or even a ratio between [Oiii] and Hβor [Nii] and Hα, which is very similar to our criteria. The fraction of AGN in the studies that require all lines to be detected will decrease with respect with the fraction found in the other studies because [Sii] and [Oi] lines are usually weaker than the other involved lines. The subtraction or not of the underlying stellar population would also affect to the fraction of AGN galaxies. With the subtraction of the stellar popula- tion is possible to detect a higher number or weak lines which are usually present in AGN galaxies, hence, an increase in the fraction of galaxies classified as AGN is ex- pected in studies where the stellar populations are subtracted from the spectra. Some differeces could also arise from the different signal to noise ratio required by different studies for a line to be considered as a detection, e.g.,Carter et al.(2001) andMiller et al.(2003) used a 2σlimit. It is important to know which of the selection lines was used to consider a galaxy as an AGN beacuse it could change the fraction of AGN, e.g.,Miller et al.(2003) use a line 1σbelow the line ofKewley et al.(2001) (althought they do not explain it in the main text of the paper, it appears in the caption of their Figure 3 and, this limit has been confirmed using their data). Finally the selection criteria used to select the different samples could affect the final fraction of AGN, e.g., a higher cut in absolute magnitudes could enhance the fraction of AGN.

The fraction of AGN galaxies found in Sorrentino et al.(2006) is∼ 2%. This values are comparable to the ones found byDressler et al.(1985, 5% in the field and 1% in clusters),Huchra & Burg(1992, 1.3%), Ivezi´c et al.(2002, 5% using SDSS data) andMaia et al.(2004, 3-4%) and it lower that the fraction found byCarter et al.

(2001,∼ 17% using data from the 15R-North galaxy redshift survey), Miller et al.

(2003, 20-40% using SDSS data) andMartinez(2008, 44-46% for compact groups).

We compared the rates obtained byMartinez(2008) with our rates. They obtained the relative fractions for the different types of nuclear activity in a sample of HCG galaxies and in a sample of compact groups from the Updated Zwicky Catalogue (Fo- cardi & Kelm 2002, UZC-CG). The compact groups samples are defined with the help of isolation requirements and are small systems of several galaxies in a compact con- figuration. Galaxies of the group are affected by strong galaxy interactions which is confirmed by their high value of the “Tidal forces estimator” (see Section2.2), hence, compact groups are perfect to test the effects of galaxy interaction, being isolated sys- tems just in the opposite extreme to isolated galaxies in terms of tidal force strength.

In Table10.3and Table10.4we show the different number and rates for the different types of galaxies in the AMIGA complete subsample and the two samples studied by Martinez(2008).

CHAPTER10.COMPARISONWITHDIFFERENTENVIRONMENTS Table 10.3: Fraction of the different types of AMIGA galaxies and compact groups galaxies fromMartinez(2008). Fractions given with respect to the total number of galaxies.

Sample AMIGA HCG UZC-CG¹

N² frac³ N² frac³ N² frac³

Total: 226 100.0 269 100.0 397 100.0

Unclassified or non-emission: 16 7.1 101 37.5 123 31.0 Classified or emission⁴: 210 92.9 168 62.5 274 69.0

SFN 126 55.8 54 20.1 83 20.9

TO 35 15.5 36 13.4 50 12.6

AGN: 49 21.7 78 29.0 141 35.5

NLAGN total: 44 19.5 76 28.3 133 33.5

LINER 20 8.9 35 13.0 11 2.8

Sy2 10 4.4 24 8.9 43 10.8

unclassified NLAGN (LLAGN) 14 6.2 17 6.3 79 19.9

Sy1 5 2.2 2 0.7 8 2.0

TO+AGN: 84 37.2 114 42.4 191 48.1

[

1] UZC-CG subsample with spectra from FAST and the SDSS.

2 Number of galaxies.

3 Figures given in percentages.

4 Classified galaxies in the AMIGA sample and galaxies with emission in the compact groups sample.