2.4. Fe II emission and narrow line Seyfert 1 galaxies
2.4.1. Fe II emission
Nearly all broad line AGNs have optical Fe II emission in their spectrum (). The blend of Fe II lines between H and H consists of lines in multiplets 26,37,38,43 and 44 (). The Fe II strength is usually measured by the quantity R4570 = Fe II 4570 / H, i.e. the relative flux in the 4570 blend measured between 4434 and 4684 (see for instance ) and in H. Typical AGNs have R4570 ~ 0.4 with 90% of objects in the range 0.1 to 1 (; ). Moderately strong Fe II emission (R4570 > 1) occurs in perhaps 5% of objects, but superstrong Fe II emission (R4570 > 2) is roughly an order of magnitude rarer (). The known superstrong Fe II emitters (; ) are listed in table 2; most are luminous/ultraluminous IR AGNs ().
The Fe II lines have the same widths as the broad H lines, suggesting that they arise in the same region (). Fe II is the single largest contributor to the emission line spectrum (). The Fe II emission in most AGNs is probably too strong to be explained by photoionization; the Fe II lines could be collisionally excited in low temperature (6 000 < T < 8 000 K), high density (Ne > 1011 cm-3) clouds (; ; ; ).
Fe II is strong in objects with weak [O III] emission and vice versa (; ). The dominant source of variation in the observed properties of broad line AGNs is a physical parameter which balances Fe II excitation against the illumination of the narrow line region; the anti-correlation could be due to an increase of the covering factor of the BLR as one moves from the strong [O III], weak Fe II objects to the weak [O III], strong Fe II objects; this sort of behaviour is thought to be dependent on the ratio of the actual accretion rate to the ``Eddington accretion rate'' (); indeed, when M >> MEdd, the disk becomes geometrically thick as radiation pressure becomes competitive with gravity ().
While, for Seyfert 1s and radio quiet QSOs, the Fe II EW distribution extends from ~ 10 to 120Å, the steep spectrum radio sources almost all lie below 20Å. BLRGs and steep spectrum QSOs have weaker optical Fe II and stronger [O III] than either flat spectrum or radio quiet objects (; ; ).
There is a strong anticorrelation between R4570 and the FWHM of the broad H component (; ; ). In fact, the Fe II strength and line width seem to be most closely connected with continuum shape in general; strong Fe II emitters have steeper optical spectra, are more X-ray quiet, have steeper X-ray spectra and weaker blue bumps, weaker [O III], and absorption features from outflowing ionized material ().
Six out of 18 QSOs with weak [O III] and strong Fe II emissions were found to exhibit a C IV BAL (broad absorption line) region which is significantly larger than the overall fraction of QSOs observed to have BAL; this suggests a covering factor of 0.33 for the BAL region ().
The relatively rare BALQSOs which show strong Mg II absorption show especially strong Fe II emission and weak [O III] lines; they are also much redder in the interval ~ 1 550-2 200 Å than the other QSOs ().
All AGNs have strong UV Fe II lines (the Fe II bump at 2 500 Å), regardless of the strength of the optical Fe II lines; the intensity ratio of UV to optical Fe II emission ranges from 4 to 12 (Wills et al. 1985; ). The main factor determining the relative strength in the two wavelength bands are the Fe II line optical depths and the Balmer opacity; when the optical thickness increases, the intensity of the optical lines relative to the UV lines increases; a ratio of the order of 1 in the relative intensities of the optical to UV lines implies a column density of about 1023 to 1024 cm-2 (; ).