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A suite of three emission line diagnostic diagrams has been used to classify the dom- inant energy source in narrow emission line galaxies [22]. These diagrams are based on the four optical line ratios: [OIII]/Hβ, [N II]/Hα, [SII]/Hα and [OI]/Hα. Although other ratios, such as [OII]/[OIII], offer cleaner separation between plan-

etaries and HII regions [22], combinations such as [N II]/Hα versus [OIII]/Hβ and

[SII]/Hα versus [OIII]/Hβ not only are very accurate in distinguishing the dom- inant component of narrow line galaxies but are also very insensitive to reddening and do not require the samples to be flux calibrated. This is due to the fact that the lines in these pairs are chosen to be close in wavelength and as a result the line ratios are accurately determined even for uncalibrated spectra. This is extremely useful in the case of our sample which is not flux calibrated due to problems associated with the set up of flats during some observations which prevented us from flux calibrating the entire sample.

We have identified a total of 30 sources with very secure detections of all 5 lines, [OIII], Hβ, [N II], Hα, [SII]. The flux in each emission line is measured us- ing IRAF’s SPLOT routine, by interactively fitting a Gaussian function to each emission feature line profile and then integrating. A table of the resulting fluxes measurements and examples of spectra used are given in Table 4.1 and Figure 4.6 respectively.

Figure 4.7 shows the [N II]/Hα versus [OIII]/Hβ diagnostic diagram for our sam- ple of 30 sources. The red line represents the most conservative AGN rejection criterion and they are the sources with very low contribution to Hα from AGN [43]. This line is similar to Kauffmann’s [166] pure starburst line which has included this line to distinguish between pure star-forming sources and composite objects whose spectra contain significant contributions from both AGN and star formation. Ev- erything below this line are classed as HII region like galaxies. The blue line is the line developed by Kewley [175]. This line uses a combination of stellar popula- tion synthesis models and detailed self consistent photoionization models to create

Figure 4.7: The [N II]/Hα versus [OIII]/Hβ diagnostic diagram for all 30 sources with available lines. The red line is the pure star formation line [166]. Blue line is the extreme starburst line [175]. The green lines is the Seyfert/LINER line [147] . Open black circles are sources with X-ray detections from the ELAIS-N1 Deep X-ray survey [222]. Red, blue and green colors represent sources fitted with a cirrus, starburst or dust torus template respectively.

a theoretical maximum starburst line. Everything that lies above this line should be AGN dominated. All sources that lie between these two lines are classed as com- posites and their spectra can be either due to a combination of star formation and a Seyfert nucleus or due to a combination of star formation and LINER emission. The green solid lines represents Ho’s classification scheme [147] where everything

below the Kewley line and left from the green line is classified as HII nuclei, upper

left represents the Seyfert nuclei sources, upper right box are LINER and low right box are the transition objects. Figure 4.8 shows the [SII]/Hα versus [OIII]/Hβ diagnostic diagram for the same sample as before. The black line separates between AGN and star forming galaxies while the red line provides an empirical division between LINER and Seyfert sources [176]. Green lines are the same as Figure 4.7.

According to the above classifications we have five different categories of sources in these two diagrams:

(i) Star forming galaxies : Those sources that lie below the pure starburst line in the [N II]/Hα versus [OIII]/Hβ diagram and below the AGN/Starburst line in the [SII]/Hα versus [OIII]/Hβ.

Figure 4.8: The [SII]/Hα versus [OIII]/Hβ diagnostic diagram for all 30 sources with available lines. The red line is the Seyfert/LINER line [176] and the black line is the AGN/starburst line [22]. Green lines and different colors and symbols same as Figure 4.7.

(ii) Seyfert : Those sources that lie above the extreme starburst line in the [N II]/Hα versus [OIII]/Hβ diagram, above the the AGN/Starburst line in the [SII]/Hα versus [OIII]/Hβ diagram and above the Seyfert/LINER line in the same diagram.

(iii) Composite Objects : Sources that lie between the pure starburst and ex- treme starburst lines in the [N II]/Hα versus [OIII]/Hβ diagram.

(iv) LINER : Objects that lie above the extreme starburst line in the [N II]/Hα versus [OIII]/Hβ diagram and below the Seyfert/LINER separation line in the [SII]/Hα versus [OIII]/Hβ diagram.

(v) Ambiguous objects : Those objects which in the [N II]/Hα versus [OIII]/Hβ diagrma appear to be composite and lie above or below the starburst/AGN separation line in the [SII]/Hα versus [OIII]/Hβ diagram.

Based on the [N II]/Hα versus [OIII]/Hβ diagram from the 30 sources in our sample we have found 14 pure starforming sources, 9 composite objects, 3 of which appear to be LINERs and 7 narrow line AGN of which 1 appears to be a LINER. In the same plot we are using different colour coding for the different infrared SED types. Sources with X-ray detections from the ELAIS-N1 Deep X-ray Survey [222]

are plotted as open black circles. According to the [SII]/Hα versus [OIII]/Hβ diagnostic 6 sources are Seyferts, 6 are LINERs and the rest appear to be star- forming galaxies. In general there is good agreement between the two diagnostics. From the 21 sources that were classified as pure star-forming or AGN dominated in the [N II]/Hα versus [OIII]/Hβ diagram there are only three cases which are classified differently in the [SII]/Hα versus [OIII]/Hβ diagnostic.

There are twelve sources in our sample best fitted with a cirrus infrared template. Ten of them are fitted with a galaxy optical template and two are fitted with a star- burst template. According to both emission line diagnostics seven of them are pure star-forming sources and one is AGN dominated. For the remaining four, emission line diagnostics are not clear whether they are star-forming, AGN or composites. For all 10 sources that are classified as star-forming or composites, SED fitting iden- tifies a strong starburst component that contributes between 20-40% to the 8µm emission which may mean that this contribution can be much higher in mid and far-infrared wavelengths. For the two cirrus sources that appear to be narrow-line AGN, infrared show no evidence of the presence of a torus.

Fifteen sources are best fitted with a starburst infrared template. Agreement be- tween SED fitting and spectroscopy for these sources is very good. For the 5 sources that were identified as star-forming from spectroscopy, template fitting finds a domi- nant star-formation component that contributes up to 65-100% to the infrared emis- sion. All six composites have composites SEDs with AGN contributing between 20 and 50% to the 8µm emission. In the case of all three narrow line AGN infrared has identified a strong torus component contributing between 35-45%. Finally, for the remaining spectroscopically ambiguous sources, SED fitting finds no evidence of a torus component.

In the case of the three sources fitted with a torus template there is an excellent agreement between spectroscopy and ImpZ code. All sources appear to be compos- ite in the infrared with AGN dominating in all three cases consistent with emission line diagnostics.

If we assume that the different spectroscopic diagnostics used here provide us with a 90% accuracy, taking into account that 3 out of 30 sources have been classified differently from the two diagrams, then X-ray achieve less than 70% since from the three luminous AGN in X-ray one appears to be either a pure star-forming galaxy

Figure 4.9: IRAC color-color plot using available data from SWIRE. Triangles represent the 123 sources with spectroscopic redshifts from GMOS/WIYN and

significant detections in all IRAC bands. Red triangles represent the sources

which are fitted with a QSO optical template. Solid line is the AGN area as defined by Lacy [185] .

or a composite object. On the other hand infrared/optical SED fitting is doing very well in isolating star forming galaxies from narrow line AGN and identifying composite objects and it reaches the same accuracy with spectroscopic diagnostics.

Although there are differences between different diagnostics it is essential to combine information from different wavelengths to be able to get a full picture of the wide range of properties of AGN and galaxies at different wavebands.