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Emission-line diagnostic analysis

The spectral characteristics of the 2dFGRS-NVSS galaxies

C A Jackson, D M Londish, PASA, 17 (3), 234.
Next Section: Summary
Title/Abstract Page: The spectral characteristics of
Previous Section: The sample of 2dFGRS-NVSS
Contents Page: Volume 17, Number 3

Emission-line diagnostic analysis



\psfig{file=pvzsel.figps,width=11cm,height=8.6cm,angle=-90}

Figure 3. Radio power as a function of redshift for the 32 narrow-emission-line 2dFGRS-NVSS galaxies. Symbols show the initial galaxy classification types: $\star$ Starburst galaxy and $\odot$ emission-line AGN.

We determine the emission line ratios for the narrow-line galaxy sample using the lines of H$\alpha$, H$\beta$, [OIII], [NII], [SII] and [OI]. Figure 3 shows the distribution of radio powers and redshifts for this sample. Comparing the distribution to Figure 1 we find we have an unbiased (P-z) sub-set of the complete sample.

Even though the 2dFGRS spectra are not flux calibrated, the pairs of emission lines were originally chosen to be close in wavelength. The line ratios are therefore accurately determined even for our uncalibrated spectra. The flux in each emission line is measured using the NOAO IRAF splot routine, fitting a Gaussian function to each emission feature. Diagnostic line ratios for [NII]$\lambda$6583/H$\alpha$, [SII]

$\lambda\lambda$6716,6731/H$\alpha$ and [OI]$\lambda$6300/H$\alpha$ are plotted in Figure 4 and the data summarised in Table 1.

\psfig{file=ratio_n2.figps,width=8cm,height=9cm,angle=-90}

\psfig{file=ratio_s2.figps,width=8cm,height=9cm,angle=-90}


\psfig{file=ratio_o1.figps,width=8cm,height=9cm,angle=-90}


$\textstyle \parbox{7cm}{{\bf Figure 4.} Diagnostic emission line diagrams for t... ...of residual night-sky features. Upper and lower limits are marked with arrows.}$






Table 1. Emission line ratios.

2dFGRS á  á  $\log_{10}$ Measured line ratios
Galaxy id bJ Redshift P

$_{1.4 \rm\thinspace GHz}\dagger$

 [OIII]/H$\beta$ [NII]/H$\alpha$ [SII]/H$\alpha$ [OI]/H$\alpha$
Initial classification SF á  á  á  á 
TGS166Z108 17.2 0.0801 21.5 0.477$\pm$0.020 0.369$\pm$0.003 0.282$\pm$0.003 0.053$\pm$0.002
TGS206Z164 14.6 0.0255 20.5 1.887$\pm$1.324 0.380$\pm$0.024 0.307$\pm$0.023 á 
TGS206Z015 17.4 0.0685 21.9 1.544$\pm$0.040 0.511$\pm$0.002 0.195$\pm$0.002 0.073$\pm$0.002
TGS232Z060 16.8 0.0590 21.2 0.443$\pm$0.046 0.514$\pm$0.007 0.342$\pm$0.007 0.069$\pm$0.005
TGS235Z125 14.8 0.0243 20.9 <0.167$\pm$0.093 0.345$\pm$0.008 0.061$\pm$0.007 0.016$\pm$0.006
TGS236Z095 17.6 0.1420 20.6 <0.187$\pm$0.048 0.390$\pm$0.005 0.262$\pm$0.004 <0.021$\pm$0.003
TGS236Z091 17.0 0.0552 21.2 0.687$\pm$0.210 <0.364$\pm$0.012 <0.320$\pm$0.011 <0.051$\pm$0.009
TGS236Z194 15.5 0.0237 20.5 0.557$\pm$0.144 0.459$\pm$0.009 0.319$\pm$0.008 0.017$\pm$0.006
TGS238Z047 16.5 0.0215 20.6 0.271$\pm$0.009 <0.391$\pm$0.001 0.338$\pm$0.001 0.016$\pm$0.001
TGS238Z180 17.2 0.1255 21.9 0.942$\pm$0.197 0.519$\pm$0.026 0.424$\pm$0.024 <0.020$\pm$0.017
TGS239Z196 14.8 0.0219 20.4 0.798$\pm$0.455 0.266$\pm$0.016 >0.093$\pm$0.014 á 
TGS318Z156 18.3 0.0687 21.3 0.799$\pm$0.047 0.789$\pm$0.004 0.305$\pm$0.003 0.071$\pm$0.002
TGN218Z230 15.9 0.0201 20.3 0.654$\pm$0.142 0.452$\pm$0.011 0.245$\pm$0.010 á 
TGN220Z065 17.3 0.0492 21.2 <4.340$\pm$1.000 >0.398$\pm$0.018 0.586$\pm$0.021 0.148$\pm$0.015
TGN222Z132 16.7 0.0505 21.1 1.633$\pm$0.329 <0.628$\pm$0.009 <0.400$\pm$0.008 á 
TGN231Z143 15.5 0.0314 20.6 0.647$\pm$0.124 0.383$\pm$0.007 0.271$\pm$0.007 á 
TGN239Z061 17.5 0.0554 21.2 1.902$\pm$0.076 0.252$\pm$0.004 0.387$\pm$0.005 0.050$\pm$0.003
TGN239Z082 18.8 0.1412 21.9 <1.050$\pm$0.054 0.455$\pm$0.007 0.298$\pm$0.006 >0.073$\pm$0.005
TGN239Z221 17.4 0.0430 21.1 0.349$\pm$0.015 0.440$\pm$0.002 0.251$\pm$0.001 0.029$\pm$0.001
TGN239Z172 16.2 0.0459 21.5 1.026$\pm$0.229 0.470$\pm$0.007 <0.191$\pm$0.005 <0.049$\pm$0.005
TGN240Z019 19.3 0.1216 22.1 1.314$\pm$0.216 0.258$\pm$0.024 0.517$\pm$0.029 á 
TGN309Z233 17.9 0.0677 21.4 0.817$\pm$0.062 0.361$\pm$0.004 0.286$\pm$0.004 0.036$\pm$0.003
á  á  á  á  á  á  á  á 
Initial classification AGN á  á  á  á 
TGS207Z011 14.2 0.0355 22.0 1.649$\pm$0.035 3.845$\pm$0.014 2.817$\pm$0.011 0.553$\pm$0.004
TGS218Z173 18.6 0.0911 22.1 7.170$\pm$0.499 0.191$\pm$0.012 >0.335$\pm$0.013 0.020$\pm$0.010
TGS220Z128 18.9 0.1298 21.9 20.817$\pm$5.495 0.391$\pm$0.042 >0.265$\pm$0.038 0.081$\pm$0.033
TGS233Z084 18.4 0.0643 21.4 2.972$\pm$0.554 0.096$\pm$0.019 0.361$\pm$0.023 á 
TGS234Z066 18.9 0.1503 22.9 <7.101$\pm$0.098 0.373$\pm$0.006 0.366$\pm$0.006 0.047$\pm$0.005
TGS313Z081 17.7 0.1220 23.4 0.306$\pm$0.271 0.334$\pm$0.039 >0.165$\pm$0.034 á 
TGS313Z100 17.1 0.1457 22.8 <3.000$\pm$0.741 0.580$\pm$0.086 0.374$\pm$0.075 á 
TGN231Z211 16.4 0.0671 21.3 7.402$\pm$1.041 0.616$\pm$0.009 0.388$\pm$0.008 0.029$\pm$0.006
TGN239Z017 18.4 0.1150 22.0 2.976$\pm$1.282 >0.814$\pm$0.064 0.511$\pm$0.054 á 
TGN238Z202 19.3 0.1602 23.3 1.323$\pm$0.103 0.311$\pm$0.011 0.301$\pm$0.011 0.049$\pm$0.009

á  á  á  á  á  á  á 

$^{\dagger}$ units W Hz-1 sr-1.


The diagnostic diagrams reveal that the majority of our narrow-line galaxy sample has [NII]$\lambda$6583/H$\alpha$, [SII]$\lambda$6716+$\lambda$6731/H$\alpha$ and [OI]$\lambda$6300/H$\alpha$ ratios which straddle the region dividing AGN and starburst galaxies. This is in contrast to optically-selected galaxy samples, whose narrow-line emission characteristics clearly delineate those with a hard ionizing radiation source (AGNs: right-hand side of curves in Figure 4) and those with hot OB stars (starburst, left-hand side of curves). Similar results to our line ratio distribution have been found for other radio-selected samples (e.g. Georgakakis et al. 1999) as well as IR-selected galaxy samples (Kewley et al. 2000).

In Table 2 we show how the galaxies are classified based on the individual emission-line ratio values. Where the errors associated with the line ratio cross the empirical dividing line, the classification is given as A? or S?. For the cases where the ratio lies right on the dividing line, the classification is given as ?.

We find 4 galaxies with different line-ratio classifications than the `eyeball' classifications of Sadler et al. (1999):

1) TGN220Z065: Starburst galaxy, line ratio diagnostics indicate it is an AGN. However, the H$\beta$ measurement is a lower limit so that the the [OIII]/H$\beta$ value is over-estimated.

2) TGN222Z132: Starburst galaxy, line ratio diagnostics indicate it is an AGN. However, the H$\alpha$ measurement is a lower limit so that the [SII]/H$\alpha$ value is over-estimated.

3) TGN238Z202: AGN galaxy, line ratio diagnostics indicate it is a starburst galaxy. Re-examining the 2dFGRS spectrum shows dominant Balmer lines characteristic of a starburst galaxy.

4) TGS313Z081: AGN galaxy, line ratio diagnostics indicate it is a starburst galaxy. Re-examining the 2dFGRS spectrum reveals strong H$\alpha$, weak H$\beta$ and the 5577ö…- line coincident with [OIII, 5007ö…]. On balance this probably should be classified as a starburst galaxy.

Thus the diagnostic line ratios confirm 30 out of 32 of the `eyeball' classifications. However, the frequency of ambiguous classifications from the line-ratio diagnostics (indicated as A?, S? and ? in Table 3) indicates that a radio-selected sample does not clearly separate into the starburst and AGN regions defined by optically-selected samples.


Table 2. Emission-line ratio classifications

á  á  á  á  Line ratio classification
2dFGRS á  á  $\log_{10}$ [NII]/ [SII]/ [OI]/
Galaxy id Mag bJ Redshift P

$_{1.4 \rm\thinspace GHz}\dagger$

 H$\alpha$ H$\alpha$ H$\alpha$
Initial classification SF á  á  á  á 
TGS166Z108 17.2 0.0801 21.5 S S S
TGS206Z164 14.6 0.0255 20.5 S? S? -
TGS206Z015 17.4 0.0685 21.9 ? S A
TGS232Z060 16.8 0.0590 21.2 S S ?
TGS235Z125 14.8 0.0243 20.9 S S S
TGS236Z095 17.6 0.1420 20.6 S S S
TGS236Z091 17.0 0.0552 21.2 S S S
TGS236Z194 15.5 0.0237 20.5 S S S
TGS238Z047 16.5 0.0215 20.6 S S S
TGS238Z180 17.2 0.1255 21.9 S S S
TGS239Z196 14.8 0.0219 20.4 S S -
TGS318Z156 18.3 0.0687 21.3 ? S A
TGN218Z230 15.9 0.0201 20.3 S S -
TGN220Z065 17.3 0.0492 21.2 A? A A
TGN222Z132 16.7 0.0505 21.1 A ? -
TGN231Z143 15.5 0.0314 20.6 S S -
TGN239Z061 17.5 0.0554 21.2 S ? ?
TGN239Z082 18.8 0.1412 21.9 S S A
TGN239Z221 17.4 0.0430 21.1 S S S
TGN239Z172 16.2 0.0459 21.5 S S S
TGN240Z019 19.3 0.1216 22.1 S A -
TGN309Z233 17.9 0.0677 21.4 S S S
á  á  á  á  á  á  á 
Initial classification AGN á  á  á  á 
TGS207Z011 14.2 0.0355 22.0 A A A
TGS218Z173 18.6 0.0911 22.1 A A S
TGS220Z128 18.9 0.1298 21.9 A A A
TGS233Z084 18.4 0.0643 21.4 S A -
TGS234Z066 18.9 0.1503 22.9 A A A
TGS313Z081 17.7 0.1220 23.4 S S -
TGS313Z100 17.1 0.1457 22.8 A? A? -
TGN231Z211 16.4 0.0671 21.3 A A ?
TGN239Z017 18.4 0.1150 22.0 A A -
TGN238Z202 19.3 0.1602 23.3 S S S

á  á  á  á  á  á 

$^{\dagger}$ units W Hz-1 sr-1.



Table 3. Star formation rates from IR and Radio flux densities.


á  á  Star-formation rate
2dFGRS $\log_{10}$

$M \ge 5M_{\odot}$ yr-1
galaxy id P

$_{1.4 \rm\thinspace GHz}\dagger$

 Radio

$_{1.4 \rm\thinspace GHz}$

 IR

$_{60 \rm\thinspace \mu m}$
TGS166Z108 21.5 9.3 7.1
TGS206Z164 20.5 0.9 0.7
TGS206Z015 21.9 26.4 77.7
TGS232Z060 21.2 5.4 4.2
TGS235Z125 20.9 2.7 2.1
TGS236Z095 20.6 1.2 1.1
TGS236Z091 21.2 4.9 2.9
TGS236Z194 20.5 1.1 1.0
TGS238Z047 20.6 1.2 1.3
TGS238Z180 21.9 23.9 11.0
TGS239Z196 20.4 0.8 0.5
TGS318Z156 21.3 6.4 8.8
TGN218Z230 20.3 0.6 0.6
TGN220Z065 21.2 5.1 2.6
TGN231Z143 20.6 1.4 1.0
TGN239Z061 21.2 4.3 4.3
TGN239Z172 21.5 9.7 7.4

á  á  á 

$^{\dagger}$ units W Hz-1 sr-1.


Further evidence as to the composite nature of the starburst galaxies can be found by comparing the star-formation rates indicated by the radio and IR flux densities. Table 3 shows the flux densities and the implied star-formation rates for massive stars ($> 5M_{\odot}$, Cram et al. (1998)), using 60$\mu$m flux density data compiled by Sadler et al. (1999) from the IRAS Point Source and Faint Source catalogues. Two galaxies stand out as having very discrepant implied star-formation rates: (1) TGS206Z015 has an IR excess and (2) TGS238Z180 has a radio excess: in both cases the UKST IIIaJ images show the galaxies are undergoing mergers which is possibly enhancing or triggering starburst activity in these objects.

Furthermore we suggest that the majority of the narrow-line galaxies whose line ratios are strung out between the two regions (starforming and AGN) are composite galaxies: The individual line ratios lie along mixing lines whose position on the diagnostic plots is determined by the admixture of AGN and starburst components as shown in Figure 5.



\psfig{file=ratio_n2res.figps,width=7cm,height=8cm,angle=-90}


$\textstyle \parbox{7cm}{{\bf Figure 5.} Diagnostic emission line ratios for [NI... ..., with increasing AGN contributions lying at higher [OIII]/H$\beta$\ values. }$












Next Section: Summary
Title/Abstract Page: The spectral characteristics of
Previous Section: The sample of 2dFGRS-NVSS
Contents Page: Volume 17, Number 3

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