The Astrophysical Journal Letters, 774:L10 (6pp), 2013 September 1 doi:10.1088/2041-8205/774/1/L10
C
2013. The American Astronomical Society. All rights reserved. Printed in the U.S.A.
THE COSMIC BPT DIAGRAM: CONFRONTING THEORY WITH OBSERVATIONS
Lisa J. Kewley
1,2
, Christian Maier
3
, Kiyoto Yabe
4
, Kouji Ohta
5
, Masayuki Akiyama
6
,
Michael A. Dopita
1,7
, and Tiantian Yuan
1
1
Australian National University, Research School for Astronomy & Astrophysics, Mount Stromlo Observatory, Cotter Road, Weston, ACT 2611, Australi a;
kewley@mso.anu.edu.au
2
Institute of Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822, USA
3
University of Vienna, Department of Astrophysics, Tuerkenschanzstrasse, 17, 1180 Vienna, Austria
4
Department of Optical and Infrared Astronomy, National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo, 181-8588, Japan
5
Department of Astronomy, Faculty of Science, Kyoto University, Kyoto, 606-8502, Japan
6
Astronomical Institute, Tohoku University, 6-3 Aramaki, Aoba-ku Sendai, Japan 980-8578
7
Department of Astronomy, King Abdulaziz University, P.O. Box 80203 Jeddah, Saudi Arabia
Received 2013 May 20; accepted 2013 June 21; published 2013 August 21
ABSTRACT
We compare a large sample of galaxies between 0.5 <z<2.6 with theoretical predictions for how the optical
diagnostic line ratios in galaxy ensembles change as a function of cosmic time. We show that star-forming galaxies
at high redshift (z>1.5) are consistent with a model in which the interstellar medium conditions are more extreme
at high redshift than seen in the global spectra of local galaxies. We speculate that global spectra of our high-redshift
galaxies may be dominated by H ii regions similar to the extreme clumpy, dense star-forming complexes in the
Antennae and M82. The transition to local-type conditions occurs between 0.8 <z<1.5. We conclude that
classification schemes developed for local samples should not be applied at high redshift (z 1.5). We use our
theoretical models to derive a new redshift-dependent classification line that utilizes the standard optical diagnostic
line ratios [O iii]/Hβ and [N ii]/Hα. Our new line can be used to separate star-forming galaxies from active galactic
nuclei (AGN) between z = 0toz ∼ 3.5. We anticipate that our redshift-dependent optical classification line will
be useful for future large surveys with near-infrared multi-object spectrographs. We apply our classification line
to a sample of gravitationally lensed galaxies at z ∼ 2.5. Although limited by small numbers, we show that our
classification line is consistent with the position of AGN that have been independently confirmed via other methods.
Key words: galaxies: abundances – galaxies: active – galaxies: starburst
Online-only material: color figures
1. INTRODUCTION
Understanding how the fundamental physical properties of
ensembles of galaxies change across cosmic time is one of the
primary probes of galaxy evolution. Spectral classification is
crucial for this work. Classification of galaxies according to
their dominant power source is required f or measuring the star
formation history of galaxies (e.g., Madau et al. 1996; Lilly
et al. 1996; Hopkins & Beacom 2006; Sobral et al. 2013, and
many others), studying the main sequence for star-forming (SF)
galaxies (Noeske et al. 2007), understanding the metallicity
history of galaxies (Kobulnicky & Kewley 2004; Zahid et al.
2011; Yuan et al. 2013), and disentangling the cosmic evolution
of the starburst versus active galactic nucleus (AGN) fraction
(Ramos Almeida et al. 2013).
Baldwin et al. (1981) first proposed the [N ii]/Hα versus
[O iii]/Hβ diagnostic diagram (now known as the BPT diagram)
to separate normal H ii regions, planetary nebulae, and objects
photoionized by a harder radiation field. A hard radiation field
can be produced by either a power-law continuum from an
AGN or from shock excitation. Veilleux & Osterbrock (1987)
extended and refined this classification scheme. They used
theoretical photoionization models to inform the shape of the
classification line between SF and AGN galaxies, and they
added two new diagnostic diagrams that exploit the [S ii]/Hα
and [O i]/Hα line ratios. Kewley et al. (2001) combined stellar
population synthesis and photoionization models to build the
first purely theoretical classification scheme for separating pure
AGN from galaxies containing star formation.
The Sloan Digital Sky Survey revolutionized optical classi-
fication schemes, allowing the shape and position of the local
SF abundance sequence and the local starburst-AGN mixing
sequence to be cleanly characterized (Kauffmann et al. 2003;
Kewley et al. 2006).
Spectral classification beyond z>0.8 has been difficult
in the past. The [N ii]/Hα ratio is redshifted into the near-
infrared (NIR) at z ∼ 0.5, while at redshifts z ∼ 1.5, all of the
standard optical diagnostic ratios are in the NIR. Fortunately,
NIR multi-object spectrographs on 8–10 m telescopes now
allow statistically significant numbers of galaxies to be classified
according to their dominant power source.
The [N ii]/Hα and [O iii]/Hβ ratios have now been observed
for small numbers of galaxies at high redshift (e.g., Erb et al.
2006; Hainline et al. 2009; Bian et al. 2010; Rigby et al. 2011;
Yabe et al. 2012 and references therein). The majority of these
galaxies exhibit larger [N ii]/Hα and [O
iii]/Hβ ratios than seen
in local SF galaxies. Brinchmann et al. (2008) used stellar
population synthesis and photoionization models to show that
local galaxies with large [N ii]/Hα and [O iii]/Hβ ratios have
a larger ionization parameter than the general SF population.
They suggest that higher electron densities and a larger escape
fraction of H-ionizing photons may be responsible for the larger
ionization parameter.
In Kewley et al. (2013), we combined the chemical evo-
lution predictions of cosmological hydrodynamic simulations
with stellar evolutionary synthesis and photoionization models
to predict how the optical emission-line ratios in SF and AGN
populations will change with redshift for four different model
1
The Astrophysical Journal Letters, 774:L10 (6pp), 2013 September 1 Kewley et al.
scenarios. We showed that the position of SF galaxies may
change significantly as a function of redshift if the interstellar
medium (ISM) conditions and/or the s tellar ionizing (EUV) ra-
diation field within the SF population changes with cosmic time.
In this Letter, we compare our new theoretical predictions
with a statistically significant sample of galaxies between
0.8 <z<3 for which NIR spectra are available. We speculate
on the SF conditions in high-redshift galaxies, and we develop
a new optical classification scheme that can be applied between
0 <z<3.5. We adopt the flat Λ-dominated cosmology from
the7yrWilkinson Microwave Anisotropy Probe experiment
(h = 0.72, Ω
m
= 0.29; Komatsu et al. 2011).
2. SAMPLE
Our sample consists of 76 galaxies from magnitude-limited
and lensed samples with z 0.5 and measurable [N ii], Hα,
[O iii], and Hβ emission lines (S/N 3σ ). These samples con-
tain the most massive (M>10
9.5
M
) actively SF galaxies at
their respective redshifts and may be missing low-mass, low-
metallicity galaxies, characterized by low [N ii]/Hα and high
[O iii]/Hβ ratios. This selection is sufficient to test the appli-
cability of the local classification scheme at intermediate and
high redshift because the most massive, actively SF galaxies
lie closest to the starburst–AGN mixing sequence of the BPT
diagram (e.g., Yuan et al. 2010). Extending our results to the
general population of high-redshift SF galaxies (i.e., fully char-
acterizing the SF abundance sequence with redshift) requires
galaxies to be sampled over ∼4 mag of stellar mass and is not
possible with current samples.
At z ∼ 0.8, we use galaxies from the zCOSMOS spec-
troscopic survey (Lilly et al. 2007, 2009)ofthe1.5deg
2
COSMOS field (Scoville et al. 2007). The zCOSMOS-bright
sample contains VIMOS spectra of ∼20,000 galaxies with
I
AB
22.5 between 0 <z<1.4, yielding [O ii], Hβ, and
[O iii] line fluxes for galaxies between 0.5 <z<0.92. NIR
Very Large Telescope ISAAC spectroscopy allowed the addi-
tional measurements of Hα and [N ii] for a sample of 18 galaxies
between 0.5 <z<0.92 and two galaxies at z ∼ 2.5(C.Maier
et al. 2013, in preparation). For our analysis of the [O iii]/Hβ
line ratio, we also use the 22 zCOSMOS-bright galaxies with
[O iii]/Hβ measurements but 3σ upper limits on [N ii]. This
supplementary sample with [N ii] upper limits allows us to
probe lower metallicity, lower stellar mass (10
9.5
–10
10.5
M
)
galaxies for our [O iii]/Hβ ratio analysis. The zCOSMOS sam-
ple excludes AGN identified with X-ray data. We supplement
the zCOSMOS data with six galaxies at z ∼ 0.8fromthe
DEEP2 Galaxy Redshift Survey (Davis et al. 2003; Faber et al.
2007) observed by Shapley et al. (2005) and Liu et al. (2008).
We also include one gravitationally lensed galaxy observed by
Christensen et al. (2012).
At z ∼ 1.5, we use 87 galaxies observed by Yabe et al. (2012,
2013) from the Subaru–XMM Deep Survey (SXDS) (Furusawa
et al. 2008) and the UKIDSS Ultra Deep Survey (UDS;
Lawrence et al. 2007). The SXDS has limiting magnitudes of
∼27 in the B, V, R
C
, and i
bands and a limiting magnitude
of 26 in the z
band, while the UDS is more shallow (limiting
magnitudes of ∼24–25 in the J,H, and K bands). Yabe et al.
selected galaxies for NIR observations to satisfy: K
s
< 23.9
mag, stellar mass M
∗
10
9.5
M
, and expected Hα flux
F
Hα
> 5.0 × 10
−17
erg s
−1
cm
−2
. The sample excludes
X-ray sources, thus avoiding l uminous AGN. Of the 87 Yabe
et al. galaxies, 20 objects have 3σ detections for all 4 BPT
emission lines. The remaining galaxies have upper limits on
either [O iii], [N ii], or Hβ, which we use for our [O iii]/Hβ
line analysis. We supplement the Yabe et al. data with six
DEEP2 galaxies between 1.36 z 1.50 observed by Shapley
et al. (2005) and Liu et al. (2008), and six gravitationally
lensed galaxies by Rigby et al. (2011), Yuan et al. (2013), and
Christensen et al. (2012).
At z ∼ 2.5, we use five galaxies from the SINS sam-
ple (F
¨
orster Schreiber et al. 2009), two zCOSMOS galaxies
(C. Maier et al. 2013, in preparation), the low-metallicity L
∗
galaxy (Q2343-BX418) from Erb et al. (2010), and 16 gravita-
tionally lensed galaxies from the compilations of Richard et al.
(2011), Yuan et al. (2013), and Jones et al. (2013). Of these,
a total of 19 galaxies have >3σ detections of the [N ii], Hα,
[O iii], and Hβ emission lines. Gravitational lensing boosts the
luminosity of galaxies by 10–30 times, allowing lower lumi-
nosity, lower metallicity galaxies to be sampled compared with
magnitude-limited surveys. The magnification (and hence the
limiting magnitude) differs for each lensing source, depend-
ing on the geometry of the lens and the background galaxy.
Although the lensing magnification factor does not affect the
emission-line ratios, the distortion of the galaxy image causes
aperture effects. Hainline et al. (2009) show that this effect is
likely to be small (0.05–0.2 dex in the [O iii]/Hβ and [N ii]/Hα
line ratios).
The combination of magnitude-limited and lensed galaxies
gives a total of 25, 32, and 19 galaxies at z = 0.8, 1.5, and
2.5 with measured [N ii]/Hα and [O iii]/Hβ line ratios. We also
analyze a further 97 galaxies from these samples with upper
limits on either [O iii], [N ii], or Hβ.
3. TESTING THE COSMIC BPT DIAGRAM
In Kewley et al. (2013), we combine the chemical evolu-
tion predictions from Dav
´
eetal.(2011) with the Starburst99
and Pegase2 evolutionary synthesis models (Leitherer et al.
1999; Fioc & Rocca-Volmerange 1999) and our Mappings IV
photoionization code (Dopita et al. 2013). Briefly, we use the
Dave et al. chemical evolution predictions for the SF gas in
M
∗
> 10
9
M
galaxies with momentum-conserving winds (vzw
in Dave et al). We use the instantaneous and continuous burst
models from Starburst99 and Pegase2 with a Salpeter initial
mass function, and the Pauldrach/Hillier stellar atmosphere
models. The resulting stellar population spectra are used as the
ionizing source in our Mappings IV photoionization code, with
a spherical nebular geometry. Mappings IV uses a Kappa tem-
perature distribution which is more suitable for a turbulent ISM
than a Stefan–Boltzmann distribution (Nicholls et al. 2012). For
AGN, we use the dusty radiation-pressure-dominated models of
Groves et al. (2004). Further details of all models used are given
in Kewley et al. (2013) and Dopita et al. (2013). Our combina-
tion of these models produces theoretical predictions for how
the optical emission-line ratios will appear for galaxy samples
at different redshifts, based on two limiting assumptions for SF
galaxies and AGN.
1. SF galaxies at high redshift (z = 3)mayhaveISM
conditions and/or an ionizing radiation field that are either
the same as local galaxies (normal ISM conditions)or
else are more extreme than local galaxies (extreme ISM
conditions). Extreme conditions in SF galaxies can be
produced by a larger ionization parameter (by a factor
of 2) and a denser ISM (by a factor of 10), and/or an
ionizing radiation field spectral energy distribution (SED)
that contains a larger fraction of photons able to ionize O
+
2
The Astrophysical Journal Letters, 774:L10 (6pp), 2013 September 1 Kewley et al.
(1)
-1.0
-0.5
0.0
0.5
1.0
LOG ([OIII]/Hβ)
HII
AGN
(a) z=0
-1.0
-0.5
0.0
0.5
1.0
LOG ([OIII]/H
β)
(b) z=0.8
-1.0
-0.5
0.0
0.5
1.0
LOG ([OIII]/H
β)
(c) z=1.5
-1.5 -1.0 -0.5 0.0 0.5
LOG ([NII]/Hα)
-1.0
-0.5
0.0
0.5
1.0
LOG ([OIII]/Hβ)
(d) z=2.5
(2)
HII
AGN
-1.5 -1.0 -0.5 0.0 0.5
LOG ([NII]/Hα)
Figure 1. The cosmic BPT diagram for four different redshift ranges (rows) and
our model scenarios (1) and (2). In each panel, solid lines show our theoretical
predictions for the position of the star-forming abundance sequence (left curves)
and the AGN mixing sequence (right curves). Black and red circles indicate
the positions of gravitationally lensed and magnitude-limited survey galaxies,
respectively. Blue dotted lines indicate the boundaries at z = 0, for comparison.
(A color version of this figure is available in the online journal.)
into O
++
(i.e., energy > 35.12 eV) relative to the number
of H-ionizing photons.
2. The AGN narrow-line region (NLR) at high redshift
may either have already reached the level of enrichment
observed in local galaxies (metal-rich), or else it has the
same metallicity as the surrounding SF gas. In the latter
case, the AGN NLR at high redshift would be more metal-
poor than local AGN NLRs. The metal-rich AGN case
is equivalent to high-z AGN galaxies containing steeper
metallicity gradients than seen in local galaxies, while the
metal-poor AGN case is equivalent to high-z AGN galaxies
containing flatter metallicity gradients than seen in local
galaxies.
These limiting assumptions are described in detail in Kewley
et al. (2013). In this work, we focus on the position of the SF
(3)
-1.0
-0.5
0.0
0.5
1.0
LOG ([OIII]/Hβ)
HII
AGN
-1.0
-0.5
0.0
0.5
1.0
LOG ([OIII]/H
β)
-1.0
-0.5
0.0
0.5
1.0
LOG ([OIII]/H
β)
-1.5 -1.0 -0.5 0.0 0.5
LOG ([NII]/Hα)
-1.0
-0.5
0.0
0.5
1.0
LOG ([OIII]/Hβ)
(4)
HII
AGN
-1.5 -1.0 -0.5 0.0 0.5
LOG ([NII]/Hα)
Figure 2. As in Figure 1, but for our model scenarios (3) and (4). Data at z>1
are most consistent with scenarios (3) and (4), in which star-forming galaxies
have more extreme ISM conditions at high redshift.
(A color version of this figure is available in the online journal.)
abundance sequence rather than the position of the AGN mixing
sequence.
In Figures 1 and 2, we compare our samples with our model
predictions. The columns indicate the four different model
scenarios for the SF galaxies and the AGN NLR that are obtained
by our two sets of limiting assumptions.
1. Normal SF/ISM conditions; metal-rich AGN NLR at
high-z (Figure 1, left panel).
2. Normal SF/ISM conditions; metal-poor AGN NLR at
high-z (Figure 1, right panel).
3. Extreme SF/ISM conditions at high-z; metal-rich AGN
NLR at high-z (Figure 2, left panel).
4. Extreme SF/ISM conditions at high-z; metal-poor AGN
NLR at high-z (Figure 2, right panel).
In Figures 1 and 2, galaxies at z ∼ 0.8 span a similar r ange
of [O iii]/Hβ ratios to local SF galaxies. However, at z>1,
all galaxies in our samples (both lensed and magnitude-limited)
3
The Astrophysical Journal Letters, 774:L10 (6pp), 2013 September 1 Kewley et al.
0 1 2 3 4
z
-1.0
-0.5
0.0
0.5
1.0
LOG ([OIII]/Hβ)
0.0
0.2
0.4
0.6
0.8
f { log ([OIII]/H
β
) > 0.6}
Figure 3. Lower panel: the [O iii]/Hβ ratio vs. redshift, including [O iii]and
Hβ upper limits. Top panel: fraction of galaxies with log([O iii]/Hβ) > 0.6for
five redshift intervals. The fraction that would be obtained if all galaxies with
[O iii]/Hβ lower limits log([O iii]/Hβ) < 0.6 truly have log([O iii]/Hβ) <
0.6 (black circles) or if they have log([O iii]/Hβ) > 0.6 (blue triangles) are
shown. The rise in log([O iii]/Hβ) > 0.6 with time from z ∼ 0 is statistically
significant for z 2.
(A color version of this figure is available in the online journal.)
are offset toward larger [O iii]/Hβ ratios than local SF galaxies
over the same [N ii]/Hα range (i.e., the same metallicity range).
This result is consistent with previous work on smaller samples
(e.g., Shapley et al. 2005; Lehnert et al. 2009; Rigby et al. 2011;
Yabe et al. 2012).
We investigate whether this offset is due to observational
detection limits in Figure 3 (lower panel), which gives the
[O iii]/Hβ ratio as a function of redshift. One advantage of
this diagram is that we can include galaxies without measurable
[N ii] and Hα, allowing us to include higher redshift samples.
We include the f our Lyman break galaxies from Pettini et al.
(2001), seven Lyman break galaxies from Maiolino et al. (2008),
and three lensed galaxies from Richard et al. (2011) with
measured [O iii]/Hβ ratios (to the 3σ level). Figure 3 also shows
galaxies with 3σ upper limits on [O iii]orHβ, where available.
The [O iii]/Hβ ratios are systematically larger for galaxies at
z>1, even when upper limits are taken into account. The
majority of galaxies (34/47; 87%) with [O iii]/Hβ limits have
[O iii] detections but no Hβ detections (i.e., lower limits on the
[O iii]/Hβ ratio), strengthening the rise of [O iii]
/Hβ with
redshift.
The [O iii]/Hβ upper envelope also rises with redshift
such that galaxies have larger [O iii]/Hβ ratios on average
at z>1 than galaxies at lower redshifts, over the same
[N ii]/Hα (or metallicity) range. The fraction of galaxies with
[O iii]/Hβ>0.6 rises with redshift (Figure 3, top panel). This
rise is not caused by detection bias because we are able to de-
tect [O iii]/Hβ for AGN with [O iii]/Hβ>0.6inthesame
observational samples at z<1.
We propose that the rise in [O iii]/Hβ with redshift is caused
by either (or a combination of) (1) a larger ionization parameter
and a denser ISM at high redshift, and/or an ionizing SED
that contains a larger fraction of O
+
-ionizing to H-ionizing
photons. The larger ionization parameter may be related to the
large specific star formation rate observed at high redshift (e.g.,
Noeske et al. 2007). We note that in the local galaxies M82
and the Antennae, extremely high ionized gas densities and
ionization parameters are found in clumpy, dense SF complexes
(Smith et al. 2006; Snijders et al. 2007). The densities and
ionization parameters measured in these dense complexes are
similar to those observed at high redshift (e.g., Rigby et al.
2011 and references therein). Snijders et al. (2007) shows that
a geometrical model in which several individual SF clumps
are embedded in a giant molecular cloud can reproduce these
extreme ISM conditions. We speculate that global spectra of
our high-redshift galaxies may be dominated by H ii regions
similar to the clumpy, dense SF regions in the Antennae and
M82. It is also possible that the H ii regions at high redshift
are predominantly matter bounded. In this case, the effective
ionization parameter from our radiation-bounded models would
appear larger (see Kewley et al. 2013, for a discussion). In
a follow-up paper (L. J. Kewley et al. in preparation), we
investigate these scenarios in detail.
4. NEW REDSHIFT-DEPENDENT
CLASSIFICATION SCHEME
Figure 2 indicates that the current optical spectral classifica-
tion schemes are not suitable for classifying galaxies at z>1.
SF galaxies are best fit by the theoretical models described in
scenarios 3 and 4. In both of these scenarios, the SF abundance
sequence can be reproduced by either (1) an ionizing radiation
field with a larger fraction of O
+
-ionizing to H-ionizing photons,
and/or (2) a combination of a larger ionization parameter and
a high electron density. We use the upper curve of our theo-
retical abundance sequence to define semi-empirically how the
position of the classification line may change with redshift:
log([O iii]/Hβ) =
0.61
(log([N ii]/Hα) − 0.02 − 0.1833 × z)
+1.2+0.03 × z, (1)
where [O iii]/Hβ uses the [O iii] λ5007 line and [N ii]/Hα uses
the [N ii] λ6584 line. Equation (1) can be used to separate purely
SF galaxies from galaxies containing AGN, and is based on the
following assumptions.
1. The shape of the ionizing radiation field and the
ISM conditions in SF galaxies evolves only marginally
(<0.1 dex) with redshift until z ∼ 1, consistent with our
findings in Figure 2. This assumption may only hold for
galaxies in the mass range of our samples (M
∗
> 10
9
M
).
The SF conditions in galaxies with lower stellar masses
may still be evolving between z = 1 and z = 0. If SF
conditions are more extreme in low stellar mass galaxies in
this redshift range, then these galaxies will lie above and to
the right of the line given by Equation (1). In this case, our
optical spectral classification line would need to include a
stellar mass term.
2. The shape of the ionizing SED and/or the ISM conditions
in SF galaxies evolves toward more extreme conditions
between 1 <z<3. Figure 2 suggests that such a
change does occur, at least in the most massive galaxies
(M
∗
> 10
9
M
). In Figures 1 and 2, we have binned
galaxies into two redshift ranges with median redshifts of
z = 1.5 and z = 2.5 to provide a sufficient number of
galaxies in each redshift interval for statistically significant
4
The Astrophysical Journal Letters, 774:L10 (6pp), 2013 September 1 Kewley et al.
-1.0
-0.5
0.0
0.5
1.0
1.5
LOG ([OIII]/Hβ)
(a) z=0
HII
AGN
-1.0
-0.5
0.0
0.5
1.0
1.5
LOG ([OIII]/Hβ)
(b) z=0.8
HII
AGN
-1.0
-0.5
0.0
0.5
1.0
1.5
LOG ([OIII]/Hβ
)
(c) z=1.5
HII
AGN
-2.0 -1.5 -1.0 -0.5 0.0 0.5
LOG ([NII]/Hα)
-1.0
-0.5
0.0
0.5
1.0
1.5
LOG ([OIII]/H
β)
(d) z=2.5
HII
AGN
Figure 4. Our new theoretical redshift-dependent classification scheme for four
redshifts. Galaxies at z ∼ 2.5 with additional evidence for AGN are shown with
a square outline.
(A color version of this figure is available in the online journal.)
conclusions. This small number of redshift intervals allows
us to coarsely fit the evolution of the abundance sequence
between 0.8 <z<2.5. A substantially larger sample
(200 galaxies) would allow one to divide the sample into
a larger number of small redshift intervals, allowing for a
more robust fit in this redshift range.
We note that our chemical evolution assumption affects the
position of the intersection between the SF abundance sequence
and the AGN mixing sequence, but does not affect the shape or
location of the classification line.
In Figure 4 we show the classification line from Equation (1)
for our four redshift ranges. The zCOSMOS-bright (z ∼ 0.8)
and Yabe (z ∼ 1.5) NIR samples excluded bright AGN using
X-ray data. The location of the SF galaxies in these samples
is consistent with our new classification line, within the errors
(±0.1dex).
Of the five z ∼ 2.5 galaxies that we classify optically
as AGN, two galaxies (MACS J09012240+1814321 and BzK
15504; square symbols in Figure 4) show additional evidence
for an AGN. UV spectroscopy led Diehl et al. (2009) to suggest
that MACS J0901 contains an AGN. Although dominant in
the UV, the AGN does not make an energetically significant
contribution to the global mid-infrared spectrum (Fadely et al.
2010). Lehnert et al. (2009) suggest that BzK15504 contains
an AGN based on its strong [O i]/Hα emission-line ratio. The
remaining three optically classified AGN are A1835 (Richard
et al. 2011), MACS J1148 (Jones et al. 2013), and Q2343-
BX389 (Lehnert et al. 2009). These three galaxies do not have
sufficient ancillary data to confirm or rule out an AGN at other
wavelengths. Follow-up of these three galaxies at X-ray, UV,
and/or mid-infrared wavelengths will facilitate testing of our
new classification scheme.
5. CONCLUSIONS
We have presented the first comparison between theoretical
predictions of the cosmic evolution of the BPT diagram and
NIR spectroscopic observations of active galaxies between
0.8 <z<2.5. We show that SF galaxies at high redshift are
consistent with a model in which the ISM conditions are more
extreme at high redshift. These extreme conditions may manifest
in either (or a combination of) a larger ionization parameter,
a larger electron density, and/or an ionizing radiation field
with a larger fraction of O
+
-ionizing to H-ionizing photons. We
speculate that the global spectra of high-redshift galaxies may
be dominated by complexes of SF clusters embedded within
giant molecular clouds, as seen in the most extreme SF regions
in the Antennae and M82.
Due to the change in ISM conditions in SF galaxies with
redshift, current optical classification methods based on local
samples are not reliable beyond z>1. We present a new
redshift-dependent optical classification line for the [O iii]/Hβ
versus [N ii]/Hα diagnostic diagram that accounts for how the
structure of the ISM changes with redshift. We show that the
position of independently confirmed AGN is consistent with our
classification line at z ∼ 2.5.
In an upcoming paper, we will further investigate the cause
of the extreme ISM conditions in high-redshift SF galaxies.
L.J.K. gratefully acknowledges the referee for useful com-
ments, the support of an ARC Future Fellowship, ARC Dis-
covery Project DP130103925, and the ANU CHELT Academic
Women’s Writing Workshop. This research used NASA’s As-
trophysics Data System Bibliographic Services and the NASA/
IPAC Extragalactic Database (NED). Based on observations ob-
tained at the European Southern Observatory (ESO) Very Large
Telescope (VLT), Paranal, Chile; ESO programs 084.B-0232A,
084.B-0312A, and 085.B-0317A.
REFERENCES
Baldwin, J. A., Phillips, M. M., & Terlevich, R. 1981, PASP, 93, 5
Bian, F., Fan, X., Bechtold, J., et al. 2010, ApJ, 725, 1877
Brinchmann, J., Pettini, M., & Charlot, S. 2008, MNRAS, 385, 769
Christensen, L., Richard, J., Hjorth, J., et al. 2012, MNRAS, 427, 1953
Dav
´
e, R., Finlator, K., & Oppenheimer, B. D. 2011, MNRAS, 416, 1354
Davis, M., Faber, S. M., Newman, J., et al. 2003, Proc. SPIE, 4834, 161
Diehl, H. T., Allam, S. S., Annis, J., et al. 2009, ApJ, 707, 686
Dopita, M. A., Sutherland, R. S., Nicholls, D. C., Kewley, L. J., & Vogt, F. P. A.
2013, ApJ, submitted
Erb, D. K., Pettini, M., Shapley, A. E., et al. 2010, ApJ, 719, 1168
Erb, D. K., Shapley, A. E., Pettini, M., et al. 2006, ApJ, 644, 813
Faber, S. M., Willmer, C. N. A., Wolf, C., et al. 2007, ApJ, 665, 265
Fadely, R., Allam, S. S., Baker, A. J., et al. 2010, ApJ, 723, 729
5
![Figure 3. Lower panel: the [O iii]/Hβ ratio vs. redshift, including [O iii] and Hβ upper limits. Top panel: fraction of galaxies with log([O iii]/Hβ) > 0.6 for five redshift intervals. The fraction that would be obtained if all galaxies with [O iii]/Hβ lower limits log([O iii]/Hβ) < 0.6 truly have log([O iii]/Hβ) < 0.6 (black circles) or if they have log([O iii]/Hβ) > 0.6 (blue triangles) are shown. The rise in log([O iii]/Hβ) > 0.6 with time from z ∼ 0 is statistically significant for z 2.](/figures/figure-3-lower-panel-the-o-iii-hb-ratio-vs-redshift-290fqi3w.webp)