Mass-Excitation (MEx) Diagram

Galaxies can host star formation, active galactic nuclei (AGN), or both. It is often difficult to distinguish between these two processes. We developed the Mass-Excitation (MEx) diagram in Juneau et al (2011) to identify AGNs in galaxies using [OIII]5007/Hb and stellar mass. Similarly, Yan et al (2011) developed a diagram based on [OIII]5007/Hb and rest-frame U-B color (see the Color-Excitation Diagram page). Both of these new diagnostics allow us to classify galaxies out to z~1 with optical spectra, beyond what was possible for traditional nebular line diagnostics such as the BPT diagrams, which are limited to z<0.4. Moreover, the BPT diagram can be applied out to z~2.5 with near-IR spectra and to yet higher redshifts (z~3.5) with the MEx diagram.

An important feature that is receiving increasingly more attention as near-IR galaxy spectra become available in greater number -- and line diagnostics can thus be applied to higher redshifts -- is the potential for evolution in the emission line ratios of galaxies. Indeed, the application of any emission line diagnostic diagram calibrated with nearby galaxies to higher redshifts can be subject to uncertainties given that the evolution of intrinsic emission line ratios and governing physical properties are not fully understood. In addition to this possible evolution, selection effects due to limited sensitivity to galaxies with intrinsically faint lines at higher redshift can create important systematic biases. Both of these aspects are now under careful investigations. Namely, the impact of emission-line selection effects is quantified, and the MEx diagram is calibrated as a function of emission line detection limits, allowing a better application to a broad range of redshifts (Juneau et al 2014; ApJ).

(May 2014): release of the new IDL script and savefiles to use the updated MEx diagram and make empirical predictions of bivariate distributions on the MEx.

MEx Dividing Lines (2014)

The dividing lines are helpful for visualization of the regions of MEx diagram. The main (upper) curve is described by (Eq. 1; Juneau et al 2014):

y = 0.375 / (x-10.5) + 1.14 ; if x < 10.0

= a₀ + a₁x + a₂x² + a₃x³ ; otherwise

where y=log([OIII]5007/Hb), x=log(M_*), and the coefficients {a₀, a₁, a₂, a₃} = {410.24, -109.333, 9.71731, -0.288244}. Using the same notation, the lower curve defining the MEx-intermediate region is given by (Eq. 2; Juneau et al 2014):

y = 0.375 / (x-10.5) + 1.14 ; if x < 9.6

= a₀ + a₁x + a₂x² + a₃x³ ; otherwise

with coefficients {a₀, a₁, a₂, a₃} = {352.066, -93.8249, 8.32651, -0.246416}.

In contrast to the original demarcations from J11, the 2014 version has been calibrated as a function of emission lines detection limit, L_line. This calibration is parameterized as an offset along stellar mass as follows (Eq. B1; Juneau et al 2014):

dx = a₀ + a₁ tan^-1 { [log(L_line) - a₂]*a₃ }

where dx= offset in log(M_*) [dex], the limiting luminosity L_line is in units of erg s^-1, and the coefficients {a₀, a₁, a₂, a₃} = {0.28988, 0.28256, 40.479, 0.82960}.

The 2011 demarcation lines are defined here.

AGN Probabilistic Classification

This page contains IDL code to calculate, given their position on the MEx diagram, the probability that galaxies belong to the following spectral classes:

• purely star-forming (‘SF’)

• composite (mixed star-formation and AGN; ‘comp’)

• LINER

• Seyfert 2 (‘Sy2’)

The probabilities are defined such that P_SF + P_comp + P_LINER + P_Sy2 = 1. When only interested in distinguishing between purely star-forming galaxies and galaxies hosting ‘any’ category of AGN, one can simply calculate P_SF = P_SF and P_AGN = P_comp + P_LINER + P_Sy2 (or, equivalently, P_AGN = 1 - P_SF).

Go here to download the code (note that the savefiles must be located in the same directory as the routines unless specified otherwise by the user in the code).

Conditions of Use

Please read the documentation provided on this page, and cite Juneau et al (2011) and Juneau et al (2014) if you make use of any of the code and/or savefiles in your work. You can also email me to register on the user list if you wish to receive updates. This code comes with no warranty or whatsoever. It is meant to be a useful tool and provided as is (although comments and suggestions are accepted).

How to use the code

The following description can also be found as the header of the code. (For the original version introduced in 2011, instead refer to this page.)

; PURPOSE:

; Calculation of galaxy/AGN classification probabilities based on

; the MEx diagnostic diagram (Juneau et al. 2011, ApJ 736, 104) and

; updated for selection effects and evolution (Juneau et al. 2014,

; ApJ, in press). Optionally, this routine includes line luminosity

; dependent AGN probabilities. As described in the 2014 article, the

; MEx dividing lines effectively shift toward higher stellar mass

; with decreasing sensitivity to emission lines (i.e., a higher line

; luminosity threshold for detection; Appendix B).

;

; SYNTAX:

; result = MEx_prob(x, y, dx, dy)

;

; INPUT:

; x = log(stellar mass) ; log_10 of stellar mass in solar masses

; y = log([OIII]5007/Hbeta) ; log_10 of flux ratio

; dx = error on log(stellar mass) - must be >0.04 dex

; dy = error on log([OIII]/Hbeta) - must be >0.025 dex

;

; OPTIONAL INPUT:

; DR7 = Use SDSS DR7 as prior sample (default)

; DR4 = Use SDSS DR4 as prior sample (2011 version)

; flux_lim = flux threshold for emission line detection (scalar) -

; if using this, must also specify the redshift keyword

; *NOTE: using default cosmology from lumdist.pro (Ho=70;

; Omega_M=0.3; Lambda0=0.7; q0=-0.55; k=0.0)

; lum_lim = luminosity threshold for emission line detection (scalar)

; z = redshift (scalar or a vector with one value per galaxy)

; silent = Avoid printing messages to screen

; evol = If set, apply characteristic L* evolution to the line

; luminosity threshold (log(1+z)^2.27 depedency by default;

; can be modified directly in the routine)

;

; OUTPUT:

; Array of 4 columns by N rows, where N is the number of elements of

; the input vectors (greater or equal to 1); the four columns give

; SDSS-based probabilities of having the following BPT classes:

; result[0,*] = P(SF) ; purely star-forming

; result[1,*] = P(comp) ; composite (SF/AGN)

; result[2,*] = P(LINER) ; LINER

; result[3,*] = P(Sy2) ; Seyfert 2

; Each number varies from 0 to 1, and the sum each row is 1. The

; values are set to -99 when the data points and errors are

; completely out-of-bound of the parameter space covered by the SDSS

; prior sample (e.g., at log(M*) < 7).

;

; OPTIONAL OUTPUT:

; None

*NOTE: If using the line flux limit and redshift (flux_lim and z keywords), it is expected that the user has the IDL routine lumdist.pro (astrolib), which is called to convert emission line fluxes to luminosities. This routine is not needed if using instead the emission line luminosity limit (lum_lim keyword).

Impact of Selection Effects

The bivariate distribution of galaxies on emission-line diagnostic diagrams depends on the intrinsic luminosities of the lines considered. Figures 2 and 3 from J14 show the effect or requiring increasing line luminosities for the [OIII] and Ha lines on the BPT and MEx diagrams, respectively. Single line selections (only [OIII], or Ha, or Hb) have also been explored in Appendix C. In all cases, more luminous lines tend to shift the locus of the mean star-forming branch toward higher values of the [OIII]/Hb ratio and/or [NII]/Ha. This feature is due to an intrinsic (physical) connection between line luminosities and ratios. In other words, galaxies with elevated [OIII]/Hb also have more luminous lines for both [OIII] and Hb (though proportionately more so for [OIII]) at low to intermediate stellar mass, though they can have lower [OIII]/Hb at high-masses when selected only on Hb (~SFR).

Stellar Masses

This code assumes that stellar masses are derived based on the Chabrier (2003) IMF. The resulting masses are very close to those obtained with the Kroupa (2001) IMF but are systematically offset from masses derived assuming different IMFs such as the Salpeter (1995) IMF. If you wish to utilize the code, it is recommended to convert the masses to a Chabrier IMF and to implement residual uncertainties from the mass conversion in the error bars on the stellar mass (the uncertainties are taken into account in the calculations of P_AGN).

MEx Download

Note that the savefiles must be located in the same directory as the routines unless specified otherwise by the user in the code.

The 2014 version of the IDL script is now released below. Please refer to the following if making use of any of the files or if applying the method described: Juneau et al 2014 (ApJ, in press)

Go here to download the 2011 version of the code. Alternatively, you can retrieve the tar file from the Attachments below.

Revision History

v2 -- Second release of IDL script and savefiles in may 2014; now using SDSS DR7 as prior sample, and including emission line luminosity detection limit (and/or line luminosity evolution) as optional keywords (J14)

v1 -- First release (also called v0 in the original tarfile name) of IDL script and savefile; using SDSS DR4 as prior sample (as described by J11)