MEx 2011

This page includes information that is specific to the first release of the MEx diagram. The main MEx diagram page contains the most up-to-date version.

MEx Dividing Lines (2011)

The dividing lines are helpful for visualization of the regions of MEx diagram. The main curve is described by Eq. 1 of Juneau et al (2011):

y = 0.37 / (x-10.5) + 1 ; if x < 9.9

= a₀ + a₁x + a₂x² + a₃x³ ; otherwise

where y=log([OIII]5007/Hb), x=log(M_*), and the coefficients {a₀, a₁, a₂, a₃} = {594.753, -167.074, 15.6748, -0.491215}. The lower curve defining the MEx-intermediate region is defined, using the same notation:

y = 800.492 - 217.328 x + 19.6431 x² - 0.591349x³

and should only be applied to the range 9.9 < x < 11.2 (Eq. 2 of Juneau et al 2011).

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) 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.

;PURPOSE:

; Calculation of galaxy/AGN classification probabilities based on

; the MEx diagnostic diagram (Juneau et al. 2011) and the prior

; classsification of SDSS galaxies at 0.05<z<0.1. The code ignores

; redshift evolution along stellar mass or [OIII]/Hbeta line ratio

; but evolution can be implemented by the user by shifting the input

; values by the expected offset, if any.

;

; SYNTAX:

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

;

; INPUT:

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

; y = log([OIII]5007/Hbeta) ; log 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

;

; 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 along each row is 1.

; 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).

Illustration of AGN probabilities

The MEx diagnostic diagram has been first calibrated using a sample of SDSS prior galaxies at 0.05<z<0.1 from the DR4.0 release. The galaxies were chosen to have valid emission line flux measurements (S/N>3) for all the required emission lines: Hbeta, [OIII]5007, Halpha, [NII]6584 and [SII]6717,6731 in order to establish their classification on BPT diagrams, yielding four mutually exclusive classes (see Section 3.3 of Juneau et al. (2011)):

• purely star-forming (‘SF’)

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

• LINER

• Seyfert 2 (‘Sy2’)

The location of these galaxies on the MEx diagram is illustrated below (Fig. 1):

So this particular galaxy would be assigned P_SF=19% and P_AGN=81% (which can be broken down into P_comp=6%, P_LINER=36% and P_Sy2=39%). This particular example was chosen to illustrate a case in a region where there is a lot of overlap between the LINER and Sy2 classes. Other diagnostics may be able to break this degeneracy (for example, the [OII]3727/Hbeta equivalent width ratio if available, see the page on the Blue Diagram). The spectral classes are more uncertain in the regions where various classes overlap. This may sometimes be a limitation when studying individual galaxies but this information should be useable nonetheless as a system of statistical weights to study global properties of statistical galaxy samples.

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.

Go here to download the 2011 version of the code.

Update: The 2014 version of the IDL script and savefiles is now released.