Updated: 14 March 2016

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Reference: Seyffert et al. (2013, ApJ, 779, 161: online, PDF)

Abstract: We have performed an analysis of over 34,000 Mg II doublets at 0.36 < z < 2.29 in Sloan Digital Sky Survey (SDSS) Data-Release 7 quasar spectra. The catalog was divided into 14 small redshift bins with roughly 2,500 doublets in each, and from Monte-Carlo simulations, we estimate 50% completeness at rest equivalent width W_r ≈ 0.8 Å. The equivalent-width frequency distribution is described well by an exponential model for all redshifts, and the distribution becomes flatter with increasing redshift, i.e., there are more strong systems relative to weak ones. Direct comparison with previous SDSS Mg II surveys reveal that we recover at least 70% of the doublets in these other catalogs, in addition to detecting thousands of new systems. We discuss how these surveys come by their different results, which qualitatively agree but, due to the extremely small uncertainties, differ by a statistically significant amount. The estimated physical cross-section of Mg II-absorbing galaxy halos increased three-fold, approximately, from z = 0.4 → 2.3, while the W_r ≥ 1 Å absorber line density grew, dN_Mg II/dX, by roughly 45%. Finally, we explore the different evolution of various absorber populations—damped Lyman-$\alpha$ absorbers, Lyman-limit systems, strong C IV absorbers, and strong and weaker Mg II systems—across cosmic time (0 < z < 6).

Note −  Summary of the most common redshift bins and data used for the various analyses. Columns 1−2 give the median, minimum, and maximum redshifts for the observed number of doublets (Column 3), and the maximum co-moving pathlength in the redshift bin is given in Column 4. The 50% completeness from the Monte Carlo tests is in Column 5. The redshift and co-moving absorber line densities for W_r ≥ 1.0 Å are in Columns 6−7. The frequency distribution was fit with an exponential ƒ(W_r) = k exp(αW_r) for N_fit absorbers with W_r ≥ 0.8 Å (Column 8), and the best-fit parameters are given in Columns 9−10. The reduced χ² from the best fit and ƒ(W_r) (in bins with ≈100 doublets each) is given in Column 11.

Go directly to: Directory Structure | Setting-up Codes | Working with Mg II Structure | Continuum Fitting, Identifying Candidates, Generating Completeness Curves

You will need to set the environment variables SDSSPATH and SDSSDR; for example, in bash:

TERM>export SDSSPATH="$HOME/Work/SDSS"

TERM>export SDSSDR="DR7_QSO"

The codes will expect the following directories under $SDSSPATH/$SDSSDR/:

• spectro/
• conti/
• abslin/
• mcmgii/

The main data processing and analysis codes are IDL-based and in the XIDL package. The majority of the custom codes are in $XIDL_DIR/SDSS/CIV. Several general programs and functions are in SDSS_FUNCTIONS.PRO, and this needs to be compiled before any XIDL SDSS codes are run. This can be done automatically at startup by adding the following lines to your .idlstartup file:

!quiet = 1 ; suppress messages

.r sdss_functions

!quiet = 0

The list and description of programs and functions in SDSS_FUNCTIONS.PRO can be seen by calling sdss_functions at the IDL prompt:

IDL>sdss_functions

Below, basic calls that other researchers may find useful are described. Other program and function options can often be seen by making the program or function calls without parameters or with the /help option. Several calls can return values in redshift (as opposed to co-moving pathlength) space when /dz is included as an option.

The absorber structure is described in SDSSCIVSTRCT__DEFINE.PRO. Here are basic ways of manipulating the structure. First, SDSS_GETCIVSTRCT() (in SDSS_FUNCTIONS.PRO) can be used to grab various sub-samples. For example, to extract the doublets with redshifts 1.5 ≤ z < 2 and rest equivalent widths W_r ≥ 1.0 Å that are more than 5000 km s⁻¹ from the background QSO (i.e., δv_QSO < −5000 km s⁻¹) and not in visual BAL QSO sightlines, the call would be:

IDL>mgiistrct = sdss_getcivstrct('sdss_mgii_seyffertetal13.fit', zlim=[1.5,2], ewlim=[1.0,!values.f_infinity], /noBAL, dvqso=-5000)

More options for SDSS_GETCIVSTRCT() can be found by using the /help option. The redshift, equivalent widths, and column densities (from the apparent optical depth method) are stored in the arrays tagged as ZABS_ORIG, EW_ORIG, and NCOLM_ORIG, with errors in the SIGZABS_ORIG, SIGEW_ORIG, and SIGNCOLM_ORIG arrays. The zeroth array element is for the Mg II 2796 line and the first is for the 2803 line. The 2796 and 2803 column density measurements can be combined into a system column density estimate as follows:

IDL>ncolm = sdss_calncolmdblt(mgiistrct, 'MgII', signcolm=signcolm, /log, ncolmflg=ncolmflg)

The column density (limit) flags can be estimated by forcing all doublets with ratio W_r,2796/W_r,2803 < (2 − combined error) to be lower limits, assuming they are saturated.

A quick way to view a lot of information about the systems is to use the rating GUI SDSS_CHKCIV.PRO, which needs the directory structure defined above. The program can accept a structure or file name as an input and needs an output variable or file:

IDL>sdss_chkciv, mgiistrct, outstrct, dblt_name='MgII'

If you have a completeness curve for 1.5 ≤ z < 2 and δv_QSO < −5000 km s⁻¹ called cmpltrec_MgII_1.5z2.0.fit (see below), you can view the completeness curve and errors, if the IDLUTILS package is installed (available e.g., here), with the following command:

IDL>cxw = sdss_getdxw('cmpltrec_MgII_1.5z2.0.fit', mgiistrct.zabs_orig[0], mgiistrct.ew_orig[0], sigewabs=mgiistrct.sigew_orig[0], sigma=sigcxw, /czw)

IDL>plot, mgiistrct.ew_orig[0], dxw, psym=4, xtitle='EW', ytitle='C(W)'

IDL>oploterror, mgiistrct.ew_orig[0], mgiistrct.sigew_orig[0], sigcxw[*,0], /lobar

IDL>oploterror, mgiistrct.ew_orig[0], mgiistrct.sigew_orig[0], sigcxw[*,1], /hibar

You can generate an equivalent width frequency distribution ƒ(W_r) as follows:

IDL>fw = sdss_calcfxw(mgiistrct, 'cmpltrec_MgII_1.5z2.0.fit', [1.5,2])

This returns a structure that can be plotted with the errors as follows:

IDL>plot, fw.ewcenter, fw.fxw, psym=4, xtitle='EW', ytitle='f(W)', /ylog

IDL>oploterror, fw.ewcenter, fw.fxw, fw.sigewcenter[*,0], fw.sigfxw[*,0], /lobar

IDL>oploterror, fw.ewcenter, fw.fxw, fw.sigewcenter[*,1], fw.sigfxw[*,1], /hibar

You can fit the 1.5 ≤ z < 2 frequency distribution with an exponential and overplot the results on the previous plot with the following calls:

IDL>sdss_fitfxw, 'cmpltrec_MgII_1.5z2.0.fit', intlim=[0.8, 8.5, 8.5], /exp, civstrct_fil=mgiistrct, ostrct_fil='fwfit_MgII_1.5z2.0.fit', psfil='fwfit_MgII_1.5z2.0.ps', /plot, sigma=1.

IDL>fwfit = sdss_calcfxwfit(fw.ewcenter, fitstrct='fwfit_MgII_1.5z2.0.fit')

IDL>oplot, fw.ewcenter, fwfit, thick=2, linestyle=2

You can generate and plot a structure with the absorber line density dN_Mg II/dX for an equivalent width range, as a function of redshift, given a completeness curve with multiple redshift bins cmpltrec_MgII_0.3z2.3cum.fit.

IDL>dndx = sdss_calcdndx(mgiistrct_all, 'cmpltrec_MgII_0.3z2.3cum.fit', [1.0, !values.f_infinity]

IDL>plot, dndx.zcenter, dndx.dndx, psym=4, xtitle='z', ytitle='dN/dX'

IDL>oploterror, dndx.zcenter, dndx.dndx, dndx.sigzcenter[*,0], dndx.sigdndx[*,0], /lobar

IDL>oploterror, dndx.zcenter, dndx.dndx, dndx.sigzcenter[*,1], dndx.sigdndx[*,1], /hibar

Continuum Fitting:

There are three flavors of continuum fits: eigen, spline, and hybrid. The latter depends on the former, and the order of the following calls will lead to the hybrid-fit. The required file is a structure with the information for the QSOs to be fit, dr7qso_MgII.fit (formatted like the Schneider et al. (2010) catalog, see example above).

1. First fit the eigen-continua, which can be accomplished by running one line in IDL or using a shell script to divide up the task between multiple "processors" via multiple IDL sessions. (Make sure the shell script is executable.) The output will be written to the conti/ directory:

   XIDL>runeigqsoconti, sdsssum='dr7qso_MgII.fit'

   or

   TERM>$XIDL_DIR/SDSS/CIV/eig_runparallelfit.sh 4 dr7qso_MgII.fit

2. Search the eigen-normalized spectra of absorption features and create the SDSSCONTISTRCT__DEFINE.PRO structure in the abslin/ directory. This code will require a text file that lists the directory to write the output to on the first line, followed by the spectra names, in the same order as the QSO structure, e.g., dr7qso_MgII.list (see example above). This again can be accomplished by a single IDL call or by running a shell script:

   IDL>sdss_fndlin, 'dr7qso_MgII.list', 'dr7qso_MgII.fit', /noBAL, only_cflg=sdss_getcflg(/eig)

   or

   TERM>$XIDL_DIR/SDSS/CIV/sdss_runparallelsrch.sh 4 dr7qso_MgII.list dr7qso_MgII.fit "/noBAL, only_cflg=sdss_getcflg(/eig)"

3. The hybrid-fitting algorithm will use the lines found in the first eigen-normalized spectra to make a mask. Whether a given spectrum is corrected by a spline-fit to the eigen-normalized spectrum is determined by the S/N, and it is faster to just query the S/N structure, generated by SDSS_MKSNRTAB. This too can be done with one IDL call or via a shell script:

   IDL>sdss_dblfitconti, 'dr7qso_MgII.list', 'dr7qso_MgII.fit', snrstrct_fil='dr7qso_MgII_SNR.fit', dblt_name='MgII'

   or

   TERM>$XIDL_DIR/SDSS/CIV/hyb_runparallelfit.sh 4 dr7qso_MgII.list dr7qso_MgII.fit "snrstrct_fil='dr7qso_MgII_SNR.fit', dblt_name='MgII'"

4. [Optional] You can also use only a b-spline to fit the continua, via an IDL call or a shell script:

   IDL>sdss_fndlin_fitallspl, 'dr7qso_MgII.list', 'dr7qso_MgII.fit', pixlim=7

   or

   TERM>$XIDL_DIR/SDSS/CIV/spl_runparallelfit.sh 4 dr7qso_MgII.list dr7qso_MgII.fit "pixlim=7"

Identifying Candidates:

To generate a candidate structure, you first must search the normalized spectra for automatically detected lines and then pair these into candidate Mg II doublets. You can generate candidates from any three of the continua stored in the abslin/ structure by changing the cflg. Focusing on the hybrid-normalized spectra:

1. To search the hybrid-normalized spectra and store the results in the abslin/ structures, use either of the following calls:

   IDL>sdss_fndlin, 'dr7qso_MgII.list', 'dr7qso_MgII.fit', /noBAL, only_cflg=sdss_getcflg(/hyb)

   or

   TERM>$XIDL_DIR/SDSS/CIV/sdss_runparallelsrch.sh 4 dr7qso_MgII.list dr7qso_MgII.fit "/noBAL, only_cflg=sdss_getcflg(/hyb)"

2. Now you can actually search for the candidates. Below, a secondary line search at the 2.5 σ level is called to find partners for un-matched, 3.5 σ lines:

   IDL>sdss_civsearch, 'dr7qso_MgII.list', 'dr7qso_MgII.fit', 'sdss_mgiicand_hyb.fit', dblt_name='MgII', lsnr2=2.5, cflg=sdss_getcflg(/hyb)

   or

   TERM>$XIDL_DIR/SDSS/CIV/sdss_runparallelsrch.sh 4 dr7qso_MgII.list dr7qso_MgII.fit sdss_mgiicand_hyb.fit "dblt_name='MgII', lsnr2=2.5, cflg=sdss_getcflg(/hyb)"

3. To rate your own candidates, you can use the GUI:

   IDL>sdss_chkciv, 'sdss_mgiicand_hyb.fit', 'sdss_mgiirate_hyb.fit', dblt_name='MgII'

Generating Completeness Curves:

There is a completeness file for each sightline tested. The information in these files need to be combined into the redshift bin or bins of interest, including any δv_QSO cut. This can be accomplished for an explicit redshift range as follows:

IDL>.r sdss_completeness

IDL>sdss_completeness_czw, 'dr7qso_MgII.list', 'dr7qso_MgII_SNR.fit', 'cmpltrec_MgII_0.3z2.3cum.fit', zbinsize=2.1, zlim=[0.3, 2.3], dvqso=-5000, /rec_param, civobs_corr=mgiistrct

Since only a subset of sightlines have Monte Carlo tests, the un-tested sightlines are accounted for by the average of the tested sightlines in the same S/N and z_QSO bin. The zbinsize= is set to something large to make just one bin in the same file, for the full redshift range 0.3 < z < 2.3. The /rec_param option signals that the redshifts and equivalent widths used for combining the completeness curves are not the input, Voigt-profile values but the ones automatically measured from the simulated spectra. The civobs_corr=mgiistrct option corrects the completeness or pathlength obscured by detected absorbers with greater equivalent width and folds in the error.

You can also combine the Monte Carlo files into little redshift bins either by forcing an approximate number per bin (first line below) or by explicitly setting the bins (second). You input the root of the output files that will be made.

IDL>sdss_completeness_cum, 2500, 'cmpltrec_MgII_', dvqso=-5000, /rec_param, dblt_name='MgII'

IDL>zlim=fltarr(10,2) & zlim[*,0] = 0.2*lindgen(10)+0.3 & zlim[*,1] = zlim[*,0] + 0.2

IDL>sdss_completeness_cum, zlim, 'cmpltrec_MgII_', /z, dvqso=-5000, /rec_param, dblt_name='MgII'

To actually run Monte Carlo tests yourself, you can use a single IDL call or the shell script:

IDL>sdss_completeness, 'dr7qso_MgII.list', 'dr7qso_MgII.fit', 1000, dblt_name='MgII', config_fil=getenv('XIDL_DIR')+'/SDSS/MgII/sdss_mgiiprof.config', vpstrct_fil='vpstrctmgii72422le500perEW.fit'

TERM>$XIDL_DIR/SDSS/CIV/sdss_runparallelcmplt.sh 4 dr7qso_MgII.list dr7qso_MgII.fit 1000 "dblt_name='MgII', config_fil=getenv('XIDL_DIR')+'/SDSS/MgII/sdss_mgiiprof.config', vpstrct_fil='vpstrctmgii72422le500perEW.fit'"

The use of the given simulated profile library (vpstrctmgii72422le500perEW.fit) is optional. The code can call SDSS_GENPROF_MC() and use the sdss_mgiiprof.config file to generate profiles on the fly, though this is slower.

igmabsorbers.info is funded by the National Science Foundation Astronomy & Astrophysics Postdoctoral Fellowship (AST-1003139).