Prospecting for C IV at high redshifts

TitleConstraints on the Universal CIV Mass Density at z~6 from Early IR Spectra Obtained with the Magellan FIRE Spectrograph
Authors: Robert A. Simcoe et al.
Affiliation: MIT-Kavli Center for Astrophysics and Space Research


In March, I went observing on Magellan with the FIRE spectrograph so I thought it would be fun to take a look at this paper, which uses the same instrument to do entirely different astrophysics.  I didn’t leave the solar neighborhood, while these authors take a look at very high redshift quasars with the goal of studying the intergalactic medium (IGM).  As the light from one of these distant objects propagates towards us, it encounters metals in the intervening IGM that superimpose absorption lines on the quasar’s spectrum.  By looking for metal absorption lines in the spectra of high redshifts quasars, we can learn about the metal content of the IGM.


This paper considers the spectra of 7 quasars with redshifts greater than 5.5.  The moderate-resolution, near-infrared spectra were obtained over the past year with the FIRE spectrograph.  The objects were additionally observed with an optical spectrograph (MagE, also on Magellan) to add data to the blue end.  Then, the authors searched each spectra for absorption features, looking for pairs that matched the expected depth and configuration of the C IV doublet.  Two false positives encountered in this process were poorly-subtracted sky and other doublets.  They also injected simulated C IV signals into their data to quantify how often these signals were recovered.

Two spectra are shown below as an example.  The red forks mark the locations of identified C IV doublets.  The dashed blue line shows where absorption from C IV at the same redshift as the quasar would appear; absorption features at relatively bluer/shorter wavelengths are the result of C IV absorption at lower redshift.

Spectra of two high redshift quasars. Red forks mark C IV absorption features while the blue dashed line shows the wavelength at which C IV absorption at the redshift of the quasar would be located.


The ~20 features detected were found to correspond 19 unique C IV systems at redshifts of 4.6 to 5.9.  There were no detections along several sight-lines.  From the absorption strength of the C IV doublet, Simcoe et al. calculate the column density of C IV in each system.  The column density is the number of atoms or molecules along some line of sight; here it is specifically the number of C IV ions in some system along our line of sight towards the quasar.  The measurements from this work were combined with work done in the northern hemisphere (Magellan is in Chile so the northern targets are not observable with FIRE), which requires adjusting previous results for differences in completeness and cosmological models. The results from various surveys are combined in the figure below.

The mass density of C IV is calculated at two redshifts, z~4.95 and z~5.66.  The density at the higher redshift is found to be four times smaller than the density at z=4.95.  Although the details are uncertain due to the paucity of data, it is clear that the density of C IV is on the decline at z>4.5.  At this point, we are left with several questions.  Is this the result of a change in the total carbon abundance or in the abundance of C IV relative to other ionization stages?  Are the C IV systems truly part of the IGM or are they found near to early galaxies?  Finally, the authors caution that a larger sample is needed.

The mass density of C IV determined from quasar absorption features as a function of redshift, showing a decline at high redshift. The results from this work are plotted in blue.

About Elisabeth Newton

Elisabeth was a Harvard graduate student and an astrobites and ComSciCon co-founder and is now a professor at Dartmouth College.

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