TY - JOUR
T1 - The population of weak Mg II absorbers. I. A survey of 26 QSO HIRES/Keck spectra
AU - Churchill, Christopher W.
AU - Rigby, Jane R.
AU - Charlton, Jane C.
AU - Vogt, Steven S.
PY - 1999/1
Y1 - 1999/1
N2 - We present a search for "weak" Mg II absorbers [those with Wr(2796) < 0.3 Å] in the HIRES/Keck spectra of 26 QSOs. We found 30, of which 23 are newly discovered. The spectra are 80% complete to Wr(2796) = 0.02 Å and have a cumulative redshift path of ∼ 17.2 for the redshift range 0.4 ≤ z ≤ 1.4. The number of absorbers per unit redshift, dN/dz, is seen to increase as the equivalent width threshold is decreased; we obtained dN/dz = 1.74 ± 0.10 for our 0.02 Wr(2796) < 0.3 Å sample. The equivalent width distribution follows a power law, N(W) ∝ W-δ, with δ ∼ 1.0; there is no turnover down to Wr(2796) = 0.02 Å at 〈z〉 = 0.9. Weak absorbers comprise at least 65% of the total Mg II absorption population, which outnumbers Lyman limit systems (LLSs) by a factor of 3.8 ± 1.1; the majority of weak Mg II absorbers must arise in sub-LLS environments. Tentatively, we predict that ∼ 5% of the Lyα forest clouds with Wr(Lyα) ≥ 0.1 Å will have detectable Mg II absorption to Wminr(2796) = 0.02 Å and that this is primarily a high-metallicity selection effect ([Z/Z⊙] ≥ - 1). This implies that Mg II absorbing structures figure prominently as tracers of sub-LLS environments where gas has been processed by stars. We compare the number density of Wr(2796) ≥ 0.02 Å absorbers with that of both high and low surface brightness galaxies and find a fiducial absorber size of 35 h-1-63 h-1 kpc, depending upon the assumed galaxy population and their absorption properties. The individual absorbing "clouds" have Wr(2796) ≤ 0.15 Å, and their narrow (often unresolved) line widths imply temperatures of ∼ 25,000 K. We measured Wr(1548) from C IV in Faint Object Spectrograph/Hubble Space Telescope archival spectra and, based upon comparisons with Fe II, found a range of ionization conditions (low, high, and multiphase) in absorbers selected by weak Mg II.
AB - We present a search for "weak" Mg II absorbers [those with Wr(2796) < 0.3 Å] in the HIRES/Keck spectra of 26 QSOs. We found 30, of which 23 are newly discovered. The spectra are 80% complete to Wr(2796) = 0.02 Å and have a cumulative redshift path of ∼ 17.2 for the redshift range 0.4 ≤ z ≤ 1.4. The number of absorbers per unit redshift, dN/dz, is seen to increase as the equivalent width threshold is decreased; we obtained dN/dz = 1.74 ± 0.10 for our 0.02 Wr(2796) < 0.3 Å sample. The equivalent width distribution follows a power law, N(W) ∝ W-δ, with δ ∼ 1.0; there is no turnover down to Wr(2796) = 0.02 Å at 〈z〉 = 0.9. Weak absorbers comprise at least 65% of the total Mg II absorption population, which outnumbers Lyman limit systems (LLSs) by a factor of 3.8 ± 1.1; the majority of weak Mg II absorbers must arise in sub-LLS environments. Tentatively, we predict that ∼ 5% of the Lyα forest clouds with Wr(Lyα) ≥ 0.1 Å will have detectable Mg II absorption to Wminr(2796) = 0.02 Å and that this is primarily a high-metallicity selection effect ([Z/Z⊙] ≥ - 1). This implies that Mg II absorbing structures figure prominently as tracers of sub-LLS environments where gas has been processed by stars. We compare the number density of Wr(2796) ≥ 0.02 Å absorbers with that of both high and low surface brightness galaxies and find a fiducial absorber size of 35 h-1-63 h-1 kpc, depending upon the assumed galaxy population and their absorption properties. The individual absorbing "clouds" have Wr(2796) ≤ 0.15 Å, and their narrow (often unresolved) line widths imply temperatures of ∼ 25,000 K. We measured Wr(1548) from C IV in Faint Object Spectrograph/Hubble Space Telescope archival spectra and, based upon comparisons with Fe II, found a range of ionization conditions (low, high, and multiphase) in absorbers selected by weak Mg II.
UR - http://www.scopus.com/inward/record.url?scp=0033417750&partnerID=8YFLogxK
UR - http://www.scopus.com/inward/citedby.url?scp=0033417750&partnerID=8YFLogxK
U2 - 10.1086/313168
DO - 10.1086/313168
M3 - Review article
AN - SCOPUS:0033417750
SN - 0067-0049
VL - 120
SP - 51
EP - 75
JO - Astrophysical Journal, Supplement Series
JF - Astrophysical Journal, Supplement Series
IS - 1
ER -