6 research outputs found
The 2dF-SDSS LRG and QSO Survey: The Star Formation Histories of Luminous Red Galaxies
We present a detailed investigation into the recent star formation histories
of 5,697 Luminous Red Galaxies (LRGs) based on the Hdelta (4101A) and [OII]
(3727A) lines. LRGs are luminous (L>3L*), galaxies which have been selected to
have photometric properties consistent with an old, passively evolving stellar
population. For this study we utilise LRGs from the recently completed 2dF-SDSS
LRG and QSO survey (2SLAQ). Equivalent widths of the Hdelta and [OII] lines are
measured and used to define three spectral types, those with only strong Hdelta
absorption (k+a), those with strong [OII] in emission (em) and those with both
(em+a). All other LRGs are considered to have passive star formation histories.
The vast majority of LRGs are found to be passive (~80 per cent), however
significant numbers of k+a (2.7 per cent), em+a (1.2 per cent) and em LRGs (8.6
per cent) are identified. An investigation into the redshift dependence of the
fractions is also performed. A sample of SDSS MAIN galaxies with colours and
luminosities consistent with the 2SLAQ LRGs is selected to provide a low
redshift comparison. While the em and em+a fractions are consistent with the
low redshift SDSS sample, the fraction of k+a LRGs is found to increase
significantly with redshift. This result is interpreted as an indication of an
increasing amount of recent star formation activity in LRGs with redshift. By
considering the expected life time of the k+a phase, the number of LRGs which
will undergo a k+a phase can be estimated. A crude comparison of this estimate
with the predictions from semi-analytic models of galaxy formation shows that
the predicted level of k+a and em+a activity is not sufficient to reconcile the
predicted mass growth for massive early-types in a hierarchical merging
scenario.Comment: Accepted for publication in MNRAS, 13 pages, 10 figure
The 2dF-SDSS LRG and QSO (2SLAQ) Survey: the z < 2.1 quasar luminosity function from 5645 quasars to g= 21.85
We have used the 2dF instrument on the AAT to obtain redshifts of a sample of
z<3, 18.0<g<21.85 quasars selected from SDSS imaging. These data are part of a
larger joint programme: the 2dF-SDSS LRG and QSO Survey (2SLAQ). We describe
the quasar selection algorithm and present the resulting luminosity function of
5645 quasars in 105.7 deg^2. The bright end number counts and luminosity
function agree well with determinations from the 2dF QSO Redshift Survey (2QZ)
data to g\sim20.2. However, at the faint end the 2SLAQ number counts and
luminosity function are steeper than the final 2QZ results from Croom et al.
(2004), but are consistent with the preliminary 2QZ results from Boyle et al.
(2000). Using the functional form adopted for the 2QZ analysis, we find a faint
end slope of beta=-1.78+/-0.03 if we allow all of the parameters to vary and
beta=-1.45+/-0.03 if we allow only the faint end slope and normalization to
vary. Our maximum likelihood fit to the data yields 32% more quasars than the
final 2QZ parameterization, but is not inconsistent with other g>21 deep
surveys. The 2SLAQ data exhibit no well defined ``break'' but do clearly
flatten with increasing magnitude. The shape of the quasar luminosity function
derived from 2SLAQ is in good agreement with that derived from type I quasars
found in hard X-ray surveys. [Abridged]Comment: MNRAS, accepted; 15 pg., 14 fig. (9 color); high res. version at
http://sdss2df.ncsa.uiuc.edu/publications/refereed/gtrichards_2SLAQ_QLF.p