galaxies have bulges that are younger and
more metal-rich than their disks. The team
describes an evolutionary process whereby
enriched gas in the spiral disk moves to the
bulge, providing the material for the last
episode of star formation in the new lenticular galaxy.
The researchers concentrate on a sample
of 21 S0 galaxies located in the Virgo Cluster. They employ a novel analysis technique,
which uses the spatial light profile to decompose the separate bulge and disk spectra for
each galaxy. This simple spatial model consists of only the bulge and disk components,
so complicated morphologies, such as dust
lanes and rings, are problematic.
In all cases, the team needs high signal-tonoise ratios to extract the distinct spectra,
which is possible in the relatively nearby Virgo Cluster and using the Gemini Multi-Object Spectrographs (GMOS) at both Gemini
North and South. Absorption lines yield age
and metallicity values, with stellar indices
measured directly and stellar models used
to derive physical properties. While different models would provide different absolute values, the sense of the relationships
remains robust, with younger and more
metal-rich stellar populations in the disks
(Figure 8) — the result of recent episodes of
star formation in enriched material.
The cluster environment, too, is likely important in the evolutionary process, which requires a traumatic event to strip the disk gas
(quenching star formation there) and funnel
it toward the galaxy’s center. The rich environments of clusters do provide such opportunities for the progenitor spiral galaxies to
interact with other galaxies and the diffuse
cluster medium. This work, however, is not
sensitive to the possible effects of environmental variations.
Full results are published in Monthly Notices
of the Royal Astronomical Society, 441: 333,
2014, and a preprint is available.
January 2015
Galaxy-wide Outflows Common
Among Quasars
New work shows that galaxy-wide outflows
are common among galaxies that host luminous quasars. The underlying energetic
source of these outflows is unclear, being related either to the accretion onto the central
supermassive black hole or star formation.
Some process to inject mass and energy into
the surroundings does, however, appear to
be an essential aspect of cosmic evolution.
In addition to depositing chemically-enriched material in the halo and larger intergalactic environment, outflows may be a
key link that provides feedback between the
growth of central black holes and star formation, which accounts for the present-day distribution of galaxy properties.
Figure 8.
Estimates of the
relative ages and
metallicities of the
bulges (blue circles)
and disks (red ellipses)
of the S0 galaxies in
the Virgo Cluster. Solid
lines link bulge and
disk stellar populations
from the same galaxy.
The general trend
shows younger, more
metal-rich bulges
relative to their
corresponding disks.
Chris Harrison (Durham University) led the
study, based on observations of 16 luminous
quasars at redshifts z < 0.2. These are all type
2 (i.e., obscured) radio-quiet quasars exhibiting ordinary rates of star formation. Selected
from a parent sample of 24,000 galaxies,
these observations yield general conclus [ۜ