Title: The Green Valley is a Red
Herring: Galaxy Zoo reveals two evolutionary pathways towards
quenching of star formation in early- and late-type galaxies
Authors: K. Schawinski, M. Urry,
B. Simmons, L. Fortson, S. Kaviraj, W. Keel, C. Lintott, K. Masters,
R. Nichol, M. Sarzi, S. Ramin, E. Treister, K. Willett, I. Wong, and
S. Yi
First Author's Institution: Institute
for Astronomy, Department of Physics, ETH Zurich
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Article Summary:
The green valley has long been thought
of as the crossroads of galaxy evolution – creating a divide
between the star-forming galaxies of the blue cloud and
passively-evolving galaxies of the red sequence. The intermediate
color of green valley galaxies is thought of as an indicator that
star formation in these samples was recently quenched, and studying
this region between the two main populations will lead to a better
understanding of the evolutionary pathways of galaxies, possibly even
predicting what may happen to our own Milky Way. Galaxies spend most
of their life on what is dubbed the Main Sequence, where there is a
tight correlation between stellar mass and star formation rate (SFR). It is believed that certain processes can cause them to leave the main sequence and begin to
travel through the green valley. Most galaxies were presumed to
follow a similar tract through this desolate region of color space,
progressing through it in a relatively short timescale to keep the scarcity of galaxies observed in this region. In recent years,
the Galaxy Zoo project has brought a new tool to the galaxy evolution
table – a plethora of morphological classifications of galaxies
imaged by the Sloan Digital Sky Survey (SDSS), allowing astronomers
to inquire as to the effects of morphology on a galaxy's transition
through the green valley. A recent study done by Schawinski et al.
using data from the SDSS, Galaxy Evolution Explorer (GALEX), and
Galaxy Zoo has found that different morphological characteristics
will alter the movement of a galaxy through the green valley into its
quiescent fate.
This study acquired a sample of
mass-limited galaxies in the local universe from the SDSS, with redshifts ranging
from z=0.02 to z=0.05. To gain a better understanding of star formation histories, ultraviolet
photometry from the GALEX was found for 71% of their sample. Galaxy Zoo
classifications were used to determine the morphology of the sample,
where morphology was assigned when volunteers agreed on the
classification at a rate of 80% or more. This resulted in the
classification of 18% early types, 34% late types, 45% intermediate
types (galaxies that did not receive at least 80% votes for early or late type), and 3% merging, where these classification follow the
morphology of Hubble's Tuning Fork (see figure 1). The science team believes the relatively
high number of intermediate types most likely results from the
abundance of systems in which the bulge or disk do not clearly
dominate rather than poor imaging data.
 |
| Figure 1. Example gri images ordered by Galaxy Zoo classifications. The left, center, and right columns show blue cloud, green valley, and red sequence galaxies, respectively. The top, center, and bottom rows show early-type, intermediate-type, and late-type galaxies, respectively. |
Though most late-type galaxies of the
sample inhabited the blue cloud and most early types were in the red
sequence, this study found that both early-type and late-type
galaxies spanned almost the entire color range, and within a given
morphological class, the green valley was nothing more than a
collection of outliers.
 |
| Figure 2. Reddening-corrected u-r color-mass diagram for the sample. This figure shows two important findings - that both early and late type galaxies span almost the entire color range, and that the green valley is only defined in the all-galaxy color panel. |
The bimodality of galaxy colors is a result
of the superposition of the two populations; late types are mostly in
the blue cloud and decrease smoothly to the red sequence, and a few
early types reach all the way to the blue cloud. Figure 2 shows a
stellar mass-color contoured plot after dust extinction corrections,
and indicates the position of the green valley in relation to the
blue cloud and red sequence.
Using
ultraviolet and optical photometry, star formation rates (SFRs) were
analyzed to find that early-type galaxies are quenched much more
rapidly than their late-type counterparts. Late-type galaxies were
still blue in the ultraviolet through the green valley, indicating
that they were still undergoing star formation as they were being
quenched. Transitions through the green valley were found to be
highly dependent on morphology, with early-type galaxies exhibiting
quenching on timescales as short as 250 Myr. If this process took
longer, like the 1-3 Gyr track estimated for late-type galaxies,
there would be a build-up of early-types in the green valley that are
not accounted for in observations. Perhaps early-type galaxies
transition through the green valley as fast as star formation would
allow.
 |
| Figure 3. UV-optical dust corrected color-color diagrams of green valley galaxies. Color-coded evolutionary tracks using different quenching timescales are overplotted on the green valley early- and late-type plots. Emission line-selected AGN in host galaxies are marked with green points. These signatures would only appear after a few megayears after the occurrence of a quenching event. |
Investigation
of local environment, gas supply for star formation, and black hole
activity was done to see if these factors could have contributed to
the very different star formation histories of late-type and
early-type galaxies. A catalogue of galaxy halo masses and
information as to whether the green valley galaxies in question were
central or satellites in their clusters was used to investigate the
differences in galactic environments. Schawinski et al. found that
there were striking differences in environments for galaxies of both
populations traveling through the green valley. Though early types
were found in both low- and high-mass haloes, late types had a
dramatic split. Blue cloud late types were mostly in low-mass
haloes, but those in the green valley and red sequence were almost
exclusively in high-mass halos. This is a possible indication that
late-type quenching is caused by environmental processes. In
accordance with the large percentage of late types in the green
valley relative to early types, it was found that late types have
large gas reservoirs relative to early types to fuel star formation
and slow the evolution through the green valley. Lastly, the growth
of supermassive black holes was investigated to see how these
culprits of quenching may differ in early and late types. Figure
3 indicates samples in the green valley that had Active Galactic
Nuclei (AGN) activity. Since
several hundred Myr or more must elapse between the end of star
formation in early types and the detection of an optical AGN, it is
likely that AGN are not responsible for the rapid quenching of star
formation in early types and rather an after-effect of the event that
triggered quenching.
Schawinski
et al. concluded with discussion on the evolutionary tracks related
to the end of star formation for late and early types. Morphological
classifications of SDSS images in Galaxy Zoo as well as ultraviolet
photometric analysis to probe star formation histories have led to
conclusions on how early- and late-type galaxies transition through
the green valley. Figure 4 and figure 5 show cartoons of the predicted
evolutionary sequence through the green valley for early- and
late-type galaxies.
This
study concluded that late-type galaxies initiate their quenching
processes when they are cut off from reservoirs of cosmic gas fueling
their ongoing star formation. This can happen when the galactic halo
reaches a critical mass that prevents further accretion or if cooling
the hot halo gas becomes inefficient. Though the star formation rate
begins to decline, the stellar mass may continue to increase as the
remaining gas reservoirs are converted to stars. Slowly, the galaxy
moves out of the blue cloud and into the green valley, with certain
physical processes possibly accelerating the gas-depletion process.
Black hole accretion may appear in late types after the galaxies have
been quenched. This process occurs over several gigayears.
 |
| Figure 4. Cartoon showing the evolution of late-type galaxies from the blue cloud, through the green valley, and into the red sequence. |
As
for early-type galaxies, quenching of star formation is triggered by
the rapid destruction of galaxy gas reservoirs, and happens too quick
to be due to gas exhaustion by star formation alone. These galaxies
immediately leave the main sequence as the SFR approaches zero and
stellar mass ceases to increase. As fast as stellar evolution
allows, these galaxies move through the green valley and into the red
sequence, typically on timescales of about 1 Gyr. Since there are
very few observed blue early types, it is thought this process is
initiated by a merging event of two late types and the morphology transforms as the
galaxy color and SFR do. After the quenching event, visible
radiation from black hole accretion can be seen, and the rapid
destruction of gas reservoirs suggests the involvement of unusually
strong stellar processes or AGN feedback.
 |
| Figure 5. Cartoon showing the evolution of early-type galaxies from the blue cloud, through the green valley, and into the red sequence. |
