117 research outputs found
Star Formation and Environmental Quenching of Group Galaxies from the GEEC2 Survey at z~1
This work presents detailed analysis from the GEEC2 spectroscopic survey of galaxy groups at 0.8 66 per cent) for eight of the eleven groups. Using an optical-NIR colour-colour diagram, the galaxies in the sample are separated with a dust insensitive method into the three categories, star-forming, quiescent, and intermediate. The strongest environmental dependence is observed in the fraction of quiescent galaxies, which is higher inside groups than in the field for all stellar masses. While intermediate galaxies represent ~15-20 per cent of the star-forming population in both the group and field, the average specific star formation rates (sSFR) of the group population is lower by a factor of ~3. The intermediate population also does not show the strong Hδ absorption that is characteristic of starburst galaxies. Inside groups, only 4.4-6.7 per cent of star-forming galaxies are starbursts, which gives additional validity to the assumption that the quenching of star-formation is the primary process in the transition from the star-forming to the quiescent state. With the use of stellar synthesis models, two possible scenarios for the origins of the intermediate population are investigated, including the quenching of star-forming galaxies via environmental processes and the rejuvenation of star formation in early-type galaxies via mixed mergers. To model the quenching scenario, we have tested the use of different exponential quenching timescales (τ_2) and different types of delays between satellite accretion and the onset of quenching. We found that the fraction of intermediate galaxies depends most strongly on the value of τ_2. The relative fractions of galaxies rule out both the no-delay scenario, which would require a long τ_2 that over-produces intermediate galaxies, as well as the constant 3 Gyr delay model, which does not produce a sufficient number of quiescent galaxies. The observed fractions are best matched with a model that includes a dynamical delay time and a τ_2=0.25 Gyr, but this model also predicts intermediate galaxies Hδ strength higher than that observed. For the rejuvenation scenario, we found that the time visible in the intermediate region is directly related to the size of the second 'burst' of star-formation, which can then be further constrained by the Hδ strength for the intermediate population. The observations are best matched to a burst size of ~1 per cent, at a rate of ~3 times per Gyr. In order to properly distinguish between the two scenarios, we will need to both increase the signal-to-noise ratio for the Hδ measurements and conduct a deeper survey of satellite galaxies both inside groups and in the field
The GEEC2 spectroscopic survey of Galaxy Groups at
We present the data release of the Gemini-South GMOS spectroscopy in the
fields of 11 galaxy groups at , within the COSMOS field. This forms
the basis of the Galaxy Environment Evolution Collaboration 2 (GEEC2) project
to study galaxy evolution in haloes with across cosmic
time. The final sample includes spectroscopically--confirmed members with
per cent complete for galaxies within the virial
radius, and with stellar mass . Including
galaxies with photometric redshifts we have an effective sample size of galaxies within the virial radii of these groups. We present group
velocity dispersions, dynamical and stellar masses. Combining with the GCLASS
sample of more massive clusters at the same redshift we find the total stellar
mass is strongly correlated with the dynamical mass, with
. This stellar
fraction of per cent is lower than predicted by some halo occupation
distribution models, though the weak dependence on halo mass is in good
agreement. Most groups have an easily identifiable most massive galaxy (MMG)
near the centre of the galaxy distribution, and we present the spectroscopic
properties and surface brightness fits to these galaxies. The total stellar
mass distribution in the groups, excluding the MMG, compares well with an NFW
profile with concentration , for galaxies beyond . This is
more concentrated than the number density distribution, demonstrating that
there is some mass segregation.Comment: Accepted for publication in MNRAS. The appendix is omitted due to
large figures. The full version will be available from the MNRAS website and
from http://quixote.uwaterloo.ca/~mbalogh/papers/GEEC2_data.pdf. Long data
tables are available from MNRAS or by contacting the first autho
The JCMT Nearby Galaxies Legacy Survey -- XI. -- Environmental Variations in the Atomic and Molecular Gas Radial Profiles of Nearby Spiral Galaxies
This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society. ©: 2017 The Author (s). Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.We present an analysis of the radial profiles of a sample of 43 HI-flux selected spiral galaxies from the Nearby Galaxies Legacy Survey (NGLS) with resolved James Clerk Maxwell Telescope (JCMT) CO and/or Very Large Array (VLA) HI maps. Comparing the Virgo and non-Virgo populations, we confirm that the HI disks are truncated in the Virgo sample, even for these relatively HI-rich galaxies. On the other hand, the H distribution is enhanced for Virgo galaxies near their centres, resulting in higher H to HI ratios and steeper H and total gas radial profiles. This is likely due to the effects of moderate ram pressure stripping in the cluster environment, which would preferentially remove low density gas in the outskirts while enhancing higher density gas near the centre. Combined with H star formation rate data, we find that the star formation efficiency (SFR/H) is relatively constant with radius for both samples, but Virgo galaxies have a lower star formation efficiency than non-Virgo galaxies.Peer reviewedFinal Published versio
The JCMT Nearby Galaxies Legacy Survey – XI. Environmental Variations in the Atomic and Molecular Gas Radial Profiles of Nearby Galaxies
We present an analysis of the radial profiles of a sample of 43 H I-flux selected spiral galaxies from the Nearby Galaxies Legacy Survey (NGLS) with resolved James Clerk Maxwell Telescope (JCMT) CO J = 3 − 2 and/or Very Large Array (VLA) H I maps. Comparing the Virgo and non-Virgo populations, we confirm that the H i discs are truncated in the Virgo sample, even for these relatively H i-rich galaxies. On the other hand, the H2 distribution is enhanced for the Virgo galaxies near their centres, resulting in higher H2 to H I ratios and steeper H2 and total gas radial profiles. This is likely due to the effects of moderate ram pressure stripping in the cluster environment, which would preferentially remove low-density gas in the outskirts while enhancing higher density gas near the centre. Combined with Hα star formation rate data, we find that the star formation efficiency (SFR/H2) is relatively constant with radius for both samples, but the Virgo galaxies have an ∼40 per cent lower star formation efficiency than the non-Virgo galaxies
A Tale of 3 Dwarfs: No Extreme Cluster Formation in Extreme Star-Forming Galaxies
Nearly all current simulations predict that outcomes of the star formation
process, such as the fraction of stars that form in bound clusters (Gamma),
depend on the intensity of star formation activity (SigmaSFR) in the host
galaxy. The exact shape and strength of the predicted correlations, however,
vary from simulation to simulation. Observational results also remain unclear
at this time, because most works have mixed estimates made from very young
clusters for galaxies with higher SigmaSFR with those from older clusters for
galaxies with lower SigmaSFR. The three blue compact dwarf (BCD) galaxies
ESO185-IG13, ESO338-IG04, and Haro11 have played a central role on the
observational side because they have some of the highest known SigmaSFR and
published values of Gamma. We present new estimates of Gamma for these BCDs in
three age intervals (1-10 Myr, 10-100 Myr, 100-400 Myr), based on age-dating
which includes Halpha photometry to better discriminate between clusters
younger and older than ~10 Myr. We find significantly lower values for Gamma
(1-10 Myr) than published previously. The likely reason for the discrepancy is
that previous estimates appear to be based on age-reddening results that
underestimated ages and overestimated reddening for many clusters, artificially
boosting Gamma (1-10 Myr). We also find that fewer stars remain in clusters
over time, with ~15-39% in 1-10 Myr, ~5-7% in 10-100 Myr, and ~1-2% in 100-400
Myr clusters. We find no evidence that Gamma increases with SigmaSFR. These
results imply that cluster formation efficiency does not vary with star
formation intensity in the host galaxy. If confirmed, our results will help
guide future assumptions in galaxy-scale simulations of cluster formation and
evolution.Comment: Accepted for publication in Ap
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