407 research outputs found
Green Valley Galaxies
The "green valley" is a wide region separating the blue and the red peaks in
the ultraviolet-optical color magnitude diagram, first revealed using GALEX UV
photometry. The term was coined by Christopher Martin in 2005. Green valley
highlights the discriminating power of UV to very low relative levels of
ongoing star formation, to which the optical colors, including u-r, are
insensitive. It corresponds to massive galaxies below the star-forming "main"
sequence, and therefore represents a critical tool for the study of the
quenching of star formation and its possible resurgence in otherwise quiescent
galaxies. This article reviews the results pertaining to morphology, structure,
environment, dust content and gas properties of green valley galaxies in the
local universe. Their relationship to AGN is also discussed. Attention is given
to biases emerging from defining the "green valley" using optical colors. We
review various evolutionary scenarios and we present evidence for a new,
quasi-static view of the green valley, in which the majority of galaxies
currently in the green valley were only partially quenched in the distant past
and now participate in a slow cosmic decline of star formation, which also
drives down the activity on the main sequence, presumably as a result of the
dwindling accretion/cooling onto galaxy disks.Comment: Invited review. 13 pages. Comments welcom
Star Formation Rate Distributions: Inadequacy of the Schechter Function
In this paper we posit that galaxy luminosity functions (LFs) come in two
fundamentally different types depending on whether the luminosity traces galaxy
stellar mass or its current star formation rate (SFR). Mass function types
reflect the older stars and therefore the stellar mass distribution, while SFR
function types arise from the young stars and hence the distribution of SFRs.
Optical and near-infrared LFs are of the mass function type, and are well fit
by a Schechter function (power law with an exponential cutoff at the bright
end). In contrast, LFs of the SFR function type are of a different form, one
that cannot be adequately described by a Schechter function. We demonstrate
this difference by generating SFR distributions for mock samples of galaxies
drawn from a Schechter stellar mass distribution along with established
empirical relations between the SFR and stellar mass. Compared with the
Schechter function, SFR distributions have a shallower decline at the bright
end, which can be traced to the large intrinsic scatter of SFRs at any given
stellar mass. A superior description of SFR distributions is given by the
"Saunders" function, which combines a power law with a Gaussian at the high
end. We show that the Schechter-like appearance of UV and H alpha LFs, although
they are LFs of SFR function type, results when luminosities are not corrected
for dust, or when average statistical corrections are used because individual
attenuation measurements are not available. We thus infer that the
non-Schechter form of the far-IR LFs is a true reflection of the underlying SFR
distribution, rather than the purported artifact of AGN contamination.Comment: Revised after a referee report. Submitted to ApJ. Compatible with B/W
printers. Comments are welcom
Dust Attenuation Curves in the Local Universe: Demographics and New Laws for Star-forming Galaxies and High-redshift Analogs
We study dust attenuation curves of 230,000 individual galaxies in the local
universe, ranging from quiescent to intensely star-forming systems, using
GALEX, SDSS, and WISE photometry calibrated on Herschel-ATLAS. We use a new
method of constraining SED fits with infrared luminosity (SED+LIR fitting), and
parameterized attenuation curves determined with the CIGALE SED fitting code.
Attenuation curve slopes and UV bump strengths are reasonably well constrained
independently from one another. We find that attenuation
curves exhibit a very wide range of slopes that are on average as steep as the
SMC curve slope. The slope is a strong function of optical opacity. Opaque
galaxies have shallower curves - in agreement with recent radiate transfer
models. The dependence of slopes on the opacity produces an apparent dependence
on stellar mass: more massive galaxies having shallower slopes. Attenuation
curves exhibit a wide range of UV bump amplitudes, from none to MW-like; with
an average strength 1/3 of the MW bump. Notably, local analogs of high-redshift
galaxies have an average curve that is somewhat steeper than the SMC curve,
with a modest UV bump that can be to first order ignored, as its effect on the
near-UV magnitude is 0.1 mag. Neither the slopes nor the strengths of the UV
bump depend on gas-phase metallicity. Functional forms for attenuation laws are
presented for normal star-forming galaxies, high-z analogs and quiescent
galaxies. We release the catalog of associated SFRs and stellar masses
(GSWLC-2).Comment: Accepted to ApJ. GSWLC-2 catalog of SED+LIR SFRs and M* to be
released Jun 1 at http://pages.iu.edu/~salims/gswlc
On the Mass-Metallicity-Star Formation Rate Relation for Galaxies at
Recent studies have shown that the local mass-metallicity (M-Z) relation
depends on the specific star formation rate (SSFR). Whether such a dependence
exists at higher redshifts, and whether the resulting M-Z-SFR relation is
redshift invariant, is debated. We re-examine these issues by applying the
non-parametric techniques of Salim et al. (2014) to ~130 galaxies
with N2 and O3 measurements from KBSS (Steidel et al. 2014). We find that the
KBSS M-Z relation depends on SSFR at intermediate masses, where such dependence
exists locally. KBSS and SDSS galaxies of the same mass and SSFR ("local
analogs") are similarly offset in the BPT diagram relative to the bulk of local
star-forming galaxies, and thus we posit that metallicities can be compared
self-consistently at different redshifts as long as the masses and SSFRs of the
galaxies are similar. We find that the M-Z-SFR relation of galaxies is
consistent with the local one at , but is offset up to -0.25 dex
at higher masses, so it is altogether not redshift invariant. This high-mass
offset could arise from a bias that high-redshift spectroscopic surveys have
against high-metallicity galaxies, but additional evidence disfavors this
possibility. We identify three causes for the reported discrepancy between N2
and O3N2 metallicities at : (1) a smaller offset that is also present
for SDSS galaxies, which we remove with new N2 calibration, (2) a genuine
offset due to differing ISM condition, which is also present in local analogs,
(3) an additional offset due to unrecognized AGN contamination.Comment: ApJ accepted. 14 pages. Comments welcom
- …