249 research outputs found
The motivation for flexible star-formation histories from spatially resolved scales within galaxies
The estimation of galaxy stellar masses depends on the assumed prior of the
star-formation history (SFH) and spatial scale of the analysis (spatially
resolved versus integrated scales). In this paper, we connect the prescription
of the SFH in the Spectral Energy Distribution (SED) fitting to spatially
resolved scales () to shed light on the systematics involved
when estimating stellar masses. Specifically, we fit the integrated photometry
of massive (log (M/M), intermediate
redshift () galaxies with , assuming both
exponentially declining tau model and flexible SFHs. We complement these fits
with the results of spatially resolved SFH estimates obtained by pixel-by-pixel
SED fitting, which assume tau models for individual pixels. These spatially
resolved SFHs show a large diversity in shapes, which can largely be accounted
for by the flexible SFHs with . The differences in the
stellar masses from those two approaches are overall in good agreement (average
difference of dex). Contrarily, the simpler tau model SFHs
typically miss the oldest episode of star formation, leading to an
underestimation of the stellar mass by dex. We further compare the
derived global specific star-formation rate (sSFR), the mass-weighted stellar
age (t), and the star-formation timescale ()
obtained from the different SFH approaches. We conclude that the spatially
resolved scales within galaxies motivate a flexible SFH on global scales to
account for the diversity of SFHs and counteract the effects of outshining of
older stellar populations by younger ones
Photometric properties of reionization-epoch galaxies in the Simba simulations
We study the photometric properties and sizes of the reionization-epoch
galaxies in high-resolution Simba cosmological hydrodynamical simulations with
box sizes of Mpc. Assuming various attenuation laws, we compute
photometry by extincting each star particle's spectrum using the line-of-sight
gas metal column density. The predicted ultraviolet luminosity function (UVLF)
generally agrees with observations at , owing to a partial cancellation
between the high metallicities of the simulated galaxies and lower
dust-to-metal ratios. The simulated UVLF is low compared to observations,
likely owing to excessive dust extinction. Simba predicts UV continuum slopes
() in agreement with the observations, with the best agreement
obtained using a Calzetti extinction law. Interestingly, the gas-phase
mass-metallicity relation in Simba is higher at than at ,
suggesting that rapid early enrichment (and dust growth) might be necessary to
match the observed . We find that is more sensitive to the dust
extinction law than the UVLF. By generating mock James Webb Space Telescope
(JWST) images and analysing in a manner similar to observations, we show that
Simba's galaxy size-luminosity relation well reproduces the current
Hubble observations. Unlike observations at lower redshifts, Simba predicts
similar rest-UV and rest-optical sizes of galaxies, owing to weak age
gradients and dust extinction in star-forming regions counteract each other to
weaken the color gradients within galaxies. These predictions will be testable
with JWST.Comment: 15 pages, first revisio
Unravelling the Dust Attenuation Scaling Relations and their Evolution
We explore the dependence of dust attenuation, as traced by the Balmer decrement, on galactic properties by using a
large sample of SDSS spectra. We use both Partial Correlation Coefficients
(PCC) and Random Forest (RF) analysis to distinguish those galactic parameters
that directly and primarily drive dust attenuation in galaxies, from parameters
that are only indirectly correlated through secondary dependencies. We find
that, once galactic inclination is controlled for, dust attenuation depends
primarily on stellar mass, followed by metallicity and velocity dispersion.
Once the dependence on these quantities is taken into account, there is no
dependence on star formation rate. While the dependence on stellar mass and
metallicity was expected based on simple analytical equations for the
interstellar medium, the dependence on velocity dispersion was not predicted
and we discuss possible scenarios to explain it. We identify a projection of
this multi-dimensional parameters space which minimises the dispersion in terms
of the Balmer decrement and which encapsulates the primary and secondary
dependences of the Balmer decrement into a single parameter defined as the
reduced mass . We show that the dependence of the Balmer decrement
on this single parameter also holds at high redshift, suggesting that the
processes regulating dust production and distribution do not change
significantly through cosmic epochs at least out to z2.Comment: 14 pages, 9 figures (+ Appendix 6 pages, 7 figures), submitted to
MNRAS, comments welcom
Reproducing the UVJ Color Distribution of Star-forming Galaxies at 0.5 < z < 2.5 with a Geometric Model of Dust Attenuation
We analyze the distribution of rest-frame U - V and V - J colors for star-forming galaxies at 0.5 < z < 2.5. Using stellar population synthesis, stochastic star formation histories, and a simple prescription for the dust attenuation that accounts for the shape and inclination of galaxies, we construct a model for the distribution of galaxy colors. With only two free parameters, this model is able to reproduce the observed galaxy colors as a function of redshift and stellar mass remarkably well. Our analysis suggests that the wide range of dust attenuation values measured for star-forming galaxies at a given redshift and stellar mass is almost entirely due to the effect of inclination; if all galaxies at a given stellar mass were observed edge-on, they would show very similar dust attenuation. This result has important implications for the interpretation of dust attenuation measurements, the treatment of UV and IR luminosity, and the comparison between numerical simulations and observations
-M* Diagram: A Valuable Galaxy Evolution Diagnostic to Complement (s)SFR-M* Diagrams
The specific star formation rate (sSFR) is commonly used to describe the
level of galaxy star formation (SF) and to select quenched galaxies. However,
being a relative measure of the young-to-old population, an ambiguity in its
interpretation may arise because a small sSFR can be either because of a
substantial previous mass build up, or because SF is low. We show, using large
samples spanning 0 < z < 2, that the normalization of SFR by the physical
extent over which SF is taking place (i.e., SFR surface density,
) overcomes this ambiguity. has
a strong physical basis, being tied to the molecular gas density and the
effectiveness of stellar feedback, so we propose -M* as
an important galaxy evolution diagram to complement (s)SFR-M* diagrams. Using
the -M* diagram we confirm the Schiminovich et al.
(2007) result that the level of SF along the main sequence today is only weakly
mass dependent - high-mass galaxies, despite their redder colors, are as active
as blue, low-mass ones. At higher redshift, the slope of the
" main sequence" steepens, signaling the epoch of bulge
build-up in massive galaxies. We also find that based
on the optical isophotal radius more cleanly selects both the starbursting and
the spheroid-dominated (early-type) galaxies than sSFR. One implication of our
analysis is that the assessment of the inside-out vs. outside-in quenching
scenarios should consider both sSFR and radial
profiles, because ample SF may be present in bulges with low sSFR (red color).Comment: 16 pages. Accepted to ApJ. Comments on content or relevant missing
references welcom
Anatomy of an ionized bubble at z=6.6: Which galaxies reionized the Universe?
Identifying the sources that drove cosmic reionization is a key goal of observational cosmology. Photons from these sources carved out ionized bubbles in the neutral intergalactic medium, and these bubbles gradually coalesced, resulting in a fully ionized Universe. The luminous z=6.6 'COLA1' galaxy lies in the epoch of reionization and shows a remarkable, double-peaked Lyman-alpha (Lya) line, the only one confirmed by multiple teams with high SNR and resolution. The detection of Lya flux bluewards of the systemic velocity means COLA1 resides in an ionized bubble. The exact velocity at which the blue Lya light is cut-off constrains the bubble size. This bubble provides a unique fortuitous, controlled environment -- since the bubble size is constrained, so is the total ionizing flux required to power it. Did COLA1 produce this ionizing flux all by itself? Or is it surrounded by large numbers of bright galaxies? Is a significant contribution from the faintest galaxies necessary? We propose to blindly identify emission-line galaxies within the ionized bubble and to obtain sensitive spectroscopy of COLA1 itself with slitless grism spectroscopy in the NIRCam F356W filter. The bubble size is well matched to the effective field of view for H-beta and the [OIII] doublet at z=6.6. We will obtain spectroscopic redshifts for all objects brighter than 0.1 L* (SFR>2 Msun/yr) and directly measure their ionizing photon production rate. We will then assess how much contribution from unseen galaxies is required. Through our detailed accounting of ionizing photons we will address the central question to reionization studies: was it bright or faint galaxies that reionized the universe
AGN Feedback in SDSS-IV MaNGA: AGNs have Suppressed Central Star Formation Rates
Despite the importance of feedback from active galactic nuclei (AGNs) in
models of galaxy evolution, observational constraints on the influence of AGN
feedback on star formation remain weak. To this end, we have compared the star
formation trends of 279 low-redshift AGN galaxies with 558 inactive control
galaxies using integral field unit spectroscopy from the SDSS-IV MaNGA survey.
With a Gaussian process-based methodology, we reconstruct nonparametric star
formation histories in spatially resolved spaxels covering the face of each
galaxy. Based on galaxy-wide star formation rates (SFRs) alone, we find no
obvious signatures of AGN feedback. However, the AGN galaxies have
significantly suppressed central (kiloparsec-scale) SFRs, lying up to a factor
of below those of the control galaxies, providing direct observational
evidence of AGN feedback suppressing star formation. The suppression of central
SFRs in the AGN galaxies began in the central regions Gyr ago
(redshift ), taking place over a few gigayears. A small subset of
the AGN galaxies were rapidly driven to quiescence shortly before being
observed (in the last Myr), potentially indicating instances of
AGN-driven feedback. More frequently, however, star formation continues in the
AGN galaxies, with suppression primarily in the central regions. This is
suggestive of a picture in which integrated (Gyr-timescale) AGN feedback can
significantly affect central star formation, but may be inefficient in driving
galaxy-wide quenching in low-redshift galaxies, instead leaving them in the
green valley.Comment: 22 pages, 15 figures. Accepted for publication in Ap
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