132 research outputs found
Reconciling the observed star-forming sequence with the observed stellar mass function
We examine the connection between the observed star-forming sequence (SFR
) and the observed evolution of the stellar mass function
between . We find the star-forming sequence cannot have a slope
0.9 at all masses and redshifts, as this would result in a
much higher number density at by
than is observed. We show that a transition in the slope of the star-forming
sequence, such that at and
({Whitaker} {et~al.} 2012) at
, greatly improves agreement with the
evolution of the stellar mass function. We then derive a star-forming sequence
which reproduces the evolution of the mass function by design. This
star-forming sequence is also well-described by a broken-power law, with a
shallow slope at high masses and a steep slope at low masses. At , it is
offset by 0.3 dex from the observed star-forming sequence, consistent
with the mild disagreement between the cosmic SFR and recent observations of
the growth of the stellar mass density. It is unclear whether this problem
stems from errors in stellar mass estimates, errors in SFRs, or other effects.
We show that a mass-dependent slope is also seen in other self-consistent
models of galaxy evolution, including semi-analytical, hydrodynamical, and
abundance-matching models. As part of the analysis, we demonstrate that neither
mergers nor hidden low-mass quiescent galaxies are likely to reconcile the
evolution of the mass function and the star-forming sequence. These results are
supported by observations from {Whitaker} {et~al.} (2014).Comment: 17 pages, 13 figures, accepted to ApJ Oct 31st 201
The superluminous supernova SN 2017egm in the nearby galaxy NGC 3191: a metal-rich environment can support a typical SLSN evolution
At redshift z=0.03, the recently-discovered SN 2017egm is the nearest Type I
superluminous supernova (SLSN) to date, and first near the center of a massive
spiral galaxy (NGC 3191). Using SDSS spectra of NGC 3191, we find a metallicity
~2 Z at the nucleus and ~1.3 Z for a star forming region at a
radial offset similar to SN 2017egm. Archival radio-to-UV photometry reveals a
star-formation rate ~15 M yr (with ~70% dust-obscured), which
can account for a Swift X-ray detection, and stellar mass ~
M. We model the early UV-optical light curves with a magnetar
central-engine model, using the Bayesian light curve fitting tool MOSFiT. The
fits indicate ejecta mass 2-4 M, spin period 4-6 ms, magnetic field
(0.7-1.7)G, and kinetic energy 1-2 erg. These
parameters are consistent with the overall distributions for SLSNe, modeled by
Nicholl et al (2017), although the derived mass and spin are towards the low
end, possibly indicating enhanced loss of mass and angular momentum before
explosion. This has two implications: (i) SLSNe can occur at solar metallicity,
although with a low fraction ~10%; and (ii) metallicity has at most a modest
effect on their properties. Both conclusions are in line with results for long
gamma-ray bursts. Assuming a monotonic rise gives an explosion date MJD
. However, a short-lived excess in the data relative to the
best-fitting models may indicate an early-time `bump'. If confirmed, SN 2017egm
would be the first SLSN with a spectrum during the bump-phase; this shows the
same O II lines seen at maximum light, which may be an important clue for
explaining these bumps.Comment: Accepted for publication in ApJ
Monte Carlo Techniques for Addressing Large Errors and Missing Data in Simulation-based Inference
Upcoming astronomical surveys will observe billions of galaxies across cosmic
time, providing a unique opportunity to map the many pathways of galaxy
assembly to an incredibly high resolution. However, the huge amount of data
also poses an immediate computational challenge: current tools for inferring
parameters from the light of galaxies take hours per fit. This is
prohibitively expensive. Simulation-based Inference (SBI) is a promising
solution. However, it requires simulated data with identical characteristics to
the observed data, whereas real astronomical surveys are often highly
heterogeneous, with missing observations and variable uncertainties determined
by sky and telescope conditions. Here we present a Monte Carlo technique for
treating out-of-distribution measurement errors and missing data using standard
SBI tools. We show that out-of-distribution measurement errors can be
approximated by using standard SBI evaluations, and that missing data can be
marginalized over using SBI evaluations over nearby data realizations in the
training set. While these techniques slow the inference process from
sec to min per object, this is still significantly faster than
standard approaches while also dramatically expanding the applicability of SBI.
This expanded regime has broad implications for future applications to
astronomical surveys.Comment: 8 pages, 2 figures, accepted to the Machine Learning and the Physical
Sciences workshop at NeurIPS 202
Hierarchical Bayesian inference of photometric redshifts with stellar population synthesis models
We present a Bayesian hierarchical framework to analyze photometric galaxy
survey data with stellar population synthesis (SPS) models. Our method couples
robust modeling of spectral energy distributions with a population model and a
noise model to characterize the statistical properties of the galaxy
populations and real observations, respectively. By self-consistently inferring
all model parameters, from high-level hyper-parameters to SPS parameters of
individual galaxies, one can separate sources of bias and uncertainty in the
data. We demonstrate the strengths and flexibility of this approach by deriving
accurate photometric redshifts for a sample of spectroscopically-confirmed
galaxies in the COSMOS field, achieving a performance competitive with
publicly-released photometric redshift catalogs based on the same data. Prior
to this work, this approach was computationally intractable in practice due to
the heavy computational load of SPS model calls; we overcome this challenge
using with neural emulators. We find that the largest photometric residuals are
associated with poor calibration for emission line luminosities and thus build
a framework to mitigate these effects. This combination of physics-based
modeling accelerated with machine learning paves the path towards meeting the
stringent requirements on the accuracy of photometric redshift estimation
imposed by upcoming cosmological surveys. The approach also has the potential
to create new links between cosmology and galaxy evolution through the analysis
of photometric datasets.Comment: 16 pages, 6 figures. To be submitted to APJ
SBI++: Flexible, Ultra-fast Likelihood-free Inference Customized for Astronomical Application
Flagship near-future surveys targeting galaxies across cosmic
time will soon reveal the processes of galaxy assembly in unprecedented
resolution. This creates an immediate computational challenge on effective
analyses of the full data-set. With simulation-based inference (SBI), it is
possible to attain complex posterior distributions with the accuracy of
traditional methods but with a increase in speed. However, it comes
with a major limitation. Standard SBI requires the simulated data to have
identical characteristics to the observed data, which is often violated in
astronomical surveys due to inhomogeneous coverage and/or fluctuating sky and
telescope conditions. In this work, we present a complete SBI-based
methodology, ``SBI,'' for treating out-of-distribution measurement
errors and missing data. We show that out-of-distribution errors can be
approximated by using standard SBI evaluations and that missing data can be
marginalized over using SBI evaluations over nearby data realizations in the
training set. In addition to the validation set, we apply SBI to
galaxies identified in extragalactic images acquired by the James Webb Space
Telescope, and show that SBI can infer photometric redshifts at least as
accurately as traditional sampling methods and crucially, better than the
original SBI algorithm using training data with a wide range of observational
errors. SBI retains the fast inference speed of 1 sec for objects
in the observational training set distribution, and additionally permits
parameter inference outside of the trained noise and data at 1 min per
object. This expanded regime has broad implications for future applications to
astronomical surveys.Comment: 12 pages, 5 figures. Code and a Jupyter tutorial are made publicly
available at https://github.com/wangbingjie/sbi_p
Forward Modeling of Galaxy Populations for Cosmological Redshift Distribution Inference
We present a forward-modeling framework for estimating galaxy redshift distributions from photometric surveys. Our forward model is composed of: a detailed population model describing the intrinsic distribution of the physical characteristics of galaxies, encoding galaxy evolution physics; a stellar population synthesis model connecting the physical properties of galaxies to their photometry; a data model characterizing the observation and calibration processes for a given survey; and explicit treatment of selection cuts, both into the main analysis sample and for the subsequent sorting into tomographic redshift bins. This approach has the appeal that it does not rely on spectroscopic calibration data, provides explicit control over modeling assumptions and builds a direct bridge between photo-z inference and galaxy evolution physics. In addition to redshift distributions, forward modeling provides a framework for drawing robust inferences about the statistical properties of the galaxy population more generally. We demonstrate the utility of forward modeling by estimating the redshift distributions for the Galaxy And Mass Assembly (GAMA) survey and the Vimos VLT Deep Survey (VVDS), validating against their spectroscopic redshifts. Our baseline model is able to predict tomographic redshift distributions for GAMA and VVDS with respective biases of Δz ≲ 0.003 and Δz ≃ 0.01 on the mean redshift—comfortably accurate enough for Stage III cosmological surveys—without any hyperparameter tuning (i.e., prior to doing any fitting to those data). We anticipate that with additional hyperparameter fitting and modeling improvements, forward modeling will provide a path to accurate redshift distribution inference for Stage IV surveys
- …