10 research outputs found
Extracting an Informative Latent Representation of High-Dimensional Galaxy Spectra
To understand the fundamental parameters of galaxy evolution, we investigated
the minimum set of parameters that explain the observed galaxy spectra in the
local Universe. We identified four latent variables that efficiently represent
the diversity of high-dimensional galaxy spectral energy distributions (SEDs)
observed by the Sloan Digital Sky Survey. Additionally, we constructed
meaningful latent representation using conditional variational autoencoders
trained with different permutations of galaxy physical properties, which helped
us quantify the information that these traditionally used properties have on
the reconstruction of galaxy spectra. The four parameters suggest a view that
complex SED population models with a very large number of parameters will be
difficult to constrain even with spectroscopic galaxy data. Through an
Explainable AI (XAI) method, we found that the region below 5000\textup{\AA}
and prominent emission lines ([O II], [O III], and H) are particularly
informative for predicting the latent variables. Our findings suggest that
these latent variables provide a more efficient and fundamental representation
of galaxy spectra than conventionally considered galaxy physical properties.Comment: 5 pages, 6 figures, accepted by NeurIPS 202
An Iterative Reconstruction Algorithm for Faraday Tomography
Faraday tomography offers crucial information on the magnetized astronomical
objects, such as quasars, galaxies, or galaxy clusters, by observing its
magnetoionic media. The observed linear polarization spectrum is inverse
Fourier transformed to obtain the Faraday dispersion function (FDF), providing
us a tomographic distribution of the magnetoionic media along the line of
sight. However, this transform gives a poor reconstruction of the FDF because
of the instrument's limited wavelength coverage. The current Faraday tomography
techniques' inability to reliably solve the above inverse problem has
noticeably plagued cosmic magnetism studies. We propose a new algorithm
inspired by the well-studied area of signal restoration, called the
Constraining and Restoring iterative Algorithm for Faraday Tomography (CRAFT).
This iterative model-independent algorithm is computationally inexpensive and
only requires weak physically-motivated assumptions to produce high fidelity
FDF reconstructions. We demonstrate an application for a realistic synthetic
model FDF of the Milky Way, where CRAFT shows greater potential over other
popular model-independent techniques. The dependence of observational frequency
coverage on the various techniques' reconstruction performance is also
demonstrated for a simpler FDF. CRAFT exhibits improvements even over
model-dependent techniques (i.e., QU-fitting) by capturing complex multi-scale
features of the FDF amplitude and polarization angle variations within a
source. The proposed approach will be of utmost importance for future cosmic
magnetism studies, especially with broadband polarization data from the Square
Kilometre Array and its precursors. We make the CRAFT code publicly available.Comment: Accepted for publication in MNRAS. 13 pages and 12 figure
Galaxy Manifold: Characterizing and understanding galaxies with two parameters
We report the discovery of a two-dimensional Galaxy Manifold within the
multi-dimensional luminosity space of local galaxies. The multi-dimensional
luminosity space is constructed using 11 bands that span from far ultraviolet
to near-infrared for redshift < 0.1 galaxies observed with GALEX, SDSS, and
UKIDSS. The two latent parameters are sufficient to express 93.2% of the
variance in the galaxy sample, suggesting that this Galaxy Manifold is one of
the most efficient representations of galaxies. The transformation between the
observed luminosities and the manifold parameters as an analytic mapping is
provided. The manifold representation provides accurate (85%) morphological
classifications with a simple linear boundary, and galaxy properties can be
estimated with minimal scatter (0.12 dex and 0.04 dex for star formation rate
and stellar mass, respectively) by calibrating with the two-dimensional
manifold location. Under the assumption that the manifold expresses the
possible parameter space of galaxies, the evolution on the manifold is
considered. We find that constant and exponentially decreasing star formation
histories form almost orthogonal modes of evolution on the manifold. Through
these simple models, we understand that the two modes are closely related to
gas content, which suggests the close relationship of the manifold to gas
accretion. Without assuming a star formation history, a gas-regulated model
reproduces an exponentially declining star formation history with a timescale
of 1.2 Gyrs on the manifold. Lastly, the found manifold suggests a
paradigm where galaxies are characterized by their mass/scale and specific SFR,
which agrees with previous studies of dimensionality reduction.Comment: Submitted to MNRAS. 15 pages and 17 figures. All comments are welcom
CO Multi-line Imaging of Nearby Galaxies (COMING) IV. Overview of the Project
Observations of the molecular gas in galaxies are vital to understanding the
evolution and star-forming histories of galaxies. However, galaxies with
molecular gas maps of their whole discs having sufficient resolution to
distinguish galactic structures are severely lacking. Millimeter wavelength
studies at a high angular resolution across multiple lines and transitions are
particularly needed, severely limiting our ability to infer the universal
properties of molecular gas in galaxies. Hence, we conducted a legacy project
with the 45 m telescope of the Nobeyama Radio Observatory, called the CO
Multi-line Imaging of Nearby Galaxies (COMING), which simultaneously observed
147 galaxies with high far-infrared flux in CO, CO, and CO
lines. The total molecular gas mass was derived using the standard
CO-to-H conversion factor and found to be positively correlated with the
total stellar mass derived from the WISE m band data. The fraction of
the total molecular gas mass to the total stellar mass in galaxies does not
depend on their Hubble types nor the existence of a galactic bar, although when
galaxies in individual morphological types are investigated separately, the
fraction seems to decrease with the total stellar mass in early-type galaxies
and vice versa in late-type galaxies. No differences in the distribution of the
total molecular gas mass, stellar mass, and the total molecular gas to stellar
mass ratio was observed between barred and non-barred galaxies, which is likely
the result of our sample selection criteria, in that we prioritized observing
FIR bright (and thus molecular gas-rich) galaxies.Comment: Accepted for publication in PASJ; 47 pages, 5 tables, 29 figures.
On-line supplementary images are available at this URL
(https://astro3.sci.hokudai.ac.jp/~radio/coming/publications/). CO data is
available at the Japanese Virtual Observatory (JVO) website
(https://jvo.nao.ac.jp/portal/nobeyama/coming.do) and the project website
(https://astro3.sci.hokudai.ac.jp/~radio/coming/data/