133 research outputs found
A Method to Distinguish Quiescent and Dusty Star-forming Galaxies with Machine Learning
Large photometric surveys provide a rich source of observations of quiescent galaxies, including a surprisingly large population at z > 1. However, identifying large, but clean, samples of quiescent galaxies has proven difficult because of their near-degeneracy with interlopers such as dusty, star-forming galaxies. We describe a new technique for selecting quiescent galaxies based upon t-distributed stochastic neighbor embedding (t-SNE), an unsupervised machine-learning algorithm for dimensionality reduction. This t-SNE selection provides an improvement both over UVJ, removing interlopers that otherwise would pass color selection, and over photometric template fitting, more strongly toward high redshift. Due to the similarity between the colors of high- and low-redshift quiescent galaxies, under our assumptions, t-SNE outperforms template fitting in 63% of trials at redshifts where a large training sample already exists. It also may be able to select quiescent galaxies more efficiently at higher redshifts than the training sample
Galaxy Zoo 1 : Data Release of Morphological Classifications for nearly 900,000 galaxies
Morphology is a powerful indicator of a galaxy's dynamical and merger
history. It is strongly correlated with many physical parameters, including
mass, star formation history and the distribution of mass. The Galaxy Zoo
project collected simple morphological classifications of nearly 900,000
galaxies drawn from the Sloan Digital Sky Survey, contributed by hundreds of
thousands of volunteers. This large number of classifications allows us to
exclude classifier error, and measure the influence of subtle biases inherent
in morphological classification. This paper presents the data collected by the
project, alongside measures of classification accuracy and bias. The data are
now publicly available and full catalogues can be downloaded in electronic
format from http://data.galaxyzoo.org.Comment: Accepted by MNRAS, 14 pages. Updated to match final version; problem
with table 7 header fixed. Full tables available at http://data.galaxyzoo.or
Predicting Future Space Near-IR Grism Surveys using the WFC3 Infrared Spectroscopic Parallels Survey
We present near-infrared emission line counts and luminosity functions from
the HST WFC3 Infrared Spectroscopic Parallels (WISP) program for 29 fields
(0.037 deg^2) observed using both the G102 and G141 grisms. Altogether we
identify 1048 emission line galaxies with observed equivalent widths greater
than 40 Angstroms, 467 of which have multiple detected emission lines. The WISP
survey is sensitive to fainter flux levels (3-5x10^-17 ergs/s/cm^2) than the
future space near-infrared grism missions aimed at baryonic acoustic
oscillation cosmology (1-4x10^-16 ergs/s/cm^2), allowing us to probe the
fainter emission line galaxies that the shallower future surveys may miss.
Cumulative number counts of 0.7<z<1.5 galaxies reach 10,000 deg^-2 above an
H-alpha flux of 2x10^-16 ergs/s/cm^2. H-alpha-emitting galaxies with comparable
[OIII] flux are roughly 5 times less common than galaxies with just H-alpha
emission at those flux levels. Galaxies with low H-alpha/[OIII] ratios are very
rare at the brighter fluxes that future near-infrared grism surveys will probe;
our survey finds no galaxies with H-alpha/[OIII] < 0.95 that have H-alpha flux
greater than 3x10^-16 ergs/s/cm^2. Our H-alpha luminosity function contains a
comparable number density of faint line emitters to that found by the NICMOS
near-infrared grism surveys, but significantly fewer (factors of 3-4 less) high
luminosity emitters. We also find that our high redshift (z=0.9-1.5) counts are
in agreement with the high redshift (z=1.47) narrow band H-alpha survey of
HiZELS (Sobral et al. 2013), while our lower redshift luminosity function
(z=0.3-0.9) falls slightly below their z=0.84 result. The evolution in both the
H-alpha luminosity function from z=0.3--1.5 and the [OIII] luminosity function
from z=0.7-2.3 is almost entirely in the L* parameter, which steadily increases
with redshift over those ranges.Comment: 18 pages, 14 figures, Accepted by Ap
A Method to Distinguish Quiescent and Dusty Star-forming Galaxies with Machine Learning
Large photometric surveys provide a rich source of observations of quiescent galaxies, including a surprisingly large population at z > 1. However, identifying large, but clean, samples of quiescent galaxies has proven difficult because of their near-degeneracy with interlopers such as dusty, star-forming galaxies. We describe a new technique for selecting quiescent galaxies based upon t-distributed stochastic neighbor embedding (t-SNE), an unsupervised machine-learning algorithm for dimensionality reduction. This t-SNE selection provides an improvement both over UVJ, removing interlopers that otherwise would pass color selection, and over photometric template fitting, more strongly toward high redshift. Due to the similarity between the colors of high- and low-redshift quiescent galaxies, under our assumptions, t-SNE outperforms template fitting in 63% of trials at redshifts where a large training sample already exists. It also may be able to select quiescent galaxies more efficiently at higher redshifts than the training sample
Galaxy Zoo: Dust in Spirals
We investigate the effect of dust on spiral galaxies by measuring the
inclination-dependence of optical colours for 24,276 well-resolved SDSS
galaxies visually classified in Galaxy Zoo. We find clear trends of reddening
with inclination which imply a total extinction from face-on to edge-on of 0.7,
0.6, 0.5 and 0.4 magnitudes for the ugri passbands. We split the sample into
"bulgy" (early-type) and "disky" (late-type) spirals using the SDSS fracdeV (or
f_DeV) parameter and show that the average face-on colour of "bulgy" spirals is
redder than the average edge-on colour of "disky" spirals. This shows that the
observed optical colour of a spiral galaxy is determined almost equally by the
spiral type (via the bulge-disk ratio and stellar populations), and reddening
due to dust. We find that both luminosity and spiral type affect the total
amount of extinction, with "disky" spirals at M_r ~ -21.5 mags having the most
reddening. This decrease of reddening for the most luminous spirals has not
been observed before and may be related to their lower levels of recent star
formation. We compare our results with the latest dust attenuation models of
Tuffs et al. We find that the model reproduces the observed trends reasonably
well but overpredicts the amount of u-band attenuation in edge-on galaxies. We
end by discussing the effects of dust on large galaxy surveys and emphasize
that these effects will become important as we push to higher precision
measurements of galaxy properties and their clustering.Comment: MNRAS in press. 25 pages, 22 figures (including an abstract comparing
GZ classifications with common automated methods for selecting disk/early
type galaxies in SDSS data). v2 corrects typos found in proof
Galaxy Zoo: Passive Red Spirals
We study the spectroscopic properties and environments of red spiral galaxies
found by the Galaxy Zoo project. By carefully selecting face-on, disk dominated
spirals we construct a sample of truly passive disks (not dust reddened, nor
dominated by old stellar populations in a bulge). As such, our red spirals
represent an interesting set of possible transition objects between normal blue
spirals and red early types. We use SDSS data to investigate the physical
processes which could have turned these objects red without disturbing their
morphology. Red spirals prefer intermediate density regimes, however there are
no obvious correlations between red spiral properties and environment -
environment alone is not sufficient to determine if a spiral will become red.
Red spirals are a small fraction of spirals at low masses, but are a
significant fraction at large stellar masses - massive galaxies are red
independent of morphology. We confirm that red spirals have older stellar popns
and less recent star formation than the main spiral population. While the
presence of spiral arms suggests that major star formation cannot have ceased
long ago, we show that these are not recent post-starbursts, so star formation
must have ceased gradually. Intriguingly, red spirals are ~4 times more likely
than normal spirals to host optically identified Seyfert or LINER, with most of
the difference coming from LINERs. We find a curiously large bar fraction in
the red spirals suggesting that the cessation of star formation and bar
instabilities are strongly correlated. We conclude by discussing the possible
origins. We suggest they may represent the very oldest spiral galaxies which
have already used up their reserves of gas - probably aided by strangulation,
and perhaps bar instabilities moving material around in the disk.Comment: MNRAS in press, 20 pages, 15 figures (v3
SDSS 0956+5128: A Broad-line Quasar with Extreme Velocity Offsets
We report on the discovery of a Type 1 quasar, SDSS 0956+5128, with a
surprising combination of extreme velocity offsets. SDSS 0956+5128 is a
broad-lined quasar exhibiting emission lines at three substantially different
redshifts: a systemic redshift of z ~ 0.714 based on narrow emission lines, a
broad MgII emission line centered 1200 km/s bluer than the systemic velocity,
at z ~ 0.707, and broad H\alpha and H\beta emission lines centered at z ~
0.690. The Balmer line peaks are 4100 km/s bluer than the systemic redshift.
There are no previously known objects with such an extreme difference between
broad MgII and broad Balmer emission. The two most promising explanations are
either an extreme disk emitter or a high-velocity black hole recoil. However,
neither explanation appears able to explain all of the observed features of
SDSS 0956+5128, so the object may provide a challenge to our general
understanding of quasar physics.Comment: ApJ, accepte
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