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Millennium Simulation

Editor’s note: Kerrin Hensley is a third-year graduate student in Boston University’s astronomy department, and she was recently selected as the inaugural AAS Media Fellow. We’re excited to welcome Kerry to the team and look forward to featuring her writing on AAS Nova regularly!

As effective laboratories for studying the impact of nature on galaxy evolution without the influence of nurture, galaxies in cosmic voids stand alone. What does the dearth of galactic neighbors mean for the morphology of galaxies in cosmic voids?

Bubbles on a Megaparsec Scale

Cosmic voids are roughly spherical regions of the cosmic web with lower-than-average density of matter. Though far less populated than dense galaxy clusters, cosmic voids aren’t empty; delicate filaments beaded with galactic pearls cut across their centers, hosting sites of galaxy formation. Because of their low density, voids represent a laboratory within which galaxy properties and evolution are largely determined independent of the influence of neighboring galaxies.

What is life like for a galaxy in the proximity of a cosmic void? To answer this question, Elena Ricciardelli (École Polytechnique Fédérale de Lausanne, Switzerland) and collaborators analyze the properties of galaxies residing in and around cosmic voids in the nearby (0.01 < z < 0.12) universe.

Exploring Void Galaxy Morphology

elliptical and spiral fractions

Fraction of elliptical and spiral galaxies as a function of absolute magnitude in and outside of voids. Voids contain a higher fraction of spirals and a lower fraction of ellipticals than the control sample. [Adapted from Ricciardelli et al. 2017]

Ricciardelli and collaborators search for the effects cosmic voids have on galaxy morphology by analyzing a sample of galaxies drawn from the Sloan Digital Sky Survey. In total, they consider roughly 6,000 void galaxies and a control sample of 200,000 galaxies from environments of average density. They use the Galaxy Zoo morphological classification tool to identify the spiral and elliptical galaxies in their sample.

Lastly, they calculate the fraction of spiral and elliptical galaxies present in their void and control samples, while correcting for the fact that faint spiral galaxies are more likely to be misclassified as ellipticals than their bright counterparts. They find that galaxies near voids are more likely to be spirals than galaxies far from voids, indicating that nearby cosmic voids have a marked effect on galaxy evolution.

galaxy fractions

Clockwise from top left: elliptical fraction, spiral fraction, star-forming fraction, and stellar mass for galaxies in and around voids out to a redshift of z = 0.065. The dashed line marks the median value for each variable for the control galaxies. The dotted lines indicate the boundaries of the zone of influence of the voids. [Ricciardelli et al. 2017]

Life in and Around the Void

The authors find that not only does a galaxy’s distance from the void affect its properties, but the size of the adjacent void has a measurable impact as well. Within the voids, they find a larger fraction of spiral galaxies compared to the control sample. This effect persists after removing the mass bias due to the fact that the low-density void environments are preferentially populated with low-mass galaxies; for a given mass or absolute magnitude, voids contain a higher proportion of spiral galaxies than the control sample.

This effect is not limited to the volume within the voids; Ricciardelli and collaborators find that the properties of void-adjacent galaxies are altered out to twice the radius of the void, with a higher fraction of spiral galaxies found closer to voids. The size of a void has an effect as well; larger-than-average voids harbor a larger fraction of spiral galaxies than smaller-than-average voids.

The authors caution that this final result depends on how the voids are defined; the effect disappears if the voids are defined using their dynamical properties rather than their size. Future research will help further disentangle the role that cosmic voids play in galaxy evolution.

Citation

Elena Ricciardelli et al 2017 ApJL 846 L4. doi:10.3847/2041-8213/aa84ad

quadruple star system

Editor’s note: In these last two weeks of 2017, we’ll be looking at a few selections that we haven’t yet discussed on AAS Nova from among the most-downloaded papers published in AAS journals this year. The usual posting schedule will resume in January.

The Age of the KIC 7177553 System

Published January 2017

 

Main takeaway:

Two scientists from the University of Delaware, James MacDonald and Dermott Mullan, recently derived the age of the quadruple star system KIC 7177553. The system appears to be younger than originally thought — it’s best modeled as being 32–36 million years old.

stellar ages

Based on stellar models and the observed radii of the stars, their ages are likely between 32 and 36 million years. [MacDonald & Mullan et al. 2017]

Why it’s interesting:

The KIC 7177553 system is intriguing because of its complex structure: it consists of two binaries (one of which is eclipsing) orbiting each other in a hierarchical structure. Observations of KIC 7177553 can teach us how hierarchical systems like this one form and evolve, but first we need to determine how old the system is so we know what stage of its evolution we’re seeing. The authors’ estimate of 32–36 million years is relatively young for stars; this age places them in the pre-main-sequence phase.

The additional intrigue of KIC 7177553:

KIC 7177553 is of further interest to astronomers because it might host a super-Jupiter-sized planet in an eccentric orbit around the system. If true, this system may provide an excellent opportunity to learn more about how planets in hierarchical star systems are born and evolve. Having an accurate determination of the age of the system is therefore especially important so that we can constrain possible planet formation scenarios.

Citation

James MacDonald and D. J. Mullan 2017 ApJ 834 99. doi:10.3847/1538-4357/834/2/99

COSMOS field

Editor’s note: In these last two weeks of 2017, we’ll be looking at a few selections that we haven’t yet discussed on AAS Nova from among the most-downloaded papers published in AAS journals this year. The usual posting schedule will resume in January.

CANDELS Multi-Wavelength Catalogs: Source Identification and Photometry in the CANDELS COSMOS Survey Field

Published January 2017

 

Main takeaway:

A publication led by Hooshang Nayyeri (UC Irvine and UC Riverside) early this year details a catalog of sources built using the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS), a survey carried out by cameras on board the Hubble Space Telescope. The catalog lists the properties of ~38,000 distant galaxies visible within the COSMOS field, a two-square-degree equatorial field explored in depth to answer cosmological questions.

Why it’s interesting:

dark matter and COSMOS

Illustration showing the three-dimensional map of the dark matter distribution in the
COSMOS field. [Adapted from NASA/ESA/R. Massey
(California Institute of Technology)]

The depth and resolution of the CANDELS observations are useful for addressing several major science goals, including the following:

  1. Studying the most distant objects in the universe at the epoch of reionization in the cosmic dawn.
  2. Understanding galaxy formation and evolution during the peak epoch of star formation in the cosmic high noon.
  3. Studying star formation from deep ultraviolet observations and studying cosmology from supernova observations.

Why CANDELS is a major endeavor:

CANDELS is the largest multi-cycle treasury program ever approved on the Hubble Space Telescope — using over 900 orbits between 2010 and 2013 with two cameras on board the spacecraft to study galaxy formation and evolution throughout cosmic time. The CANDELS images are all publicly available, and the new catalog represents an enormous source of information about distant objects in our universe.

Citation

H. Nayyeri et al 2017 ApJS 228 7. doi:10.3847/1538-4365/228/1/7

Titan's atmosphere

Editor’s note: In these last two weeks of 2017, we’ll be looking at a few selections that we haven’t yet discussed on AAS Nova from among the most-downloaded papers published in AAS journals this year. The usual posting schedule will resume in January.

Carbon Chain Anions and the Growth of Complex Organic Molecules in Titan’s Ionosphere

Published July 2017

 

Main takeaway:

Titan atmosphere chemical reactions

Graphic depicting some of the chemical reactions taking place in Titan’s atmosphere, leading to the generation of organic haze particles. [ESA]

In a recently published study led by Ravi Desai (University College London), scientists used data from the Cassini mission to identify negatively charged molecules known as “carbon chain anions” in the atmosphere of Saturn’s largest moon, Titan.

Why it’s interesting:

Carbon chain anions are the building blocks of more complex molecules, and Titan’s thick nitrogen and methane atmosphere might mimic the atmosphere of early Earth. This first unambiguous detection of carbon chain anions in a planet-like atmosphere might therefore teach us about the conditions and chemical reactions that eventually led to the development of life on Earth. And if we can use Titan to learn about how complex molecules grow from these anion chains, we may be able to identify a universal pathway towards the ingredients for life.

What we’ve learned so far:

Cassini measured fewer and fewer lower-mass anions the deeper in Titan’s ionosphere that it looked — and at the same time, an increase in the number of precursors to larger aerosol molecules further down. This tradeoff strongly suggests that the anions are indeed involved in building up the more complex molecules, seeding their eventual growth into the complex organic haze of Titan’s lower atmosphere.

Citation

R. T. Desai et al 2017 ApJL 844 L18. doi:10.3847/2041-8213/aa7851

AIJ example plot

Editor’s note: In these last two weeks of 2017, we’ll be looking at a few selections that we haven’t yet discussed on AAS Nova from among the most-downloaded papers published in AAS journals this year. The usual posting schedule will resume in January.

AstroImageJ: Image Processing and Photometric Extraction for Ultra-Precise Astronomical Light Curves

Published January 2017

 

AIJ

The AIJ image display. A wide range of astronomy specific image display options and image analysis tools are available from the menus, quick access icons, and interactive histogram. [Collins et al. 2017]

Main takeaway:

AstroImageJ is a new integrated software package presented in a publication led by Karen Collins (Vanderbilt University, Fisk University, and University of Louisville). It enables new users — even at the level of undergraduate student, high school student, or amateur astronomer — to quickly start processing, modeling, and plotting astronomical image data.

Why it’s interesting:

Science doesn’t just happen the moment a telescope captures a picture of a distant object. Instead, astronomical images must first be carefully processed to clean up the data, and this data must then be systematically analyzed to learn about the objects within it. AstroImageJ — as a GUI-driven, easily installed, public-domain tool — is a uniquely accessible tool for this processing and analysis, allowing even non-specialist users to explore and visualize astronomical data.

Some features of AstroImageJ:

(as reported by Astrobites)

  • Image calibration: generate master flat, dark, and bias frames
  • Image arithmetic: combine images via subtraction, addition, division, multiplication, etc.
  • Stack editing: easily perform operations on a series of images
  • Image stabilization and image alignment features
  • Precise coordinate converters: calculate Heliocentric and Barycentric Julian Dates
  • WCS coordinates: determine precisely where a telescope was pointed for an image by PlateSolving using Astronomy.net
  • Macro and plugin support: write your own macros
  • Multi-aperture photometry with interactive light curve fitting: plot light curves of a star in real time

Citation

Karen A. Collins et al 2017 AJ 153 77. doi:10.3847/1538-3881/153/2/77

GJ 1132 b

Editor’s note: In these last two weeks of 2017, we’ll be looking at a few selections that we haven’t yet discussed on AAS Nova from among the most-downloaded papers published in AAS journals this year. The usual posting schedule will resume in January.

Detection of the Atmosphere of the 1.6 M ⊕ Exoplanet GJ 1132 b

Published March 2017

 

Main takeaway:

An atmosphere was detected around the roughly Earth-size exoplanet GJ 1132 b using a telescope at the European Southern Observatory in Chile. A team of scientists led by John Southworth (Keele University) found features indicating the presence of an atmosphere in the observations of this 1.6-Earth-mass planet as it transits an M-dwarf host star. This is the lowest-mass planet with a detected atmosphere thus far.

Why it’s interesting:

M dwarfs are among the most common stars in our galaxy, and we’ve found many Earth-size exoplanets in or near the habitable zones around M-dwarf hosts. But M dwarfs are also more magnetically active than stars like our Sun, suggesting that the planets in M-dwarf habitable zones may not be able to support life due to stellar activity eroding their atmospheres. The detection of an atmosphere around GJ 1132 b suggests that some planets orbiting M dwarfs are able to retain their atmospheres — which means that these planets may be an interesting place to search for life after all.

How the atmosphere was detected:

GJ 1132 b radius

The measured planetary radius for GJ 1132 b as a function of the wavelength used to observe it. [Southworth et al. 2017]

When measuring the radius of GJ 1132 b based on its transits, the authors noticed that the planet appeared to be larger when observed in some wavelengths than in others. This can be explained if the planet has a “surface radius” of ~1.4 Earth radii, overlaid by an atmosphere that extends out another few tenths of an Earth radius. The atmosphere, which may consist of water vapor or methane, is transparent to some wavelengths and absorbs others — which is why the apparent size of the planet changes across wavelength bands.

Citation

John Southworth et al 2017 AJ 153 191. doi:10.3847/1538-3881/aa6477

SDSS

Editor’s note: In these last two weeks of 2017, we’ll be looking at a few selections that we haven’t yet discussed on AAS Nova from among the most-downloaded papers published in AAS journals this year. The usual posting schedule will resume in January.

Sloan Digital Sky Survey IV: Mapping the Milky Way, Nearby Galaxies, and the Distant Universe

Published June 2017

 

Main takeaway:

The incredibly prolific Sloan Digital Sky Survey has provided photometric observations of around 500 million objects and spectra for more than 3 million objects. The survey has now entered its fourth iteration, SDSS-IV, with the first public data release made in June 2016. A publication led by Michael Blanton (New York University) describes the facilities used for SDSS-IV, its science goals, and its three core programs.

Why it’s interesting:

Since data collection began in 2000, SDSS has been one of the premier surveys providing imaging and spectroscopy for objects in both the near and distant universe. SDSS has measured spectra not only for the stars in our own Milky Way, but also for galaxies that lie more than 7 billion light-years distant — making it an extremely useful and powerful tool for mapping our universe.

What SDSS-IV is looking for:

MaNGA target

SDSS image of an example MaNGA target galaxy (left), with some of the many things we can learn about it shown in the right and bottom panels: stellar velocity dispersion, stellar mean velocity, stellar population age, metallicity, etc. [Blanton et al. 2017]

SDSS-IV contains three core programs:

  1. Apache Point Observatory Galactic Evolution Experiment 2 (APOGEE-2) provides high-resolution near-infrared spectra of hundreds of thousands of Milky-Way stars with the goal of improving our understanding of the history of the Milky Way and of stellar astrophysics.
  2. Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) obtains spatially resolved spectra for thousands of nearby galaxies to better understand the evolutionary histories of galaxies and what regulates their star formation.
  3. Extended Baryon Oscillation Spectroscopic Survey (eBOSS) maps the galaxy, quasar, and neutral gas distributions at redshifts out to z = 3.5 to better understand dark matter, dark energy, the properties of neutrinos, and inflation.

Citation

Michael R. Blanton et al 2017 AJ 154 28. doi:10.3847/1538-3881/aa7567

TRAPPIST-1

Editor’s note: In these last two weeks of 2017, we’ll be looking at a few selections that we haven’t yet discussed on AAS Nova from among the most-downloaded papers published in AAS journals this year. The usual posting schedule will resume in January.

The Threatening Magnetic and Plasma Environment of the TRAPPIST-1 Planets

Published July 2017

 

Main takeaway:

Models of the magnetic environment surrounding the seven planets of the TRAPPIST-1 system suggest that this is not a pleasant place to be for life. In particular, the simulations run by Cecilia Garraffo (Harvard-Smithsonian Center for Astrophysics) and collaborators indicate that all planets in the system are bombarded by a stellar wind with a pressure that’s 1,000 to 100,000 times the pressure of what we experience on Earth.

Why it’s interesting:

magnetic field lines

Simulations of the magnetic environment around the planet TRAPPIST-1 f, for a variety of different assumed planetary magnetic fields. Red field lines are those that have connected between the star and the planet. [Garraffo et al. 2017]

The discovery of seven Earth-sized planets in the nearby TRAPPIST-1 system — particularly given many of the planets’ apparent location in the star’s habitable zone — gave us hope that these planets might be an interesting place to look for life. But the issue of habitability is more complicated than whether or not the planets can support liquid water. Garraffo and collaborators’ models suggest that these planets likely have their atmospheres eroded or completely stripped by the stellar wind, rendering prospects for life on these planets low.

Why the TRAPPIST-1 system is still awesome:

We may be bummed that the magnetically active host star impedes chances for life on the TRAPPIST-1 planets, but the environment it produces is still pretty awesome. According to the authors’ models, the planets pass through wildly changing wind pressure changes as they orbit. In the process, their magnetospheres are compressed, and their magnetic field lines connect with the stellar field lines over much of the planets’ surfaces, causing the stellar wind particles to funnel directly onto the planets’ atmospheres. The result is an exciting and dynamic environment definitely worth studying further.

Citation

Cecilia Garraffo et al 2017 ApJL 843 L33. doi:10.3847/2041-8213/aa79ed

Milky Way

Editor’s note: In these last two weeks of 2017, we’ll be looking at a few selections that we haven’t yet discussed on AAS Nova from among the most-downloaded papers published in AAS journals this year. The usual posting schedule will resume in January.

Machine-Learned Identification of RR Lyrae Stars from Sparse, Multi-Band Data: The PS1 Sample

Published April 2017

 

Main takeaway:

A sample of RR Lyrae variable stars was built from the Pan-STARRS1 (PS1) survey by a team led by Branimir Sesar (Max Planck Institute for Astronomy, Germany). The sample of 45,000 stars represents the widest (three-fourths of the sky) and deepest (reaching 120 kpc) sample of RR Lyrae stars to date.

Why it’s interesting:

It’s challenging to understand the overall shape and behavior of our galaxy because we’re stuck on the inside of it. RR Lyrae stars are a useful tool for this purpose: they can be used as tracers to map out the Milky Way’s halo. The authors’ large sample of RR Lyrae stars from PS1 — combined with proper-motion measurements from Gaia and radial-velocity measurements from multi-object spectroscopic surveys — could become the premier source for studying the structure, kinematics, and the gravitational potential of our galaxy’s outskirts.

How they were found:

RR Lyrae sample

The black dots show the distribution of the 45,000 probable RR Lyrae stars in the authors’ sample. [Sesar et al. 2017]

The 45,000 stars in this sample were selected not by humans, but by computer. The authors used machine-learning algorithms to examine the light curves in the Pan-STARRS1 sample and identify the characteristic brightness variations of RR Lyrae stars lying in the galactic halo. These techniques resulted in a very pure and complete sample, and the authors suggest that this approach may translate well to other sparse, multi-band data sets — such as that from the upcoming Large Synoptic Survey Telescope (LSST) galactic plane sub-survey.

Citation

Branimir Sesar et al 2017 AJ 153 204. doi:10.3847/1538-3881/aa661b

planetary system

A few weeks ago, Astrobites reported on a Neptune-sized planet discovered orbiting a star in the Hyades cluster. A separate study submitted at the same time, however, reveals that there may be even more planets lurking in this system.

Thanks, Kepler

Kepler K2

Artist’s impression of the Kepler spacecraft and the mapping of the fields of the current K2 mission. [NASA]

As we learn about the formation and evolution of planets outside of our own solar system, it’s important that we search for planets throughout different types of star clusters; observing both old and young clusters, for instance, can tell us about planets in different stages of their evolutionary histories. Luckily for us, we have a tool that has been doing exactly this: the Kepler mission.

In true holiday spirit, Kepler is the gift that just keeps on giving. Though two of its reaction wheels have failed, Kepler — now as its reincarnation, K2 — just keeps detecting more planet transits. What’s more, detailed analysis of past Kepler/K2 data with ever more powerful techniques — as well as the addition of high-precision parallaxes for stars from Gaia in the near future — ensures that the Kepler data set will continue to reveal new exoplanet transits for many years to come.

Hyades cluster

Image of the Hyades cluster, a star cluster that is only ~800 million years old. [NASA/ESA/STScI]

Hunting in the Young Hyades

Two studies using K2 data were recently submitted on exoplanet discoveries around EPIC 247589423 in the Hyades cluster, a nearby star cluster that is only 800 million years old. Astrobites reported on the first study in October and discussed details about the newly discovered mini-Neptune presented in that study.

The second study, led by Andrew Mann (University of Texas at Austin and NASA Hubble Fellow at Columbia University), was published this week. This study presented a slightly different outcome: the authors detect the presence of not just the one, but three exoplanets orbiting EPIC 247589423.

New Discoveries

Mann and collaborators searched through the K2 light curves of young stars as part of the ZEIT (Zodiacal Exoplanets in Time) Survey. Using these data, they identified the presence of three planets in the EPIC 247589423 system:

  1. a roughly Earth-sized planet (~1.0 Earth radii) with a period of ~8.0 days,
  2. the mini-Neptune identified in the other study, with a size of ~2.9 Earth radii and period of ~17 days, and
  3. a super-Earth, with a size of ~1.5 Earth radii and period of ~26 days.
K2 light curves

Light curve of EPIC 247589423 from K2, with the lower panels showing the transits of the three discovered planets. [Mann et al. 2018]

The smallest planet is among the youngest Earth-sized planets ever discovered, allowing us a rare glimpse into the history and evolution of planets similar to our own.

But these planetary discoveries are additionally exciting because they’re orbiting a bright star that’s relatively quiet for its age — making the system an excellent target for dedicated radial-velocity observations to determine the planet masses.

Since most young star clusters are much further away, they lie out of range of radial-velocity follow-up, rendering EPIC 247589423 a unique opportunity to explore the properties of young planets in detail. With more discoveries like these from Kepler’s data, we can hope to soon learn more about planets in all their stages of evolution.

Citation

Andrew W. Mann et al 2018 AJ 155 4. doi:10.3847/1538-3881/aa9791

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