A new look at the largest known disk galaxy

Combination of 4 NGVS images of Malin 1, obtained with MegaCam camera on CFHT. An indication of the size is given in the figure to show the amazing size of the disk of the galaxy (in comparison, the Milky Way has only a diameter around 30 kpc). Image Credit: Boissier/A&A/ESO/CFHT. Hi-res image

Left: The curve with the errorbars shows the variation with radius of the color between 2 GALEX bands (FUV and NUV). A blue colour indicates the presence of young stellar populations. The blue and red curve shows the model used in the paper. It is in agreement with the observation. On the other hand, the yellow line show the color of stars that would have been formed during a interaction 1.4 billions year ago, or a star formation event that would have expanded from the center in the distant past to the outer regions in the more recent period (stars). The expected colors for this model do not fit the data. 

Right: History of the star formation rate (SFR) in the the giant disk of Malin 1 according to the model that reproduces correctly the stellar surface density and the colors of the galaxy. This history suggests that star formation proceeded at a regular rate for several billion years (the error-bar indicates an estimate of the recent SFR published in another study). Figures adapted from Boissier et al. 2016. Hi-res image

In a publication recently accepted in Astronomy and Astrophysics, an international team involving French researchers from the Laboratoire d’Astrophysique de Marseille and Canadian researchers from NRC Herzberg and Queens University have studied Malin 1, a nearby galaxy that has been known only since the eighty's and that shows an extremely large disk of gas and stars. The new observations of Malin 1, a prototype of the class of "giant low surface brightness galaxies" allowed the team to obtain new results in contradiction with one of the hypotheses concerning the formation of this type of galaxies.

Because they are very diffuse and of low surface brightness, giant low surface brightness galaxies, yet massive, are difficult to observe and are still poorly known. They could represent a significant percentage of the galaxies in the universe, especially because we could have missed such objects in our galaxy surveys. It is thus important do study them and understand their formation and evolution. This is now possible owing to the new generation of telescopes and modern detectors, with higher sensitivity to low surface brightness than in the past.

This paper presents for the first time deep images obtained at 6 different wavelengths, from the UV of the GUViCS project to the optical and near-infrared obtained in the context of the Next Generation Virgo Survey with MegaCam on CFHT. Originally, these large observational campaigns were planned to study the Virgo cluster, but they also allow us to study background objects like Malin 1. The images offer us a new view of this spectacular galaxy, the largest galactic disk known, with a diameter above 250 kilo-parsec (in comparison, our Milky Way is only about 30 kpc wide).

The team of researchers extracted from these data the variation of the luminosity with the distance to the center of the galaxy, as well as the variation of the colors (corresponding to the ratios between the luminosity at various wavelengths). These colors strongly depends on the star formation history. The comparison of the observations with predictions of various numerical models allowed the team to estimate for the first time what must have been the history of star formation in the giant disk of Malin 1. It suggests that the giant disk has been in place for several Gyr, and that star formation proceed at a regular long-term rhythm despite the very low density.

This result is important as it clearly contradicts a scenario proposed a few years ago predicting that these giant galaxies are formed during violent interactions. Moreover, in the context of the cosmological formation of galaxies, numerous fusions and interaction should have perturbed the disk of Malin 1. The formation of such a structure and its survival for very long time offers then a challenge for cosmological simulations of the formation of galaxies.

What is the future of Malin 1? The giant disk contains a large quantity of gas in which star formation will keep proceeding at a low rate for billions of years, increasing progressively the stellar mass of the galaxy. Unless another galaxy comes in the picture to interact with Malin 1 and totally change its destiny. Few galaxies, however, may play this role as Malin 1 is a relatively isolated galaxy.

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Additional information

• Scientific paper.
• Official press release from INSU (in French).

Contact

Dr. Samuel Boissier
Laboratoire d'Astrophysique de Marseille (AMU, CNRS).
Phone number: +33 4 91 05 59 37
samuel.boissier@lam.fr

Source: Canada-France-Hawaii Telescope (CFHT)

X Marks the Spot for Milky Way Formation

Researchers used data from NASA's Wide-field Infrared Survey Explorer (WISE) mission to highlight the X-shaped structure in the bulge of the Milky Way. Credit: NASA/JPL-Caltech/D.Lang.   › Full image and caption

To reveal the X shape in the Milky Way's central bulge, researchers took WISE observations and subtracted a model of how stars would be distributed in a symmetrical bulge. Credit: NASA/JPL-Caltech/D.Lang.  › Larger image

A new understanding of our galaxy's structure began in an unlikely way: on Twitter. A research effort sparked by tweets led scientists to confirm that the Milky Way's central bulge of stars forms an "X" shape. The newly published study uses data from NASA's Wide-field Infrared Survey Explorer (WISE) mission.

The unconventional collaboration started in May 2015 when Dustin Lang, an astronomer at the Dunlap Institute of the University of Toronto, posted galaxy maps to Twitter, using data from WISE's two infrared surveys of the entire sky in 2010. Infrared light allows astronomers to see the structures of galaxies in spite of dust, which blocks crucial details in visible light. Lang was using the WISE data in a project to map the web of galaxies far outside our Milky Way, which he made available through an interactive website.

But it was the Milky Way's appearance in the tweets that got the attention of other astronomers. Some chimed in about the appearance of the bulge, a football-shaped central structure that is three-dimensional compared to the galaxy's flat disk. Within the bulge, the WISE data seemed to show a surprising X structure, which had never been as clearly demonstrated before in the Milky Way. Melissa Ness, a postdoctoral researcher at the Max Planck Institute for Astronomy in Heidelberg, Germany, recognized the significance of the X shape, and contacted Lang.

The two met a few weeks later at a conference in Michigan, and decided to collaborate on analyzing the bulge using Lang's WISE maps. Their work resulted in a new study published in the Astronomical Journal confirming an X-shaped distribution of stars in the bulge.

"The bulge is a key signature of formation of the Milky Way," said Ness, the study's lead author. "If we understand the bulge we will understand the key processes that have formed and shaped our galaxy."

The Milky Way is an example of a disk galaxy -- a collection of stars and gas in a rotating disk. In these kinds of galaxies, when the thin disk of gas and stars is sufficiently massive, a "stellar bar" may form, consisting of stars moving in a box-shaped orbit around the center. Our own Milky Way has a bar, as do nearly two-thirds of all nearby disk galaxies.

Over time, the bar may become unstable and buckle in the center. The resulting "bulge" would contain stars that move around the galactic center, perpendicular to the plane of the galaxy, and in and out radially. When viewed from the side, the stars would appear distributed in a box-like or peanut-like shape as they orbit. Within that structure, according to the new study, there is a giant X-shaped structure of stars crossing at the center of the galaxy.

A bulge can also form when galaxies merge, but the Milky Way has not merged with any large galaxy in at least 9 billion years.

"We see the boxy shape, and the X within it, clearly in the WISE image, which demonstrates that internal formation processes have driven the bulge formation," Ness said. "This also reinforces the idea that our galaxy has led a fairly quiet life, without major merging events since the bulge was formed, as this shape would have been disrupted if we had any major interactions with other galaxies."

The Milky Way's X-shaped bulge had been reported in previous studies. Images from the NASA Cosmic Background Explorer (COBE) satellite's Diffuse Infrared Background Experiment suggested a boxy structure for the bulge. In 2013, scientists at the Max Planck Institute for Extraterrestrial Physics published 3-D maps of the Milky Way that also included an X-shaped bulge, but these studies did not show an actual image of the X shape. Ness and Lang's study uses infrared data to show the clearest indication yet of the X shape.

Additional research is ongoing to analyze the dynamics and properties of the stars in the Milky Way's bulge.

Collaborating on this study was unusual for Lang -- his expertise is in using computer science to understand large-scale astronomical phenomena, not the dynamics and structure of the Milky Way. But he was able to enter a new field of research because he posted maps to social media and used openly accessible WISE data.

"To me, this study is an example of the interesting, serendipitous science that can come from large data sets that are publicly available," he said. "I'm very pleased to see my WISE sky maps being used to answer questions that I didn't even know existed."

NASA's Jet Propulsion Laboratory, Pasadena, California, manages and operates WISE for NASA's Science Mission Directorate in Washington. The spacecraft was put into hibernation mode in 2011, after it scanned the entire sky twice, thereby completing its main objectives. In September 2013, WISE was reactivated, renamed NEOWISE and assigned a new mission to assist NASA's efforts to identify potentially hazardous near-Earth objects.

For more information on WISE, visit:  http://www.nasa.gov/wise


News Media Contact

Elizabeth Landau
Jet Propulsion Laboratory, Pasadena, Calif.
818-354-6425
elizabeth.landau@jpl.nasa.gov

Source: JPL-Caltech/news

The diversity of stellar halos in massive disk galaxies

Figure 1: The observed stellar halo of NGC 474 showing the complex substructure present in the outer regions of this large elliptical galaxy. Shells and tidal stellar streams are formed as a consequence of the interactions with and accretion of smaller galaxies. Image credit: P.-A. Duc (CEA), J.-C. Cuillandre (CFHT) et al. 2013, IAUS, 295, 358

The stellar halos of galaxies are diffuse and faint components which provide scientists with a window into the assembling history of galaxies. A research team at MPA has investigated the properties of stellar halos in large disk galaxies by using both observations and state-of-the-art simulations of galaxy formation. They find a great diversity in the halo properties for galaxies that are – otherwise – alike in terms of morphology, mass, and luminosity. Observed properties, such as a mean metallicity as a function of galactocentric distance, can be reproduced by the simulations if they are analyzed in the same way as the data. 

Stellar halos are diffuse and faint components surrounding most large galaxies. They form largely from the accretion and disruption of smaller satellite galaxies, although their inner regions also contain stars which were formed in the disk of their host galaxy. Stellar halos are highly important to understand galaxy formation and evolution since they preserve a record of the accretion history of galaxies and serve as probes of the total matter distribution of galaxies. However, stellar halos are far less luminous than the main galactic disk. Observing them is therefore an extremely challenging task.

Over the past decades, different approaches have been developed to reveal this diffuse component of galaxies. Observations of galaxies with very long exposure times have produced stunning images, an example of which is shown in Figure 1. These observations helped characterize the role of accretion and mergers in shaping the outskirts of galaxies as well as unveil their vast extent, reaching distances larger than 100 kpc from their centre. 

Figure 2: Optical image of one of the galaxies in the GHOSTS sample, NGC 4945. Yellow squares indicate Hubble Space Telescope (HST) observations obtained in the stellar halo of this galaxy. © GHOSTS project

To study the properties of stellar halos in detail, however, the astronomers need to observe their constituent stars. By doing so, it is possible to estimate the mean age and chemical composition (called metallicity, when all elements but hydrogen and helium are considered) of the halo stellar population as a function of distance from the galactic centre. Until recently, studies of halo stars were confined largely to our own Milky Way and the neighbouring Andromeda galaxy, due to the proximity of their stars. However, during the past years the GHOSTS survey has extended this study to nearby disk galaxies, thanks to the exquisite resolution of the Hubble Space Telescope (Figure 2).

In her recent work, MPA scientist Antonela Monachesi has studied individual stars in the halos of nearby galaxies similar to the Milky Way using GHOSTS observations. This study showed that there is a great diversity in the metallicity of the observed stellar halos. In addition, the way this metallicity depends on the distance from the galactic centre also varies from galaxy to galaxy. In half the currently analyzed galactic halos the mean metallicity decreases with galactocentric distance, whereas the other half shows no significant variation with distance.

Moreover, the results from the GHOSTS observations showed that stellar halo properties differ greatly among galaxies that are otherwise alike in morphology, mass, and luminosity. The process of merger and accretion of satellites is rather stochastic and the properties of the stellar halos reflect the properties of the different accreted satellite galaxies.

The variety in the observed metallicity behaviour of stellar halos, with half galaxies presenting flat metallicity trends with distance, appeared to be at odds with previous results from hydrodynamical simulations, which had predicted large metallicity variations with distance as ubiquitous features. In order to investigate this seeming discrepancy, the research team at MPA used state-of-the art hydrodynamical cosmological simulations called “Auriga”. These simulations include the main physical processes responsible for the formation and evolution of galaxies in the universe and self-consistently follow the evolution of gas, stars and dark matter over time.

Figure 3: Metallicity trends of stellar halos as a function of galactocentric distance for two simulated galaxies using the Auriga simulations. This figure shows how the metallicity behaviour of stellar halos differs as a function of galactocentric distance if the metallicities are measured by averaging the properties of stars in concentric spherical shells (black), by projecting along a direction that is perpendicular to the galactic disk (blue), or on the disk plane (red). Dashed lines are linear fits to the black and blue lines to show trends more clearly. The blue lines are much flatter than the other two, showing that the metallicity variations will be much less, when averaging perpendicular to the disk – the method preferred by observations. © MPA

The MPA group analyzed the metallicity trends in stellar halos of simulated galaxies with similar mass and morphology as the Milky Way and the GHOSTS galaxies. As the main result, the scientists find from this analysis that the behaviour of the mean stellar halo metallicity with galactocentric distance depends strongly on the way it is measured. If the mean metallicities are derived by spherically averaging the properties of halo stars around the galactic center, as done in previous simulations, large variations in the mean metallicity with distance are obtained. However, if the mean metallicities are derived along a direction that is perpendicular to the galactic disk, the metallicity tends to be much more uniform (Figure 3). In observational studies, the latter direction is preferred since it allows the scientists to minimize contamination from galactic disk stars.

The MPA group has thus clearly highlighted the crucial importance of preforming careful comparisons between models and observations of halo metallicity distributions. This is the only way to alleviate tension between theory and observations.

 In future work, the MPA group will perform a very careful and detailed analysis of the simulations to compare against observations and to interpret the observations. This will allow them to decipher the accretion history of the GHOSTS galaxies.

Antonela Monachesi, Facundo Gomez and Guinevere Kauffmann

Source:  Max Planck Institute for Astrophysics