The latest finding with the Hubble space telescope:

Observable Universe contains ten times more galaxies
than previously thought

Astronomers using data from the NASA/ESA Hubble Space Telescopes and other telescopes have performed an accurate census of the number of galaxies in the Universe. The group came to the surprising conclusion that there are at least 10 times as many galaxies in the observable Universe as previously thought. The results have clear implications for our understanding of galaxy formation, and also help solve an ancient astronomical paradox — why is the sky dark at night?

Since Edwin Hubble discovered that the Milky Way is not the only galaxy in the Universe astronomers try to find out how many of them are there. This new Hubblecast focusses on the question “How many galaxies are there?” including the new numbers achieved in 2016.

One of the most fundamental questions in astronomy is that of just how many galaxies the Universe contains. The Hubble Deep Field images, captured in the mid 1990s, gave the first real insight into this. Myriad faint galaxies were revealed, and it was estimated that the observable Universe contains about 100 billion galaxies [1]. Now, an international team, led by Christopher Conselice from the University of Nottingham, UK, have shown that this figure is at least ten times too low.

This animation demonstrates the evolution of galaxy size and number over cosmic time. It starts with the modern Universe with rather few and large galaxies — as they can be seen in our cosmic neighbourhood — and ends with a view of the early Universe with many tiny galaxies. Credit: ESA/Hubble, M. Kornmesser

Conselice and his team reached this conclusion using deep space images from Hubble, data from his team’s previous work, and other published data [2]. They painstakingly converted the images into 3D, in order to make accurate measurements of the number of galaxies at different times in the Universe’s history. In addition, they used new mathematical models which allowed them to infer the existence of galaxies which the current generation of telescopes cannot observe. This led to the surprising realisation that in order for the numbers to add up, some 90% of the galaxies in the observable Universe are actually too faint and too far away to be seen — yet.

“It boggles the mind that over 90% of the galaxies in the Universe have yet to be studied. Who knows what interesting properties we will find when we observe these galaxies with the next generation of telescopes,” explains Christopher Conselice about the far-reaching implications of the new results.

In analysing the data the team looked more than 13 billion years into the past. This showed them that galaxies are not evenly distributed throughout the Universe’s history. In fact, it appears that there were a factor of 10 more galaxies per unit volume when the Universe was only a few billion years old compared with today. Most of these galaxies were relatively small and faint, with masses similar to those of the satellite galaxies surrounding the Milky Way.

This animation demonstrates the lookback into the distant, early Universe. While the modern Universe contains in average larger galaxies, the early Universe is dominated by many tiny galaxies. Credit: ESA/Hubble, M. Kornmesser

These results are powerful evidence that a significant evolution has taken place throughout the Universe’s history, an evolution during which galaxies merged together, dramatically reducing their total number. “This gives us a verification of the so-called top-down formation of structure in the Universe,” explains Conselice.

The decreasing number of galaxies as time progresses also contributes to the solution of Olbers’ paradox — why the sky is dark at night [3]. The team came to the conclusion that there is such an abundance of galaxies that, in principle, every point in the sky contains part of a galaxy. However, most of these galaxies are invisible to the human eye and even to modern telescopes, owing to a combination of factors: redshifting of light, the Universe’s dynamic nature and the absorption of light by intergalactic dust and gas, all combine to ensure that the night sky remains mostly dark.

This animation starts with a lookback into the early Universe. The local, modern Universe with large and evolved galaxies can be seen to the left. The distant, early Universe with many tiny and primordial galaxies can be seen to the right. These galaxies grew through mergers to the galaxies we see today. The animation slowly turns by 90 degree and ends with a view similar to the Hubble Deep fields. Credit: ESA/Hubble, M. Kornmesser

Notes

[1] The limited speed of light and the age of the Universe mean that the entire Universe cannot be seen from Earth. The part visible within our cosmological horizon is called the observable Universe.

[2] The study uses data from Perez-Gonzalez et al. (2008), Kajisawa et al. (2009), Fontanta et al. (2004, 2006), Caputi et al. (2011), Pozzetti et al. (2009), Mortlock et al. (2011), Muzzin et al. (2013), Mortlock et al. (2015), Duncan et al. (2014), Grazian et al. (2015), Tomczak et al. (2014) and Song et al. (2015).

[3] The astronomer Heinrich Olbers argued that the night sky should be permanently flooded by light, because in an unchanging Universe filled with an infinite number of stars, every single part of the sky should be occupied by a bright object. However, our modern understanding of the Universe is that it is both finite and dynamic — not infinite and static.

This video pans across the southern field of the Great Observatories Origins Deep Survey (GOODS). GOODS is a large galaxy census, a deep-sky study by several observatories to trace the formation and evolution of galaxies. Credit: NASA, ESA/Hubble, Music: Johan B. Monell (www.johanmonell.com)

The latest report from the European Southern Observatory (ESO):

The Milky Way’s Ancient Heart
VISTA finds remains of archaic globular star cluster

Ancient stars, of a type known as RR Lyrae, have been discovered in the centre of the Milky Way for the first time, using ESO’s infrared VISTA telescope. RR Lyrae stars typically reside in ancient stellar populations over 10 billion years old. Their discovery suggests that the bulging centre of the Milky Way likely grew through the merging of primordial star clusters. These stars may even be the remains of the most massive and oldest surviving star cluster of the entire Milky Way.

The video, based on observations made in the infrared with the VISTA infrared survey telescope shows the central region of the Milky Way galaxy. Several variable stars within the field of views are marked with circles. They can also be identified as they brighten and fade in regular intervals. At the end the video zooms on one of the newly discovered RR Lyrae stars (marked with a red circle), which are too faint to be seen clearly in the large field of view. Credit: ESO/VVV Survey/D. Minniti

A team led by Dante Minniti (Universidad Andrés Bello, Santiago, Chile) and Rodrigo Contreras Ramos (Instituto Milenio de Astrofísica, Santiago, Chile) used observations from the VISTA infrared survey telescope, as part of the Variables in the Via Lactea (VVV) ESO public survey, to carefully search the central part of the Milky Way. By observing infrared light, which is less affected by cosmic dust than visible light, and exploiting the excellent conditions at ESO’s Paranal Observatory, the team was able to get a clearer view of this region than ever before. They found a dozen ancient RR Lyrae stars at the heart of the Milky Way that were previously unknown.

Our Milky Way has a densely populated centre — a feature common to many galaxies, but unique in that it is close enough to study in depth. This discovery of RR Lyrae stars provides compelling evidence that helps astronomers decide between two main competing theories for how nuclear bulges form [1].

This video sequence starts from a wide field view of the Milky Way and closes near the galactic centre. Here astronomers using the VISTA infrared survey telescope discovered several ancient stars, of a type known as RR Lyrae. RR Lyrae stars typically reside in ancient stellar populations over 10 billion years old. Their discovery suggests that the bulging centre of the Milky Way likely grew through the merging of primordial star clusters. Credit: ESO, Digitized Sky Survey 2, N. Risinger (skysurvey.org. Acknowledgment: Davide De Martin and S. Guisard (www.eso.org/~sguisard), Music: Johan B. Monell (www.johanmonell.com)

RR Lyrae stars are typically found in dense globular clusters. They are variable stars, and the brightness of each RR Lyrae star fluctuates regularly. By observing the length of each cycle of brightening and dimming in an RR Lyrae, and also measuring the star’s brightness, astronomers can calculate its distance [2].

Unfortunately, these excellent distance-indicator stars are frequently outshone by younger, brighter stars and in some regions they are hidden by dust. Therefore, locating RR Lyrae stars right in the extremely crowded heart of the Milky Way was not possible until the public VVV survey was carried out using infrared light. Even so, the team described the task of locating the RR Lyrae stars in amongst the crowded throng of brighter stars as “daunting”.

This video zooms across a part of the sky close to the centre of our Milky Way, which was observed with the VISTA infrared survey telescope. Within this part of the sky astronomers discovered a dozen new variable stars. Their discovery suggests that the bulging centre of the Milky Way likely grew through the merging of primordial star clusters. Credit: ESO/VVV Survey/D. Minniti. Music: 5th Dimension

Their hard work was rewarded, however, with the identification of a dozen RR Lyrae stars. Their discovery indicate that remnants of ancient globular clusters are scattered within the centre of the Milky Way’s bulge.

Rodrigo Contreras Ramos elaborates:

“This discovery of RR Lyrae Stars in the centre of the Milky Way has important implications for the formation of galactic nuclei. The evidence supports the scenario in which the nuclear bulge was originally made out of a few globular clusters that merged.”

The theory that galactic nuclear bulges form through the merging of globular clusters is contested by the competing hypothesis that these bulges are actually due to the rapid accretion of gas. The unearthing of these RR Lyrae stars — almost always found in globular clusters — is very strong evidence that part of the Milky Way’s nuclear bulge did in fact form through merging. By extension, all other similar galactic bulges may have formed the same way.

Not only are these stars powerful evidence for an important theory of galactic evolution, they are also likely to be over 10 billion years old — the dim, but dogged survivors of perhaps the oldest and most massive star cluster within the Milky Way.

Notes

[1] The nuclear stellar bulge is the compact component in the innermost regions of the Milky Way (and other galaxies) extending to a size of about 400 light-years.

[2] RR Lyrae stars, like some other regular variables such as Cepheids, show a simple relationship between how quickly they change in brightness and how luminous they are. Longer periods mean brighter stars. This period-luminosity relationship can be used to deduce the distance of a star from its period of variation and its apparent brightness.