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COLOR VARIATIONS IN BULGES AND DISKS: NEW INSIGHTS INTO THE SECULAR EVOLUTION OF GALAXIES

An analysis of nearby galaxies is shedding new light on how spiral galaxies age and transform. By analyzing the color variations of the bulges and disks of more than 3,000 nearby galaxies, the researchers from two French labs draw a coherent scenario of slow morphological evolution of disks of galaxies over billions of years, with dust playing a key role in revealing how bulges grow. Part of these results are confirmed with 2,500 distant galaxies observed with Euclid, illustrating the potential of Euclid for understanding the complex physical processed driving the evolution of galaxies.

The year 2026 marks the 100th anniversary of the Edwin Hubble’s discovery that galaxies can be classified by their shapes and structure into three main classes: ellipticals, lenticulars, and spirals (the so-called Hubble sequence, see Figure 1). Various lines of evidence indicate that it is an inverse sequence of galaxy evolution, in which spirals likely evolve into the lenticulars and ellipticals (see previous IAP's science highlight). However, which processes are operating to shape these transformations has been a matter of debate for nearly a century.

Figure 1: The three major types of the Hubble sequence of galaxy morphological types: elliptical, lenticular, spiral. Lenticulars and spirals have a central dense bulge and a surrounding disk, whereas ellipticals are dominated by a bulge-like component.

Figure 1: The three major types of the Hubble sequence of galaxy morphological types: elliptical, lenticular, spiral. Lenticulars and spirals have a central dense bulge and a surrounding disk, whereas ellipticals are dominated by a bulge-like component.
Credit: Valérie de Lapparent, Louis Quilley, Matthew Lehnert, AstrOmatic.net, Sloan Digital Sky Survey.

Spirals and lenticulars have two luminous components. A very dense central part called the bulge which contains predominately old stars, and a disk which surrounds the bulge and contain stars that are either old, as in the lenticular galaxies, or that have formed more recently, as in the spiral galaxies (see Figure 1). Galaxies form and evolve by successive mergers with predominantly smaller galaxies, as well as by accretion of gas from a reservoir of gas in which galaxies are embedded (the circumgalactic and intergalactic media). Any observation that provides new constraints on the importance of these processes is essential for understanding how galaxies have evolved over the billions of years over which the Universe has expanded.

Previous studies have found that as spiral galaxies age, their disks form stars at a slower rate, and their bulges grow significantly, leading them to probably transform into lenticulars. This has been interpreted as evidence of feedback powered by supermassive black holes in the centers of the bulges, a process in which both the efficiency in forming stars and the rate at which galaxies accumulate gas is lowered significantly. Whatever the interpretation, the predominance of the bulge light compared to that of the disk, and their respective colors indicate the evolutionary state of galaxies.

Figure 2: Color images of two spiral galaxies, PGC0023028 (top panels), and PGC0033792 (bottom panels), showing that their spiral arms are bluer than their inner disk, and than their red bulges. For each object, the whole galaxy is shown (left), as well as a zoom-in of the central region (right). The zoomed-in images show that the bulges have color changes as well, which we interpret as reddening and scattering of the bulge light by infalling dusty gas clouds.

Figure 2: Color images of two spiral galaxies, PGC0023028 (top panels), and PGC0033792 (bottom panels), showing that their spiral arms are bluer than their inner disk, and than their red bulges. For each object, the whole galaxy is shown (left), as well as a zoom-in of the central region (right). The zoomed-in images show that the bulges have color changes as well, which are interpreted as reddening and scattering of the bulge light by infalling dusty gas clouds.
Credit: Valérie de Lapparent, Louis Quilley, Matthew Lehnert, AstrOmatic.net, Sloan Digital Sky Survey.

In order to explore these processes, Louis Quilley (CNES postdoctoral fellow) and Matthew Lehnert (senior researcher), both at the Centre de Recherche Astrophysique de Lyon, and Valérie de Lapparent (senior researcher) at the Institut d’Astrophysique de Paris, have fitted the light profiles of the bulges and disks of 3106 nearby galaxies at different optical wavelengths, which allowed them to measure the apparent colors of their emitted starlight. They measure that disks are bluer than bulges in spiral galaxies (see Figure 2), whereas both components have similar colors in lenticular galaxies, with a very progressive reddening of disks as one moves from small spiral galaxies to more massive spirals, all the way to lenticulars. Gradients are also found within spiral disks, which are redder in their inner parts than in their outskirts (voir Figure 2), and this effect is stronger for older and larger spirals along the Hubble sequence. The fact that stars appear redder in the inner disks may be due to them being older, while stars in the outer disk could appear bluer because they are younger. Part of the color gradient between the inner and outer disks could also be due to stars of the same age but formed from gas with different chemical compositions, characterized by the abundances of atoms heavier than hydrogen and helium. It is difficult to distinguish unambiguously between the effects of the age of stars and those of their richness in heavy elements.

Finding such color differences between the inner and outer disks of spirals may support the widely held theoretical view that galaxies are continuously forming their disks from gas accreted from the circumgalactic gas found around all galaxies, and that this gas is mostly accreted in the outer disk regions. As galaxies age, external and internal tidal processes are driving gas rich in heavy elements and old stars into the inner regions of galaxies.

Moreover, these fits of the bulges and disks of nearby galaxies reveal an unprecedented property of spiral bulges: they appear to have very red colors (see Figure 2), in fact, redder than the lenticulars which are known to contain populations of old, red stars. These very red colors can only be explained by the presence of dust which is known to redden light emission. In addition, bulges exhibit spatial variations in color (see Figure 2), which the researchers interpret as due to large clouds of gas and dust. These characteristics imply that we are witnessing the growth of the bulges through star formation. Gas and dust are being swept up from the disk and fall into the central bulges, which supports the formation of stars, and therefore the growth of the bulges. These results provide a new picture for the aging of galaxies via a simple scenario of secular morphological transformation across cosmic times, over billions of years.

Figure 3: Euclid color image of a spiral galaxy (upper-left), by combining the optical and infrared light from the VIS and NISP intruments, at the NISP angular resolution of 0.3 arcseconds per pixel. The light emitted by the disk, that hosts the spirals arms, is markedly bluer than the central bulge appearing in yellow, an effect seen in thousands of other galaxies in this field with weak to moderate bulges. Moreover, the reddening of the galaxy disk towards the center and around the bulge, is visible by comparing with the even bluer external parts of the disk and spiral arms. Bright blue regions of intense star formation are also visible along the spiral arms. The upper-right image is the optical VIS image alone, with its three times finer resolution of 0.1 arcsec per pixel, showing with more details the structure of the spiral arms and their regions of strong star formation (appearing as white spots), which are unseen with NISP. The lower images shows the same galaxy as it would be observed in the reddest NISP band (lower-left), and with VIS (lower-right), but from a ground telescope with a minimal atmospheric blurring of 0.6 arcseconds. The outskirts of the galaxy and the details along the spiral arms are detected neither in the infrared nor in the optical, due to the strong noise generated by the bright night sky from the Earth's atmosphere, and which affects all astronomical images taken from ground-based telescopes. The same details as in the Euclid upper left image could be detected from the ground (with similar observing times) using a telescope with a 30 to 50 meter diameter mirror (compared to 1.2 meter for Euclid).

Figure 3: Euclid color image of a spiral galaxy (upper-left), by combining the optical and infrared light from the VIS and NISP intruments, at the NISP angular resolution of 0.3 arcseconds per pixel. The light emitted by the disk, that hosts the spirals arms, is markedly bluer than the central bulge appearing in yellow, an effect seen in thousands of other galaxies in this field with weak to moderate bulges. Moreover, the reddening of the galaxy disk towards the center and around the bulge, is visible by comparing with the even bluer external parts of the disk and spiral arms. Bright blue regions of intense star formation are also visible along the spiral arms. The upper-right image is the optical VIS image alone, with its three times finer resolution of 0.1 arcsec per pixel, showing with more details the structure of the spiral arms and their regions of strong star formation (appearing as white spots), which are unseen with NISP. The lower images shows the same galaxy as it would be observed in the reddest NISP band (lower-left), and with VIS (lower-right), but from a ground telescope with a minimal atmospheric blurring of 0.6 arcseconds. The outskirts of the galaxy and the details along the spiral arms are detected neither in the infrared nor in the optical, due to the strong noise generated by the bright night sky from the Earth's atmosphere, and which affects all astronomical images taken from ground-based telescopes. The same details as in the Euclid upper left image could be detected from the ground (with similar observing times) using a telescope with a 30 to 50 meter diameter mirror (compared to 1.2 meter for Euclid).
Credit: Valérie de Lapparent, Louis Quilley, Matthew Lehnert, ESA / Euclid / Euclid Consortium / NASA, AstrOmatic.net. Image processing by J.-C. Cuillandre (CEA Paris-Saclay).

These results are confirmed in a study of the shapes of more distant galaxies observed with the Euclid space telescope, as part of the Early Release Observations (Early Release Observations). Galaxies in the background of the nearby Perseus galaxy cluster (located 380 million light years away) were observed as they were up to about 10 billion years after the Big Bang, or more than half the age of the universe. By their shapes and color, these galaxies appear to be look-a-likes of the nearby and present time galaxies, examined in the current study, hence demonstrating the power of this new telescope. With one and a half to four times better sharpness than in the images from the best ground-based telescopes over a remarkably large field of view for a space telescope (about the size of the moon or half a square degree in size), and the absence of emission from the Earth's atmosphere, Euclid provides a spectacular view of distant galaxies (see Figure 3).

Modeling the bulges and disks of the 2,500 brightest distant galaxies seen in the visible and near-infrared light by Euclid's VIS and NISP instruments (using the same tools as for the nearby galaxies SourceXtractor++, voir Bertin, E. et al. 2020, Kümmel, M. et al. 2020), revealed the average color of their stars: the predominant bulges are on average of the same color as the disks at the center of which they are located, while around the weaker bulges (containing a small fraction of the light of the galaxy), the disks are on average bluer than the bulges (see Figure 3). These images of these distant spirals also reveal the same remarkable property of disks as seen in the nearby galaxies: variations in the distribution of colors across their disks (see Figure 3).

No matter what the exact processes are, these new data are showing the real potential and promise of Euclid to substantially further our understanding of the complex physical processes that drive the evolution of galaxies over time. Such a study would be impossible on these distant galaxies with existing telescopes on Earth, due to the blurring by the atmosphere, combined with its emission known as the night sky background, which mask the external parts of the disks and the fine details in their interior (see Figure 3). With its exquisite image sharpness and wide survey area, a factor of about 4,000 more galaxies have now been observed (Data Release 1), and another factor of 15 at the end of 2028. Altogether 65 million galaxies bright enough to have their internal structure resolved in detail,will be observed out of the billions in total. These will allow the researchers to study the temporal variations in the fractions of galaxies with different morphologies and masses, and to establish links with the colors and color gradients of their disks and bulges. One will then directly observe the evolution of bulges and disks of galaxies over cosmic time.

Links

puce The article in Astronomy and Astrophysics: L. Quilley, M. D. Lehnert, V. de Lapparent,  “Color dichotomy and gradients in the bulges and disks of EFIGI galaxies along the Hubble sequence” (public version)

puce The article in Astronomy and Astrophysics: L. Quilley, V. de Lapparent, M. Bolzonella, M. Baes, I. Damjanov, B. Häußler, F. R. Marleau, A. Nersesian, T. Saifollahi, D. Scott, J. G. Sorce, C. Tortora, M. Urbano et al., “Euclid: Galaxy morphology and photometry from bulge-disk decomposition of Early Release Observations” (public version)

puce News of INSU/CNRS, scientific result in the theme « Univers », June 25, 2026: “Colours of galaxies: new clues on secular evolution”

puce IAP's science highlight on galaxy morphologies and evolution, in 2022: “The changing faces of galaxies: what their mophologies tell us about their evolution over cosmic time”

Acknowledgements

This work has made use of the Sloan Digital Sky Survey III, and of the Early Release Observations (ERO) data from the Euclid mission of the European Space Agency (ESA), 2024.

Louis Quilley was funded by a Centre National d’Etudes Spatiales (CNES) Postdoctoral fellowship.

Writing and contact

Layout: Jean Mouette

June 2026

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