Unique Binary Globular Cluster Delivered By The Sagittarius Dwarf Spheroidal Galaxy, M53 and NGC5053

Unique Binary Globular Cluster Delivered By The Sagittarius Dwarf Spheroidal Galaxy, M53 and NGC5053

Rudy E. Kokich, Alexandra J. Kokich, Andrea I. Hudson

December 22, 2021

About 59 dwarf galaxies, each containing between several thousand and several billion stars, have been identified in the cosmic neighborhood of the Milky Way. Until recently they were assumed to be satellites in decaying orbits captured by our large galaxy over billions of years. However, the third data release from the European Space Agency's GAIA astrometric space telescope revealed a surprise. It showed with extreme precision that most of the dwarf galaxies are moving much too fast to be orbiting the Milky Way, and are relatively recent newcomers to our cosmic environment. The motion of these galaxies relative to the Milky Way is due to our collective peculiar velocities through space rather than to the expansion of space. The discovery parallels one made some 20 years ago about the Large Magellanic Cloud (LMC), which was found to be a passing traveller instead of a constant companion.

https://iopscience.iop.org/article/10.3847/1538-4357/ac27a8

Many of these dwarf galaxies will fly by, and escape into the void, while some will be captured and merged into the Milky Way. The course of events depends on the nearest approach distance, relative speed, galaxy masses, the distribution of dark matter halos, among other factors. Although pertinent physical theories are fairly sound, they are not yet convincingly predictive. For one thing, the mass of the Milky Way is not precisely known. Best estimates vary by a factor of 2.  

There is reliable evidence based on stellar kinematics that the Milky Way has already absorbed a number of dwarf galaxies during its long lifespan. For example, dwarf galaxy Gaia-Enceladus merged with the Milky Way about 9 billion years ago. GAIA astrometric space telescope has identified its stars based on their common eccentric orbits and high orbital speeds. 4 to 6 billion years ago, approximately while the Solar System was forming, another small galaxy, Sagittarius Dwarf Spheroidal (Sgr dSph, Sgr Dwarf), was captured by the Milky Way. After several orbits, immense gravitational forces dispersed it into a stellar stream which follows an oval trajectory, passing nearly perpendicularly through the outer part of the Galactic disk, about 50,000 light years from the Milky Way core. Johnston et al. (1999) estimated its orbital period at 550 to 750 million years, and suggested that the mass in its deformed core is presently two to three times lower than the original. Coherence of its stellar stream so long after the initial merger implies an unusually high dark matter concentration in that galaxy. Since 2018, data from the GAIA project revealed rippling perturbations in the motion of stars near the Milky Way core, and major bursts of star formation in the disk caused by the repeated passage of Sgr Dwarf through our Galaxy. Sgr Dwarf stellar stream is the single largest structure in the Milky Way halo.

(NOTE:When capitalized, word Galaxy refers to the Milky Way)

Fig. 1: Sagittarius Dwarf Spheroidal Galaxy core stretched by tidal interaction with the Milky Way (GAIA, visible band). The true size of the core, detectable in the infrared band, is much larger, extending over 60*

In 2003, Majewski et al. reported the all-sky distribution of Sgr Dwarf core and stellar stream debris by plotting positions of spectral class M giant stars detected in the infrared band by the Two Micron All-Sky Survey (2MASS). Such stars are of the most ancient type, usually found near the core of a galaxy, and only very rarely in the Galactic spiral arm disk or the halo. The premise was that most M giants located at a distance from the Milky Way center are of extra-Galactic origin, stripped from disrupted cores of absorbed satellite galaxies. The M giant population belonging to the stretched main body of the Sgr Dwarf was found to extend over 60 degrees, substantially more than previously measured or assumed. Prominent stellar streams of tidal debris were detected arcing across entire South and North Galactic Hemispheres, following a well-defined orbital plane around the Milky Way center. The Sun lies within a kiloparsec of that plane, leading to a probability that former Sgr Dwarf stars are within the solar neighborhood.

https://arxiv.org/abs/astro-ph/0304198

Fig. 2: An equatorial coordinate plot of Sgr Dwarf class M giant stars showing its main body and north and south tidal tails. The region around the Milky Way center which contains numerous Galactic M giants is excluded for clarity. Since more than 75% of high latitude M giants in the Milky Way originated in the Sgr Dwarf core, they are reliable tracers of its tidal debris.

The original mass of Sgr Dwarf is estimated at roughly 1000 times lower than the Milky Way's. When captured, the dwarf galaxy contributed about half a billion ancient Population II stars to the baryonic matter of the Milky Way. These stars are among the first to have formed in the early Universe, before later generations of supernovae enriched interstellar gas with heavy elements. Their spectra have extremely low metallicity, showing only minimal traces of elements heavier than primordial hydrogen and helium. The galaxy also brought in a relatively large number of globular clusters. According to Minniti et al. (2021), based on the latest GAIA EDR3 release, the confirmed number is 29, with 8 more potential candidates.

https://arxiv.org/abs/2106.03605

There is even a speculative hypothesis that the Solar System originated in the Sgr Dwarf. Findings which support the hypothesis are that the Sun's orbit around the Milky Way is substantially inclined to the Galactic plane, and that the Sun lies within 1 Kpc of the Sgr Dwarf stellar stream orbital plane. http://cosmology.com/RogueEarth1.html

However, the hypothesis is disputable on several points. First, due to extreme age, Sgr Dwarf contains very little interstellar dust and no detectable neutral hydrogen. Under such conditions new star formation is essentially impossible, making it far more likely that the Sun was born within the Milky Way. Second, Sgr Dwarf is an ancient galaxy with a very low metallicity environment, inconsistent with the formation of a high metallicity star like the Sun. Third, if the Sun originated in the Sgr Dwarf, it would be expected to follow its stellar stream, orbiting the Milky Way in a plane nearly perpendicular to the Galactic disk. That said, the possibility does remain that Solar System formation was triggered by the collapse of Milky Way interstellar gas clouds gravitationally perturbed by the initial entanglement with Sgr Dwarf. The two events do appear to have occurred in the same cosmological epoch, about 5 billion years ago.

Perhaps the most unusual object delivered by Sgr Dwarf is a binary globular cluster, which is so far unique in the entire Milky Way galaxy. Located in the constellation of Coma Berenices, it is composed of the well organized Messier 53 (M53, NGC 5024) and the peculiar globular cluster NGC 5053, which has apparently suffered dramatic gravitational deformation during the galaxy merger.

Fig. 3: Unique binary globular cluster NGC 5053 and M53 (NGC 5024) delivered to the Milky Way in a merger with the Sagittarius Dwarf Spheroidal galaxy. No other binary globular clusters have so far been identified in our Galaxy. Image taken by the authors with a TSapo65q astrograph.

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According to Arakelyan et al. (2021) and a number of other studies, based on position, 3-dimensional velocities, motions of neighboring stars, and metallicity, there is high probability that both clusters belong to the Sgr Dwarf stellar stream.

https://arxiv.org/abs/2105.09850

Angular separation between the clusters is 58 arcmin, and the difference in estimated heliocentric distances is 4,500 ly. From this we calculate that the clusters are separated from each other by approximately 4,600 ly. Since the clusters are relatively close, have similar proper motions, and since NGC 5053 shows clear signs of major disorganization, it is interesting to look for further evidence of gravitational interaction between the two.

Using photometric, near-infrared spectroscopic, and GAIA DR2 data, Sang-Hyun Chun et al (2020) first determined the tidal radius for each cluster, representing the perimeter of the "cluster proper" within which stars orbit the cluster's center of gravity. They then plotted an isodensity contour map of extra-tidal stars which have escaped individual clusters, but still follow orbits defined by the center of gravity of both clusters. A common prediction among various computer models is that the clusters were originally much larger and more massive than presently observed. Various simulations estimate star loss between 50 and 90%. These "lost stars", stripped from the clusters by gravitational disruption and interaction, form a tidal bridge between them and a common envelope around them. Analysis of these structures leads to a better understanding of the dynamic relationships between the clusters. Like the rest of the Sgr Dwarf, the structures might contain minimal interstellar dust, but no detectable neutral hydrogen gas.

https://arxiv.org/abs/2008.10410

(NOTE: Stars lost by the Milky Way globular cluster system are thought to constitute around 10% of the total Galactic halo stellar population.)

Fig. 4: Wide angle isodensity contour map of extra-tidal stars around M53 and NGC 5053. The presence of a  tidal bridge and a common envelope connote a binary system with long-standing gravitational interaction.

Multiple lines of evidence indicate that the two clusters are not merely in a temporary line-of-sight vicinity. Measured physical proximity, similar proper motion within the Sgr Dwarf stream, clear signs of gravitational disorganization in NGC 5053, and the presence of a tidal bridge and a common envelope of stars strongly suggest that the clusters form a persistent binary system. In addition, spectroscopic Doppler shift analysis shows that M53 is approaching us at 63 km/s (z = -0.000210), while NGC 5053 is receding at 43 km/s (z = +0.000143). That observation is consistent with the two clusters orbiting each other.

Situated within the Sgr Dwarf stellar stream, about 60,000 ly above the Galactic plane, the pair are among the more remote members of the Milky Way globular cluster system. Both are easy photographic targets for modest telescopes and even telephoto lenses. For example, the image in Fig.3 was taken using a small TSapo65q astrograph with an aperture of only 65mm and focal length of 420mm. 4 to 5 inch telescopes are required for visual appreciation of NGC 5053, and much larger still for resolution into stars.

Spectroscopic chemical analysis revealed that, just like the Sgr Dwarf stellar stream, stellar populations of both clusters are very poor in "metals", or elements heavier than hydrogen and helium. This indicates they formed in the young Universe, about 12.5 billion years ago, before the early generations of supernovae enriched the primordial gas clouds with heavier elements. Over the last five decades, a number of spectroscopic studies were carried out on the giant stars of NGC 5053 to determine their [Fe/H] metallicity, or the ratio of iron to hydrogen compared to the same ratio in the Sun.  Depending on the method used and the star population selected for testing, these values range between -2.29 dex (10^-2.29 = 1/195 solar) and -2.62 dex (10^-2.62 = 1/417 solar). In the 1984 edition of the Zinn and West Catalog of Globular Cluster Properties, NGC 5053 was listed as by far the most metal-poor globular cluster in our Galaxy, based on the then current [Fe/H] metallicity of -2.58 dex. It was originally presumed the cluster must have formed high in the Galactic halo.

https://iopscience.iop.org/article/10.1086/382903/fulltext/

A more recent study of the cluster's red giant branch stars by Boberg et al. (2015) derived an average spectroscopic metallicity value of -2.45 dex (10^-2.45 = 1/282 solar). This keeps NGC 5053 in the population of the lowest metallicity globular clusters known, and also keeps it in line with metallicities of the Sgr Dwarf stellar stream.

https://arxiv.org/abs/1504.01791

According to Lamb et al. (2015), the average spectroscopic metallicity for M53 (NGC  5024) is slightly higher at -2.06 dex, which would imply a somewhat younger age. However, based on the metallicities of the most ancient individual stars in each cluster, M53 may be older than NGC 5053 by several hundred million years. Their ages are estimated at 12.67 and 12.29 billion years respectively.

https://arxiv.org/abs/1001.4289

(NOTE: When comparing metallicity data in the literature, it is important to distinguish between metallicity measured spectroscopically and metallicity measured by color-magnitude photometry. The two values can differ by more than 20% at spectroscopic metallicities lower than -2 dex.)

https://www.aanda.org/articles/aa/full_html/2011/07/aa16998-11/aa16998-11.html

Like all other globular clusters, M53 and NGC 5053 are composed of diverse stellar populations. The best method to illustrate different stellar types is to construct a Hertzsprung-Russell Diagram (HR diagram or HRD) by plotting for individual stars their spectral class or color-index against their absolute magnitudes. Since the spectral class and color-index values mainly reflect stellar surface temperature, the plot in effect shows the relationship between a star's surface temperature and its intrinsic luminosity.

https://www.cloudynights.com/articles/cat/articles/introduction-to-stellar-spectroscopy-r3006

However, within the same type of stellar population, a star's position on the HR diagram also depends on its metallicity, so that stars of lower metallicity tend to have higher surface temperatures. The effect is clearly visible in Fig. 5, a superimposed HR Diagram of 14 globular clusters which widely range in metallicity. The lowest metallicity clusters marked in yellow, such as the binary system under consideration, are composed of demonstrably hotter stars

Fig. 5: A plot of superimposed HR diagrams of 14 globular clusters which range in metallicity from the lowest to the highest (based on GAIA DR2).

Different stellar populations in each cluster can be identified according to their position on the HR diagram, depending on the stellar mass, metallicity, and the evolutionary stage of the population. Generally speaking, stars of similar initial mass and metallicity pass through very similar evolutionary stages, resulting in very similar modifications in physical properties such as size, temperature, spectral class, color index, and luminosity. As stars transition from one evolutionary stage to another, they shift their position on the HR diagram from one population type to another, sometimes quite rapidly.

Stellar evolution is a complex subject which is beyond the scope of this article. A summary can be found here:

https://www.accessscience.com/content/stellar-evolution/654000

We will briefly discuss the topic only to mention large features in the HR diagram and major stellar populations critical for the study of globular clusters.

Stars begin their life cycle on the main sequence, which extends diagonally from the left upper corner on the HR diagram. More massive stars are more luminous, and occupy higher levels on the main sequence, but have exponentially shorter lifespans, on the order of millions of years. Since globular clusters are ancient structures, their massive stars have perished eons ago, leaving only the bottom third of the original main sequence populated by small stars of low mass, whose lifetimes can extend into hundreds of billions of years. Main sequence stars derive their energy from nuclear fusion of hydrogen to helium, which takes place only in the stellar core. With some exceptions, the core material does not generally mix with the visible surface of a star, and there is no appreciable nuclear processing in the outer layers. Consequently, stellar surfaces can preserve their original chemical composition, or metallicity, for billions of years.

Blue stragglers are stars which remain on the main sequence longer than expected by the standard theory of stellar evolution. They are much hotter, bluer, and two to three times more massive than the remaining main sequence stars. They should have evolved into the red giant stage long ago had they been formed concurrently with other stars in the cluster. Therefore, they are presumed to have formed more recently by collisions between stars, or mergers within binary star systems. The impact severely disrupts the two stars, mixes new hydrogen into the stellar core, and allows hydrogen to helium fusion to continue. Blue stragglers are most commonly found in old and very dense stellar systems such as globular clusters, dwarf galaxies, some open clusters, but also, rarely, in the field.

The turnoff point (or the main sequence turnoff, or the "knee") is the region on the HR diagram at which a main sequence star begins to change into a red giant after exhausting hydrogen fuel in its core. As a cluster becomes older, progressively smaller main sequence stars turn toward the red giant stage, the knee shifts downward on the HR diagram, which makes it possible to estimate the age of a cluster by the height, or the luminosity level  of the knee. Notice in Fig.5 that, for a given mass, lower metallicity stars burn hotter, and turn toward the giant stage sooner. This observation led to the formulation of the age-metallicity relation (AMR) derived from the width of the knee at the inflection level. These relations give fair estimates in a relative sense - when comparing the age of one cluster to another. In general, stellar age is inversely proportional to metallicity. But, there is no simple solution for accurately calibrating the relation since there is no independent method for precisely measuring age.

The brightest stars in globular clusters are the red giants. These are very luminous stars of enormous dimensions, but low to intermediate mass (0.3 - 5 solar), which are in the later stages of stellar evolution. Since the progress of stellar evolution is inversely proportional to stellar mass, red giants are more massive than the remaining main sequence population. A red giant is formed after a main sequence star exhausts the hydrogen supply in the core, but continues to fuse hydrogen to helium in an ever increasing shell around the inert helium core. As the shell grows, radiation pressure causes the diffuse outer envelope of the star to expand to enormous dimensions, up to 1,000 times the diameter of the Sun. Increase in size results in a decrease in density, a decrease in surface temperature between 5,000 and 2,500K, a change in spectral classification to K or M respectively, and a change in color from yellow to orange-red. Although surface brightness per unit area becomes lower because of lower temperature, absolute magnitude of the star increases dramatically due to the increase in the size of the luminous surface. The brightest stars in globular clusters are predominantly population II red giants. They are also found in galactic cores and, at much lower concentrations, in  galactic halos. Class M red giants, gravitationally stripped from the cores of absorbed dwarf galaxies, photographed in the infrared band, are used as bright markers to trace the course of the residual stellar streams around the Milky Way.

It is estimated that stars spend only about 1% of their life cycle in the red giant stage. During this time, the burning hydrogen shell continues to deposit helium into the inert helium core. Gravitational contraction of the core results in increasing density and temperature. When the temperature reaches approximately 100 million K, the core ignites in a thermonuclear reaction which fuses helium to carbon and oxygen, The process causes rearrangement of the star's internal structure and hydrostatic equilibrium whereby the star decreases in size, but increases in temperature, and rapidly moves into the horizontal branch population.

After helium in the core is exhausted, nuclear fusion again stops, and the star begins to contract gravitationally. This increases internal pressure and temperature, reigniting nuclear fusion in a helium shell around the inert carbon-oxygen core, and a hydrogen shell around the helium shell. Radiation pressure causes the outer stellar envelope to expand, and the star evolves into the asymptotic red giant phase where it actually becomes even more luminous than it had been in the first red giant phase.

When intermediate-mass stars finally run out of nuclear fuel, they gravitationally collapse, rebound, and expel their outer layers in the form of a planetary nebula, leaving behind in the center a small, but long-lived white dwarf star, whose source of heat is derived from gradual gravitational contraction.

Of particular interest in the study of globular clusters are the RR Lyrae variable stars, identified in the mid-1890s exclusively within globular clusters, and originally named cluster variables. By 1900, several were discovered in the field, including the prototype V* RR Lyrae. It was nearly three decades before they were recognized as a separate class of variable stars, distinct from classical Cepheids on the basis of their short periods, minimal metallicity, and locations within the Galactic halo. They were observed more recently in the halo and globular clusters of the Andromeda galaxy. Although their variability is somewhat irregular in the visible band, in the infrared 2.2 um K-band they display a strict period-luminosity relationship, making them useful as "standard candles" for measuring globular cluster distances. RR Lyrae are ancient, low metallicity population II stars which are passing through the instability gap of the horizontal branch toward the end of their core helium burning phase. Their mass is typically 0.5-0.8 solar, luminosity 40-50 times solar, spectral class A or F, and age over 10 billion years.

(NOTE: Metallicity can only be directly measured in the luminous surface layers of a star, which very closely resemble the chemical composition of the primordial gas cloud in which the star was born. Stars have much higher metallicity in the interior where stellar nucleosynthesis of heavier elements takes place. However, with some exceptions, interior layers of stars do not mix with or contaminate visible surface layers even after billions of years.)

Due to the density of stars in most globular clusters, it is difficult to distinguish different stellar populations even with the largest telescopes. For example, photometric measurements of RR Lyrae apparent magnitudes are brought into question because they can usually not be isolated from the dense field of surrounding stars. On small scale images taken at short focal lengths, the only population identifiable with any confidence is the red giant branch.

Fig. 6: NGC 5053 and M53 with colors oversaturated in order to emphasize red giant stellar populations. Notice a disorganized state of red giants in NGC 5053, and a number of extra-tidal red giants around it. 12 most luminous stars near the center of NGC 5053 are blue stragglers.

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Although we can reliably state that the two globular clusters form an enduring binary system, their appearance and some physical properties are quite dissimilar.

M53 (NGC 5024) is a well organized globular cluster in Coma Berenices, discovered in 1775 by German astronomer Johann Bode, then independently discovered by Messier in 1777, and described as a "nebula". William Herschel was the first to resolve it into stars using a larger telescope. He documented it as, "...one of the most beautiful objects I remember to have seen in the heavens." With angular diameter of 13 arcmin, and integrated apparent magnitude of 8.3 (V), it is easily observed in small telescopes as an oval nebulosity, but requires larger apertures for resolution. Its brightest stars are listed as magnitude 13.8, and are predominantly population II red giants. Its lowest metallicity stars indicate the cluster started forming around 12.67 billion years ago. From its estimated mass of 826,000 solar, we can approximate its tidal diameter of nearly 1,600 light years, and well over a million member stars. In its central region, the stars are on average only 0.3 light years apart. The cluster lies at a heliocentric distance of 58,000 ly, and is approaching us at 63 km/s. Situated within the Sgr Dwarf stellar stream, about 60,000 ly above the Galactic plane, along with its binary companion NGC 5053, it is one of the more outlying globular clusters. Considering its well preserved structural coherence during a turbulent history, it is not unreasonable to hypothesize the presence of a central black hole population, or a dense subhalo envelope of dark matter.

Fig. 7: Messier 53 (NGC 5024) image taken by the authors with a 4 inch TSapo100q astrograph.

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NGC 5053 is a very peculiar globular cluster in Coma Berenices, first documented by W. Herschel in 1784. Visually, it is very faint, irregularly oval in shape, gradually brighter toward the center. Compared with its spectacular binary companion, M53, it has only modest stellar content, low luminosity of 40,000 solar, a relatively small physical diameter of 160 ly, and a smaller tidal diameter around 580 ly. Because of loose appearance, low stellar density, absence of a concentrated bright nucleus, and low stellar velocity dispersion, the nature of this cluster as a globular has been doubted for a long time. However, the color-magnitude diagram (CMD) and the HR diagram show a population of blue straggler stars, ten RR Lyrae "cluster variables", and a "knee" between the main sequence and the giant branch characteristic of globular clusters. Tidal disruptions, relatively low total mass, and absence of stabilizing black holes or a dark matter envelope might explain the cluster's peculiar morphological features. Its angular size is 10.5 arcmin, integrated apparent magnitude 9.96 (V), and estimated heliocentric distance 53,500 ly, receding at 43 km/sec. Its brightest red giant stars are of apparent magnitude 14, and horizontal-branch stars average around 16.7. While it is accessible to small apertures photographically, substantial telescopes are required for visual observation. The cluster is remarkable for its extremely low average spectroscopic metallicity of -2.45 dex, among the lowest for Galactic globular clusters. However, based on the age of its oldest individual stars, it seems to have started forming several hundred million years after M53, approximately 12.29 billion years ago.

Fig. 8: NGC 5053 photographed by the authors with a 6 inch Takahashi TOA 150 astrograph.

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It is irresistible to imagine the environment within a tightly organized globular cluster. The night sky would be sublime with a million visible stars, and a bird's-eye view of the entire Milky Way galaxy. How much earlier would astronomy and associated technology develop among an intelligent species living on a world graced with such inspiration? Unfortunately, complex life in globular clusters is extremely unlikely due to virtual absence of heavier elements. It is not even known if rocky planets can form in that environment. Therefore, such spectacles are probably unseen by intelligent eyes, and must remain confined to our imagination.

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