Astronomical and Imaging Data

Useful Informations

The globular cluster Palomar 4 (Pal 4) is a faint, low-density stellar system located in the far reaches of the Milky Way’s outer halo. It is considered an unusual or “anomalous” globular cluster due to its unique combination of properties.

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Location and Physical Characteristics

Palomar 4 is one of the most distant globular clusters in the Milky Way, at a distance of approximately 100 kpc (∼330,000 light-years) from the Sun.

• Initial Classification: It was initially mistaken for a dwarf galaxy, given the name “Ursa Major Dwarf,” before being confirmed as a globular cluster.
• Size and Mass: Palomar 4 is a relatively low-density cluster with a current mass estimated to be around 30,000 M_⊙​ (solar masses). Its effective radius is quite large for a globular cluster, around 20 pc.
• Metallicity and Age: It exhibits a low-to-intermediate metallicity and is classified as a “second-parameter” cluster. Its color-magnitude diagram suggests it is significantly younger than typical inner-halo globular clusters like M3 or M15, with an age estimated to be about 1.5 to 2 Gyr younger.

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Dynamical State and Evolution

Palomar 4’s dynamical state is its most intriguing feature, as it exhibits signs of a dynamically evolved system despite its large size and remote location:

• Long Relaxation Time: Its two-body relaxation time, the timescale for a cluster to undergo significant dynamical evolution through stellar encounters, is roughly on the order of a Hubble time (the age of the universe). This long timescale means that standard internal dynamical processes alone, like two-body relaxation, should not have produced the observed state.
• Mass Segregation: Contrary to the expectation of a non-evolved cluster, Palomar 4 shows clear evidence of mass segregation. This means more massive stars have sunk towards the cluster’s center, while lower-mass stars have moved outwards.
• Stellar Mass Function (SMF): The cluster’s SMF is depleted of low-mass stars and is described as “flattened.”
• Eccentric Orbit: N-body simulations suggest that Palomar 4 is most likely on a highly eccentric orbit with an eccentricity of e ≈ 0.9 and a small pericentric distance (Rp ​≈ 5 kpc). This eccentric orbit leads to strong tidal shocking during its close passages to the Galactic center, which can rapidly remove stars, particularly low-mass stars, and drive its expansion.
• Formation Scenario: To explain both its present-day mass segregation and flattened SMF despite its long relaxation time, models suggest that Palomar 4 likely had a high degree of primordial mass segregation and a flattened initial mass function (IMF). The dynamical evolution, driven by tidal shocking on its eccentric orbit, then shaped it into its current unusual state.

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