[Paper Review] RR Lyrae stars in the newly discovered ultra-faint dwarf galaxy Centaurus I

C. E. Martínez-Vázquez, W. Cerny+23 more|arXiv (Cornell University)|Jul 12, 2021

Stellar, planetary, and galactic studies93 references12 citations

TL;DR

This study reports the detection of three RR Lyrae stars—two RRc and one RRab—in the ultra-faint dwarf galaxy Centaurus I using multi-epoch DECam imaging. The stars' spatial clustering at large distances from the Galactic halo provides strong evidence (4.7σ significance) for membership in Cen I, yielding a precise distance modulus of μ₀ = 20.354 ± 0.002 mag (D⊙ = 117.7 ± 0.1 kpc), and confirming Cen I as a genuine ultra-faint dwarf galaxy with a metallicity consistent with other UFDs.

ABSTRACT

We report the detection of three RR Lyrae (RRL) stars (two RRc and one RRab) in the ultra-faint dwarf (UFD) galaxy Centaurus I (CenI) and two Milky Way (MW) $\delta$ Scuti/SX Phoenicis stars based on multi-epoch $giz$ DECam observations. The two RRc stars are located within 2 times the half-light radius (r$_h$) of Cen I, while the RRab star (CenI-V3) is at $\sim6$ r$_h$. The presence of three distant RRL stars clustered this tightly in space represents a 4.7$\sigma$ excess relative to the smooth distribution of RRL in the Galactic halo. Using the newly detected RRL stars, we obtain a distance modulus to Cen I of $\mu_0 = 20.354 \pm 0.002$ mag ($\sigma=0.03$ mag), a heliocentric distance of D$_\odot = 117.7 \pm 0.1$ kpc ($\sigma=1.6$ kpc), with systematic errors of $0.07$ mag and $4$ kpc. The location of the Cen I RRL stars in the Bailey diagram is in agreement with other UFD galaxies (mainly Oosterhoff II). Finally, we study the relative rate of RRc+RRd (RRcd) stars ($f_{cd}$) in UFD and classical dwarf galaxies. The full sample of MW dwarf galaxies gives a mean of $f_{cd} = 0.28$. While several UFD galaxies, such as Cen I, present higher RRcd ratios, if we combine the RRL populations of all UFD galaxies, the RRcd ratio is similar to the one obtained for the classical dwarfs ($f_{cd}$ $\sim$ 0.3). Therefore, there is no evidence for a different fraction of RRcd stars in UFD and classical dwarf galaxies.

Motivation & Objective

• To confirm the membership of RR Lyrae stars in the newly discovered ultra-faint dwarf galaxy Centaurus I.
• To measure the distance to Centaurus I using RR Lyrae stars as standard candles.
• To assess the RRc+RRd (RRcd) star fraction in ultra-faint dwarfs relative to classical dwarfs.
• To evaluate the role of UFDs in contributing to the Galactic halo's RR Lyrae population.

Proposed method

• Multi-epoch giz-band imaging with the DECam on the Blanco 4m telescope was used to detect variable stars in the field of Centaurus I.
• Time-series photometry and light curve analysis were applied to identify and classify variable stars, including RR Lyrae and δ Scuti/SX Phoenicis types.
• Spatial clustering and proper motion analysis were used to distinguish member stars of Cen I from foreground Galactic halo contaminants.
• Distance modulus was derived using the Period-Luminosity relation for RR Lyrae stars, with systematic uncertainties accounted for via photometry, reddening, [Fe/H], and [α/Fe] errors.
• The RRcd fraction (fcd) was calculated and compared across UFD and classical dwarf galaxies to assess potential differences in pulsation properties.
• The Bailey diagram was used to classify the detected RR Lyrae stars and compare their period-luminosity-color relations with those of other UFDs.

Experimental results

Research questions

• RQ1 Are the detected RR Lyrae stars in Centaurus I genuine members of the galaxy, or are they foreground contaminants from the Galactic halo?
• RQ2 What is the precise distance to Centaurus I based on RR Lyrae standard candles?
• RQ3 Does the fraction of RRcd stars in ultra-faint dwarfs differ significantly from that in classical dwarf galaxies?
• RQ4 How do the pulsation properties of RR Lyrae stars in Centaurus I compare to those in other ultra-faint and classical dwarf galaxies?
• RQ5 To what extent do ultra-faint dwarfs contribute to the long-period tail of RR Lyrae stars in the Galactic halo?

Key findings

• Three RR Lyrae stars—two RRc and one RRab—were detected in Centaurus I, with two RRc stars located within 2 times the half-light radius and the RRab star (CenI-V3) at ∼6 times rh.
• The spatial clustering of these three RR Lyrae stars represents a 4.7σ excess over the smooth Galactic halo distribution, strongly supporting their membership in Cen I.
• The distance modulus to Centaurus I is measured as μ₀ = 20.354 ± 0.002 mag (σ = 0.03 mag), corresponding to a heliocentric distance of D⊙ = 117.7 ± 0.1 kpc (σ = 1.6 kpc), with systematic errors of 0.07 mag and 4 kpc.
• The RRcd fraction (fcd) in Centaurus I is consistent with other UFDs and classical dwarfs, with a combined UFD sample showing fcd ∼ 0.3, similar to the classical dwarf mean of 0.28.
• The location of the Cen I RR Lyrae stars in the Bailey diagram aligns with other UFDs, particularly those of Oosterhoff type II, indicating consistency in pulsation properties.
• The results confirm that Centaurus I is a genuine ultra-faint dwarf galaxy with a metallicity of [Fe/H] = −1.8 dex and a stellar population older than 12.85 Gyr.