[Paper Review] Tip of the red giant branch distance to the nearby dwarf galaxy [TT2009] 25 in the NGC 891 group

Oliver Müller, Rodrigo Ibata+2 more|HAL (Le Centre pour la Communication Scientifique Directe)|Aug 8, 2019

Galaxies: Formation, Evolution, Phenomena71 references4 citations

TL;DR

This study measures the tip of the red giant branch (TRGB) distance to the dwarf galaxy [TT2009] 25 in the NGC 891 group using deep Subaru Suprime-Cam $V$ and $i$-band photometry, yielding a distance of $10.28^{+1.17}_{-1.73}$ Mpc. The result confirms [TT2009] 25 as a member of the NGC 891 group, consistent with its distance and structural properties matching those of Local Group dwarf galaxies.

ABSTRACT

Dwarf galaxies are key objects for small-scale cosmological tests like the abundance problems or the planes-of-satellites problem. It is therefore a crucial task to get accurate information for as many nearby dwarf galaxies as possible. Using extremely deep, ground-based $V$ and $i$-band Subaru Suprime Cam photometry with a completeness of $i=27$ mag, we measure the tip of the red giant branch distance for the dwarf galaxy [TT2009] 25. This dwarf resides in the field around the Milky Way-analog NGC 891. By using a Bayesian approach, we measure a distance of $10.28^{+1.17}_{-1.73}$ Mpc, which is consistent with the distance of NGC 891, thus we confirm it as a member of NGC 891. The dwarf galaxy follows the scaling relations defined by the Local Group dwarfs. We do not find an extended stellar halo around [TT2009] 25. In the small field of view of 100 kpc covered by the survey, only one bright dwarf galaxy and the giant stream are apparent. This is comparable to the Milky Way, where one bright dwarfs reside in the same volume, as well as the Sagittarius stream - excluding satellites which are farther away but would be projected in the line-of-sight. It is thus imperative to survey for additional dwarf galaxies in a larger area around NGC 891 to test the abundance of dwarf galaxies and compare it to the number of satellites around the Milky Way.

Motivation & Objective

• To determine the precise distance to the faint dwarf galaxy [TT2009] 25 in the NGC 891 group using deep optical photometry.
• To assess whether [TT2009] 25 is a physical member of the NGC 891 group by comparing its distance to that of NGC 891.
• To evaluate the dwarf galaxy's structural and photometric properties in comparison to Local Group dwarfs.
• To investigate the abundance of dwarf galaxies in the NGC 891 field and compare it to the Milky Way's satellite system.
• To test the consistency of [TT2009] 25 with scaling relations observed in dwarf galaxies and to search for extended stellar halos or tidal features.

Proposed method

• Acquired deep $V$ and $i$-band imaging with the Subaru Suprime-Cam, reaching a photometric completeness of $i = 27$ mag.
• Performed PSF-fitting photometry using DAOPHOT on the deep imaging data, calibrated against the Pan-STARRS DR2 catalog.
• Applied quality cuts based on sharpness and $χ^2$ parameters to reject non-stellar objects and outliers.
• Used a Bayesian Markov Chain Monte Carlo (MCMC) method to measure the TRGB magnitude from the $i$-band luminosity function.
• Calibrated the TRGB magnitude using theoretical and empirical calibrations, accounting for extinction ($A_i = 0.09$ mag).
• Derived the distance modulus and distance using the absolute $I$-band magnitude of the TRGB, assuming a standard value of $M_I^{ ext{TRGB}} = -4.03$ mag.

Experimental results

Research questions

• RQ1 Is the dwarf galaxy [TT2009] 25 physically associated with the NGC 891 group, based on its distance?
• RQ2 How does the structural and photometric properties of [TT2009] 25 compare to those of Local Group dwarf galaxies?
• RQ3 What is the total number of resolved dwarf galaxies within a 100 kpc field around NGC 891, and how does this compare to the Milky Way's satellite system?
• RQ4 Does [TT2009] 25 exhibit signs of a stellar halo or tidal debris, indicating past interactions?
• RQ5 What is the implication of the observed dwarf galaxy abundance for the 'planes of satellites' problem and the $Λ$CDM model?

Key findings

• The TRGB distance to [TT2009] 25 is $10.28^{+1.17}_{-1.73}$ Mpc, consistent with the distance to NGC 891, confirming its membership in the NGC 891 group.
• The distance modulus of [TT2009] 25 is $(m-M)_0 = 30.06^{+0.23}_{-0.40}$ mag, with a $V$-band absolute magnitude of $M_r = -13.18^{+0.24}_{-0.40}$ mag.
• The effective radius of [TT2009] 25 is $649^{+76}_{-111}$ pc, and its Sérsic index is $n = 1.01 \pm 0.01$, indicating a disk-like or exponential profile.
• The galaxy shows no evidence of an extended stellar halo; only one bright dwarf and a giant stream are visible within the 100 kpc field.
• The field around NGC 891 contains only one resolved dwarf galaxy, comparable to the Milky Way’s satellite system within 50 kpc, which hosts two bright classical dwarfs.
• The observed dwarf galaxy population in the field is consistent with expectations from the Milky Way’s satellite system, but deeper surveys over larger areas are needed to test the full abundance of faint and low-surface-brightness dwarfs.