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Post Info TOPIC: NGC 3109
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NGC 3109
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Title: Perseus I and the NGC 3109 association in the context of the Local Group dwarf galaxy structures
Author: Marcel S. Pawlowski, Stacy S. McGaugh

The recently discovered dwarf galaxy Perseus I appears to be associated with the dominant plane of non-satellite galaxies in the Local Group (LG). We predict its velocity dispersion and those of the other isolated dSphs Cetus and Tucana to be 6.5, 8.2, and 5.5 km/s, respectively. The NGC 3109 association, including the recently discovered dwarf galaxy Leo P, aligns with the dwarf galaxy structures in the LG such that all known nearby non-satellite galaxies in the northern Galactic hemisphere lie in a common thin plane (rms height 53 kpc; diameter 1.2 Mpc). This plane has an orientation similar to the preferred orbital plane of the Milky Way (MW) satellites in the vast polar structure. Five of seven of these northern galaxies were identified as possible backsplash objects, even though only about one is expected from cosmological simulations. This may pose a problem, or instead the search for local backsplash galaxies might be identifying ancient tidal dwarf galaxies expelled in a past major galaxy encounter. The NGC 3109 association supports the notion that material preferentially falls towards the MW from the Galactic south and recedes towards the north, as if the MW were moving through a stream of dwarf galaxies.

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Title: The Araucaria Project. A Wide-Field Photometric Survey for Cepheid Variables in NGC 3109
Authors: G. Pietrzynski, W. Gieren, A. Udalski, I. Soszynski, F. Bresolin, R.P. Kudritzki, R. Mennickent, M. Szymanski

Researchers have obtained mosaic images of NGC 3109 in the V and I bands on 74 nights, spanning approximately one year. From these data, they have conducted an extensive search for Cepheid variables over the entire field of the galaxy, resulting in the discovery of 113 variables with periods ranging from 3.4 to 31.4 days. In this sample, 76 Cepheids, including many long-period variables, were not known before. For the previously known 45 Cepheids in this galaxy, their data proved that reported periods were wrong for 14 objects; for nearly all other previously known Cepheid variables they were able to significantly improve on the periods.
The researchers construct period-luminosity relations from their data and obtain reddening-corrected distance moduli of 25.72 ±0.05 mag in V, and 25.66 ±0.04 mag in I. The distance modulus derived form the reddening-independent V-I Wesenheit index turns out to be significantly shorter (25.54 ±0.05 mag), which indicates that in addition to the foreground extinction of E(B-V) = 0.05 mag, there is an intrinsic to NGC 3109 reddening of about 0.05 mag.
Their distance obtained based on the reddening-free Wesenheit magnitudes is consistent with earlier distance determinations of NGC 3109 from Cepheids, and the tip of the red giant branch. The researchers will improve on their distance and extinction determination combining their optical data with the follow-up near-infrared observations of a subsample of NGC 3109 Cepheids.

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Position(2000): RA 10 : 03.1 Dec. -26 : 09
The location of the observed field in NGC 3109 on the DSS blue plate. The field
of view was about 35 x 35 arcmin. North is up and East is to the left.

NGC 3109 is a small 10th magnitude irregular galaxy between 4.5 and 4.9 million light years away in the direction of the constellation of Hydra. It is the most prominent member of a Local Group subgroup. Other names for this galaxy include h 3221 and GC 2003.

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