The Extraordinary “Superthin” Spiral Galaxy UGC 7321. I. Disk Color Gradients and Global Properties from Multiwavelength Observations

Abstract

We present B-and R-band optical imaging and photometry, Hα narrow-band imaging, near-infrared H-band imaging, and H i 21-cm spectroscopy of the nearby (V h =407 km/s), Sd spiral galaxy UGC 7321. UGC 7321 exhibits a remarkably thin stellar disk with no discernible bulge component. The galaxy has a very diffuse, low surface brightness disk, which appears to suffer relatively little internal extinction in spite of its nearly edge-on geometry (i ≈88 • ). If seen face-on, UGC 7321 would have an observed central B-band surface brightness of only ∼23.4 mag arcsec −2 .The UGC 7321 disk shows significant B − R color gradients in both the radial and vertical directions: ∆(B − R) ≥0.80 magnitudes along the galaxy major axis, and ∆(B − R) as large as 0.45 magnitudes is observed parallel to the galaxy minor axis. These color gradients cannot be explained solely by dust and are indicative of changes in the mix of stellar ages and/or metallicity as a function of both radius and height above the galaxy plane. The outer regions of the UGC 7321 disk are too blue to be explained by low metallicity alone (B − R ≤ 0.6), and must be relatively young. However, the galaxy also contains stellar populations with B − R >1.1, indicating it is not a young or recently-formed galaxy.The disk of UGC 7321 is not a simple exponential, but exhibits a light excess at small radii, as well as distinct surface brightness zones. Despite its organized disk structure, many of the global properties of UGC 7321 (M B = −17.0; M HI =1.1×10 9 M ⊙ ;

The dark matter halo shape of edge-on disk galaxies

“…Observations by Matthews et al (1999) that found a significant fraction of young stars support our results. Detailed studies of the vertical disk structure indicate multiple disk components (Matthews 2000).…”

“…We found the projected central R-band surface brightness to be 22.0 mag arcsec −2 , with a scalelength of 4.0 ± 0.3 kpc, in agreement with that measured by Pohlen et al (2003). The projected central surface brightness is consistent with the B and R-band measurement by Matthews et al (1999) when the internal extinction model that Matthews et al used is factored in. The projected surface brightness shows a small nuclear feature smaller than 1 kpc, and an exponential profile from 0.5 to 6.5 kpc, declining steeply after 7 kpc.…”

“…Due to its northern location, UGC 7321 was not in our initial southern galaxy sample for which we measured near-IR and optical stellar photometry at Siding Spring Observatory. However, VLA HI data observed by Lyn Matthews (Matthews et al 1999) was available for this galaxy and Michael Pohlen (Pohlen et al 2002) kindly supplied R-band photometry which allowed us to derive the stellar luminosity density necessary to analyse the halo density distribution.…”

Modeling the Interstellar Medium of Low Surface Brightness Galaxies: Constraining Internal Extinction, Disk Color Gradients, and Intrinsic Rotation Curve Shapes

“…However, even then, the amount of reddening due to dust will be insufficient to explain the large radial color gradients now observed in a number of LSB galaxies (de Blok, van der Hulst, & Bothun 1995 ;MGvD99 ;Matthews et al 2000 ;Bell et al 2000). Thus some LSB galaxies must have large intrinsic stellar population and/or metallicity gradients in their disks.…”

The dark matter halo shape of edge-on disk galaxies

“…Other explanations for localised regions of increased thickness are infalling material from a companion or disruption by a companion, however UGC 7321 is an isolated galaxy. The high symmetry of the flaring on both sides of the centre, and the high stellar disk major-to-minor axis ratio (a/b = 10.3 at μ R = 25.0 mag arcsec −2 , Matthews et al 1999) render disruption by an undetected neighbouring galaxy rather implausible. 3 We thank Lia Athanassoula for pointing this out.…”