The Redshift and Classification of IGR J20187+4041: A Low-Luminosity Seyfert Galaxy Behind the Galactic Plane in Cygnus

ATel #1623; T. S. Goncalves, D. C. Martin (Caltech), J. P. Halpern (Columbia U.), M. Eracleous, G. G. Pavlov (Penn State)
on 22 Jul 2008; 01:25 UT
Credential Certification: Jules Halpern (jules@astro.columbia.edu)

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A spectrum of 2MASX J20183871+4041003, the galaxy counterpart of IGR J20187+4041, was obtained on 2008 May 5 UT using DEIMOS on the Keck II telescope, covering the wavelength range 6656-9275 Å at 1.2 Å resolution. Seeing was ~0.5", and the nucleus was isolated using a 0.7" wide slit. Narrow (FWHM ~300 km/s) emission lines of Hα, [N II] 6583, [S II] 6716,6730, and [S III] 9068 are detected at z = 0.0144. In addition, stellar photospheric absorption in [Ca II] 8498,8542,8662 is seen at the same redshift; the near-IR continuum is evidently dominated by starlight. These properties are consistent with the I-band image of the galaxy shown in ATel #1498. We tentatively assign a Seyfert 2 classification based on the line intensities and the absence of broad Hα emission. However, weak, broad Hα could be difficult to see as it would overlap the end of the spectrum. An earlier spectrum of lower resolution and lower signal-to-noise, obtained with the LRS on the Hobby-Eberly Telescope on 2006 July 14, is consistent with the Keck spectrum, but it could not unambiguously determine the redshift or classification.

The z = 0.0144 redshift of IGR J20187+4041 is slightly smaller than the mean of 0.021 for Seyfert galaxies detected by INTEGRAL (Bassani et al. 2006, ApJ, 636, L65). The 0.5-10 keV flux of 4.3e-12 ergs/cm/s reported by Pandel et al. (ATel #1595) from an XMM-Newton observation on 2008 June 1 corresponds to a luminosity of 1.9e42 ergs/s. Similar luminosities were measured using the Swift XRT (Bykov et al. 2006, ApJ, 649, L21; Halpern, ATel #1498). More recent XRT measured fluxes (not included here) range within a factor of 2. The 20-40 keV flux measured by INTEGRAL (1.4e-11 ergs/cm2/s: Bykov et al.) corresponds to 6.2e42 ergs/s. These X-ray properties are typical of low-luminosity Seyfert galaxies, as is the nuclear radio power of IGR J20187+4041, log P(6 cm) = 20.95 W/Hz (Dubner et al., ATel #1518). See Ho & Peng (2001, ApJ, 555, 650) for a Seyfert radio luminosity distribution. The VLBI observation of Trejo et al. (ATel #1597) yields a core brightness temperature of only ~2.e6 K at 1.6 GHz, which does not require relativistic beaming (with the caveat that interstellar scattering may affect this measurement).

That IGR J20187+4041 is an unremarkable Seyfert galaxy, as well as its positional offset from either the EGRET source 3EG J2020+4017 or its possible AGILE counterpart AGL2021+4029 (Chen et al., ATel #1585), argue against its association with these high-energy gamma-ray sources.