Astronomy and Astrophysics – Astronomy
Scientific paper
Jun 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996apj...463..818h&link_type=abstract
Astrophysical Journal v.463, p.818
Astronomy and Astrophysics
Astronomy
8
Errata, Addenda
Scientific paper
In the paper "Probing the Interstellar Medium along the Lines of Sight to Supernovae SN 1994D and SN 19941" by Luis C. Ho and Alexei V. Filippenko (ApJ, 444,165 [1995]; hereafter HF95), SN 1994D was assumed to be on the approaching side of its host galaxy, NGC 4526. The authors wish to correct this assumption. Long-slit optical spectra (resolution 6-7 A) obtained on 1994 March 18 UT with the 3 m Shane reflector at Lick Observatory exhibit Hα and [N II] λλ6548, 6583 emission lines that clearly show that SN 1994D is on the receding side of the galaxy, at a heliocentric velocity of ~850 km s^-1^ (The heliocentric systemic velocity of NGC 4526, measured at the nucleus, is ~620 km s^-1^, higher than the value of ~450 km s^-1^ often cited in the literature [e.g., the Revised Shapley-Ames Catalog of Bright Galaxies].) Thus, "system 6" in the high-resolution Keck spectrum of SN 1994D (Table 1 of HF95), at v = 709 km s^-1^, is almost certainly produced by gas in the outer regions of the gas/dust disk of NGC 4526, along the line of sight to the supernova (which is assumed to be within, or close to, the disk). Its velocity is not "anomalous," and it is unlikely to be of intergalactic origin, contrary to the previous conclusion of the paper. The revised value for the velocity of SN 1994D also suggests that systems 2-5 in Table 1 of HF95, all of which have heliocentric velocities in the range 200-250 km s^-1^, are not produced in the disk of NGC 4526; instead, they almost certainly correspond to "high-velocity clouds" (HVCs) associated with the Milky Way. Hence, they are similar to systems 3-5 along the line of sight to SN 1994I in M51 (HF95). In support of this, the authors note that Kumar & Thonnard (AJ, 88,260 [1983]) detect H I emission centered on heliocentric velocity 215 km s^-1^ at several different positions toward NGC 4526, in good agreement with the average velocity of the Na I absorption lines in our Keck spectrum; they conclude that the H I emission is produced by a Galactic HVC. Moreover, Meyer & Roth (ApJ, 383, L41 [1991]) detect weak absorption lines due to HVCs at comparable velocities in spectra of SN 1991T, only 5^deg^ away from SN 1994D. Systems 2-5 of SN 1994D should therefore be added to Table 2 of HF95, which lists known HVCs along the lines of sight to extragalactic objects; their velocities relative to the local standard of rest are 207, 218, 232, and 252 km s^-1^, respectively. High-resolution spectra of SN 1994D obtained by King et al. (A&A, 300,881 [1995]) with the William Herschel Telescope (La Palma) reveal the same sets of Na i absorption lines. King et al. independently conclude that system 6 (in the notation of HF95) is produced by interstellar gas in NGC 4526, while systems 2-5 originate in a complex of Galactic HVCs. Additional high-dispersion spectra of SN 1994D, obtained with the New Technology Telescope at the European Southern Observatory (La Silla), are presented by Patat et al. (MNRAS, 278, 111 [1996]). They, too, suggest that system 6 is associated with NGC 4526, and they adopt the interpretation of King et al. (1995) for systems 2-5. All three studies agree that system 1 in the spectrum of SN 1994D is produced by Galactic low-velocity gas.
Filippenko Alexei V.
Ho Luis C.
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