Galaxies observed in the EUV?

[Nereid]

Can anyone provide references for papers reporting observations of galaxies (other than the MW) in the EUV?

By EUV I mean at wavelengths shorter than the Lyman limit (912Å), but not so short as to be called soft x-rays.

I expect that the hydrogen in the MW disk, locally, makes this part of the electromagnetic spectrum opaque to extragalactic objects. However, IIRC, it's not perfect; there are some holes through which some EUV from beyond the MW disk can get through ...

[ngc3314]

AFAIK all nearby galaxies detected in the EUV are AGN. Quickly leafing through some atlases of optical counterparts (for the ROSAT wide-field camera here and EUVE here), significant detections are
PKS 2155-304 (BL Lac object)
Mkn 478 (Seyfert 1/QSO)
NGC 7213 (Seyfert 1)
1H1430+423 (BL Lac object)
Mkn 501 (BL Lac object)
Mkn 279 (Seyfert 1)
Mkn 421 (BL Lac object)
NGC 4051 (Seyfert 1)
3C 273 (QSO)
1H0419-577 (AGN - I don't know that one...)
KUV 00549-2239 (AGN, ditto)

(I omit a few random galaxies which are candidate identifications based on position alone, but are faint and undistinguished elsewhere in the spectrum).

The non-usual-suspectness of the list testifies to the dominant role of structure in the Milky Way's H I absorption in determining where we can see out. Quasars can ionize their entire host galaxy ISM, so its common for their spectra to continue with barely a blip past the Lyman limit. As a result, composite spectra (to average out foreground absorbing clouds) go all the way down past 350 A in the emitted frame (examples from HST and FUSE). A handful of high-z quasars are unabsorbed down to 228 A in the emitted frame making them useful for the He II Gunn-Peterson test to tell when helium was fully ionized in the intergalactic medium.

Minor exception - starburst galaxies at significant redshift have been examined to see what fractio of the Lyman continum might leak out (as a controbutor to ionizing the IGM).

[Nereid]

Thanks ngc3314.

I was not sufficiently clear, in the OP (sorry); I am interested in any observations of extragalactic objects in the observer frame EUV.

AFAIK, the detectors on FUSE did not really go beyond the Lyman limit. In fact, the only mission which did observe in the EUV is the EUVE (Extreme Ultraviolet Explorer). While there are several papers on the detection, and measurement, of extragalactic objects (galaxies/AGN, clusters) with EUVE instruments, as far as I've been able to discover, they are all in the shortest wavelength band(s), centred on ~90Å (long wavelength limit ~175Å).

In the rest/emitted frame, the closest (lowest z) detected objects with EUV emission would be those which move the EUV emission into FUSE's spectral windows. Per that mission's Technical Documentation, that redshift should be negative (e.g. "FUSE was designed to provide high resolution spectra (λ/Δλ ≥ 20,000) with large effective area (20 -70 cm²) across the 905 - 1187 Å FUV spectral bandpass.")! However, reading that document in some more detail (but still not thoroughly), I can't find anything on how well the ~905-920Å region was, in fact, covered.

I'll go search the published FUSE literature, to see if there's anything on EUV (i.e. blueward of the Lyman limit) detections of extragalactic objects ...

[ngc3314]

Ohhhh. Detections just shortward of 912 A in the observer's frame. That's a different matter. Here is the list of detections as far as I know:



(this page deliberately left blank)




The lowest foreground column density of H I in the Milky Way found over any significant region is the Lockman Hole (popular for surveys for this reason) in Ursa Major with column density around 4x10¹⁹ atoms/cm². That still gives optical depth tau=250 (!) at the Lyman edge, dropping as wavelength^-3 shortward. Optical depth unity would then be reached near 140 A, fitting with detection of extragalactic targets only in the shortest bands of EUVE (and the roughly comparable ROSAT WFC).

[EigenState]

Greetings,

Astrophysics in the Extreme Ultraviolet (Proceedings of Colloquium No. 152, International Astronomical Union) [1] might prove useful.

See Google books.

Best regards,
ES

1. Astrophysics in the Extreme Ultraviolet (Proceedings of Colloquium No. 152, International Astronomical Union), Stuart Bowyer and Roger F. Malina, Eds., Kluwer Academic Publishers (1996). ISBN-13: 978-0792339083.

[Nereid]

Ohhhh. Detections just shortward of 912 A in the observer's frame. That's a different matter. Here is the list of detections as far as I know:



(this page deliberately left blank)




The lowest foreground column density of H I in the Milky Way found over any significant region is the Lockman Hole (popular for surveys for this reason) in Ursa Major with column density around 4x10¹⁹ atoms/cm². That still gives optical depth tau=250 (!) at the Lyman edge, dropping as wavelength^-3 shortward. Optical depth unity would then be reached near 140 A, fitting with detection of extragalactic targets only in the shortest bands of EUVE (and the roughly comparable ROSAT WFC).

Thanks!

That's exactly what I was after!!

The bit that I was missing was a) the minimum column density (in the Lockman Hole), and b) the fact observations (i.e. nothing extragalactic seen, by the appropriate missions) match theory (i.e. most of the local EUV band - which is, very roughly, three times as wide as the very narrow optical band/window through which we learned most of our extragalactic astronomy up until ~50 years' ago [1] - is a desert so far as extragalactic astronomy is concerned).

[1] 905/140 ~= 3 x 700/350

[ngc3314]

Yeah, the only glimpses we get are objects at such high redshift that their emitted EUV gets past the 912-A ionization edge of local hydrogen. In practice this means only luminous AGN, and very patchy information on a few starburst galaxies just shortward of 912. Such radiation gets out of its own galaxy only if the galaxy's ISM is almost fully ionized.

For these reasons, doing EUV astronomy was a pretty hard sell to begin with, since it was almost bound to be limited to a small piece of our galactic neighborhood. The very patchy distribution of H I matters a lot (I keep making the same argument in proposals about dust extinction at longer wavelengths...) One important proof of concept used a detector mounted on the service module of the ASTP Apollo run by a group at Berkeley, which at least showed that enough hot stars could be seen to make a case for a full-up mission (EUVE, around 17 years later).