ASTRO: Arp 2 A very faint galaxy

Arp 2 is an SB dwarf galaxy of the Magellanic class in the constellation
of Hercules. Arp classed it as Spiral galaxy: low surface brightness.
It is indeed. I used twice my normal exposure time on this object and
it was still exceedingly difficult to pull out of the noise. This is
one challenging object needing very dark skies.

It is also known as UGC 10310 and is thought to be about 15.8 mega
parsecs away or about 50 million light-years away. Though redshift
alone would indicate a closer distance of 10.5 mega parsecs or 34
million light-years. But at this close distance redshift alone is often
a very poor distance indicator. One note at NED is shown below.

"This Magellanic barred spiral has rather poor H{alpha} emission, but
exhibits a very bright giant HII region to the SE. Some emission can be
seen in the north part of the bar and in the spiral arms starting at
both ends. The scarcity of the emission makes the drawing of isovelocity
lines difficult. It is also hard to derive a reliable rotation curve,
although both sides are in fairly good agreement and suggest that a
maximum around 70 km s^-1^ is reached at about 50 arcsec from the
center. The WHISP HI data show that the neutral gas is mainly
concentrated in the two spiral arms. Our H{alpha} velocities are
consistent with the HI velocity field, as confirmed by the good
agreement between our rotation curve and the HI curve derived by Swaters
(1999)."

Some of that weak H alpha emission at the north part of the bar may be
why the upper end of the galaxy has a slightly reddish hue. The HII
region to the SE is very obvious in my image as the blue blob.
Stretching the image has greatly increased its size same as a star gets
increased. In the original data stars in the image have a FWHM of 2.8"
(better than average for my location) but this "star" was 4.6" FWHM
showing it wasn't a star at all. Arp made the following comment about
the galaxy and this object: "Low surface brightness dwarf. Large bright
knot in arm appears almost stellar." Some of the other blue "stars" may
also be HII regions in the galaxy. Their distribution would seem to
suggest this is the case.

Annotated image shows galaxies, two quasars and a quasar candidate's
distance in billions of light-years. One galaxy cluster, MaxBCG
J243.92214+47.16785 at about 2.5 billion light-years has the core
galaxies marked as GC. NED says it has 14 members. No diameter is
given but I assume most if not all the reddened galaxies in this part of
the image belong to the cluster. Only the 2 marked galaxies have a red
shift given however. Oddly they result in a distance of 2.4 billion
light-years. This may be the more accurate figure. So that is what I
used in the annotated image.

To give an indication of how deep this image goes I've marked the
magnitude of one star on the SDSS survey they list at 22.1 magnitude.
Oddly the galaxy beside it isn't listed! I see this quite often but
don't know the reason.

There was a horrid flare from a K0 star just out of frame at the
southwest corner. While I reduced it, it likely covered up several
distant galaxies. This is why that corner is rather bare of features.

14" LX200R @ f/10, L=8x10' RGB=4x10', STL-11000XM, Paramount ME, image
scale 1" per pixel.

Rick

--
Correct domain name is arvig and it is net not com. Prefix is correct.
Third character is a zero rather than a capital "Oh".

Great picture Rick. Thanks to the longer than usual exposure time the image
looks nice smooth in spite of the faint object. And detail is very good too.

Stefan

"Rick Johnson" schrieb im Newsbeitrag
ster.com...
Arp 2 is an SB dwarf galaxy of the Magellanic class in the constellation
of Hercules. Arp classed it as Spiral galaxy: low surface brightness.
It is indeed. I used twice my normal exposure time on this object and
it was still exceedingly difficult to pull out of the noise. This is
one challenging object needing very dark skies.

It is also known as UGC 10310 and is thought to be about 15.8 mega
parsecs away or about 50 million light-years away. Though redshift
alone would indicate a closer distance of 10.5 mega parsecs or 34
million light-years. But at this close distance redshift alone is often
a very poor distance indicator. One note at NED is shown below.

"This Magellanic barred spiral has rather poor H{alpha} emission, but
exhibits a very bright giant HII region to the SE. Some emission can be
seen in the north part of the bar and in the spiral arms starting at
both ends. The scarcity of the emission makes the drawing of isovelocity
lines difficult. It is also hard to derive a reliable rotation curve,
although both sides are in fairly good agreement and suggest that a
maximum around 70 km s^-1^ is reached at about 50 arcsec from the
center. The WHISP HI data show that the neutral gas is mainly
concentrated in the two spiral arms. Our H{alpha} velocities are
consistent with the HI velocity field, as confirmed by the good
agreement between our rotation curve and the HI curve derived by Swaters
(1999)."

Some of that weak H alpha emission at the north part of the bar may be
why the upper end of the galaxy has a slightly reddish hue. The HII
region to the SE is very obvious in my image as the blue blob.
Stretching the image has greatly increased its size same as a star gets
increased. In the original data stars in the image have a FWHM of 2.8"
(better than average for my location) but this "star" was 4.6" FWHM
showing it wasn't a star at all. Arp made the following comment about
the galaxy and this object: "Low surface brightness dwarf. Large bright
knot in arm appears almost stellar." Some of the other blue "stars" may
also be HII regions in the galaxy. Their distribution would seem to
suggest this is the case.

Annotated image shows galaxies, two quasars and a quasar candidate's
distance in billions of light-years. One galaxy cluster, MaxBCG
J243.92214+47.16785 at about 2.5 billion light-years has the core
galaxies marked as GC. NED says it has 14 members. No diameter is
given but I assume most if not all the reddened galaxies in this part of
the image belong to the cluster. Only the 2 marked galaxies have a red
shift given however. Oddly they result in a distance of 2.4 billion
light-years. This may be the more accurate figure. So that is what I
used in the annotated image.

To give an indication of how deep this image goes I've marked the
magnitude of one star on the SDSS survey they list at 22.1 magnitude.
Oddly the galaxy beside it isn't listed! I see this quite often but
don't know the reason.

There was a horrid flare from a K0 star just out of frame at the
southwest corner. While I reduced it, it likely covered up several
distant galaxies. This is why that corner is rather bare of features.

14" LX200R @ f/10, L=8x10' RGB=4x10', STL-11000XM, Paramount ME, image
scale 1" per pixel.

Rick

--
Correct domain name is arvig and it is net not com. Prefix is correct.
Third character is a zero rather than a capital "Oh".

Great picture Rick. Thanks to the longer than usual exposure time the image
looks nice smooth in spite of the faint object. And detail is very good too.

Stefan

"Rick Johnson" schrieb im Newsbeitrag
ster.com...
Arp 2 is an SB dwarf galaxy of the Magellanic class in the constellation
of Hercules. Arp classed it as Spiral galaxy: low surface brightness.
It is indeed. I used twice my normal exposure time on this object and
it was still exceedingly difficult to pull out of the noise. This is
one challenging object needing very dark skies.

It is also known as UGC 10310 and is thought to be about 15.8 mega
parsecs away or about 50 million light-years away. Though redshift
alone would indicate a closer distance of 10.5 mega parsecs or 34
million light-years. But at this close distance redshift alone is often
a very poor distance indicator. One note at NED is shown below.

"This Magellanic barred spiral has rather poor H{alpha} emission, but
exhibits a very bright giant HII region to the SE. Some emission can be
seen in the north part of the bar and in the spiral arms starting at
both ends. The scarcity of the emission makes the drawing of isovelocity
lines difficult. It is also hard to derive a reliable rotation curve,
although both sides are in fairly good agreement and suggest that a
maximum around 70 km s^-1^ is reached at about 50 arcsec from the
center. The WHISP HI data show that the neutral gas is mainly
concentrated in the two spiral arms. Our H{alpha} velocities are
consistent with the HI velocity field, as confirmed by the good
agreement between our rotation curve and the HI curve derived by Swaters
(1999)."

Some of that weak H alpha emission at the north part of the bar may be
why the upper end of the galaxy has a slightly reddish hue. The HII
region to the SE is very obvious in my image as the blue blob.
Stretching the image has greatly increased its size same as a star gets
increased. In the original data stars in the image have a FWHM of 2.8"
(better than average for my location) but this "star" was 4.6" FWHM
showing it wasn't a star at all. Arp made the following comment about
the galaxy and this object: "Low surface brightness dwarf. Large bright
knot in arm appears almost stellar." Some of the other blue "stars" may
also be HII regions in the galaxy. Their distribution would seem to
suggest this is the case.

Annotated image shows galaxies, two quasars and a quasar candidate's
distance in billions of light-years. One galaxy cluster, MaxBCG
J243.92214+47.16785 at about 2.5 billion light-years has the core
galaxies marked as GC. NED says it has 14 members. No diameter is
given but I assume most if not all the reddened galaxies in this part of
the image belong to the cluster. Only the 2 marked galaxies have a red
shift given however. Oddly they result in a distance of 2.4 billion
light-years. This may be the more accurate figure. So that is what I
used in the annotated image.

To give an indication of how deep this image goes I've marked the
magnitude of one star on the SDSS survey they list at 22.1 magnitude.
Oddly the galaxy beside it isn't listed! I see this quite often but
don't know the reason.

There was a horrid flare from a K0 star just out of frame at the
southwest corner. While I reduced it, it likely covered up several
distant galaxies. This is why that corner is rather bare of features.

14" LX200R @ f/10, L=8x10' RGB=4x10', STL-11000XM, Paramount ME, image
scale 1" per pixel.

Rick

--
Correct domain name is arvig and it is net not com. Prefix is correct.
Third character is a zero rather than a capital "Oh".