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ASTR1001
Zog: The Second Data Release
Results to date
In a recent article in Scientific Zoggian, Prof Paul Franciz
suggests that the astronomical community seems to have
reached a consensus on some issues.
Virtually everyone seems to agree that the Greater Milkstain
is a collection of around 10 million stars, clustered in the
centre. Our own star seems to lie in the outer regions of this
vast star cluster, which we are calling a “galaxy”.
It is also widely (but not universally) accepted that at least
some fuzzballs are other “galaxies” much like our own.
There have been a few attempts to estimate the size of the
GMS, and distances to other fuzzballs, but there is no
consensus yet on these values.
Fuzzballs show both red- and blue shifts. No clear pattern
has yet emerged, and the blue spots remain an enigma.
Wagner, Bach and Hayden (IAP)
This group have been trying to measure a distance to the blue spots.
They asked for and were awarded time on the Bubble Space Telescope
to look for parallax in the blue spots. No parallax was found: the blue
spots must therefore be more than about fifty light-years away.
Many individual stars in the Greater Milk Stain were also included in their
image of the North Blue Spot. These stars also show no measurable
parallax. They typically have measured fluxes of around 10-16 W m-2 nm-1
in the V band.
Gilbert and Sullivan
This group asked for long exposure images of the blue spots
with the Bubble Space Telescope. The time assignment
committee considered their request to be sensible, as many
astronomers are facinated by these mysterious objects, and
allocated 40 orbits of exposure to each blue spot.
Up close, both blue spots look quite similar to how they appear
unmagnified. Neither breaks up into stars (at the 0.1 arcsecond
resolution of the Bubble Space Telescope), though the North Blue spot
image is full of stars from the Greater Milk Stain.
One surprise: under magnification, the North Blue Spot (the one within
the Greater Milk Stain) has jets of fuzzballs, just like the South Blue
Spot.
Another new result: many new jets of fuzzballs were found around both
blue spots: jets too faint and small to have been seen before. These
faint jets are slightly bluer in colour than the well known bright ones.
Diaz, Heston and Smythe (Ozford Uni)
This team, together with many collaborators, have been
mapping the whole sky, using a special pair of wide field
telescopes.
Such telescopes are called Schmidt telescopes, and use a
special combination of lenses, mirrors and photographic
plates to take photographs of a whopping 36 square degrees
of the sky in one go. Two such telescope, the Palomarz and
Anglo-Auztralian Schmidts, have been photographing the
whole sky for ten years. They have taken these photographs,
digitised them, and have used them to construct a complete
digital map of the sky on 100 cd-roms.
The Anglo-Auztralian Schmidt
The first result concerns the jets. With the all sky digital map
they have been able to show that they extend out from the
North Blue Spot as well as the South one: the northern jets
have, until now, been lost in the midst of the Great Milk Stain.
Furthermore, the Jets seem to extend further out from the
blue spots than anyone previously expected. As they get
further from the spots, the gaps between fuzzballs get very
large, but they can trace some jets out to five degrees from
the blue spots!
The fuzzballs that lie in the jets are always very faint ones:
they never see the famous bright fuzzballs like M23 or M86
in these chains. The jets with bright first members (the
fuzzball furthest from the blue spot) tend to have a bigger
gap between the first and second members. In the table
below they’ve measured the declinations of the first four
members of two jets. The first jet has the brighter first
member.
Galaxy
Order
First
Declination: First Jet
Declination: Second Jet
85.0
-87.0
Second
88.57
-89.40
Third
89.16
-89.67
Forth
89.41
-89.77
They have counted fuzzballs as a function of their
brightness. After calibrating their photographic map, they
came up with a list of over a million fuzzballs: all the
fuzzballs in the sky with fluxes greater than 10-19 W m-2 nm-1,
anywhere in the sky.
The approximate number of fuzzballs as a function of their
flux is listed in the table below.
Total Flux (W m-2 nm-1) as measured Number of Fuzzballs
on Zog.
Flux > 10-15
1
10-15 >Flux > 10-16
28
10-16 >Flux > 10-17
1,150
10-17 >Flux > 10-18
42,000
10-18 >Flux > 10-19
1,550,000
The number of bright fuzzballs (Flux > 10-17 W m-2 nm-1)
per unit area seems to be relatively uniform across the sky
(though they do seem to be concentrations of fuzzballs in a
few places). Fainter fuzzballs, however, are more common
near declination +90 and -90. Near declination zero, the
very faintest fuzzballs are only half as common as they are
at the celestial poles.
Carter and Thoris (Helium Institute)
These researchers managed to persuade the Space
Telescope Science Institute to take a really deep exposure of
a random part of the sky. A really deep exposure takes a lot of
Bubble time, so they were only given time to image one region
of the sky. Furthermore, their data was made generally
available to everyone as soon as it was taken: publicised as
the Bubble Deep Field.
40 orbits of Bubble time were used to image a small region of
the sky at right ascension 0, declination 0, through each of
three filters: B (0.39-0.5 m), V(0.45-0.55 m) and R (0.550.75 m). These were combined to produce a colour image of
this region.
The Bubble Deep Field: 120 orbits exposure with the Wide
Field Planetary Camera 2.
They have counted fuzzballs as a function of their
brightness. They then extrapolated their counts to the
whole sky, assuming that the average density of fuzzballs
in the BDF extends over the whole sky. Their field of view
is too small to measure the space density of brighter
galaxies, and the error bars on the number of galaxies in
the first row is large.
Total Flux (W m-2 nm-1) as measured Number of Fuzzballs
on Zog.
10-16 >Flux > 10-17
800
10-17 >Flux > 10-18
37,000
10-18 >Flux > 10-19
1,200,000
10-19 >Flux > 10-20
51,000,000
10-20 >Flux > 10-21
2,700,000,000
Verdi and Puccini (Venesia Instiute)
Hearing of the recent remarkable discovery of jets around
the North Blue Spot, this group used the William Herzchel
Telescope to get spectra of the fuzzballs in one of these jets.
They obtained spectra of four fuzzballs from one of the
biggest jets extending from the Northern Blue Spot, as
shown below.
B1
B2
B3
B4
Relative Flux
All four fuzzballs had similar spectra: spectra resembling
those of typical stars.
Observed Wavelength (nm)
The only significant differences between the spectra were
that the lines were shifted. All four fuzzballs were
blueshifted - the blueshifts are listed below.
Fuzzball name Blueshift
B1
0.001
B2
0.0043
B3
0.0077
B4
Nearby Stars
0.011
0
Strittmatter and Shu, Zteward Observatory
These two have led a consortium of 73 astronomers from
fifteen countries in doing a massive X-ray and radio survey of
the whole sky.
The radio observations were made with the Auztralia
Telescope Compact Array in the south, and the Very Large
Array in the north. Both groups combined to do an X-ray
survey of the whole sky using the XMM satellite (X-rays do not
penetrate the atmosphere).
The Compact Array
The VLA (Very Large Array)
The X-ray Multi-Mirror (XMM) satellite.
The radio maps
detected thousands of
sources, most of them
looking something like
this. Blue is an optical
image. Red is the radio
map: showing twin jets
extending away from a
small faint fuzzball.
Most sources have
radio fluxes of less than
half a Jansky. The one
spectacular exception is
fuzzball M12, which has
a colossal flux of 11
Janskys.
A Jansky is 10-26 W m-2Hz-1.
Here is an optical image of M12: far and away the most
powerful radio source in the sky. Looks much like a normal
fuzzball. It lies at coordinates RA 236.88, Dec 37.13.
In the radio it looks quite different, as can be seen in these
three images, taken at different resolutions. It seems to
have a jet of relativistic particles squirting out in both
directions.
The second most powerful X-ray and radio source in the
sky was Galaxy NFC64, an optically rather boring fuzzball
that had been observed with the BST by Group 1 in the
first round of observations. XMM detected 27 X-rays per
second from it.
It was also a double radio source, though the two jets were
of more similar brightness than those of M12.
The two blue spots were not strong X-ray or radio sources.
However, all the fuzzballs in one jet sticking out of the
Southern Blue Spot were strong X-ray and radio sources.
The same applies to the Northern blue spot: all the fuzzballs
in one jet sticking out of it were strong X-ray and radio
sources.
The Radio and X-ray Jet
The other jets radiating from the blue spots did not emit strong
radio or X-ray flux. No new jets were discovered, travelling in
any direction. Published images were checked, and this jet
seems similar to all the others optically. In the radio, all
sources in both chains are double radio sources, similar to
M12 and NFC64. All the radio axes point in the same way
(roughly perpendicular to the direction of the jets).
Name
A
B
C
D
E
Right
Ascension
236.88
236.88
236.88
236.88
236.88
Declination X-rays per
second
-81.26
5.5
-84.42
2.2
-85.90
1.2
-86.76
0.8
-87.33
0.5
Details of the Southern Radio/X-ray Jet
Here are the details of the Northern Jet. As with the Southern
Jet, the brightest source, which in both cases is the furthest
from the Blue Spot, is called ‘A’, and the others are numbered
in order as they approach the blue spots. There are many
more members of both jets - only those from which more than
0.5 X-rays per second are detected are listed.
Name
A
B
C
D
E
Right
Ascension
236.88
236.88
236.88
236.88
236.88
Declination X-rays per
second
+78.0
10.2
+83.2
3.3
+85.3
1.6
+86.4
0.9
+87.1
0.6
Details of the Northern Radio/X-ray Jet
De Canis et al.
This group have been slowly and painstakingly searching for
variable stars in the central regions of the Greater Milk Stain.
This is very difficult work as these stars are faint - the power
of the Very Large Telescope (VLT), with its four 8m mirrors
was required.
Stars pulsing with 2 hour periods were found.
They further seached for such pulsing stars in two of the
brightest fuzzballs in the sky: M23 and M86. This
observation required the Bubble Space Telescope. Once
again, they were successful in finding stars with 2 hour
pulsation periods.
The Very Large Telescope
Here is a table of the average peak brightness of the 2-hour
pulsing stars in the three targets.
Target
GMS
Average Peak V-band
Flux (Wm-2nm-1)
1.0x10-16
M23
2.1x10-22
M86
7.3x10-23