1. ALFALFA and the Hunt for Dark Galaxies
V < 500 km/s
600 < V < 1300
1400 < V < 2000
V > 2000
X-ray image of the Virgo cluster of galaxies in the hard band from the ROSAT All-Sky Survey (RASS). This nearby cluster, at a distance of about 20 Mpc, extends over a large region in the sky and can be traced in X-rays in the RASS for about 8 degrees in north-south and 5 degrees in east-west direction. The cluster consist of a compact, X-ray luminous core region centered on M87 and an irregular extended part which shows the cluster as a dynamically young object.
Chung et al 2005,
Bohringer et al. 1994
Kristine Spekkens
ALFALFA Undergrad Workshop – July 13th, 2006

2. ALFALFA and the Hunt for Dark Galaxies
Outline:
Why we expect them, how we detect them
Hunting for HI-rich, low-mass galaxies with ALFALFA
Case studies: VirgoHI21 and a new HI cloud complex in the Virgo Cluster
V < 500 km/s
600 < V < 1300
1400 < V < 2000
V > 2000
X-ray image of the Virgo cluster of galaxies in the hard band from the ROSAT All-Sky Survey (RASS). This nearby cluster, at a distance of about 20 Mpc, extends over a large region in the sky and can be traced in X-rays in the RASS for about 8 degrees in north-south and 5 degrees in east-west direction. The cluster consist of a compact, X-ray luminous core region centered on M87 and an irregular extended part which shows the cluster as a dynamically young object.
Chung et al 2005,
Bohringer et al. 1994
Kristine Spekkens
ALFALFA Undergrad Workshop – July 13th, 2006

3. “Dim” and “Dark” Galaxies
MALIN 1
Most of our galaxy catalogs stem from optical surveys…
Is there a significant population of “dim” or “dark” galaxies that have been missed?
David Malin

4. Do We Expect Dark Galaxies?
What’s in the Universe (today):
NASA/WMAP Science Team
Galaxy and cluster growth:
Virgo Consortium
“cold dark matter” drives galaxy formation and evolution

5. Do We Expect Dark Galaxies?
Number density of “cold
dark matter” halos
cold dark matter predicts large numbers of low-mass dark galaxies
Low mass
Log(Number density)
Number density of
galaxies detected in the optical
High mass
Marinoni & Hudson 2002
Luminosity

6. Detecting Dark Galaxies
Gravitational lensing
J.P. Kneib
RESULT:
Strong and weak lensing models constrain satellite distribution in clusters (Natarajan & Springel 2004)
Good agreement with theory at higher masses
number
inferred from lensing
theory
Natarajan & Springel 2004
Comparison of the substructure mass function of the stacked clusters A2210, A2390, and Cl0054 (shaded histogram) with N-body simulations of a rich cluster of galaxies (filled histogram), carried out at four different numerical resolutions.
Tadpole galaxy - A spiral galaxy that has been grossly distorted by a small interloper – a nearby, blue compact galaxy visible. The Tadpole lies about 420 million light-years away in the constellation Draco. Strong gravitational forces from the interaction have created an incredibly long tail of stars and gas stretching out more than 280,000 light-years. Numerous young blue stars and star clusters, spawned by the galaxy collision, are seen in the spiral arms, as well as in the long tidal tail of stars. Each of these clusters contains up to a million stars and will redden with age to become in time globular clusters similar to those found in essentially all halos of large galaxies, including our own Milky Way. Two prominent clumps of young bright blue stars are visible in the tidal tale and are separated by a gap. These will likely become dwarf galaxies that orbit in the Tadpole's halo. See also galaxy interaction.
- Caption: Artist's Impression of the brown dwarf Gl 229B (with the low mass star Gl 229A) sitting about 40 AU away. This brown dwarf is a few billion years old, with a surface temperature of about 1000K. Its mass is in the jupiter mass range (or 3-5% of our Sun). The dull red color is what your eye would see, there is more luminosity coming out in the near infrared. The dark bands are hypothetical; although dust does form in the atmospheres of brown dwarfs, it is unclear how non-uniform their appearance is. The banding is reasonable, since they usually rotate very fast (in hours; like Jupiter). A couple of magnetic "spots" are also shown; there is evidence of magnetic fields at least on the younger brown dwarfs (although it doesn't give rise to the same atmospheric heating and activity as on stars). Gl 229B is about 100,000 times fainter than our Sun (and almost 1000 times fainter than its low mass stellar companion).

7. Detecting Dark Galaxies
Gravitational lensing
J.P. Kneib
Isolated interacting galaxies
RESULT:
Very few (0.5%) optically isolated galaxies are disturbed (Karachentsev et al. 2005)
BUT: can’t always see small companions…
Tadpole galaxy - A spiral galaxy that has been grossly distorted by a small interloper – a nearby, blue compact galaxy visible. The Tadpole lies about 420 million light-years away in the constellation Draco. Strong gravitational forces from the interaction have created an incredibly long tail of stars and gas stretching out more than 280,000 light-years. Numerous young blue stars and star clusters, spawned by the galaxy collision, are seen in the spiral arms, as well as in the long tidal tail of stars. Each of these clusters contains up to a million stars and will redden with age to become in time globular clusters similar to those found in essentially all halos of large galaxies, including our own Milky Way. Two prominent clumps of young bright blue stars are visible in the tidal tale and are separated by a gap. These will likely become dwarf galaxies that orbit in the Tadpole's halo. See also galaxy interaction.
- Caption: Artist's Impression of the brown dwarf Gl 229B (with the low mass star Gl 229A) sitting about 40 AU away. This brown dwarf is a few billion years old, with a surface temperature of about 1000K. Its mass is in the jupiter mass range (or 3-5% of our Sun). The dull red color is what your eye would see, there is more luminosity coming out in the near infrared. The dark bands are hypothetical; although dust does form in the atmospheres of brown dwarfs, it is unclear how non-uniform their appearance is. The banding is reasonable, since they usually rotate very fast (in hours; like Jupiter). A couple of magnetic "spots" are also shown; there is evidence of magnetic fields at least on the younger brown dwarfs (although it doesn't give rise to the same atmospheric heating and activity as on stars). Gl 229B is about 100,000 times fainter than our Sun (and almost 1000 times fainter than its low mass stellar companion).

8. Detecting Dark Galaxies
Gravitational lensing
J.P. Kneib
Isolated interacting galaxies
Brown dwarfs/white dwarfs
- Isn’t feasible yet (need ~2 days
on-source!)
Tadpole galaxy - A spiral galaxy that has been grossly distorted by a small interloper – a nearby, blue compact galaxy visible. The Tadpole lies about 420 million light-years away in the constellation Draco. Strong gravitational forces from the interaction have created an incredibly long tail of stars and gas stretching out more than 280,000 light-years. Numerous young blue stars and star clusters, spawned by the galaxy collision, are seen in the spiral arms, as well as in the long tidal tail of stars. Each of these clusters contains up to a million stars and will redden with age to become in time globular clusters similar to those found in essentially all halos of large galaxies, including our own Milky Way. Two prominent clumps of young bright blue stars are visible in the tidal tale and are separated by a gap. These will likely become dwarf galaxies that orbit in the Tadpole's halo. See also galaxy interaction.
- Caption: Artist's Impression of the brown dwarf Gl 229B (with the low mass star Gl 229A) sitting about 40 AU away. This brown dwarf is a few billion years old, with a surface temperature of about 1000K. Its mass is in the jupiter mass range (or 3-5% of our Sun). The dull red color is what your eye would see, there is more luminosity coming out in the near infrared. The dark bands are hypothetical; although dust does form in the atmospheres of brown dwarfs, it is unclear how non-uniform their appearance is. The banding is reasonable, since they usually rotate very fast (in hours; like Jupiter). A couple of magnetic "spots" are also shown; there is evidence of magnetic fields at least on the younger brown dwarfs (although it doesn't give rise to the same atmospheric heating and activity as on stars). Gl 229B is about 100,000 times fainter than our Sun (and almost 1000 times fainter than its low mass stellar companion).

9. Detecting Dark Galaxies
Neutral hydrogen
Gravitational lensing
J.P. Kneib
Isolated interacting galaxies
Brown dwarfs/white dwarfs
Tadpole galaxy - A spiral galaxy that has been grossly distorted by a small interloper – a nearby, blue compact galaxy visible. The Tadpole lies about 420 million light-years away in the constellation Draco. Strong gravitational forces from the interaction have created an incredibly long tail of stars and gas stretching out more than 280,000 light-years. Numerous young blue stars and star clusters, spawned by the galaxy collision, are seen in the spiral arms, as well as in the long tidal tail of stars. Each of these clusters contains up to a million stars and will redden with age to become in time globular clusters similar to those found in essentially all halos of large galaxies, including our own Milky Way. Two prominent clumps of young bright blue stars are visible in the tidal tale and are separated by a gap. These will likely become dwarf galaxies that orbit in the Tadpole's halo. See also galaxy interaction.
- Caption: Artist's Impression of the brown dwarf Gl 229B (with the low mass star Gl 229A) sitting about 40 AU away. This brown dwarf is a few billion years old, with a surface temperature of about 1000K. Its mass is in the jupiter mass range (or 3-5% of our Sun). The dull red color is what your eye would see, there is more luminosity coming out in the near infrared. The dark bands are hypothetical; although dust does form in the atmospheres of brown dwarfs, it is unclear how non-uniform their appearance is. The banding is reasonable, since they usually rotate very fast (in hours; like Jupiter). A couple of magnetic "spots" are also shown; there is evidence of magnetic fields at least on the younger brown dwarfs (although it doesn't give rise to the same atmospheric heating and activity as on stars). Gl 229B is about 100,000 times fainter than our Sun (and almost 1000 times fainter than its low mass stellar companion).

10. Hunting for Dark Galaxies in HI
Low surface brightness galaxies are gas-rich
Feasible to go to low halo masses
WHY?
Zwaan et al. 2003
PLOT: top: HI mass as a function of distance for optically selected Fisher-Tully catalog (1981). Solid black line: HI detection limit for FT sample.
Dashed line: completeness limit for M<10^8.2Mo : 10 Mpc. N=number gals, omega=solid angle. Bottom two panels are same plots but for surveys targeting LSB galaxies: all lie below the detection limit for local survey, implying that there is not a significant population of dim galaxies nearby.

11. Searching for HI clouds in Virgo
High concentration of galaxies  better chance of finding a dark one
BUT: Virgo is a rough place for HI clouds!
Worry about:
Tidal interactions
Mergers
Ram pressure stripping
“Harassment” (lots of little, fast interactions)
X-ray image of the Virgo cluster of galaxies in the hard band from the ROSAT All-Sky Survey (RASS). This nearby cluster, at a distance of about 20 Mpc, extends over a large region in the sky and can be traced in X-rays in the RASS for about 8 degrees in north-south and 5 degrees in east-west direction. The cluster consist of a compact, X-ray luminous core region centered on M87 and an irregular extended part which shows the cluster as a dynamically young object.

12. Hunting for dark galaxies: VirgoHI21
You are here
X-ray image of the Virgo cluster of galaxies in the hard band from the ROSAT All-Sky Survey (RASS). This nearby cluster, at a distance of about 20 Mpc, extends over a large region in the sky and can be traced in X-rays in the RASS for about 8 degrees in north-south and 5 degrees in east-west direction. The cluster consist of a compact, X-ray luminous core region centered on M87 and an irregular extended part which shows the cluster as a dynamically young object.

13. Hunting for dark galaxies: VirgoHI21
Slide from B. Kent

14. Hunting for dark galaxies: VirgoHI21
Single dish detection VIRGOHI21: dark galaxy?
Minchin et al. 2005
Slide from B. Kent

15. Hunting for dark galaxies: VirgoHI21
Image at higher resolution… part dark galaxy, part tidal feature?
Minchin et al (preprint)
Slide from B. Kent

16. ALFALFA detects VIRGOHI21
Slide from B. Kent

17. ALFALFA detects VIRGOHI21
ALFALFA detects a huge stream connecting VIRGOHI21 and
NGC 4254
Slide from B. Kent

18. A New HI Complex in Virgo
X-ray image of the Virgo cluster of galaxies in the hard band from the ROSAT All-Sky Survey (RASS). This nearby cluster, at a distance of about 20 Mpc, extends over a large region in the sky and can be traced in X-rays in the RASS for about 8 degrees in north-south and 5 degrees in east-west direction. The cluster consist of a compact, X-ray luminous core region centered on M87 and an irregular extended part which shows the cluster as a dynamically young object.
You are here

19. A New HI Complex in Virgo
ALFALFA Spring 2005 run:
150 kpc
30’
At Virgo distance, 1’=5kpc
Giovanelli et al 2006

20. A New HI Complex in Virgo
OPTICAL
Uncatalogued A1
ALFALFA Spring 2005 run:
150 kpc
30’
Giovanelli et al 2006

21. A New HI Complex in Virgo
ALFALFA Spring 2005 run:
150 kpc
30’
VCC 1357
Faint!
OPTICAL A2
150 kpc
30’
Giovanelli et al 2006

22. A New HI Complex in Virgo
ALFALFA Spring 2005 run:
optical
Cortes et al. 2006
150 kpc
30’
A6 = NGC 4424
Disturbed stellar and gaseous morphologies
Need both a merger and a cluster “wind” to explain this HI
Chung 2005 2’
150 kpc
30’
Giovanelli et al 2006

23. A New HI Complex in Virgo
Follow up with higher resolution observations…
150 kpc
30’
Two detections: A1 and A2
- A6 is 210 kpc in projection away from the main clump, ~100km/s in velocity
Giovanelli et al 2006

24. Disordered morphology, kinematics
HI Morphology – A1
HI Intensity
Velocity
1020 cm-2
1.2
2.4
3.6
km/s
Disordered morphology, kinematics

25. Coherent motion across A2?
HI Morphology – A2
Intensity
Velocity
1020 cm-2
1.0
1.5
1.9
2.4
km/s
Coherent motion across A2?

26. HI Morphology – A2
7 kpc
95% zoom in acrobat A2

27. What could it be?
X-ray image of the Virgo cluster of galaxies in the hard band from the ROSAT All-Sky Survey (RASS). This nearby cluster, at a distance of about 20 Mpc, extends over a large region in the sky and can be traced in X-rays in the RASS for about 8 degrees in north-south and 5 degrees in east-west direction. The cluster consist of a compact, X-ray luminous core region centered on M87 and an irregular extended part which shows the cluster as a dynamically young object.

28. A New HI Complex in Virgo
Gas stripped from NGC 4424?
Group of dwarf galaxies?
Dark galaxies?
150 kpc
30’
At Virgo distance, 1’=5kpc
Giovanelli et al 2006

29. Conclusions
The “cold dark matter paradigm” predicts many dark, low mass galaxies
They might be detectable in HI with ALFALFA
ALFALFA is finding many interesting low-mass clouds…
The hunt for dark galaxies is underway!
B. Kent
A. Saintonge
S. Stierwalt