ALFALFA:
The Arecibo Legacy Fast ALFA
Extragalactic HI Survey:
(The Search for
Low-Mass, Gas-Rich Halos)
Martha Haynes (Cornell University)
for the ALFALFA team
ALFA
a car…
Itisisnot
a radio
“camera”
Arecibo L-band Feed Array
..on Arecibo 305m telescope
ALFALFA as a Legacy Survey
• One of several major surveys currently ongoing at
Arecibo, exploiting its new multibeam capability
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•
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An extragalactic spectral line survey
Covers 7000 sq deg of high galactic latitude sky
1345-1435 MHz (-2000 to +17500 km/s for HI line)
5 km/s resolution
2-pass, drift mode (total int. time per beam ~ 40 sec)
1.5-2 mJy rms
4000 hrs of telescope time, 6-7 years
started Feb 2005; as of end of 2006, 1/3 complete
http://egg.astro.cornell.edu/alfalfa
The Void Phenomenon
Peebles (2000)
•Cosmic voids are filled with
low mass dark matter haloes
vrot>55km/s
• ~1000 haloes with M < 109M
and vrot< 20 km/s in a 20 h-1 Mpc
void are predicted
Halo mass function in voids : Gottlöber et al (2003)
Luminosity function of void galaxies
• Void LF has a faint M*
but a similar faint-end
slope, compared to the
overall LF
• Void galaxies are blue,
disk-like and have high
H equivalent width
Void galaxies in the SDSS : Hoyle et al (2005)
The “Missing Satellite Problem”
• Models/simulations predict large numbers
of satellites
• Kauffmann et al. (1993)
• Klypin et al. (1999)
• The current census does not count them.
• Faint end slope of the optical LF
• Faint end slope of the HIMF
• Willman et al. (2005) suggest that
more than half of the MW satellites
have not been identified because of
extinction, surface brightness limits
• Nothing to detect?
• Baryon loss during reionization ( e.g., Efstathiou
1992; Barkana & Loeb 1999; Shaviv & Dekel 2003)
• Can they (ever) form stars? (Verde et al. 2002)
Dwarf galaxies
Leo A
Sagittarius
LGS3
•
•
•
•
•
•
•
dE, dSph, dIrr
Low mass: detected only nearby
Dark matter dominated
Low abundances
Stellar mass: 106-108 M
Blue Luminosity: 106-108 L (MB> -15)
Dynamical mass: 107-109 M
Where they are gas-rich:
• HI mass: 106-108 M
• Sometimes, extensive HI
• Evidence for dark matter
“Dwarf galaxies of the Local Group” Mateo 1998 ARAA
Carina
IZw18
SBS0335-052
Substructure in the Local Group
• Galaxies mainly clustered
around the two principal
galaxies MW & M31
• Morphological
segregation evident
• dE/dSph near large
galaxies
• dI at larger distances
Diagram from Grebel 1999
Giant spirals
dSph (+dEll)
dIrr
dIrr/dSph
The Search for Low Mass Halos
• Do large numbers of low mass “halos” exist?
• If so, do they contain baryons?
• If so, could they be “starless” but gas-rich?
=> Extragalactic HI survey
• If so, could they be found preferentially in some
environments but not in others?
=> “Fair sample”
ALFALFA is designed to detect low mass gas-rich halos
The HI Mass Function
• Previous surveys
have included few (if
any) objects with HI
masses less than 108
M.
?
• At lowest masses,
differ by 10X:
Rosenberg &
Schneider (2000)
versus
Zwaan et al. (1997)
Parkes HIPASS survey: Zwaan et al. 2003
Environment & the HIMF
Previous studies based only on Virgo have suggested that the
HIMF in Virgo is shallower than in the field
• Only a single cluster
• Very small number statistics
• Is this just HI deficiency?
• Watch out for morphological biases
Springob, Haynes & Giovanelli (2005)
• Much larger sample, but optically targeted
• Used PSCz density field out to 6000 km/s
• Low mass end of HIMF in high density regions flatter
and M* lower
• Cannot be explained simply by morphology or HI
deficiency.
Zwaan et al. (2005): HIPASS seems inconsistent
ALFALFA will provide needed statistics and be HI blind.
Previous surveys for HI in voids
for example:
• Weinberg et al 1991 (PPS & foreground void)
• Szoromu et al 1994
• Szomoru et al 1993 (HI in Bootes void galaxy)
• Szomoru et al 1996a (Bootes void)
• Szomoru et al 1996b
• Pustilnik et al 2002 (Blue compact galaxies)
HI in “famous” voids
VLA surveys:
• Szomoru et al (1996): Bootes void
HI in “famous” voids
VLA surveys:
• Szomoru et al (1996): Bootes void
HI in “famous” voids
VLA surveys:
• Szomoru et al (1994): PPS & foreground void
Void
PPS
# fields
30
12
Int. time/field
40 min
210 min
rms/beam
1 mJy
0.4 mJy
# detections
0
17
Serious limitations:
• Relatively small volume sampled
• Bandwidth only 6.25 MHz (1200 km/s)
• Velocity resolution ~42 km/s (too wide)
ALFALFA Survey
High
galactic
latitude sky
visible from
AO
• Commensal with
TOGS HI
• Does not compete
with galactic
plane surveys
Supergalactic plane
Virgo
Leo
ALFALFA Science Goals
1. Determination of the faint end of the HI Mass
Function and the abundance of low mass gas rich
halos
2. Environmental variation in the HI Mass Function
3. Blind survey for HI tidal remnants
4. Determination of the HI Diameter Function
5. The low HI column density environment of galaxies
6. The nature of HVC’s around the MW (and beyond?)
7. HI absorbers and the link to Ly  absorbers
8. OH Megamasers at intermediate redshift 0.16 < z < 0.25
ALFALFA as a Blind HI Survey
• HI mass and distribution (for
extended objects)
• Normal, star-forming disks
• Potential for future star
formation (HI content)
• HI deficiency in clusters
• History of tidal events
• Low mass, LSB dwarfs
• HI absorption: optical depth
• Link to Ly-α absorbers
• Redshifts
• Rotational velocities
• Dark matter
• Distances via Tully-Fisher
relation
HI in M31
Credit: R. Braun
∫ SdV
V
∆V
HI mass
Distance
Mass
ALFALFA as a Blind HI Survey
• HI mass and distribution (for
extended objects)
• Normal, star-forming disks
• Potential for future star
formation (HI content)
• HI deficiency in clusters
• History of tidal events
• Low mass, LSB dwarfs
• HI absorption: optical depth
• Link to Ly-α absorbers
• Redshifts
• Rotational velocities
• Dark matter
• Distances via Tully-Fisher
relation
Credit: A. Chung
ALFALFA as a Blind HI Survey
• HI mass and distribution (large
objects)
• Normal, star-forming disks
• Potential for future star
formation (HI content)
• HI deficiency in clusters
• History of tidal events
• Low mass, LSB dwarfs
• HI absorption: optical depth
• Link to Ly-α absorbers
• Redshifts
• Rotational velocities
• Dark matter
• Distances via Tully-Fisher
relation
Durrell &
DeCesar;
+Yun 1994
ALFALFA as a Blind HI Survey
• HI mass and distribution (large
objects)
• Normal, star-forming disks
• Potential for future star
formation (HI content)
• HI deficiency in clusters
• History of tidal events
• Low mass, LSB dwarfs
• HI absorption: optical depth
• Link to Ly-α absorbers
• Redshifts
• Rotational velocities
• Dark matter
• Distances via Tully-Fisher
relation
I Zw 18
van Zee et al. 1998
Are there totally “dark” galaxies?
Arecibo map outer extent [Hoffman et al. 1993]
DDO154
MH
= 2.5 x 108 M
Mstars = 5.0 x 107 M
MDyn = 3.0 x 109 M
Extent of
Optical image
Carignan & Beaulieu 1989
VLA D HI
HI 1225+01
Optical galaxy
M
> 200
L
Giovanelli, Williams & Haynes 1989
The ALFALFA Team
• ALFALFA is a collaboration of >50 people, from 34
institutions in 13 countries.
• ALFALFA is an open collaboration: anybody with a valid
scientific interest and a hardworking disposition can join.
• Legacy survey on a national
telescope used not only for
radio astronomy but also radar
studies of Solar System
objects and the Earth’s upper
atmosphere
• Heavy student involvement
• Broad institutional
representation
Participating Institutions
•Cornell University
•NAIC
•NRAO
•Lafayette College
•Union College
•Wesleyan University
•Harvard/Smithsonian CfA
•U. of Michigan
•Indiana U.
•Rutgers U.
•U. of Minnesota
•U. of Wisconsin
•St. Lawrence U.
•Humboldt St. U.
•Colgate U.
•Georgia Southern U.
•NRL
•NASA/GSFC
•U. of Crete
•U. of Tel-Aviv
•U. of Rome
•U. of Milan II
•Obs. Of Brera
•U. of Provence
•Arcetri Astrophys. Obs.
•Lab. D’Astrophysique/Marseille
•Cardiff U.
•U. de Barcelona
•U. de Cordoba (Argentina)
•NCRA/GMRT (India)
•P. U. Cat. de Chile
•Special Astrophys. Obs. (Russia)
•U. of Kiev
ALFALFA: a wide area survey
Beam dilution
Giovanelli et al.
2005, AJ130, 2598
To sample more
deeply, you have
to integrate A
LOT longer!
…
For low masses:
larger solid angle
ALFALFA strategy: Keep it simple!
ALFA beams are
3.3’x3.8’
“Almost” fixed
azimuth drift mode
2nd pass offset
from the first to
give 1.05’ sampling
Automated signal detection
●
Signal extraction in the Fourier domain by Amélie Saintonge
–
Match filter over a range of
widths of the template
●
–
–
e.g. 10 km/s – 600km/s
Choose the width for which
the convolution is maximized -> position of the signal
Calculate the amplitude of the
signal from the width
• Run on 3-D datacubes after completion of “tile”
• Once signals identified, further interactive analysis
Slide: Amelie Saintonge
Exploit VO tools during data processing
• Data processing tools
developed here at CU
are now running at 11
other institutions
• VO tools incorporated
to allow access to
external datasets
during data processing
• DSS, DSS2, Sloan,
2MASS, NVSS images
can be fetched
• NED and other on-line
catalogs, including our
own, can be accessed
and overplotted.
VO portal
• ALFALFA data &
products will be
made public as soon
as possible.
• Targeted HI survey
and precursor data
already available
• Remember: we need
a complete dataset –
both passes – to
make a grid.
• Issue is serving data
volume/local cpu:
“google ALFALFA”
http://arecibo.tc.cornell.edu/hiarchive
Current Status (by 15 Dec 2006)
Total # blocks
177
145
Total hours
1070
690
In map region
1010
625
07h30 – 16h30
22h00 – 03h00
+04 to +16 deg
+12 to +16 deg
+24 to +32 deg
• 1/3 of survey covered with 2 passes (by 15Jan07)
• Processing requires complete sky coverage
• Riccardo will present only 4% of final survey…
Comparison of blind HI surveys
Survey
Beam
arcmin
Area
rms
min MHI
Ndet
65
265
4315
(?)
31
129
sq. deg. (mJy @ 18 km/s) @ 10 Mpc
AHISS
ADBS
HIPASS
HIJASS
J-Virgo
HIDEEP
3.3
3.3
15.
12.
12
15
13
430
30,000
(TBD)
32
32
0.7
3.3
13
13
4
3.2
2.0x106
9.6x106
3.6x107
3.6x107
1.1x107
8.8x106
ALFALFA
3.5
7,000
1.7
4.4x106 20,000+
ts
sec
Nlos
var
17,000
12
500,000
460 1.9x106
3500 (TBD)
3500
3200
9000 2000
30
7x106
ALFALFA will be ~ 1 order of magnitude more
sensitive than HIPASS with 4X better angular
resolution, 3X better spectral resolution, and
1.6X total spectral bandwidth
ALFALFA Precursor
Giovanelli et al 2005 AstronJ 130, 2598 & 2613
* Aug-Sep 2004
* Candidate Detections Confirmation Run Jan-Feb 2005
* 36 hours of ALFA data
166 confirmed HI sources :
- 25 with HI mass > 1010 M
- 4 with HI mass < 107 M (twice as many as all of HIPASS)
- high positional accuracy => optical counterparts ID’d
- slightly better detection rate than expected (high side),
i.e. our ability to reliably dig in low S/N territory is high
- system hardware performance, “hands-off” bandpass
calibration and baselining (IDL processing pipeline) yield
EXCELLENT data quality
Precursor (ALFA commissioning phase) results
HIPASS Completeness Limit
HIPASS Detection Limit
HIPASS would have detected
only a handful; and none of
the low mass ones.
F
Integrated Flux of 1 Jy km/s
In 36 hours, we detected 4X more
lowest mass objects than all of HIPASS
First ALFALFA catalog
Giovanelli et al (2006, Astron. J. submitted)
Northern Virgo cluster region
11h44m < R.A. < 14h00m
+12º < DecJ < +16º
716 detections of good/excellent quality
Median redshift 7000 km/s
In the same sky region, HIPASS detected only 40 objects.
HIJASS Virgo survey: region of maximum sensitivity:
ALFALFA: 193 detections
HIJASS: 13
ALFALFA: Hunting in/around voids
ALFALFA is designed to hunt for low mass systems efficiently
• High sensitivity (Arecibo = 1/10th SKA!)
• Efficient: 97% “open shutter” time
• Large solid angle (7000 sq deg)
• Moderate angular resolution (~3.5’) => optical i.d. - or none!
• High spectral resolution (5 km/s)
• “Minimum intrusion” => high data quality
• Dual pol/2 pass coverage (confirmation)
• Automated signal detection
• HI mass function to low masses, and its environmental dependence
• HI detections + HST distances => shapes and kinematics of voids
Tikhonov & Karachentsev 2006 astro-ph/0609109
• Evolutionary history of isolated (never interacting!) galaxies
Riccardo will talk next, after tea