ALFALFA: The A recibo L egacy F ast ALFA Survey The Search for - - PowerPoint PPT Presentation

ALFALFA:

The Arecibo Legacy Fast ALFA Survey

The Search for (Almost) Dark Galaxies and their Space Distribution

Martha P. Haynes Cornell University, USA For the ALFALFA team

Venezia Aug 2007

ALFALFA: A 2nd generation HI survey

ALFALFA:

• Designed to explore the HI mass function over a

cosmologically significant volume

• Higher sensitivity than previous surveys
• Higher spectral resolution => low mass halos
• Higher angular resolution => most probable optical

counterparts

• Deeper: 3X HIPASS median redshift => volume
• Wider area than surveys (other than HIPASS) =>

nearby volumes for lowest MHI

• In comparison with opt/IR, the HI view is largely

immature

• HIMF based only only few thousand objects (HIPASS)

The HI Mass Function N=1000

?

Parkes HIPASS survey: Zwaan et al. 2003

• 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)

• Statistics
• Systematics

HIMF Challenges

• Need better statistics: larger, more sensitive surveys
• At the faint end, all the galaxies are nearby
• Redshift distances are highly unreliable
• LSS affects accuracy of flow models

Masters, H & G 2004, ApJ 607 L115

• Need a “fair sample”
• Σ(1/Vmax) corrections must account for LSS
• Not just that space density varies with distance
• Fractional volume of space occupied by regions of a

particular density do too Springob, H & G 2005, ApJ 621, 215

Statistics, statistics, statistics

Rosenberg & Schneider 2002 Springob et al. 2005 (optically selected) N=2800 N= 265

HIMF Challenges

• Need better statistics: bigger surveys
• At the faint end, all the galaxies are nearby
• Redshift distances are highly unreliable
• LSS affects accuracy of flow models

Masters, H & G 2004, ApJ 607 L115

• Need a “fair sample”
• Σ(1/Vmax) corrections much account for LSS
• Not just that space density varies with distance
• Fractional volume of space occupied by regions of a

particular density do too Springob, H & G 2005, ApJ 621, 215

Problem with Virgo/foreground Distances

• Deficit of galaxies

with MHI < 107.5 M in Virgo

• Might some lie in

foreground?

HIMF Challenges

• Need better statistics: bigger surveys
• At the faint end, all the galaxies are nearby
• Redshift distances are highly unreliable
• LSS affects accuracy of flow models

Masters, H & G 2004, ApJ 607 L115

• Need a “fair sample”
• Σ(1/Vmax) corrections much account for LSS
• Not just that space density varies with distance
• Fractional volume of space occupied by regions of a

particular density do also Springob, H & G 2005, ApJ 621, 215

ALFALFA Today

If we covered a similar slice in the opposite part of the sky (coming….) we would see a very DIFFERENT redshift distribution => LSS!!! At these distances, 540 square degrees is not enough.

Sorry, folks. We are not done yet! 7h30m < R.A. < 16h30m, +12° < Dec < +16°

HIMF Challenges

• Need better statistics: bigger surveys
• At the faint end, all the galaxies are nearby
• Redshift distances are highly unreliable
• LSS affects accuracy of flow models

Masters, H & G 2004, ApJ 607 L115

• Need a “fair sample”
• Σ(1/Vmax) corrections much account for LSS
• Not just that space density varies with distance
• Fractional volume of space occupied by regions of a

particular density do also Springob, H & G 2005, ApJ 621, 215

=> => ALFALFA! (…but not yet!)

ALFALFA & “missing satellites”?

• HIPASS result: no cosmologically significant

population of HI-rich dark galaxies: ALFALFA agrees… but HIPASS MHI > 108 M

• ALFALFA is specifically designed (wide area, high

velocity resolution) to detect hundreds of objects with MHI < 107.5 M – Low HI mass – Narrow HI line width + exclude face-on objects – Will only be detected nearby ALFALFA has already detected more objects with log MHI < 7.5 than all other previous blind HI surveys combined

Lowest HI mass objects

ALFALFA has already detected more objects with log MHI < 7.5 than all other previous blind HI surveys combined

log MHI < 7.2

WISE (N.Brosch)

Hα R V B U Saintonge, Begum et al. (in prep)

WIYN0.9m (L. vanZee)

log(O/H)+12 ~ 7.4

AGC 112521

Radius ~ 400 pc

• MHI/L ~ 2.2 so MHI ~ M*
• GMRT HI map analysis underway

cz 274 km/s W 26 km/s F 0.65 Jy km/s D 7.2 Mpc (N672 group) Log MHI 6.9 M

Giovanelli et al 2005 Saintonge et al 2007

Clustering of HI galaxies

ξ(r) for HIPASS

• Meyer et al (2007):
• HI rich galaxies extremely weakly clustered
• Clustering scale depends on Vrot
• Basilakos et al (2007):
• Massive HIPASS galaxies show same clustering as
• ptically-selected sample
• Low mass systems (MHI < 109 M) show nearly uniform

distribution Inconsistency: Symptoms of inadequate volume?

Environment & the HIMF

Previous studies based only on Virgo have suggested that the HIMF in Virgo is flatter than in the field

• Only a single cluster
• Very small number statistics/systematics vs

comparison

• Is this just HI deficiency?
• Watch out for morphological biases

Kovač, Oosterloo & van der Hulst (2005): CanVen

• Similar to Virgo (low mass slow flatter)

BUT…….. Zwaan et al. (2005): HIPASS

• Higher density regions => more low masses

Inconsistency: Symptom of inadequate volume?

Springob, Haynes & Giovanelli (2005)

• Much larger sample, optically targeted
• HI flux and diameter limited subsample

(N = 2200 objects)

• PSCz density field out to 6000 km/s
• Low mass end of HIMF in high density

regions flatter and M* lower

• Cannot be just morphology or HI

deficiency Environment & the HIMF

• Agreement between optically selected and

HI blind HIMFs no worse than internal agreement among HI blind surveys

• Need larger sample to discriminate whether

HIMF shape is dependent on morphology and environment separately (as done for 2dFGRS LF, e.g. Croton et al 2006)

Springob et al 2005 ApJ 621 215

Peebles (2000)

Halo mass function in voids : Gottlöber et al (2003)

• Cosmic voids are filled with

low mass dark matter haloes

• ~1000 haloes with M < 109M

and vrot< 20 km/s in a 20 h-1 Mpc void are predicted

vrot>55km/s

ALFALFA & the Void Problem

Mare Nostrum simulation

Void galaxies in the SDSS : Hoyle et al (2005)

• Void LF has a faint M*

but a similar faint-end slope, compared to the

• verall LF
• Void galaxies are blue,

disk-like and have high Hα equivalent width

Luminosity function of void galaxies

HI galaxies in “famous” voids

VLA surveys:

• Szomoru et al (1996): Bootes void

MHI/LB = 1.1

HI in PPS + foreground void

VLA surveys:

• Szomoru et al (1994): PPS & foreground void

17 # detections 0.4 mJy 1 mJy rms/beam 210 min 40 min

• Int. time/field

12 30 # fields PPS Void Serious limitations:

• Relatively small volume sampled
• Bandwidth only 6.25 MHz (1200 km/s)
• Velocity resolution ~42 km/s (too wide for

low mass halos)

Amélie Saintonge, Ph.D. thesis, Cornell U. Saintonge et al. 2007, submitted

(Very) preliminary ALFALFA result

• 460 Mpc-3 in PPS foreground void at v~2200 km/s
• Simulations of Gottlöber et al. (2003) with

dark:HI = 10:1 predict 38 HI sources

• ALFALFA finds no objects

But only 2% of ALFALFA volume STAY TUNED…..

Highest mass objects: future SKA

ALFALFA has already detected more than twice as many objects with log MHI > 10.4 than all

• ther previous blind HI surveys combined
• A prime science driver of the SKA is a HI

“billion galaxy” survey (Abdalla & Rawlings 2004)

• Previous HI surveys detect very few objects

with MHI > 1010 M; HIMF not well constrained at highest masses either.

Highest mass objects: future SKA

ALFALFA has already detected more than twice as many objects with log MHI > 10.4 than all

• ther previous blind HI surveys combined

Mass assembly: gas content

• Highest HI mass ALFALFA detections show a range of

morphologies/optical surface brightnesses – (Most) appear to be luminous disk systems – Some have MHI/L > 2 => Mgas ~ M* – Some have M* > 3 x 1010 M (“transition mass”) – Fraction of AGN (TBD)

• Direct measure of gas content in z ~ 0 “transition objects”

– High mass objects – GASS (GALEX-Arecibo-SDSS survey: Schminovich et al.)

• 1000 galaxies, chosen by colors, spectra
• 0.025 < z < 0.06 (matches ALFALFA range)
• Low gas mass fraction Mgas/M* ~ 0.01

ALFALFA: (Almost) Dark Galaxies

• ALFALFA is detecting objects as low as log MHI ~6 and as high as log

MHI ~ 10.8 => robust HIMF (T,ρ), ξ(r) and b at z ~ 0

• ALFALFA will detect hundreds of objects with log MHI < 8
• How many are there?
• Where are they?
• Why are they? M/L, SFR, SFH
• Only 3% of all extragalactic sources and <1% of HI detections with

log MHI > 9.5 cannot be identified with a stellar component

• In one nearby void (2% ALFALFA), too few void galaxies
• Highest HI mass objects also exhibit range of morphologies and
• ptical SB => massive disks => future SKA surveys

STAY TUNED……

With the current survey coverage, ALFALFA is seriously affected by LSS and statistics, but…

http://arecibo.tc.cornell.edu/hiarchive http://egg.astro.cornell.edu/alfalfa

ALFALFA sprouts!