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

ALFALFA: The A recibo L egacy F ast ALFA Survey The Search for (Almost) Dark Galaxies and their Space Distribution Martha P. Haynes Cornell University, USA Venezia For the ALFALFA team Aug 2007

ALFALFA: A 2 nd generation HI survey • In comparison with opt/IR, the HI view is largely immature • HIMF based only only few thousand objects (HIPASS) 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 M HI

The HI Mass Function • Previous surveys N=1000 have included few (if ? any) objects with HI masses less than 10 8 M � . • At lowest masses, differ by 10X: Rosenberg & Schneider (2000) versus Zwaan et al. (1997) • Statistics • Systematics Parkes HIPASS survey: Zwaan et al. 2003

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

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/V max ) 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 M HI < 10 7.5 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/V max ) 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 7 h 30 m < R.A. < 16 h 30 m , +12° < Dec < +16° 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!

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/V max ) 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 M HI > 10 8 M � • ALFALFA is specifically designed (wide area, high velocity resolution) to detect hundreds of objects with M HI < 10 7.5 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 M HI < 7.5 than all other previous blind HI surveys combined

Lowest HI mass objects log M HI < 7.2 ALFALFA has already detected more objects with log M HI < 7.5 than all other previous blind HI surveys combined

Giovanelli et al 2005 AGC 112521 Saintonge et al 2007 H α R V WISE (N.Brosch) U B cz 274 km/s W 26 km/s F 0.65 Jy km/s D 7.2 Mpc (N672 group) WIYN0.9m (L. vanZee) Log M HI 6.9 M � log(O/H)+12 ~ 7.4 Radius ~ 400 pc • M HI /L ~ 2.2 so M HI ~ M * • GMRT HI map analysis underway Saintonge, Begum et al. ( in prep )

Clustering of HI galaxies ξ (r) for HIPASS • Meyer et al (2007): • HI rich galaxies extremely weakly clustered • Clustering scale depends on V rot • Basilakos et al (2007): • Massive HIPASS galaxies show same clustering as optically-selected sample • Low mass systems (M HI < 10 9 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? Inconsistency: • Watch out for morphological biases Symptom of inadequate volume? Kova č , 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

Environment & the HIMF 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 • Agreement between optically selected and HI blind HIMFs no worse than internal agreement among HI blind surveys Springob et al 2005 ApJ 621 215 • 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)

ALFALFA & the Void Problem Peebles (2000) • Cosmic voids are filled with low mass dark matter haloes Mare Nostrum simulation v rot >55km/s • ~1000 haloes with M < 10 9 M � and v rot < 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 α equivalent width Void galaxies in the SDSS : Hoyle et al (2005)

HI galaxies in “famous” voids VLA surveys: • Szomoru et al (1996): Bootes void M HI /L B = 1.1

HI in PPS + foreground void 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 for low mass halos)

(Very) preliminary ALFALFA result Amélie Saintonge, Ph.D. thesis, Cornell U. Saintonge et al. 2007, submitted • 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 • A prime science driver of the SKA is a HI “billion galaxy” survey (Abdalla & Rawlings 2004) • Previous HI surveys detect very few objects with M HI > 10 10 M � ; HIMF not well constrained at highest masses either. ALFALFA has already detected more than twice as many objects with log M HI > 10.4 than all other previous blind HI surveys combined

Highest mass objects: future SKA ALFALFA has already detected more than twice as many objects with log M HI > 10.4 than all other 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 M HI /L > 2 => M gas ~ M * – Some have M * > 3 x 10 10 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 M gas /M * ~ 0.01

ALFALFA: (Almost) Dark Galaxies With the current survey coverage, ALFALFA is seriously affected by LSS and statistics, but… • ALFALFA is detecting objects as low as log M HI ~6 and as high as log M HI ~ 10.8 => robust HIMF (T, ρ ), ξ (r) and b at z ~ 0 • ALFALFA will detect hundreds of objects with log M HI < 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 M HI > 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 optical SB => massive disks => future SKA surveys STAY TUNED……