Observations of the evolution of
HI in galaxies across different
environments.
D.J. Pisano
(West Virginia University)
Why study galaxy environment?
• Nature vs. Nurture
– Can they be distinguished?
– What does the environment do to a galaxy?
– What would a galaxy look like without any
environmental influence?
• Study phenomena that strongly depend on the
environment.
– cold mode accretion, tidal interactions, ram
pressure stripping
Physical Processes affecting HI content
• Inflow
– Gas accretion (cold mode, hot mode)
– Minor/major interactions & mergers
• Consumption (star formation)
• Outflow
– fly-by encounters, tidal stripping
– ram-pressure stripping
– AGN, SF feedback
The importance of each process depends, in part, on
environment.
Physical Processes affecting HI content
• Inflow
– Gas accretion (cold mode, hot mode)
– Minor/major interactions & mergers
Galaxy Density
⇐low, high
⇐moderate
• Consumption (star formation)
• Outflow
– fly-by encounters, tidal stripping
– ram-pressure stripping
– AGN, SF feedback
⇐high (clusters)
⇐moderate-high
⇐moderate-high,
any
The importance of each process depends, in part, on
environment.
Hot/Cold Flows
• Many simulations predict that
gas is accreted by galaxies in two
forms (e.g. Birnboim & Dekel
2003, Keres et al. 2005, 2009).
• At z=0, hot mode accretion
should be dominant in high mass
halos, and in high density
environments.
• Cold mode should be dominant
for Mhalo ≤ 1011-12 M and in low
density environments.
• To find cold mode accretion,
must search in low density
environments.
z=5.52
Green =cold
z=3.24
Green =hot
Keres et al. 2005
How do we define environment?
Local Environment
• Isolated
– no companions brighter/more massive than a
certain level within a given distance.
• Galaxy pairs & triplets
• Compact Groups
– 4 or more galaxies within 3 mag of brightest
galaxy.
– no similar galaxies within 3 Rg
– mean surface brightness < 26 mag/sq. arcsec (300108 Mpc-2)
Global Environment
• “Field” vs. Cluster
• Voids vs. Filaments
• Voids  Groups (loose, poor, rich) 
Clusters
• Groups can be within both voids and clusters.
A galaxy has both a local & global environment.
Both the local and global environment affect
galaxy properties.
HI properties vs. Global
Environment
A statistical look:
• HI mass function
• Circular velocity function (halo mass
function).
• HI mass fraction, distribution
HI Mass Function
Martin et al. 2010
• Similar global HIMF between ALFALFA & HIPASS
• Low MHI slope flattens as density decreases.
Zwaan et al. 2005
HIMF
Freeland et al. 2009
Springob et al. 2005
• Springob et al. (2005) found that
the slope flattens in Virgo core.
• All other studies show flat low
mass slope in groups (of varying
densities).
Kilborn et al. 2009
HIMF
Martin et al. 2010
Pisano et al. 2011
• Similar global HIMF between ALFALFA & HIPASS
• Pisano et al. (2011) found that Local Group analogs had flat
low mass slope.
Circular Velocity Function
• Little evidence of variation with environment.
• Slightly lower in underdense regions.
Zwaan et al. 2010
Papastergis et al. 2011
Circular Velocity Function
• Similar shape in groups as well (Pisano et al. 2011,
confirmed by Abramson et al. 2014).
• Desai et al. (2004) found clusters follow the expected
CDM power law.
HI mass fraction
Fabello et al. 2012
• Based on HI stacking of
ALFALFA data for
GASS galaxies.
• As Mgroup (N) increases,
the HI mass fraction
decreases.
• Evidence for ram
pressure stripping in
moderate density
environments.
HI content in
groups
• Hess & Wilcots (2013) found that
as N increases, radius of HI
detections increases, implying
removal of HI in groups.
• Number of HI detections drops as
group mass increases.
• Kilborn et al. (2009) found that HI
deficiency in groups increases with
X-ray luminosity implying ram
pressure stripping important in
groups with X-ray IGM.
Hess & Wilcots 2013
Kilborn et al. 2009
HI properties vs. environment
• HI mass function appears to flatten as galaxy
density drops.
• Circular velocity (halo mass) function is
unaffected by environment; only agrees with
CDM predictions in clusters.
• Decrease in fgas, Ndetections and increase in HI
distribution in groups (with group mass)
suggests ram pressure stripping is occurring
even in moderate density environments.
Local vs. Global Environment
• Isolated vs. Void galaxies
• Compact groups vs. Loose Groups
• Different regions of Clusters
Isolated Galaxies
UGC 260
UGC 11152
29% (12/41) of isolated galaxies have companions or
signatures of recent interaction/accretion.
IC 5078
Total HI Intensity Contours on Optical
1,5,10,50,100 x 1019 cm-2.
NGC 6368
Pisano et al. 2003
AMIGA Isolated Galaxies
CIG 292 (Portas et al.
2011)
CIG 340 (Scott et al. 2014)
All of of the AMIGA isolated galaxies that have been
mapped show signatures of interactions (maybe
accretion?). Some of these are in small, low density
groups.
Only 2% of AMIGA HI profiles
are
“highly
CIG
96
(Espada
et al. 2005)
CIG 85 (Sengupta et al. 2012)
asymmetric” compared to 10-20% of field galaxies
(Espada et al. 2011)
Problem with Profiles
HI distribution
HI profile
It is impossible to unambiguously find signatures of
interactions, asymmetries, or a companion with just an HI
profile.
HI in void galaxies
Of the 55 void galaxies imaged by Kreckel et al. (2012),
about 50% have strongly disturbed HI morphology or
kinematics.
A much higher rate than seen in isolated galaxies.
This suggests an ongoing interaction or accretion from
the IGM.
Kreckel et al. 2011, 2012
NGC 6946
• NGC 6946 is a void galaxy in a
group. Local galaxy density is
0.07 Mpc-3 (Tully 1988)
• Optical in blue, WSRT HI data
in green, GBT HI data in red.
• GBT can detect HI down to NHI
= 1018 cm-2.
• Filament has peak NHI = 2x1018
cm-2 and FWHM = 48 km/s.
• The filament smoothly connects
in position and velocity with
NGC 6946 and companions.
• Some emission from filament
could be due to stray radiation.
• Filament could be a cold flow,
but is more likely to be a tidal
stream. No visible stellar
counterpart.
Pisano (2014)
Boomsma et al. (2008)
NGC 2403
• A group galaxy with local
galaxy density of 0.3 Mpc-3.
• Fraternali et al. 2001 found
an anomalous velocity HI
cloud in their WSRT data.
• Our GBT data (de Blok et al.
2014) reveal a more diffuse,
extended cloud connected
spatially and kinematically.
• This may have a tidal or
accretion origin.
Compact Groups
Verdes-Montenegro
et al. 2001
• As group assembles, HI
is removed from
galaxies.
• Either HI becomes
diffuse envelope in
group (e.g. Borthakur et
al. 2010, Stevens et al.
2004), or turns into hot
gas.
• Interactions are very
important.
Loose groups
• Loose groups are larger than compact groups (and could contain them).
• NGC 2997 group (Pisano et al. 2011; left) is a Local Group analog.
• USGC U451 group (Freeland et al. 2009; right) is a more massive X-ray
bright group containing two compact groups (NGC 4076, 4098).
HI at the 1017 cm-2 level
in the Local Group
NHI = 0.5 – 20 x 1018 cm-2
Thilker et al. 2004; Braun & Thilker 2004
But is this really diffuse?
Wolfe et al. 2013
The GBT data shows that this feature is much clumpier than
previous data suggested. We think that they are part of a
condensing intergalactic filament.
Cloud #4 may have a stellar counterpart (Martin et al. 2013) and
appears to be interacting with a CGM or IGM.Wolfe et al. 2013
It is not associated with nearby dwarf
galaxies.
Wolfe et al. 2013
The clouds are at different velocities than M31’s HVCs
and its dwarf companions, so they are a different
phenomenon.
VIVA observations of the Virgo Cluster
• HI deficient galaxies in center of cluster.
• Tidal interactions occur in outskirts of cluster.
• Signs of ram-pressure stripping as galaxies
pass through cluster.
• More details in the next talk.
Chung et al. 2009
Summary
• Studying different environments sheds light on different
physical processes.
• A good definition of the environment is important as both local
and global environment matter.
• Void galaxies show more signatures of interaction/accretion
than isolated galaxies.
• Signatures of accretion present around galaxies in voids and
low density groups.
• Evidence for processing of HI through tidal interactions and
ram pressure stripping in even moderate density groups.
Future Work
• What will surveys like WALLABY and
MHONGOOSE reveal about HI across
environments and at better sensitivity?
• Can we find conclusive evidence for cold
mode accretion in low density environments in
future surveys?