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 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. Galaxy Density ⇐ low, high ⇐ moderate ⇐ high (clusters) ⇐ moderate-high ⇐ moderate-high, any

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 M halo ≤ M  and in low density environments. To find cold mode accretion, must search in low density environments. Keres et al z=5.52 z=3.24 Green =cold Green =hot

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 R g – mean surface brightness < 26 mag/sq. arcsec ( 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 Similar global HIMF between ALFALFA & HIPASS Low M HI slope flattens as density decreases. Martin et al Zwaan et al. 2005

HIMF 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). Springob et al Freeland et al Kilborn et al. 2009

HIMF Similar global HIMF between ALFALFA & HIPASS Pisano et al. (2011) found that Local Group analogs had flat low mass slope. Martin et al Pisano et al. 2011

Circular Velocity Function Little evidence of variation with environment. Slightly lower in underdense regions. Papastergis et al Zwaan et al. 2010

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.

Fabello et al HI mass fraction Based on HI stacking of ALFALFA data for GASS galaxies. As M group (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 f gas, N detections 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 Total HI Intensity Contours on Optical 1,5,10,50,100 x cm -2. Pisano et al UGC IC 5078 UGC 260 NGC % (12/41) of isolated galaxies have companions or signatures of recent interaction/accretion.

CIG 340 (Scott et al. 2014)CIG 292 (Portas et al. 2011) CIG 85 (Sengupta et al. 2012) CIG 96 (Espada et al. 2005) AMIGA Isolated Galaxies 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 asymmetric” compared to 10-20% of field galaxies (Espada et al. 2011)

Problem with Profiles HI distributionHI profile It is impossible to unambiguously find signatures of interactions, asymmetries, or a companion with just an HI profile.

Kreckel et al. 2011, 2012 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.

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 N HI = cm -2. Filament has peak N HI = 2x10 18 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 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.

Verdes-Montenegro et al Compact Groups 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 cm -2 level in the Local Group Thilker et al. 2004; Braun & Thilker 2004N HI = 0.5 – 20 x cm -2

But is this really diffuse? 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. The clouds are at different velocities than M31’s HVCs and its dwarf companions, so they are a different phenomenon. Wolfe et al. 2013

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?