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Does Gas Cool From the Hot Phase?(onto galaxies)
The MPA/ESO/MPE/USM 2007 Joint Astronomy Conference
Gas Accretion and Star Formation in Galaxies
Joel Bregman (Univ. of Michigan)
Hot Gas in/around Ellipticals: Lessons Learned
Hot Gas Environment of the Milky Way and Local Group: Limits on Accretion
Missing Baryons in Galaxies and Galaxy Groups
Let’s Look at Galaxies with a lot of Hot Gas that is Cooling
• Gas masses of 1E8-1E10 Msun• T = 3-8 E6 K; Lx ~ 1E41 erg/s• Cooling rate of 0.1-1 Msun/yr
This is known as
Cooling Flows
X-ray contours
How to Speak “Cooling Flow”
• Two types of hot gas situations– Hot gas in clusters of galaxies– Hot gas in early-type galaxies
• Clusters of Galaxies – Most of the baryons are gaseous (not galaxies)– 2-10x107 K– Cooling: Free-Free (X-Rays)– Last Millennia: cooling rates 100-1000 Msun/yr– This Millennia: cooling rates of 1-30 Msun/yr
The last Cooling Flow Meeting
Cooling Flow Ellipticals
• Most Ellipticals are not X-ray bright– Bright ones in groups/clusters (1041 erg/s)
• “Lots” of hot gas (~109.5 Msun)• Gas is bound• X-rays mainly due to line emission
– X-ray faint galaxies (1040 erg/s)• LMXBs + a little hot gas (~108 Msun)• Galactic Winds
• Bright Ellipticals are well-studied
Metallicity of the Hot Gas
X-Ray Observations (XMM-Newton)
Fe is about Solar ([Fe] = 0)
[O/Fe] = -0.3
Metallicity like stars
Not like Cluster/group [Fe] = -0.5
Gas from Stellar Mass Loss
Not dominated by accretion from cluster/group
XMM RGS spectrum of NGC 4636 (Xu et al. 2003)
Does this Gas Cool?• Observe OVIII (hot, ambient;
5E6 K)• OVI often detected in X-ray
bright galaxies– 3E5 K gas; evidence for cooling
from hotter gas
Far Ultraviolet Spectroscopic Explorer (RIP)
OVI is detected in about 40% of galaxies
Some cooling gas at 3x105 K
Cooling rates of 0.1-0.5 Msun/yr
In central region; not distributed.
Bregman et al. (2005)
Where Does The Cooled Gas Go?
• Es in the RSA Catalog (Hogg et al. 1990; Bregman et al. 1992)
• 104 K gas detected – Same metallicity as stars and hot gas– Not much mass (< 1E5 Msun)
• HI and H2 rarely detected– M(HI) < 3E7 Msun for many ellipticals; 5% detection rate– M(H2 ) < 2E8 Msun; 0% detected– Limits the mass of HI HVC
• Low levels of star formation, if present– Probably less than 0.1 Msun/yr
Lessons Learned From Ellipticals
• Ellipticals appear to have much more hot gas than Spirals (total ISM mass similar)
• Cooling flows in Es don’t lead to detectable masses of cooled HI (< 3E7 Msun)
• Not much evidence for accretion from surrounding group/cluster medium
• Radially distributed cooling should not occur– Local Thermal Instabilities suppressed in near-
hydrostatic situations (Balbus 1995)– Even most non-linear disturbances are suppressed
(Reale et al. 1991)
Hot Gas Around Spirals and in Galaxy Groups
• Stellar evolution models for the Galaxy– Need to resolve the G-dwarf problem (Z > 0.2)
– Accrete 1-2 Msun/yr of gas with [Fe/H] < -3
• Lx = 4E40 (Mdot/1Msun/yr) (T/3E6 K) erg/s
• Milky Way has Lx ~ 1039.3 erg/sec
• Other similar spirals have Lx ~ 1039.5 erg/s
• No obvious support for accretion with rates exceeding 0.1 Msun/yr
NGC 891 in X-raysChandra ACIS-S; 108 ksec of cleaned data
Point sources removed; smoothed
Hot gas easily seen to a height of 4.5 kpc from disk (1.6’) Fainter emission goes to a height of 9 kpc (3’)
Extent along disk similar to Halpha, FIR
Oosterloo et al. 2007; NGC 891 HI
Chandra 0.3 keV gas; 8’ box
Thermal emission; 4E39 erg/s
Mdot = 0.1 Msun/yr
Give Up The Idea Of Accreting Pristine Gas Onto MW
• Every Galaxy Group with good X-ray data– 0.0 > [Fe/H] > -0.7
• Sightlines out of the MW show OVII and OVIII in absorption (metals are present)
• Need to adopt a different approach to solving the G-dwarf problem– pollute the gas with metals before accretion
(Binney and Merrifield; Galactic Astronomy)
A Census of 106 -107 K Gas in the Milky Way and Local Group
Z = 0.1 Solar
N = 1019 cm-2
OVII, OVIII have X-ray resonance lines; best sensitivity
Detect Hot Gas by X-ray Absorption Lines
Galactic Halo Model: distribution largely spherical around the MWcolumn densities similar in all directionsmight see evidence for the shape of the Galaxy
Local Group ModelLocal Group is elongated along the MW-M31 axis
columns greater along this axis and especially toward M31 (the long way through the LG)
Concern: M31 may have its own extended halo
Discriminating Between a Galactic Halo and Local Group Model
Group simulation like the Local Group (Moore) shows elongation of matter .
Column enhancement along major axis ~2-3x perpendicular to axis
26 Target AGNs; mean EW = 22 mÅ; 17 with rms < 10 mÅ
These are the 4 best.
Bregman and Lloyd-Davies (2007; arXiv:0707.1699)
Toward Bulge
An AGN toward M31 (long axis of Local Group)
One of the smallest columns, not one of the largest
Local Group Model Prediction
The OVII data don’t fit a Local Group Model
Prediction of Local Group Model
A Galactic (Halo) Model Works Better
Central Line Velocities close to MW
Correlation with Galactic ¾ keV X-Ray Background
Supports a Galatic Halo Origin (10-100 kpc)
Gas Mass of 108 – 1010 Msun
Compare to NGC 5746 (Rasmussen & Ponman 2004): 109 Msun for similar metallicity
Other Evidence for Hot Gas In Local Group
• Nearby LG dwarfs have no gas but distant ones have gas (Blitz & Robishaw 2000; Grcevich et al. 2007)
– Ram pressure stripping – n = 2.5E-5 cm-3 at d = 200 kpc (less than
column inferred from OVII line by 4x)
– 1E10 Msun of gas
– Cooling time longer than Hubble time
X-Ray Shadowing: CHVC and a Magellanic Stream Cloud
This would reveal the fraction of X-ray emission beyond these clouds
Help determine the hot gas component of the Local Group
JNB, Birgit Otte, J. Irwin, M. Putman, E. Lloyd-Davies, C. Breuns (2007)
We see a brightening, not a shadow toward both clouds.
Clouds are interacting with a hot medium within 100 kpc of the MW.
Density/Temp of the hot medium is model-dependent.
Mini-Summary
• There is hot gas around the Milky Way and in the Local Group– Masses are not large (0.1-10E9 Msun)– Cooling times are long (3E8-1E10 yr)
• Observed Lx is consistent with only 0.05 Msun/yr of accretion onto MW (and other spirals)
• HVC probably not dominated by condensations from the dilute hot gas
When the Demons Visit at Night• Where did the Baryons go?
– Cosmological value is 17%– Rich Clusters have not lost their baryons– Spirals (like MW) are missing 2/3 of baryons– Galaxy groups (T < 1 keV) also missing most of their baryons within r1000
• Good News – Bad News– Lots of gas available for accretion– … but it’s nowhere in sight (unbound)
• Make the galaxy, then blow out the baryons in a “superwind” (this must occur, but when?)– Pollutes the surroundings– Need to drive the gas way away from galaxy (to the outer parts of
the groups)
• Other Possibility: The Gas Never Fell In– Entropy floor (preheating is 0.4 keV; 5E6K)
• Need about 1 SNe per 500 Msun of gas
– Enrich the metals by distributed SNe (Pop III)• 0.2 Solar metals is also 1 SNe per 500 Msun of gas
– Naturally solves the G-dwarf problem (accrete enriched gas to make the disk)
– Need to form some parts of galaxy before preheating (halo stars; dwarf galaxies)
– Predict that this SNe heating occurs at z > 2 (Pop III ?)• Disk is 10 Gyr old
– Has a significant effect on modeling (e.g., Davé): gas supply is hot
• How are galaxies so smart?– Tight relationship as more baryons are lost– Clever feedback/formation scheme?
5E6 K
Poor Groups