Sharp Images of Galaxy Groups: Chandra and XMM

Uncover New Intricacies

J. M. Vrtilek1, E. J. O’Sullivan1, T. J. Ponman2, L. P. David1,D. E. Harris1, W. Forman1, C. Jones1, N. Soker3, W. M. Lane4, N. Kassim4

1 CfA2 Univ. of Birmingham3 Technion4 Naval Research Lab.

Why observe Groups (in X-rays)?• Location of many (most?) galaxies?

Geller & Huchra 1983: CfA Redshift Survey; density contrast ≥ 20

Nolthenius & White 1987: comparison with numerical modelsand numerous following papers

• Prerequisite for understanding formation and evolution of structure:galaxy => group => cluster hierarchye.g., Blumenthal, Faber, Primack, Rees 1983 and large subsequent enterprise

• Cool (~1 keV) => strong lines of O, Si, S, as well as of FeKaastra, Kahn, Paerels, Peterson, et al.: XMM RGS

• “Simpler” environment than rich clusters for examining heavy element enrichment?

O’Sullivan et al. 2003, 2004: MKW 4, AWM 4Buote, Lewis, Brighenti, Mathews, et al.: NGC 5044

Opportunities from Chandra and XMM Effective areas

Angular resolutions

XMM

Chandra

ROSAT

ASCA

(XMM Users’ Handbook)

(Pareschi et al. 2003)

Opportunities from Chandra and XMM (#2)

• Extraction of physical parameters

Deprojection techniques

• Examination of disturbancesBubbles, shocks, …

• Comparison with features observed at other wavelengths

Extended radio features

• PROBLEMS:groups are faint (Lx < 1043-44 erg s-1 — about 2 orders of magnitudeless than rich clusters), therefore inaccessible at high z

groups are extended, therefore nearby ones fill the field-of-view, leading to issues with understanding of the outer regions and with background removal

Sketch of groups observed with Chandra and XMM

52 groups; 8 Chandra alone, 15 XMM alone; 29 both.

HCG 62: • X-ray brightest and one of the most

intrinsically luminous of the 100 Hickson compact groups;

Lx ≈ 1043 erg s-1

• Central galaxies: 2 ellipticals/S0’s very similar (∆m ≈ 0.5);within larger loose group (Rood & Struble 1994; Tovmassian 2001)

• Nearby: 59 Mpc for h = 0.7,giving 1’=17 kpc

• Mgas ≈ 1012 Msun within “ROSAT radius” (~20’: Ponman & Bertram 1993)

• Recent X-ray data include50 ks Chandra ACIS-S12 ks XMM

4’

DSS image

HCG 62: a gallery of X-ray images

XMM MOS1+2, adaptively-smoothed

8’

Chandra ACIS-S

8’4’

Wavelet-smoothed Chandra image

HCG 62: radial dependences of physical parameters

5 kpc 50 kpc

Data from XMM: Cycle 1 GTO program

Radial profile of temperature from ROSAT(Ponman & Bertram 1993)

50 kpc

HCG 62annular and deprojected abundance profiles

(left) Radial temperature distribution. Free parameters: temperature, metalAbundance, neutral hydrogen column density, and normalization.(right) As to the left, but showing metallicity as a function of radius.

HCG 62: temperature variations

4’

Adaptively-smoothed hardness ratioImage (Chandra ACIS S3): [1.2 - 2 keV]/[0.4 - 0.8 keV].

0.6 keV 1.2 keV

Adaptively-binned temperature map(Chandra ACIS S3)

Multiphase gas?• So far, have discussed models that describe gas with a single set of

physical parameters at each point

generally varying as a function of r only, with spherical or ellipsoidal symmetry

• However: gas could be mixed on very fine spatial scales (multiphase)(Mathews, Brighenti, & Buote 2004; Arabadjis & Bautz 2003;

Buote et al. 2002, 2003,…)• Evidence for multiphase gas:

complex appearance of intensity, temperature at high resolutiondetails of spectral fitting (e.g., NGC 5044)

• Consequences of multiphase gas:Affects determination of metal abundancesImplications for regulation of cooling: e.g., AGNs, heat conduction, small-scale inhomogeneities

HCG 62: Chandra + VLA

Color: Chandra ACIS S3 50 ksContours: VLA 1.4 GHz

X-ray image has been wavelet-smoothed.Circles indicate “cavities” of reduced X-ray surface brightness ~10 kpc to NE and SW of X-ray peak.Radio map has a clean beam size of 18x12 arcsec and an rms noise level of 80 µJy;Contours are spaced by factors of 2 with lowest at 0.3 mJy/beam.

HCG 62: cavities and radio emission• Only unresolved 5 mJy (1.4 GHz) core radio source originally known;

motivated VLA observations:3.3 hrs at 1.4 GHz in CnB configuration (12x18 arsec beam)9 hrs at 330 MHz in BnA configuration (8x15 arcsec beam)

• Detection only at 1.4 GHz in SW “tail”: 1.2±0.4 mJy• Where detected, the equipartition magnetic field pressure is a factor of

a few less than the thermal pressure, but could be increased by adjustments in filling factor, presence of relativistic protons…

Timescales and energetics of cavities

• Parameters of HCG 62 cavities are very typical, though radio luminosity, ~few x 1038 erg s-1, is at the very low end of the range (cf. Birzan et al. 2004)

• Cavity ages:by time to rise at sound speed, time to rise buoyantly,and time to refill displaced volume: ~ (1.5 - 3)x107 yr

• “Energy content” (work done on surrounding medium to produce a cavity) is ~few x 1056 erg

• Current radio source is far too weak to produce cavities in required time. But a very modest AGN (1041 - 1042 erg s-1) would suffice.

• Cavities are seen in ~20% of clusters. Is the same true for groups?

NGC 741 group

• Close pair of early-type galaxies

• z = 0.019D = 81 Mpc (1’ = 24 kpc

• Core of approx. 41-member group (Zabludoff & Mulchaey 1998), with r ~ 430 km s-1

9’

DSS image

2MASS image

4’

NGC 742

NGC 741

NGC 741 group

• Narrow-angle tail radio source; bright, complex morphology

• Angular extent of features well matched to ACIS detector

• Well-studied previously in X-ray, optical, and radio bands -- but X-ray angular resolution poor

• How does sharply-bent jet structure arise in poor group with low IGM density?6cm VLA map

8’

NGC 741

2’

• Chandra images (31 ks): rawcounts above, “true color” below

• NGC 741 dominates X-ray structure (and radio?)

• X-ray structure immediately surrounding NGC 741 is strongly asymmetric

• Weak X-ray deficient “bubble” found to SW in region of radio tail

• What is the nature of the two “bridges” joining NGC 741 and NGC 742, and extending to the north?

“Taffy galaxies” in X-rays?

• Proposed by Condon et al. (1993) for UGC 12914/12915: radio bridge explained in terms of gas and magnetic fields from postcollision galaxies

• Could NGC 741/742 constitute such a pair?

Relative narrowness of bridge due to greater concentration of gas in ellipticals into dense cores (in comparison with spirals)?

Age: ~few x 107 yrs since (nearly) head-on collision.

Jet pointed approximately from NGC 741 to NCG 742 confined

by enhanced magnetic field?

• Related phenomena of wakes (Sakelliou 2000; Acreman et al. 2003) and tidal tails may also be interesting here...

Summary

• Chandra and XMM support numerous observational advances in the study of hot gas in groups:

Associations of X-ray with radio structures

Detection of cavities and other faint/small featuresAssociation of X-ray features with optical galaxiesTracing of temperatures and abundances

Unmatched angular resolution of Chandra….

• Areas old and new accessible to detailed analysis:“Isolated” galaxies <> Groups <> Clusters: evolution and

scaling

Hydrodynamic equilibrium vs. nonthermal pressure support

Temperature structure in central regions; multiphase?

Nature/regulation of cooling cores