Sharp Images of Galaxy Groups: Chandra and XMM Uncover New Intricacies J. M. Vrtilek 1, E. J. O’Sullivan 1, T. J. Ponman 2, L. P. David 1,D. E. Harris 1, W. Forman 1, C. Jones 1, N. Soker 3, W. M. Lane 4, N. Kassim 4 1 CfA 2 Univ. of Birmingham 3 Technion 4 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 models and numerous following papers Prerequisite for understanding formation and evolution of structure: galaxy => group => cluster hierarchy e.g., Blumenthal, Faber, Primack, Rees 1983 and large subsequent enterprise Cool (~1 keV) => strong lines of O, Si, S, as well as of Fe Kaastra, 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 4 Buote, 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 disturbances Bubbles, shocks, … Comparison with features observed at other wavelengths Extended radio features PROBLEMS: groups are faint (L x < erg s -1 — about 2 orders of magnitude less 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; L x ≈ 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 ≈ Msun within “ROSAT radius” (~20’: Ponman & Bertram 1993) Recent X-ray data include 50 ks Chandra ACIS-S 12 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 kpc50 kpc Data from XMM: Cycle 1 GTO program Radial profile of temperature from ROSAT (Ponman & Bertram 1993) 50 kpc

HCG 62 annular and deprojected abundance profiles (left) Radial temperature distribution. Free parameters: temperature, metal Abundance, 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 ratio Image (Chandra ACIS S3): [ keV]/[ 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 resolution details of spectral fitting (e.g., NGC 5044) Consequences of multiphase gas: Affects determination of metal abundances Implications for regulation of cooling: e.g., AGNs, heat conduction, small-scale inhomogeneities

HCG 62: Chandra + VLA Color: Chandra ACIS S3 50 ks Contours: 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 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: ~ ( )x10 7 yr “Energy content” (work done on surrounding medium to produce a cavity) is ~few x erg Current radio source is far too weak to produce cavities in required time. But a very modest AGN ( 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 = D = 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 10 7 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 features Association of X-ray features with optical galaxies Tracing 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