A panoramic VISTA of the stellar halo of NGC 253

Context. Outskirts of large galaxies contain important information about galaxy formation and assembly. Resolved star count studies can probe the extremely low surface brightness of the outer halos. Aims. NGC 253 is a nearly edge-on disk galaxy in the Sculptor group where we resolved the halo stars from ground-based images, with the aim of studying its stellar population content, the structure and the overall extent of the halo. Methods. We use Z and J-band images from the VIRCAM camera mounted on the VISTA telescope to construct the spatially resolved J vs. Z-J colour-magnitude diagrams (CMDs). The very deep photometry and the wide area covered allows us to trace the red giant branch (RGB) and asymptotic giant branch (AGB) stars that belong to the halo of NGC 253 out to 50 kpc along the galaxy minor axis. Results. We confirm the existence of an extra planar stellar component of the disk, with a very prominent southern shelf and a symmetrical feature on the north side. The only additional visible sub-structure is an overdensity in the north-west part of the halo 28 kpc distant from the plane and extending over 20 kpc parallel with the disk. We measure the transition from the disk to the halo at a radial distance of about 25 kpc with a clear break in the number density profile. The isodensity contours show that the inner halo is a flattened structure that blends with a more extended, di use, rounder outer halo. Such external structure can be traced to the very edge of our image out to 50 kpc from the disk plane. The number density profile of the stars in the stellar halo follows a power law with index 1:6, as function of radius. The CMD shows a very homogeneous stellar population across the field; by comparing with isochrones we conclude that the RGB stars are 8 Gyr old or more, while the AGB stars trace a population of about 2 10 8 M formed from 0:5 to a few Gyr ago. Surprisingly, part of this latter population appears scattered over a wide area. We explore several ideas to explain the origin of this relatively young component in the inner halo of NGC 253.

[1]  M. Irwin,et al.  THE LARGE-SCALE STRUCTURE OF THE HALO OF THE ANDROMEDA GALAXY. I. GLOBAL STELLAR DENSITY, MORPHOLOGY AND METALLICITY PROPERTIES , 2013, 1311.5888.

[2]  S. Warren,et al.  Suppression of star formation in the galaxy NGC 253 by a starburst-driven molecular wind , 2013, Nature.

[3]  C. Maraston,et al.  CALIBRATING STELLAR POPULATION MODELS WITH MAGELLANIC CLOUD STAR CLUSTERS , 2013, 1305.3617.

[4]  O. Fèvre,et al.  On-sky characterisation of the VISTA NB118 narrow-band filters at 1.19 μm , 2013, 1305.0262.

[5]  J. Bailin,et al.  TESTING GALAXY FORMATION MODELS WITH THE GHOSTS SURVEY: THE COLOR PROFILE OF M81's STELLAR HALO , 2013, 1302.2626.

[6]  M. Irwin,et al.  Unearthing Foundations of a Cosmic Cathedral: Searching the Stars for M33's Halo , 2012, 1210.4114.

[7]  M. Hilker,et al.  ESO VISTA Public Surveys - A Status Overview , 2012 .

[8]  D. Fantinel,et al.  Resolved Stellar Population of Distant Galaxies in the ELT Era , 2012, 1206.0909.

[9]  E. Zackrisson,et al.  Unlocking the secrets of stellar haloes using combined star counts and surface photometry , 2011, 1112.1696.

[10]  J. Schaye,et al.  Global structure and kinematics of stellar haloes in cosmological hydrodynamic simulations , 2011, 1111.1747.

[11]  A. Renzini,et al.  Stellar Populations: A User Guide from Low to High Redshift , 2011 .

[12]  M. Irwin,et al.  Quantifying the faint structure of galaxies , 2011, 1109.2625.

[13]  S. Majewski,et al.  A PETAL OF THE SUNFLOWER: PHOTOMETRY OF THE STELLAR TIDAL STREAM IN THE HALO OF MESSIER 63 (NGC 5055) , 2011, 1109.0019.

[14]  J. Bailin,et al.  THE GHOSTS SURVEY. I. HUBBLE SPACE TELESCOPE ADVANCED CAMERA FOR SURVEYS DATA , 2011 .

[15]  N. Arimoto,et al.  Stellar structures in the outer regions of M33 , 2011, 1106.4704.

[16]  R. Beaton,et al.  THE LUMINOSITY PROFILE AND STRUCTURAL PARAMETERS OF THE ANDROMEDA GALAXY , 2011, 1106.3564.

[17]  J. Bailin,et al.  THE RESOLVED STELLAR HALO OF NGC 253, , 2011, 1105.0005.

[18]  M. Westmoquette,et al.  Spatially resolved optical integral field unit spectroscopy of the inner superwind of NGC 253 , 2011, 1103.1775.

[19]  R. Dettmar,et al.  Tidal streams around galaxies in the SDSS DR7 archive. I. First results , 2011, 1102.2905.

[20]  Claudio Dalla Vecchia,et al.  Cosmological simulations of the formation of the stellar haloes around disc galaxies , 2011, 1102.2526.

[21]  A. Helmi,et al.  SUBSTRUCTURE IN THE STELLAR HALOS OF THE AQUARIUS SIMULATIONS , 2011, 1101.2544.

[22]  T. Davidge.  SHAKEN, NOT STIRRED: THE DISRUPTED DISK OF THE STARBURST GALAXY NGC 253 , 2010, 1011.3006.

[23]  A. Dotter,et al.  THE ACS SURVEY OF GALACTIC GLOBULAR CLUSTERS. X. NEW DETERMINATIONS OF CENTERS FOR 65 CLUSTERS , 2010, 1008.2755.

[24]  Taylor S. Chonis,et al.  STELLAR TIDAL STREAMS IN SPIRAL GALAXIES OF THE LOCAL VOLUME: A PILOT SURVEY WITH MODEST APERTURE TELESCOPES , 2010, 1003.4860.

[25]  M. Irwin,et al.  VISTA Science Verification — The Galactic and Extragalactic Mini-surveys , 2010 .

[26]  R. Ibata,et al.  A PANORAMIC VIEW OF THE MILKY WAY ANALOG NGC 891 , 2010, 1002.0461.

[27]  F. Courbin,et al.  Direct detection of galaxy stellar halos: NGC 3957 as a test case , 2010, 1001.3067.

[28]  A. Helmi,et al.  Galactic stellar haloes in the CDM model , 2009, 0910.3211.

[29]  M. Irwin,et al.  The remnants of galaxy formation from a panoramic survey of the region around M31 , 2009, Nature.

[30]  J. Kalirai,et al.  STRUCTURE AND POPULATION OF THE ANDROMEDA STELLAR HALO FROM A SUBARU/SUPRIME-CAM SURVEY , 2009, 0908.0245.

[31]  H. Kaneda,et al.  AKARI DETECTION OF FAR-INFRARED DUST EMISSION IN THE HALO OF NGC 253 , 2009, 0905.3800.

[32]  C. Brook,et al.  THE DUAL ORIGIN OF STELLAR HALOS , 2009, 0904.3333.

[33]  R. Ibata,et al.  The stellar population content of the thick disc and halo of the Milky Way analogue NGC 891 , 2009, 0903.4211.

[34]  R. Ibata,et al.  An HST/ACS investigation of the spatial and chemical structure and sub-structure of NGC 891, a Milky Way analogue* , 2009, 0903.4209.

[35]  Hugo van Woerden,et al.  MAPPING THE GALACTIC HALO. VIII. QUANTIFYING SUBSTRUCTURE , 2009, 0903.3043.

[36]  C. Grillmair.  FOUR NEW STELLAR DEBRIS STREAMS IN THE GALACTIC HALO , 2008, 0811.3965.

[37]  S. Majewski,et al.  The Ghost of a Dwarf Galaxy: Fossils of the Hierarchical Formation of the Nearby Spiral Galaxy NGC 5907 , 2008, 0805.1137.

[38]  S. Majewski,et al.  DISCOVERY OF A GIANT STELLAR TIDAL STREAM AROUND THE DISK GALAXY NGC 4013 , 2008, 0801.4657.

[39]  Belgium,et al.  Evolution of asymptotic giant branch stars. II. Optical to far-infrared isochrones with improved TP- , 2007, 0711.4922.

[40]  D. York,et al.  Two stellar components in the halo of the Milky Way , 2007, Nature.

[41]  Zeljko Ivezic,et al.  The Accretion Origin of the Milky Way’s Stellar Halo , 2007, 0706.0004.

[42]  R. Ibata,et al.  The Haunted Halos of Andromeda and Triangulum: A Panorama of Galaxy Formation in Action , 2007, 0704.1318.

[43]  Princeton,et al.  The Field of Streams: Sagittarius and Its Siblings , 2006, astro-ph/0605025.

[44]  David Schlegel,et al.  The Milky Way Tomography with SDSS. I. Stellar Number Density Distribution , 2005, astro-ph/0510520.

[45]  H. Ferguson,et al.  Halos of Spiral Galaxies. III. Metallicity Distributions , 2005, astro-ph/0510255.

[46]  H. Ferguson,et al.  Halos of Spiral Galaxies. I. The Tip of the Red Giant Branch as a Distance Indicator , 2005, astro-ph/0510253.

[47]  M. Steinmetz,et al.  Stars beyond galaxies: the origin of extended luminous haloes around galaxies , 2005, astro-ph/0506659.

[48]  F. Walter,et al.  The Temperature Distribution of Dense Molecular Gas in the Center of NGC 253 , 2005, astro-ph/0505143.

[49]  D. Schiminovich,et al.  GALEX Observations of the Ultraviolet Halos of NGC 253 and M82 , 2004, astro-ph/0411309.

[50]  F. Fraternali,et al.  Extra-planar H I in the starburst galaxy NGC 253 , 2004, astro-ph/0410055.

[51]  A. Helmi.  Is the dark halo of our Galaxy spherical , 2003, astro-ph/0309579.

[52]  J. Torra,et al.  A bright early-type star in the halo of NGC 253: Runaway or in situ formation? , 2003 .

[53]  P. W. Hodge,et al.  Distances to nearby galaxies in Sculptor , 2003, astro-ph/0302045.

[54]  D. Lamb,et al.  A Low-Latitude Halo Stream around the Milky Way , 2003, astro-ph/0301029.

[55]  R. Rich,et al.  Age and Metallicity Distribution of the Galactic Bulge from Extensive Optical and Near-IR Stellar Photometry , 2002, astro-ph/0210660.

[56]  T. M. Heckman,et al.  Chandra Observations of NGC 253. II. On the Origin of Diffuse X-Ray Emission in the Halos of Starburst Galaxies , 2001, astro-ph/0111511.

[57]  R. Mendez,et al.  Possible star formation in the halo of NGC 253 , 2001 .

[58]  K. Freeman,et al.  Surface Brightness Fluctuation Distances to Dwarf Elliptical Galaxies in the Sculptor Group , 1998, astro-ph/9809046.

[59]  D. M. Kelly,et al.  The Nuclear Starburst in NGC 253 , 1998, astro-ph/9805153.

[60]  F. Prada,et al.  The Stellar and Gaseous Kinematics in NGC 253 , 1997, astro-ph/9711262.

[61]  D. Malin,et al.  HI in Shell Galaxies and Other Merger Remnants , 1997, Publications of the Astronomical Society of Australia.

[62]  William E. Harris,et al.  A Catalog of Parameters for Globular Clusters in the Milky Way , 1996 .

[63]  K. Wakamatsu,et al.  Dark Filaments in the Galaxy NGC 253: A Boiling Galactic Disk , 1994, astro-ph/9407074.

[64]  M. Irwin,et al.  A dwarf satellite galaxy in Sagittarius , 1994, Nature.

[65]  Marcia J. Lebofsky,et al.  NGC 253 and a proposed sequence for nuclear starbursts , 1988 .

[66]  G. Trinchieri,et al.  The Complex X-ray Emission of NGC 253 , 1984 .

[67]  Rodger I. Thompson,et al.  The nature of the nuclear sources in M82 and NGC 253 , 1980 .

[68]  S. Tsygankov,et al.  Quenching of the accretion disk strong aperiodic variability at the magnetospheric boundary , 2009 .

[69]  M. J. Irwin,et al.  AUTOMATED STAR COUNTS IN THE SOUTHERN GLOBULAR-CLUSTER NGC-6809(M55) , 1984 .