STAR FORMATION IN THE CENTRAL 400 PC OF THE MILKY WAY: EVIDENCE FOR A POPULATION OF MASSIVE YOUNG STELLAR OBJECTS

The central kpc of the Milky Way might be expected to differ significantly from the rest of the Galaxy with regard to gasdynamics and the formation of young stellar objects (YSOs). We probe this possibility with mid-infrared observations obtained with Infrared Array Camera and Multiband Imaging Photometer on Spitzer and with Midcourse Space Experiment. We use color-color diagrams and spectral energy distribution (SED) fits to explore the nature of YSO candidates (including objects with 4.5 {mu}m excesses possibly due to molecular emission). There is an asymmetry in the distribution of the candidate YSOs, which tend to be found at negative Galactic longitudes; this behavior contrasts with that of the molecular gas, approximately 2/3 of which is at positive longitudes. The small-scale height of these objects suggests that they are within the Galactic center region and are dynamically young. They lie between two layers of infrared dark clouds and may have originated from these clouds. We identify new sites for this recent star formation by comparing the mid-IR, radio, submillimeter, and methanol maser data. The methanol masers appear to be associated with young, embedded YSOs characterized by 4.5 {mu}m excesses. We use the SEDs of these sources to estimate their physicalmore » characteristics; their masses appear to range from {approx}10 to {approx}20 M{sub sun}. Within the central 400 x 50 pc (|l| < 1.{sup 0}3 and |b| < 10') the star formation rate (SFR) based on the identification of Stage I evolutionary phase of YSO candidates is about 0.14 M{sub sun} yr{sup -1}. Given that the majority of the sources in the population of YSOs are classified as Stage I objects, we suggest that a recent burst of star formation took place within the last 10{sup 5} yr. This suggestion is also consistent with estimates of SFRs within the last {approx}10{sup 7} yr showing a peak around 10{sup 5} yr ago. Lastly, we find that the Schmidt-Kennicutt Law applies well in the central 400 pc of the Galaxy. This implies that star formation does not appear to be dramatically affected by the extreme physical conditions in the Galactic center region.« less

[1]  D. Frail,et al.  Three New Supernova Remnant OH Masers Near the Galactic Center: Evidence for Large-Scale Maser Emission from Supernova Remnants , 1999, astro-ph/9907370.

[2]  A. Stark,et al.  The AST/RO Survey of the Galactic Center Region. I. The Inner 3 Degrees , 2002, astro-ph/0211025.

[3]  K. Sellgren,et al.  A mid-infrared survey of the inner 2 ? 1.5 degrees of the Galaxy with Spitzer/IRAC , 2005 .

[4]  N. Panagia,et al.  Some physical parameters of early-type stars , 1973 .

[5]  James Liebert,et al.  The Two Micron All Sky Survey (2MASS): Overview and Status , 1997 .

[6]  R. Indebetouw,et al.  Interpreting Spectral Energy Distributions from Young Stellar Objects. I. A Grid of 200,000 YSO Model SEDs , 2006, astro-ph/0608234.

[7]  A. Winnberg,et al.  A new sample of OH/IR stars in the Galactic center , 1998 .

[8]  H Germany,et al.  Recent star formation in the inner Galactic Bulge seen by ISOGAL. I. Classification of bright mid-IR , 2006, astro-ph/0603576.

[9]  Pavla Maršálková.  A comparison catalogue ofHii-regions , 1974 .

[10]  Eugene Serabyn,et al.  THE GALACTIC CENTER ENVIRONMENT , 1996 .

[11]  R. Indebetouw,et al.  A CATALOG OF EXTENDED GREEN OBJECTS IN THE GLIMPSE SURVEY: A NEW SAMPLE OF MASSIVE YOUNG STELLAR OBJECT OUTFLOW CANDIDATES , 2008, 0810.0530.

[12]  James J. Condon,et al.  Radio Emission from Normal Galaxies , 1992 .

[13]  NTT infrared imaging of star cluster candidates towards the central parts of the Galaxy , 2003, astro-ph/0306608.

[14]  V. Slysh,et al.  The Parkes survey of methanol masers at 44.07 GHz , 1994 .

[15]  M. Schultheis,et al.  OH/IR stars in the inner bulge detected by ISOGAL ?;?? , 2002 .

[16]  S. Sharpless.  A Catalogue of H II Regions. , 1959 .

[17]  L. Mundy,et al.  THE MID-INFRARED EXTINCTION LAW IN THE OPHIUCHUS, PERSEUS, AND SERPENS MOLECULAR CLOUDS , 2008, 0809.1106.

[18]  M. Wolff,et al.  Two-dimensional Radiative Transfer in Protostellar Envelopes. I. Effects of Geometry on Class I Sources , 2003, astro-ph/0303479.

[19]  R. Indebetouw,et al.  A Multiwavelength Study of M17: The Spectral Energy Distribution and PAH Emission Morphology of a Massive Star Formation Region , 2007 .

[20]  C. Brogan,et al.  Extended Green Objects (EGOs) In The GLIMPSE Survey: A New Sample Of Massive Young Stellar Object Outflow Candidates , 2009 .

[21]  G. Rieke,et al.  A CATALOG OF 24 μm SOURCES TOWARD THE GALACTIC CENTER , 2008, 0811.1557.

[22]  Andreas Wicenec,et al.  Galactic planetary nebulae and their central stars. I. An accurate and homogeneous set of coordinates , 2003 .

[23]  J. Rathborne,et al.  MASSIVE PROTOSTARS IN THE INFRARED DARK CLOUD MSXDC G034.43 00.24 , 2005, astro-ph/0508458.

[24]  R. Indebetouw,et al.  Interpreting Spectral Energy Distributions from Young Stellar Objects. II. Fitting Observed SEDs Using a Large Grid of Precomputed Models , 2006, astro-ph/0612690.

[25]  V. Slysh,et al.  Detection of new sources of methanol emission at 95 GHz with the Mopra telescope , 2000, astro-ph/0003207.

[26]  STAR FORMATION IN GALAXIES ALONG THE HUBBLE SEQUENCE , 1998, astro-ph/9807187.

[27]  Robert H. Becker,et al.  A 5-GHz VLA Survey of the Galactic Plane (Becker+ 1994) , 1994 .

[28]  D. Padgett,et al.  MIPSGAL: A Survey of the Inner Galactic Plane at 24 and 70 μm , 2005 .

[29]  Mark R. Morris,et al.  The center of the galaxy , 1989 .

[30]  K. Flaherty,et al.  Infrared Extinction toward Nearby Star-forming Regions , 2007, astro-ph/0703777.

[31]  W. Goss,et al.  VLA Radio Recombination Line Observations of Ionized Gas in the -30 Kilometers per Second Molecular Cloud (G0.04+0.03) near the Galactic Center. I. The Discrete Radio Sources , 1993 .

[32]  CAUGHT IN THE ACT: THE ONSET OF MASSIVE STAR FORMATION , 2005, astro-ph/0510512.

[33]  E. Serabyn,et al.  Quiescent Giant Molecular Cloud Cores in the Galactic Center , 2001 .

[34]  C. Lada.  The Formation of Low Mass Stars , 1999 .

[35]  R. Wilson,et al.  Galactic center molecular clouds. II. Distribution and kinematics , 1988 .

[36]  Garry Robinson,et al.  Studies of ultracompact H ii regions — II. High-resolution radio continuum and methanol maser survey , 1998 .

[37]  Simulations of mixed-morphology supernova remnants with anisotropic thermal conduction , 2006, astro-ph/0604474.

[38]  T. Geballe,et al.  HOT AND DIFFUSE CLOUDS NEAR THE GALACTIC CENTER PROBED BY METASTABLE H~3^+ , 2005, astro-ph/0507463.

[39]  B. Weiner,et al.  DETERMINING STAR FORMATION RATES FOR INFRARED GALAXIES , 2008, 0810.4150.

[40]  K. Menten,et al.  VLA Observations of the Sagittarius D Star-forming Region , 1998 .

[41]  P. Kroupa.  On the variation of the initial mass function , 2000, astro-ph/0009005.

[42]  M. Morris,et al.  Water masers in the direction of the Galactic center. I: Results from initial observations , 1993 .

[43]  J. Carlstrom,et al.  SUBMILLIMETER CONTINUUM SURVEY OF THE GALACTIC CENTER , 1994 .

[44]  J. Rho,et al.  DR 21: A Major Star Formation Site Revealed by Spitzer , 2004 .

[45]  T. Handa,et al.  Dense Molecular Clouds in the Galactic Center Region. I. Observations and Data , 1999 .

[46]  W. Cotton,et al.  A 20 Centimeter Survey of the Galactic Center Region. I. Detection of Numerous Linear Filaments , 2004, astro-ph/0409292.

[47]  D. Padgett,et al.  Is the Cepheus E Outflow Driven by a Class 0 Protostar? , 2004, astro-ph/0506710.

[48]  B. Draine,et al.  Infrared Emission from Interstellar Dust Ii. the Diffuse Interstellar Medium , 2000 .

[49]  G. Helou,et al.  A physical model of the infrared-to-radio correlation in galaxies , 1993 .

[50]  C. Lada,et al.  Spectral evolution of young stellar objects , 1986 .

[51]  W. Goss,et al.  The Sagittarius B2 Star-forming Region. I. Sensitive 1.3 Centimeter Continuum Observations , 1995 .

[52]  Ipac,et al.  The Mid-Infrared Colors of the Interstellar Medium and Extended Sources at the Galactic Center , 2008, 0804.4491.

[53]  Mark Wardle,et al.  On the Formation of Compact Stellar Disks around Sagittarius A* , 2008, 0805.3274.

[54]  K. Menten,et al.  44 GHz Methanol Masers and Quasi-Thermal Emission in Sagittarius B2 , 1997 .

[55]  A spitzer space telescope infrared survey of supernova remnants in the inner galaxy , 2005, astro-ph/0510630.

[56]  W. Cotton,et al.  Green Bank Telescope Multiwavelength Survey of the Galactic Center Region , 2008, 0801.4294.

[57]  William D. Vacca,et al.  The Lyman-Continuum Fluxes and Stellar Parameters of O and Early B-Type Stars , 1996 .

[58]  T. Bania.  Carbon monoxide in the inner galaxy , 1977 .

[59]  G. Jacoby,et al.  Radio observations of new galactic bulge planetary nebulae , 2001, astro-ph/0102336.

[60]  A. Walsh,et al.  Discovery of molecular hydrogen line emission associated with methanol maser emission , 2001 .

[61]  A. H. Barrett,et al.  High-resolution surveys of the Sagittarius A molecular cloud complex in ammonia, carbon monoxide, and isocyanic acid , 1985 .

[62]  K. Rice,et al.  Protostars and Planets V , 2005 .

[63]  Wm. A. Wheaton,et al.  Absolute Calibration and Characterization of the Multiband Imaging Photometer for Spitzer. II. 70 μm Imaging , 2007, 0704.2196.

[64]  T. Do,et al.  A magnetic torsional wave near the Galactic Centre traced by a ‘double helix’ nebula , 2006, Nature.

[65]  Linda J. Smith,et al.  SPITZER SAGE SURVEY OF THE LARGE MAGELLANIC CLOUD. III. STAR FORMATION AND ∼1000 NEW CANDIDATE YOUNG STELLAR OBJECTS , 2008 .

[66]  Jessica R. Lu,et al.  Measuring Distance and Properties of the Milky Way’s Central Supermassive Black Hole with Stellar Orbits , 2008, 0808.2870.

[67]  P. Ho,et al.  The Nature of the Molecular Environment within 5 Parsecs of the Galactic Center , 2004, astro-ph/0409271.

[68]  Elizabeth Ledwosinska,et al.  The SCUBA Legacy Catalogues: Submillimeter-Continuum Objects Detected by SCUBA , 2008, 0801.2595.

[69]  A. Miyazaki,et al.  A Large-Scale CO Survey of the Galactic Center , 1998 .

[70]  Outflows from Massive Young Stellar Objects as Seen with the Infrared Array Camera , 2006, astro-ph/0603404.

[71]  Hubble Space Telescope/NICMOS Observations of Massive Stellar Clusters near the Galactic Center , 1999, astro-ph/9906299.

[72]  M. Egan,et al.  Submillimeter Observations of Midcourse Space Experiment Galactic Infrared-Dark Clouds , 2000 .

[73]  S. Deguchi,et al.  SiO Maser Sources toward the Sagittarius B2 Molecular Cloud , 1997 .

[74]  M. Schultheis,et al.  High mass‐loss asymptotic giant branch stars detected by the Midcourse Space Experiment in the ‘intermediate’ and ‘outer’ Galactic bulge , 2007 .

[75]  Antony A. Stark,et al.  Understanding the kinematics of Galactic Centre gas , 1991 .

[76]  J. Caswell.  A GALACTIC CENTRE SURVEY FOR 6.6-GHZ METHANOL MASERS , 1996 .

[77]  M. Morris,et al.  HST/NICMOS observations of massive stellar clusters near the galactic center , 1999 .

[78]  G. Fazio,et al.  A far-infrared survey of the galactic center , 1984 .

[79]  G. Rieke,et al.  Stellar Kinematics and the Black Hole in the Galactic Center , 1996 .

[80]  C. Flynn,et al.  A SCHMIDT–KENNICUTT LAW FOR STAR FORMATION IN THE MILKY WAY DISK , 2008, 0810.1656.

[81]  K. Menten,et al.  Infrared Space Observatory Long Wavelength Spectrometer Observations of a Cold Giant Molecular Cloud Core near the Galactic Center , 1998 .

[82]  H. Liszt.  Radio images of Sagittarius. I: Overview, Sagittarius D and Sagittarius E , 1992 .

[83]  Robert H. Becker,et al.  A catalog of small-diameter radio sources in the Galactic plane , 1990 .

[84]  Paolo Persi,et al.  The Impact of Large Scale Near-IR Sky Surveys , 1997 .

[85]  L. Kewley,et al.  An Initial Look at the Far-Infrared-Radio Correlation within Nearby Star-forming Galaxies Using the Spitzer Space Telescope , 2005, astro-ph/0510227.

[86]  T. Jenness,et al.  A deep submillimeter survey of the Galactic center , 2000 .

[87]  Martin G. Cohen,et al.  A Population of Cold Cores in the Galactic Plane , 1998 .

[88]  A. Stark,et al.  On the fate of Galactic Centre molecular clouds , 1991 .

[89]  C. Lang,et al.  A Radio Polarimetric Study of the Galactic Center Threads , 1999, astro-ph/9906336.

[90]  T. Joseph W. Lazio,et al.  High-Resolution, Wide-Field Imaging of the Galactic Center Region at 330 MHz , 2003 .

[91]  A. Robin,et al.  ISOGAL : A deep survey of the obscured inner Milky Way with ISO at 7 mu m and 15 mu m and with DENIS in the near-infrared , 2003, astro-ph/0304383.

[92]  R. Laureijs,et al.  IRAS OBSERVATIONS OF THE GALACTIC CENTER , 1989 .

[93]  T. Paumard,et al.  The Two Young Star Disks in the Central Parsec of the Galaxy: Properties, Dynamics, and Formation , 2006, astro-ph/0601268.

[94]  J. Rathborne,et al.  STAR FORMATION ACTIVITY OF CORES WITHIN INFRARED DARK CLOUDS , 2009 .

[95]  D. Lis,et al.  The Origin of Diffuse X-Ray and γ-Ray Emission from the Galactic Center Region: Cosmic-Ray Particles , 2006, astro-ph/0608710.

[96]  Planetary nebulae near the Galactic center: Identifications , 2004, astro-ph/0402280.

[97]  Martin G. Cohen,et al.  TWO-DIMENSIONAL RADIATIVE TRANSFER IN PROTOSTELLAR ENVELOPES. II. AN EVOLUTIONARY SEQUENCE , 2003, astro-ph/0309007.

[98]  H. Habing,et al.  The ATCA/VLA OH 1612 MHz survey. I. Observations of the galactic bulge Region , 1997 .

[99]  Brian Babler,et al.  The Bubbling Galactic Disk. II. The Inner 20° , 2007 .

[100]  Richard G. Arendt,et al.  Point Sources from a Spitzer IRAC Survey of the Galactic Center , 2007, 0709.3113.

[101]  Jessica R. Lu,et al.  Orbits and Origins of the Young Stars in the Central Parsec , 2006 .

[102]  C. Law,et al.  X-Ray Observations of Stellar Clusters Near the Galactic Center , 2004 .

[103]  K. Menten,et al.  Detection of widespread strong methanol masers at 44 GHz , 1990 .

[104]  J. Ott,et al.  Spectral imaging of the Sagittarius B2 region in multiple 3-mm molecular lines with the Mopra telescope , 2007, 0712.0218.

[105]  M. Morris,et al.  G0. 18-0. 04 - interaction of thermal and nonthermal radio structures in the arc near the galactic center , 1987 .

[106]  J. Rathborne,et al.  The Characterization and Galactic Distribution of Infrared Dark Clouds , 2006 .

[107]  Karl M. Menten,et al.  The discovery of a new, very strong, and widespread interstellar methanol maser line , 1991 .

[108]  M. Morris,et al.  Large, highly organized radio structures near the galactic centre , 1984, Nature.

[109]  K. Menten,et al.  Interstellar Hydroxyl Masers in the Galaxy. I. The VLA Survey , 2000 .

[110]  H. J. Habing,et al.  Circumstellar envelopes and Asymptotic Giant Branch stars , 1996 .