Star Formation in the Central 400 Pc of the Milky Way: Evidence for a Population of Massive Young Stellar Objects

Abstract

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 midinfrared 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 μm 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 μm excesses. We use the SEDs of these sources to estimate their physical characteristics; their masses appear to range from ∼10 to ∼20 M . Within the central 400 × 50 pc (|l| < 1.• 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 yr −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 5 yr. This suggestion is also consistent with estimates of SFRs within the last ∼10 7 yr showing a peak around 10 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.

Star Formation in the Central Molecular Zone of the Milky Way

“…During the later half of the simulation, most of the star formation takes place in the CMZ, thus the obtained SFR values of 0.03-0.04 M yr −1 can be regarded as the SFR in the CMZ. These values are very close to the recent SFR estimated from mid-infrared observations by Yusef-Zadeh et al (2009), 0.04-0.08 M yr −1 .…”

“…We find that the gas accumulated in the CMZ by a bar potential of the inner bulge effectively turns into stars, supporting the idea that the stellar cusp inside the central 200 pc is a result of the sustained star formation in the CMZ. The obtained star formation rate in the CMZ, 0.03-0.1 M , is consistent with the recent estimate based on the mid-infrared observations by Yusef-Zadeh et al (2009). …”

Multiwavelength features of Fermi bubbles as signatures of a Galactic wind

“…The current rate of star formation in the central molecular zone of Milky Way is of order 0.1 M⊙ yr −1 . Mid-infrared observations by Yusuf-Zadeh et al (2009) have led to an estimate of SFR ranging between 0.007-0.14 M⊙ yr −1 , over the last 10 Gyr. Observations of young stellar objects in the central molecular zone (CMZ) in the 5-38 µm band with Spitzer allowed Immer et al (2011) to estimate a SFR of ∼ 0.08 M⊙ yr −1 .…”

Conversion of gas into stars in the Galactic center

“…Yusef-Zadeh et al 2009, Immer et al 2012a, Longmore et al 2013a). These studies have reached different conclusions about whether the gas in the CMZ is consistent with the predictions of galactic-scale star formation relations, leading to some confusion.…”