Journal Article

Cold streams in early massive hot haloes as the main mode of galaxy formation

Goerdt, T., Mumcuoglu, M., Neistein, E., Pichon, C., Freundlich, J., Birnboim, Y., Engel, G., Zinger, E., Dekel, A., Teyssier, R.

2009

Overview

Early star formation: steady progress Recent observations suggest that the massive galaxies that were at the height of their star-forming activity in the young Universe ten billion years ago formed their stars at surprisingly high rates. While such rates are commonly attributed to violent galaxy mergers, many of these galaxies are rotating discs, as extended as today's Milky Way, a structure that is incompatible with such a history. A new cosmological simulation suggests that these galaxies were 'stream fed', acquiring the material that was needed to fuel star formation as a steady flow of cold gas from the extended dark-matter haloes surrounding the galaxies. It is the rarer submillimetre galaxies, which form stars even more intensely, that are largely merger-induced starbursts. Massive galaxies in the young universe (ten billion years ago) formed stars at surprising intensities. Although this is commonly attributed to violent mergers, the properties of many of these galaxies are incompatible with mergers. This paper reports that they are 'stream-fed galaxies', growing via steady, narrow, cold gas streams. Unlike destructive mergers, the smoother flows are likely to keep the rotating disc configuration intact. Massive galaxies in the young Universe, ten billion years ago, formed stars at surprising intensities 1 , 2 . Although this is commonly attributed to violent mergers, the properties of many of these galaxies are incompatible with such events, showing gas-rich, clumpy, extended rotating disks not dominated by spheroids 1 , 2 , 3 , 4 , 5 . Cosmological simulations 6 and clustering theory 6 , 7 are used to explore how these galaxies acquired their gas. Here we report that they are ‘stream-fed galaxies’, formed from steady, narrow, cold gas streams that penetrate the shock-heated media of massive dark matter haloes 8 , 9 . A comparison with the observed abundance of star-forming galaxies implies that most of the input gas must rapidly convert to stars. One-third of the stream mass is in gas clumps leading to mergers of mass ratio greater than 1:10, and the rest is in smoother flows. With a merger duty cycle of 0.1, three-quarters of the galaxies forming stars at a given rate are fed by smooth streams. The rarer, submillimetre galaxies that form stars even more intensely 2 , 12 , 13 are largely merger-induced starbursts. Unlike destructive mergers, the streams are likely to keep the rotating disk configuration intact, although turbulent and broken into giant star-forming clumps that merge into a central spheroid 4 , 10 , 11 . This stream-driven scenario for the formation of discs and spheroids is an alternative to the merger picture.

• Publisher: Nature Publishing Group UK,Nature Publishing Group
• Subject: Astronomy / Astrophysics / Characteristics and properties of external galaxies and extragalactic objects / Clusters / Cosmology and Extra-Galactic Astrophysics / Dark matter / Earth, ocean, space / Exact sciences and technology / Galaxies / Humanities and Social Sciences / letter / multidisciplinary / Observations / Origin, formation, evolution, age, and star formation / Physics / Science / Science (multidisciplinary) / Sciences of the Universe / Star & galaxy formation / Stars / Stars & galaxies / Stellar systems. Galactic and extragalactic objects and systems. The universe / Universe