REVIEW
Luminous Compact Blue Galaxies up to z~1 in the HST Ultra Deep Field: I. Small galaxies, or blue centers of massive disks?
Astrophysics · Submitted 28 Feb 2006 · 44 references
Pith's verdict: UDF-depth profiles settle the structural question—most LCBGs are truly small—but the evolutionary claim rests on an untested no-growth assumption.
T0 · AI review
Why Pith says this
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Referee report
The paper
The paper claims
Most Luminous Compact Blue Galaxies at z~1 are small, not massive disk nuclei.
The reading
What carries the argument
The central mechanism is two-component surface photometry decomposition: separating a compact, blue, star-forming excess from a fainter, roughly exponential extended component. The diagnostic is the exponential scale length of that extended component, the e-folding radius of its surface brightness profile. By applying two independent fitting methods (an adaptive-mask profile technique and a point-spread-function-corrected two-exponential decomposition) and comparing the resulting scale lengths with those of local disk and dwarf galaxy samples, the paper can test whether a large disk lurks underneath the starburst.
Extended reading notes
Core claim
For a sample of 26 Luminous Compact Blue Galaxies at redshifts 0.21 to 1.25, the paper measures surface brightness profiles that reach fainter than rest-frame 26.5 B mag per square arcsecond even for compact objects near z~1. Most galaxies show a bright, irregular component coincident with ongoing star formation, plus an extended component with roughly exponential profile and colors implying stellar ages of at least 100 Myr to a few Gyr. The exponential scale lengths of these extended components are mostly about 2 kpc or smaller, a factor of 1.5 to 2 smaller than the scale length of the Milky Way and other large disks; larger, very low surface brightness disks can be excluded down to the depth of the images. One or two of the 26 are large disk galaxies with bright central nuclei, possibly forming bulges. The paper concludes that more than 90% of intermediate-redshift LCBGs are intrinsically small galaxies that will fade into small disk galaxies, low-mass irregulars, or spheroidals under passive evolution, while no more than about 10% of this sample represents nuclear starbursts in massive disks.
Load-bearing premise
The conclusion that these galaxies will not become large present-day disks assumes that their extended stellar components do not grow significantly in size after the current starburst fades; if strong disk growth or inside-out star formation happens after z~1, the same compact measurements could still end up as large disks.
Editorial extensions
If this is right
- Most LCBGs at z~0.2-1.3 are small stellar systems rather than massive disks with bright nuclei.
- Large low-surface-brightness disks would have been detected down to rest-frame about 26 B mag per square arcsecond, so their absence is a real property of this sample.
- The extended components already contain stars older than about 100 Myr before the current starburst, so these galaxies are not forming entirely from cold gas at z~1.
- Passive fading of the extended components naturally produces local small disk galaxies, irregulars, and low-mass spheroidals.
- The roughly 10% of LCBGs that are large disk-like galaxies with bright nuclei support a separate channel where some LCBGs are bulge formation in massive disks.
Reading between the lines
- If high-redshift LCBGs are mostly small, then the strong decline in LCBG number density from z~1 to z=0 reflects the fading of small starburst systems, not the transformation of small galaxies into massive spirals; this shifts the burden of mass assembly onto other galaxy populations.
- Applying the same two-component decomposition to rest-frame near-infrared images, where the old stellar component dominates, could test the no-disk-growth assumption directly by measuring scale lengths of the stellar mass rather than blue light.
- Spatially resolved nebular line kinematics of the small LCBGs would confirm whether emission lines really sample most of their velocity fields, as the paper infers from the extent of irregular blue emission.
- If a future survey found a population of LCBGs with large but very low-surface-brightness disks just below the current detection limit, the 90% estimate would shrink; the paper's stance predicts that such disks are rare.
Original abstract
We analyze 26 Luminous Compact Blue Galaxies (LCBGs) in the HST/ACS Ultra Deep Field (UDF) at z ~ 0.2-1.3, to determine whether these are truly small galaxies, or rather bright central starbursts within existing or forming large disk galaxies. Surface brightness profiles from UDF images reach fainter than rest-frame 26.5 B mag/arcsec^2 even for compact objects at z~1. Most LCBGs show a smaller, brighter component that is likely star-forming, and an extended, roughly exponential component with colors suggesting stellar ages >~ 100 Myr to few Gyr. Scale lengths of the extended components are mostly >~ 2 kpc, >1.5-2 times smaller than those of nearby large disk galaxies like the Milky Way. Larger, very low surface brightness disks can be excluded down to faint rest-frame surface brightnesses (>~ 26 B mag/arcsec^2). However, 1 or 2 of the LCBGs are large, disk-like galaxies that meet LCBG selection criteria due to a bright central nucleus, possibly a forming bulge. These results indicate that >~ 90% of high-z LCBGs are small galaxies that will evolve into small disk galaxies, and low mass spheroidal or irregular galaxies in the local Universe, assuming passive evolution and no significant disk growth. The data do not reveal signs of disk formation around small, HII-galaxy-like LCBGs, and do not suggest a simple inside-out growth scenario for larger LCBGs with a disk-like morphology. Irregular blue emission in distant LCBGs is relatively extended, suggesting that nebular emission lines from star-forming regions sample a major fraction of an LCBG's velocity field.
The compiled record
The paper's own sentences, arranged, kept beside the review and separate from it. Each label is drawn from a closed vocabulary; the number beside it is the probability the classifier gave that label. Numbers in the tables are the authors', checked by code. No sentence in this record was written by a model.
The abstract, sentence by sentence
We analyze 26 Luminous Compact Blue Galaxies (LCBGs) in the HST/ACS Ultra Deep Field (UDF) at z ~ 0.2-1.3, to determine whether these are truly small galaxies, or rather bright central starbursts within existing or forming large disk galaxies. Surface brightness profiles from UDF images reach fainter than rest-frame 26.5 B mag/arcsec^2 even for compact objects at z~1. Most LCBGs show a smaller, brighter component that is likely star-forming, and an extended, roughly exponential component with colors suggesting stellar ages >~ 100 Myr to few Gyr. Scale lengths of the extended components are mostly >~ 2 kpc, >1.5-2 times smaller than those of nearby large disk galaxies like the Milky Way. Larger, very low surface brightness disks can be excluded down to faint rest-frame surface brightnesses (>~ 26 B mag/arcsec^2). However, 1 or 2 of the LCBGs are large, disk-like galaxies that meet LCBG selection criteria due to a bright central nucleus, possibly a forming bulge. These results indicate that >~ 90% of high-z LCBGs are small galaxies that will evolve into small disk galaxies, and low mass spheroidal or irregular galaxies in the local Universe, assuming passive evolution and no significant disk growth. The data do not reveal signs of disk formation around small, HII-galaxy-like LCBGs, and do not suggest a simple inside-out growth scenario for larger LCBGs with a disk-like morphology. Irregular blue emission in distant LCBGs is relatively extended, suggesting that nebular emission lines from star-forming regions sample a major fraction of an LCBG's velocity field.
background contribution method result interpretation main claim
| # | sentence | role | the authors' own work | states the main conclusion | strength |
|---|---|---|---|---|---|
| s0 | We analyze 26 Luminous Compact Blue Galaxies (LCBGs) in the HST/ACS Ultra Deep Field (UDF) at z ~ 0.2-1.3, to determine whether these are truly small galaxies, or rather bright central starbursts within existing or forming large disk galaxies. | contribution 0.97 | 0.92 | 0.05 | unclear 0.51 |
| s1 | Surface brightness profiles from UDF images reach fainter than rest-frame 26.5 B mag/arcsec^2 even for compact objects at z~1. | method 0.72 | 0.87 | 0.02 | asserted 1.00 |
| s2 | Most LCBGs show a smaller, brighter component that is likely star-forming, and an extended, roughly exponential component with colors suggesting stellar ages >~ 100 Myr to few Gyr. | result 0.99 | 0.93 | 0.06 | hedged possibility 0.86 |
| s3 | Scale lengths of the extended components are mostly >~ 2 kpc, >1.5-2 times smaller than those of nearby large disk galaxies like the Milky Way. | result 1.00 | 0.94 | 0.05 | asserted 0.98 |
| s4 | Larger, very low surface brightness disks can be excluded down to faint rest-frame surface brightnesses (>~ 26 B mag/arcsec^2). | result 0.99 | 0.90 | 0.04 | asserted 1.00 |
| s5 | However, 1 or 2 of the LCBGs are large, disk-like galaxies that meet LCBG selection criteria due to a bright central nucleus, possibly a forming bulge. | result 0.96 | 0.93 | 0.05 | hedged possibility 0.83 |
| s6 | These results indicate that >~ 90% of high-z LCBGs are small galaxies that will evolve into small disk galaxies, and low mass spheroidal or irregular galaxies in the local Universe, assuming passive evolution and no significant disk growth. | interpretation 0.99 | 0.84 | 0.85 | hedged possibility 0.91 |
| s7 | The data do not reveal signs of disk formation around small, HII-galaxy-like LCBGs, and do not suggest a simple inside-out growth scenario for larger LCBGs with a disk-like morphology. | result 0.63 | 0.93 | 0.09 | asserted 0.81 |
| s8 | Irregular blue emission in distant LCBGs is relatively extended, suggesting that nebular emission lines from star-forming regions sample a major fraction of an LCBG's velocity field. | result 0.62 | 0.85 | 0.03 | hedged possibility 0.98 |
What was examined, and what was not
- arXiv listing abstract v1
- full text
- figures and tables
- references and citing sentences
- relations to other papers
- P abstract
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Figures
Figures reproduced from arXiv:astro-ph/0602610 by the authors.
References
How the literature uses it
1 arXiv paper cite this in 7 sentences
Papers citing it · 5 sentences without a confident label
Uses its method or tool 1 paper
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There are various definitions of LCBGs in the literature, but for our purposes we follow [ref] requirements that $B-V < 0.6$, $B < -18.5$, and $r_h<3.5$ kpc.
The WHIQII Survey: Metallicities and Spectroscopic Properties of Luminous Compact Blue Galaxies · AGN Contamination and Sub-Samples · 0.88
Compares with it 1 paper
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These fractions are slightly lower than previous results [ref] , likely resulting from a slightly lower median redshift combined with the fact that LCBGs clearly evolve very strongly at $z ≲1$.
The WHIQII Survey: Metallicities and Spectroscopic Properties of Luminous Compact Blue Galaxies · LCBG Fraction · 1.00
Discussion
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