156 research outputs found
The growth and assembly of a massive galaxy at z ~ 2
We study the stellar mass assembly of the Spiderweb Galaxy (MRC 1138-262), a
massive z = 2.2 radio galaxy in a protocluster and the probable progenitor of a
brightest cluster galaxy. Nearby protocluster galaxies are identified and their
properties are determined by fitting stellar population models to their
rest-frame ultraviolet to optical spectral energy distributions. We find that
within 150 kpc of the radio galaxy the stellar mass is centrally concentrated
in the radio galaxy, yet most of the dust-uncorrected, instantaneous star
formation occurs in the surrounding low-mass satellite galaxies. We predict
that most of the galaxies within 150 kpc of the radio galaxy will merge with
the central radio galaxy by z = 0, increasing its stellar mass by up to a
factor of ~ 2. However, it will take several hundred Myr for the first mergers
to occur, by which time the large star formation rates are likely to have
exhausted the gas reservoirs in the satellite galaxies. The tidal radii of the
satellite galaxies are small, suggesting that stars and gas are being stripped
and deposited at distances of tens of kpc from the central radio galaxy. These
stripped stars may become intracluster stars or form an extended stellar halo
around the radio galaxy, such as those observed around cD galaxies in cluster
cores.Comment: 12 pages, accepted for publication in MNRA
Imprints of Environment on Cluster and Field Late-type Galaxies at z~1
We present a comparison of late-type galaxies (Sa and later) in intermediate
redshift clusters and the field using ACS imaging of four cluster fields:
CL0152-1357, CL1056-0337 (MS1054), CL1604+4304, and CL1604+4321. Concentration,
asymmetry, and clumpiness parameters are calculated for each galaxy in blue
(F606W or F625W) and red (F775W or F814W) filters. Galaxy half-light radii,
disk scale lengths, color gradients, and overall color are compared. We find
marginally significant differences in the asymmetry distributions of spiral and
irregular galaxies in the X-ray luminous and X-ray faint clusters. The massive
clusters contain fewer galaxies with large asymmetries. The physical sizes of
the cluster and field populations are similar; no significant differences are
found in half-light radii or disk scale lengths. The most significant
difference is in rest-frame color. Late-type cluster galaxies are
significantly redder, magnitudes at rest-frame , than their
field counterparts. Moreover, the intermediate-redshift cluster galaxies tend
to have blue inward color gradients, in contrast to the field galaxies, but
similar to late-type galaxies in low redshift clusters. These blue inward color
gradients are likely to be the result of enhanced nuclear star formation rates
relative to the outer disk. Based on the significant rest-frame color
difference, we conclude that late-type cluster members at are not a
pristine infalling field population; some difference in past and/or current
star formation history is already present. This points to high redshift
``groups'', or filaments with densities similar to present-day groups, as the
sites where the first major effects of environment are imprinted.Comment: updated titl
Galaxy protocluster candidates around z ~ 2.4 radio galaxies
We study the environments of 6 radio galaxies at 2.2 < z < 2.6 using
wide-field near-infrared images. We use colour cuts to identify galaxies in
this redshift range, and find that three of the radio galaxies are surrounded
by significant surface overdensities of such galaxies. The excess galaxies that
comprise these overdensities are strongly clustered, suggesting they are
physically associated. The colour distribution of the galaxies responsible for
the overdensity are consistent with those of galaxies that lie within a narrow
redshift range at z ~ 2.4. Thus the excess galaxies are consistent with being
companions of the radio galaxies. The overdensities have estimated masses in
excess of 10^14 solar masses, and are dense enough to collapse into virizalised
structures by the present day: these structures may evolve into groups or
clusters of galaxies. A flux-limited sample of protocluster galaxies with K <
20.6 mag is derived by statistically subtracting the fore- and background
galaxies. The colour distribution of the protocluster galaxies is bimodal,
consisting of a dominant blue sequence, comprising 77 +/- 10% of the galaxies,
and a poorly populated red sequence. The blue protocluster galaxies have
similar colours to local star-forming irregular galaxies (U -V ~ 0.6),
suggesting most protocluster galaxies are still forming stars at the observed
epoch. The blue colours and lack of a dominant protocluster red sequence
implies that these cluster galaxies form the bulk of their stars at z < 3.Comment: Accepted for publication in MNRA
The Nascent Red Sequence at z~2
We present new constraints on the evolution of the early-type galaxy
color-magnitude relation (CMR) based on deep near-infrared imaging of a galaxy
protocluster at z=2.16 obtained using NICMOS on-board the Hubble Space
Telescope. This field contains a spectroscopically confirmed space-overdensity
of Lyman-alpha and H-alpha emitting galaxies which surrounds the powerful radio
galaxy MRC 1138-262. Using these NICMOS data we identify a significant
surface-overdensity (= 6.2x) of red J-H galaxies in the color-magnitude diagram
(when compared with deep NICMOS imaging from the HDF-N and UDF). The
optical-NIR colors of these prospective red-sequence galaxies indicate the
presence of on-going dust-obscured star-formation or recently formed (<~ 1.5
Gyr)stellar populations in a majority of the red galaxies. We measure the slope
and intrinsic scatter of the CMR for three different red galaxy samples
selected by a wide color cut, and using photometric redshifts both with and
without restrictions on rest-frame optical morphology. In all three cases both
the rest-frame slope and intrinsic color scatter are considerably higher
than corresponding values for lower redshift galaxy clusters. These results
suggest that while some relatively quiescent galaxies do exist in this
protocluster both the majority of the galaxy population and hence the
color-magnitude relation are still in the process of forming, as expected.Comment: 8 pages, 7 figures, accepted for publication in ApJ (to appear June
1, 2008, v679n2
Sub-millimeter galaxies as progenitors of compact quiescent galaxies
Three billion years after the big bang (at redshift z=2), half of the most
massive galaxies were already old, quiescent systems with little to no residual
star formation and extremely compact with stellar mass densities at least an
order of magnitude larger than in low redshift ellipticals, their descendants.
Little is known about how they formed, but their evolved, dense stellar
populations suggest formation within intense, compact starbursts 1-2 Gyr
earlier (at 3<z<6). Simulations show that gas-rich major mergers can give rise
to such starbursts which produce dense remnants. Sub-millimeter selected
galaxies (SMGs) are prime examples of intense, gas-rich, starbursts. With a
new, representative spectroscopic sample of compact quiescent galaxies at z=2
and a statistically well-understood sample of SMGs, we show that z=3-6 SMGs are
consistent with being the progenitors of z=2 quiescent galaxies, matching their
formation redshifts and their distributions of sizes, stellar masses and
internal velocities. Assuming an evolutionary connection, their space densities
also match if the mean duty cycle of SMG starbursts is 42 (+40/-29) Myr
(consistent with independent estimates), which indicates that the bulk of stars
in these massive galaxies were formed in a major, early surge of
star-formation. These results suggests a coherent picture of the formation
history of the most massive galaxies in the universe, from their initial burst
of violent star-formation through their appearance as high stellar-density
galaxy cores and to their ultimate fate as giant ellipticals.Comment: ApJ (in press
Massive Ellipticals at High Redshift: NICMOS Imaging of Z~1 Radio Galaxies
We present deep, continuum images of eleven high-redshift (0.811 < z < 1.875)
3CR radio galaxies observed with NICMOS. Our images probe the rest-frame
optical light where stars are expected to dominate the galaxy luminosity. The
rest-frame UV light of eight of these galaxies demonstrates the well-known
``alignment effect''. Most of the radio galaxies have rounder, more symmetric
morphologies at rest-frame optical wavelengths. Here we show the most direct
evidence that in most cases the stellar hosts are normal elliptical galaxies
with de Vaucouleurs law light profiles. For a few galaxies very faint traces of
the UV-bright aligned component are also visible in the infrared images. We
derive both the effective radius and surface-brightness for nine of eleven
sample galaxies by fitting surface-brightness models to them. We find their
sizes are similar to those of local FRII radio source hosts and are in general
larger than other local galaxies. The derived host galaxy luminosities are very
high and lie at the bright end of luminosity functions constructed at similar
redshifts. The galaxies in our sample are also brighter than the rest-frame
size--surface-brightness locus defined by the low-redshift sources. Passive
evolution roughly aligns the z ~ 1 galaxies with the low-redshift samples. The
optical host is sometimes centered on a local minimum in the rest-frame UV
emission, suggesting the presence of substantial dust obscuration. We also see
good evidence of nuclear point sources in three galaxies. Overall, our results
are consistent with the hypothesis that these galaxies have already formed the
bulk of their stars at redshifts greater than z >~ 2, and that the AGN
phenomenon takes place within otherwise normal, perhaps passively evolving,
galaxies. (abridged)Comment: 28 pages, 14 figures, accepted to ApJ. Uses AASTEX and emulateapj
HST and Spitzer imaging of red and blue galaxies at z~2.5: A correlation between size and star formation activity from compact quiescent galaxies to extended star forming galaxies
We present HST NICMOS+ACS and Spitzer IRAC+MIPS observations of 41 galaxies
at 2<z<3.5 in the FIRES MS1054 field with red and blue rest-frame optical
colors. About half of the galaxies are very compact (effective radii r_e < 1
kpc) at rest-frame optical wavelengths, the others are extended (1< r_e < 10
kpc). For reference, 1 kpc corresponds to 0.12 arcsec at z=2.5 in the adopted
cosmology. We separate actively star forming galaxies from quiescent galaxies
by modeling their rest-frame UV-NIR SEDs. The star forming galaxies span the
full range of sizes, while the quiescent galaxies all have r_e<2kpc. In the
redshift range where MIPS 24 micron imaging is a sensitive probe of re-radiated
dust emission (z<2.5), the 24 micron fluxes confirm that the light of the small
quiescent galaxies is dominated by old stars, rather than dust-enshrouded star
formation or AGN activity. The inferred surface mass densities and velocity
dispersions for the quiescent galaxies are very high compared to those in local
galaxies. The galaxies follow a Kormendy relation (between surface brightness
and size) with approximately the same slope as locally, but shifted to brighter
surface brightnesses, consistent with a mean stellar formation redshift of
z_f~5. This paper demonstrates a direct relation between star formation
activity and size at z~2.5, and the existence of a significant population of
massive, extremely dense, old stellar systems without readily identifiable
counterparts in the local universe.Comment: Accepted for publication in Ap
Mass-Selection and the Evolution of the Morphology-Density Relation from z=0.8 to z=0
We examined the morphology-density relations for galaxy samples selected by
luminosity and by mass in each of five massive X-ray clusters from z=0.023 to
0.83 for 674 spectroscopically-confirmed members. Rest-frame optical colors and
visual morphologies were obtained primarily from Hubble Space Telescope images.
Morphology-density relations (MDR) are derived in each cluster from a complete,
luminosity-selected sample of 452 galaxies with a magnitude limit M_V <
M^{*}_{V} + 1. The change in the early-type fraction with redshift matches
previous work for massive clusters of galaxies. We performed a similar
analysis, deriving MDRs for complete, mass-selected samples of 441 galaxies
with a mass-limit of 10^{10.6} M_{\sun}. Our mass limit includes faint objects,
the equivalent of =~1 mag below L^{*} for the red cluster galaxies, and
encompasses =~70% of the stellar mass in cluster galaxies. The MDRs in the
mass-selected sample at densities of Sigma > 50 galaxies Mpc^{-2} are similar
to those in the luminosity-selected sample but show larger early-type
fractions. However, the trend with redshift in the fraction of elliptical and
S0 galaxies with masses > 10^{10.6} M_{\sun} differs significantly between the
mass- and luminosity-selected samples. The clear trend seen in the early-type
fraction from z=0 to z=~ 0.8 is not found in mass-selected samples. The
early-type galaxy fraction changes much less, and is consistent with being
constant at 92% +/- 4% at \Sigma> 500 galaxies Mpc^{-2} and 83 +/- 3% at 50 <
\Sigma < 500 galaxies Mpc^{-2}. This suggests that galaxies of mass lower than
> 10^{10.6} M_{\sun} play a significant role in the evolution of the early-type
fraction in luminosity-selected samples. (Abstract abridged)Comment: 18 pages in emulate ApJ format, with 10 color figures, Accepted to
ApJ. Version updated to reflect published version, includes new references
and a correction to table
Spectroscopy of z ∼ 6 i-dropout galaxies : frequency of Lyα emission and the sizes of Lyα-emitting galaxies
We report on deep spectroscopy, using LRIS on Keck I and FORS2 on the VLT, of a sample of 22 candidate z similar to 6 Lyman break galaxies (LBGs) selected by the (i)775 - z(850) > 1: 3 dropout criterion. Redshifts could be measured for eight objects. These redshifts are all in the range z = 5: 5-6.1, confirming the efficiency of the i775 - z850 color selection technique. Six of the confirmed galaxies show Ly alpha emission. Assuming that the 14 objects without redshifts are z similar to 6 LBGs that lack detectable Ly alpha emission lines, we infer that the fraction of Ly alpha-emitting LBGs with Ly alpha equivalent widths greater than 20 angstrom among z similar to 6 LBGs is approximate to 30%, similar to that found at z similar to 3. Every Ly alpha-emitting object in our sample is compact, with half-light radii r(hl) 97% confidence. We speculate that the small sizes of the Ly alpha-emitting LBGs are due to these objects being less massive than other LBGs at z similar to 6
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